Everyday Science Notes

8/9/2019 Everyday Science Notes

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Eye (anatomy)

I -INTRODUCTION

Eye (anatomy), light-sensitive organ of vision in animals. The eyes of various speciesvary from simple structures that are capable only of differentiating between light anddark to complex organs, such as those of humans and other mammals, that can

distinguish minute variations of shape, color, brightness, and distance. The actualprocess of seeing is performed by the brain rather than by the eye. The function ofthe eye is to translate the electromagnetic vibrations of light into patterns of nerveimpulses that are transmitted to the brain.

II -THE HUMAN EYE

The entire eye, often called the eyeball, is a spherical structure approximately .! cm(about " in) in diameter with a pronounced bulge on its forward surface. The outerpart of the eye is composed of three layers of tissue. The outside layer is the sclera,a protective coating. #t covers about five-sixths of the surface of the eye. $t the frontof the eyeball, it is continuous with the bulging, transparent cornea. The middle layerof the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris,which lies at the front of the eye. The innermost layer is the light-sensitive retina.

The cornea is a tough, five-layered membrane through which light is admitted to theinterior of the eye. %ehind the cornea is a chamber filled with clear, watery fluid, thea&ueous humor, which separates the cornea from the crystalline lens. The lens itselfis a flattened sphere constructed of a large number of transparent fibers arranged inlayers. #t is connected by ligaments to a ringlike muscle, called the ciliary muscle,which surrounds it. The ciliary muscle and its surrounding tissues form the ciliarybody. This muscle, by flattening the lens or making it more nearly spherical, changesits focal length.The pigmented iris hangs behind the cornea in front of the lens, and has a circularopening in its center. The si'e of its opening, the pupil, is controlled by a muscle

around its edge. This muscle contracts or relaxes, making the pupil larger or smaller,to control the amount of light admitted to the eye.%ehind the lens the main body of the eye is filled with a transparent, ellylikesubstance, the vitreous humor, enclosed in a thin sac, the hyaloid membrane. Thepressure of the vitreous humor keeps the eyeball distended.

The retina is a complex layer, composed largely of nerve cells. The light-sensitivereceptor cells lie on the outer surface of the retina in front of a pigmented tissuelayer. These cells take the form of rods or cones packed closely together like matchesin a box. irectly behind the pupil is a small yellow-pigmented spot, the maculalutea, in the center of which is the fovea centralis, the area of greatest visual acuityof the eye. $t the center of the fovea, the sensory layer is composed entirely ofcone-shaped cells. $round the fovea both rod-shaped and cone-shaped cells are

present, with the cone-shaped cells becoming fewer toward the periphery of thesensitive area. $t the outer edges are only rod-shaped cells.

*here the optic nerve enters the eyeball, below and slightly to the inner side of thefovea, a small round area of the retina exists that has no light-sensitive cells. Thisoptic disk forms the blind spot of the eye.

III -FUNCTIONING OF THE EYE

#n general the eyes of all animals resemble simple cameras in that the lens of the


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eye forms an inverted image of obects in front of it on the sensitive retina, whichcorresponds to the film in a camera.+ocusing the eye, as mentioned above, is accomplished by a flattening or thickening(rounding) of the lens. The process is known as accommodation. #n the normal eyeaccommodation is not necessary for seeing distant obects. The lens, when flattenedby the suspensory ligament, brings such obects to focus on the retina. +or nearer

obects the lens is increasingly rounded by ciliary muscle contraction, which relaxesthe suspensory ligament. $ young child can see clearly at a distance as close as .cm (.! in), but with increasing age the lens gradually hardens, so that the limits ofclose seeing are approximately "! cm (about in) at the age of and / cm ("in) at the age of !. #n the later years of life most people lose the ability toaccommodate their eyes to distances within reading or close working range. Thiscondition, known as presbyopia, can be corrected by the use of special convex lensesfor the near range.

0tructural differences in the si'e of the eye cause the defects of hyperopia, orfarsightedness, and myopia, or nearsightedness. 0ee Eyeglasses1 2ision.$s mentioned above, the eye sees with greatest clarity only in the region of thefovea1 due to the neural structure of the retina. The cone-shaped cells of the retinaare individually connected to other nerve fibers, so that stimuli to each individual cellare reproduced and, as a result, fine details can be distinguished. The rodshapedcells, on the other hand, are connected in groups so that they respond to stimuliover a general area.

The rods, therefore, respond to small total light stimuli, but do not have the ability toseparate small details of the visual image. The result of these differences in structureis that the visual field of the eye is composed of a small central area of greatsharpness surrounded by an area of lesser sharpness. #n the latter area, however,the sensitivity of the eye to light is great. $s a result, dim obects can be seen atnight on the peripheral part of the retina when they are invisible to the central part.

The mechanism of seeing at night involves the sensiti'ation of the rod cells by meansof a pigment, called visual purple or rhodopsin, that is formed within the cells.2itamin $ is necessary for the production of visual purple1 a deficiency of this vitaminleads to night blindness. 2isual purple is bleached by the action of light and must bereformed by the rod cells under conditions of darkness. 3ence a person who stepsfrom sunlight into a darkened room cannot see until the pigment begins to form.*hen the pigment has formed and the eyes are sensitive to low levels ofillumination, the eyes are said to be dark-adapted.

$ brownish pigment present in the outer layer of the retina serves to protect thecone cells of the retina from overexposure to light. #f bright light strikes the retina,granules of this brown pigment migrate to the spaces around the cone cells,sheathing and screening them from the light. This action, called light adaptation, has

the opposite effect to that of dark adaptation.0ubectively, a person is not conscious that the visual field consists of a central 'oneof sharpness surrounded by an area of increasing fu''iness.

The reason is that the eyes are constantly moving, bringing first one part of thevisual field and then another to the foveal region as the attention is shifted from oneobect to another. These motions are accomplished by six muscles that move theeyeball upward, downward, to the left, to the right, and obli&uely. The motions of theeye muscles are extremely precise1 the estimation has been made that the eyes can


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be moved to focus on no less than ", distinct points in the visual field. Themuscles of the two eyes, working together, also serve the important function ofconverging the eyes on any point being observed, so that the images of the two eyescoincide. *hen convergence is nonexistent or faulty, double vision results. Themovement of the eyes and fusion of the images also play a part in the visualestimation of si'e and distance.

IV -PROTECTIVE STRUCTURES

0everal structures, not parts of the eyeball, contribute to the protection of the eye.The most important of these are the eyelids, two folds of skin and tissue, upper andlower, that can be closed by means of muscles to form a protective covering over theeyeball against excessive light and mechanical inury.The eyelashes, a fringe of short hairs growing on the edge of either eyelid, act as ascreen to keep dust particles and insects out of the eyes when the eyelids are partlyclosed. #nside the eyelids is a thin protective membrane, the conunctiva, whichdoubles over to cover the visible sclera. Each eye also has a tear gland, or lacrimalorgan, situated at the outside corner of the eye. The salty secretion of these glandslubricates the forward part of the eyeball when the eyelids are closed and flushesaway any small dust particles or other foreign matter on the surface of the eye.4ormally the eyelids of human eyes close by reflex action about every six seconds,but if dust reaches the surface of the eye and is not washed away, the eyelids blinkoftener and more tears are produced. 5n the edges of the eyelids are a number ofsmall glands, the 6eibomian glands, which produce a fatty secretion that lubricatesthe eyelids themselves and the eyelashes. The eyebrows, located above each eye,also have a protective function in soaking up or deflecting perspiration or rain andpreventing the moisture from running into the eyes. The hollow socket in the skull inwhich the eye is set is called the orbit. The bony edges of the orbit, the frontal bone,and the cheekbone protect the eye from mechanical inury by blows or collisions.

V -COMPARATIVE ANATOMY

The simplest animal eyes occur in the cnidarians and ctenophores, phyla comprising

the ellyfish and somewhat similar primitive animals. These eyes, known as pigmenteyes, consist of groups of pigment cells associated with sensory cells and oftencovered with a thickened layer of cuticle that forms a kind of lens. 0imilar eyes,usually having a somewhat more complex structure, occur in worms, insects, andmollusks.

Two kinds of image-forming eyes are found in the animal world, single andcompound eyes. The single eyes are essentially similar to the human eye, thoughvarying from group to group in details of structure. The lowest species to developsuch eyes are some of the large ellyfish. 7ompound eyes, confined to the arthropods(see $rthropod), consist of a faceted lens, each facet of which forms a separateimage on a retinal cell, creating a moasic field. #n some arthropods the structure ismore sophisticated, forming a combined image.

The eyes of other vertebrates are essentially similar to human eyes, althoughimportant modifications may exist. The eyes of such nocturnal animals as cats, owls,and bats are provided only with rod cells, and the cells are both more sensitive andmore numerous than in humans. The eye of a dolphin has 8 times as many rodcells as a human eye, enabling it to see in deep water. The eyes of most fish have aflat cornea and a globular lens and are hence particularly adapted for seeing closeobects. %irds9 eyes are elongated from front to back, permitting larger images ofdistant obects to be formed on the retina.


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VI -EYE DISEASES

Eye disorders may be classified according to the part of the eye in which thedisorders occur.

The most common disease of the eyelids is hordeolum, known commonly as a sty,

which is an infection of the follicles of the eyelashes, usually caused by infection bystaphylococci. #nternal sties that occur inside the eyelid and not on its edge aresimilar infections of the lubricating 6eibomian glands. $bscesses of the eyelids aresometimes the result of penetrating wounds. 0everal congenital defects of theeyelids occasionally occur, including coloboma, or cleft eyelid, and ptosis, a droopingof the upper lid. $mong ac&uired defects are symblepharon, an adhesion of the innersurface of the eyelid to the eyeball, which is most fre&uently the result of burns.Entropion, the turning of the eyelid inward toward the cornea, and ectropion, theturning of the eyelid outward, can be caused by scars or by spasmodic muscularcontractions resulting from chronic irritation.The eyelids also are subect to several diseases of the skin such as ec'ema and acne,and to both benign and malignant tumors. $nother eye disease is infection of theconunctiva, the mucous membranes covering the inside of the eyelids and theoutside of the eyeball. 0ee 7onunctivitis1 Trachoma.isorders of the cornea, which may result in a loss of transparency and impairedsight, are usually the result of inury but may also occur as a secondary result ofdisease1 for example, edema, or swelling, of the cornea sometimes accompaniesglaucoma.

The choroid, or middle coat of the eyeball, contains most of the blood vessels of theeye1 it is often the site of secondary infections from toxic conditions and bacterialinfections such as tuberculosis and syphilis. 7ancer may develop in the choroidaltissues or may be carried to the eye from malignancies elsewhere in the body. Thelight-sensitive retina, which lies ust beneath the choroid, also is subect to the sametype of infections. The cause of retrolental fibroplasia, however:a disease of

premature infants that causes retinal detachment and partial blindness:is unknown.;etinal detachment may also follow cataract surgery.


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experiments in preserving human corneas by free'ing give hope of extending itsuseful life for months. Eye banks also preserve and distribute vitreous humor, theli&uid within the larger chamber of the eye, for use in treatment of detached retinas.The first eye bank was opened in 4ew ?ork 7ity in "@/!. The Eye-%ank $ssociation of$merica, in ;ochester, 4ew ?ork, acts as a clearinghouse for information.

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I -INTRODUCTION

+ingerprinting, method of identification using the impression made by the minuteridge formations or patterns found on the fingertips. 4o two persons have exactly thesame arrangement of ridge patterns, and the patterns of any one individual remainunchanged through life. To obtain a set of fingerprints, the ends of the fingers areinked and then pressed or rolled one by one on some receiving surface. +ingerprintsmay be classified and filed on the basis of the ridge patterns, setting up anidentification system that is almost infallible.

II -HISTORY

The first recorded use of fingerprints was by the ancient $ssyrians and 7hinese forthe signing of legal documents. Brobably the first modern study of fingerprints wasmade by the 7'ech physiologist =ohannes Evengelista Burkine, who in ">proposed a system of classification that attracted little attention. The use offingerprints for identification purposes was proposed late in the "@th century by the%ritish scientist 0ir +rancis Calton, who wrote a detailed study of fingerprints inwhich he presented a new classification system using prints of all ten fingers, whichis the basis of identification systems still in use. #n the ">@s the police in %engal,#ndia, under the %ritish police official 0ir Edward ;ichard 3enry, began usingfingerprints to identify criminals. $s assistant commissioner of metropolitan police,3enry established the first %ritish fingerprint files in " million people. #n "@@@ the +%# began fulloperation of the #ntegrated $utomated +ingerprint #dentification 0ystem (#$+#0), acomputeri'ed system that stores digital images of fingerprints for more than


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million individuals, along with each individual9s criminal history if one exists. Dsing#$+#0, authorities can conduct automated searches to identify people from theirfingerprints and determine whether they have a criminal record. The system alsogives state and local law enforcement agencies the ability to electronically transmitfingerprint information to the +%#. The implementation of #$+#0 represented abreakthrough in crimefighting by reducing the time needed for fingerprint

identification from weeks to minutes or hours.AAAAAAAAAAAAAAAAAAThe difference between a successful person and others is not a lack of strength,notlack of knowledge, but rather in a lack of will.

#magination is more important than knowledge. +or knowledge is limited to all wenow know and understand, while imagination embraces the entire world, and allthere ever will be to know and understand.$lbert EinsteinIn$"a"e% Ra%aton

#nfrared ;adiation, emission of energy as electromagnetic waves in the portion of thespectrum ust beyond the limit of the red portion of visible radiation (seeElectromagnetic ;adiation). The wavelengths of infrared radiation are shorter thanthose of radio waves and longer than those of light waves. They range betweenapproximately "- and "- (about ./ and ./ in).

#nfrared radiation may be detected as heat, and instruments such as bolometers areused to detect it. 0ee ;adiation1 0pectrum.#nfrared radiation is used to obtain pictures of distant obects obscured byatmospheric ha'e, because visible light is scattered by ha'e but infrared radiation isnot. The detection of infrared radiation is used by astronomers to observe stars andnebulas that are invisible in ordinary light or that emit radiation in the infraredportion of the spectrum.

$n opa&ue filter that admits only infrared radiation is used for very precise infraredphotographs, but an ordinary orange or light-red filter, which will absorb blue andviolet light, is usually sufficient for most infrared pictures. eveloped about ">>,infrared photography has today become an important diagnostic tool in medicalscience as well as in agriculture and industry. Dse of infrared techni&ues revealspathogenic conditions that are not visible to the eye or recorded on F-ray plates.;emote sensing by means of aerial and orbital infrared photography has been usedto monitor crop conditions and insect and disease damage to large agricultural areas,and to locate mineral deposits. 0ee $erial 0urvey1 0atellite, $rtificial. #n industry,infrared spectroscopy forms an increasingly important part of metal and alloyresearch, and infrared photography is used to monitor the &uality of products. 0eealso BhotographyG Bhotographic +ilms.

#nfrared devices such as those used during *orld *ar ## enable sharpshooters to seetheir targets in total visual darkness. These instruments consist essentially of aninfrared lamp that sends out a beam of infrared radiation, often referred to as blacklight, and a telescope receiver that picks up returned radiation from the obect andconverts it to a visible image.AAAAAAAAAAAAAAAAAAThe difference between a successful person and others is not a lack of strength,notlack of knowledge, but rather in a lack of will.


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#magination is more important than knowledge. +or knowledge is limited to all wenow know and understand, while imagination embraces the entire world, and allthere ever will be to know and understand.$lbert Einstein

Deo&y"'on*e A%

I -INTRODUCTIONeoxyribonucleic $cid (4$), genetic material of all cellular organisms and mostviruses. 4$ carries the information needed to direct protein synthesis andreplication. Brotein synthesis is the production of the proteins needed by the cell orvirus for its activities and development. ;eplication is the process by which 4$copies itself for each descendant cell or virus, passing on the information needed forprotein synthesis. #n most cellular organisms, 4$ is organi'ed on chromosomeslocated in the nucleus of the cell.

II -STRUCTURE

$ molecule of 4$ consists of two chains, strands composed of a large number ofchemical compounds, called nucleotides, linked together to form a chain. Thesechains are arranged like a ladder that has been twisted into the shape of a windingstaircase, called a double helix. Each nucleotide consists of three unitsG a sugarmolecule called deoxyribose, a phosphate group, and one of four different nitrogen-containing compounds called bases. The four bases are adenine ($), guanine (C),thymine (T), and cytosine (7). The deoxyribose molecule occupies the center positionin the nucleotide, flanked by a phosphate group on one side and a base on the other.The phosphate group of each nucleotide is also linked to the deoxyribose of theadacent nucleotide in the chain. These linked deoxyribose-phosphate subunits formthe parallel side rails of the ladder. The bases face inward toward each other, formingthe rungs of the ladder.

The nucleotides in one 4$ strand have a specific association with the correspondingnucleotides in the other 4$ strand. %ecause of the chemical affinity of the bases,nucleotides containing adenine are always paired with nucleotides containingthymine, and nucleotides containing cytosine are always paired with nucleotidescontaining guanine. The complementary bases are oined to each other by weakchemical bonds called hydrogen bonds.#n "@! $merican biochemist =ames . *atson and %ritish biophysicist +rancis 7rickpublished the first description of the structure of 4$. Their model proved to be soimportant for the understanding of protein synthesis, 4$ replication, and mutationthat they were awarded the "@ 4obel Bri'e for physiology or medicine for theirwork.

III -PROTEIN SYNTHESIS

4$ carries the instructions for the production of proteins. $ protein is composed ofsmaller molecules called amino acids, and the structure and function of the protein isdetermined by the se&uence of its amino acids. The se&uence of amino acids, in turn,is determined by the se&uence of nucleotide bases in the 4$. $ se&uence of threenucleotide bases, called a triplet, is the genetic code word, or codon, that specifies aparticular amino acid. +or instance, the triplet C$7 (guanine, adenine, and cytosine)is the codon for the amino acid leucine, and the triplet 7$C (cytosine, adenine, andguanine) is the codon for the amino acid valine. $ protein consisting of " aminoacids is thus encoded by a 4$ segment consisting of nucleotides. 5f the two


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polynucleotide chains that form a 4$ molecule, only one strand contains theinformation needed for the production of a given amino acid se&uence. The otherstrand aids in replication.

Brotein synthesis begins with the separation of a 4$ molecule into two strands. #n aprocess called transcription, a section of one strand acts as a template, or pattern, to

produce a new strand called messenger ;4$ (m;4$). The m;4$ leaves the cellnucleus and attaches to the ribosomes, speciali'ed cellular structures that are thesites of protein synthesis. $mino acids are carried to the ribosomes by another typeof ;4$, called transfer ;4$ (t;4$). #n a process called translation, the amino acidsare linked together in a particular se&uence, dictated by the m;4$, to form aprotein.

$ gene is a se&uence of 4$ nucleotides that specify the order of amino acids in aprotein via an intermediary m;4$ molecule. 0ubstituting one 4$ nucleotide withanother containing a different base causes all descendant cells or viruses to have thealtered nucleotide base se&uence. $s a result of the substitution, the se&uence ofamino acids in the resulting protein may also be changed. 0uch a change in a 4$molecule is called a mutation. 6ost mutations are the result of errors in thereplication process. Exposure of a cell or virus to radiation or to certain chemicalsincreases the likelihood of mutations.

IV -REP+ICATION#n most cellular organisms, replication of a 4$ molecule takes place in the cellnucleus and occurs ust before the cell divides. ;eplication begins with the separationof the two polynucleotide chains, each of which then acts as a template for theassembly of a new complementary chain. $s the old chains separate, each nucleotidein the two chains attracts a complementary nucleotide that has been formed earlierby the cell. The nucleotides are oined to one another by hydrogen bonds to form therungs of a new 4$ molecule. $s the complementary nucleotides are fitted intoplace, an en'yme called 4$ polymerase links them together by bonding the

phosphate group of one nucleotide to the sugar molecule of the adacent nucleotide,forming the side rail of the new 4$ molecule. This process continues until a newpolynucleotide chain has been formed alongside the old one, forming a new double-helix molecule.

V -TOO+S AND PROCEDURES

0everal tools and procedures facilitate are used by scientists for the study andmanipulation of 4$. 0peciali'ed en'ymes, called restriction en'ymes, found inbacteria act like molecular scissors to cut the phosphate backbones of 4$ moleculesat specific base se&uences. 0trands of 4$ that have been cut with restrictionen'ymes are left with single-stranded tails that are called sticky ends, because theycan easily realign with tails from certain other 4$ fragments. 0cientists takeadvantage of restriction en'ymes and the sticky ends generated by these en'ymes to

carry out recombinant 4$ technology, or genetic engineering. This technologyinvolves removing a specific gene from one organism and inserting the gene intoanother organism.

$nother tool for working with 4$ is a procedure called polymerase chain reaction(B7;). This procedure uses the en'yme 4$ polymerase to make copies of 4$strands in a process that mimics the way in which 4$ replicates naturally withincells. 0cientists use B7; to obtain vast numbers of copies of a given segment of4$.


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4$ fingerprinting, also called 4$ typing, makes it possible to compare samples of4$ from various sources in a manner that is analogous to the comparison offingerprints. #n this procedure, scientists use restriction en'ymes to cleave a sampleof 4$ into an assortment of fragments. 0olutions containing these fragments areplaced at the surface of a gel to which an electric current is applied. The electric

current causes the 4$ fragments to move through the gel. %ecause smallerfragments move more &uickly than larger ones, this process, called electrophoresis,separates the fragments according to their si'e. The fragments are then marked withprobes and exposed on F-ray film, where they form the 4$ fingerprint:a pattern ofcharacteristic black bars that is uni&ue for each type of 4$.$ procedure called 4$ se&uencing makes it possible to determine the precise order,or se&uence, of nucleotide bases within a fragment of 4$. 6ost versions of 4$se&uencing use a techni&ue called primer extension, developed by %ritish molecularbiologist +rederick 0anger.#n primer extension, specific pieces of 4$ are replicated and modified, so that each4$ segment ends in a fluorescent form of one of the four nucleotide bases. 6odern4$ se&uencers, pioneered by $merican molecular biologist


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is that vultures of the $mericas are more closely related to storks than to thevultures of Europe, $sia, or $frica (see 7lassification).

Techni&ues of 4$ manipulation are used in farming, in the form of geneticengineering and biotechnology. 0trains of crop plants to which genes have beentransferred may produce higher yields and may be more resistant to insects. 7attle

have been similarly treated to increase milk and beef production, as have hogs, toyield more meat with less fat.

VII -SOCIA+ ISSUES

espite the many benefits offered by 4$ technology, some critics argue that itsdevelopment should be monitored closely. 5ne fear raised by such critics is that 4$fingerprinting could provide a means for employers to discriminate against membersof various ethnic groups. 7ritics also fear that studies of people9s 4$ could permitinsurance companies to deny health insurance to those people at risk for developingcertain diseases. The potential use of 4$ technology to alter the genes of embryosis a particularly controversial issue.

The use of 4$ technology in agriculture has also sparked controversy. 0ome people&uestion the safety, desirability, and ecological impact of genetically altered cropplants. #n addition, animal rights groups have protested against the geneticengineering of farm animals.

espite these and other areas of disagreement, many people agree that 4$technology offers a mixture of benefits and potential ha'ards. 6any experts alsoagree that an informed public can help assure that 4$ technology is used wisely.AAAAAAAAAAAAAAAAAAThe difference between a successful person and others is not a lack of strength,notlack of knowledge, but rather in a lack of will.

#magination is more important than knowledge. +or knowledge is limited to all we

now know and understand, while imagination embraces the entire world, and allthere ever will be to know and understand.$lbert Einstein

B*oo%

I -INTRODUCTION%lood, vital fluid found in humans and other animals that provides importantnourishment to all body organs and tissues and carries away waste materials.0ometimes referred to as Hthe river of life,I blood is pumped from the heart througha network of blood vessels collectively known as the circulatory system.

$n adult human has about ! to liters (" to gal) of blood, which is roughly 8 to >percent of total body weight. #nfants and children have comparably lower volumes ofblood, roughly proportionate to their smaller si'e. The volume of blood in anindividual fluctuates. uring dehydration, for example while running a marathon,blood volume decreases. %lood volume increases in circumstances such aspregnancy, when the mother9s blood needs to carry extra oxygen and nutrients tothe baby.

II -RO+E OF B+OOD


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%lood carries oxygen from the lungs to all the other tissues in the body and, in turn,carries waste products, predominantly carbon dioxide, back to the lungs where theyare released into the air. *hen oxygen transport fails, a person dies within a fewminutes. +ood that has been processed by the digestive system into smallercomponents such as proteins, fats, and carbohydrates is also delivered to the tissuesby the blood. These nutrients provide the materials and energy needed by individual

cells for metabolism, or the performance of cellular function. *aste productsproduced during metabolism, such as urea and uric acid, are carried by the blood tothe kidneys, where they are transferred from the blood into urine and eliminatedfrom the body. #n addition to oxygen and nutrients, blood also transports specialchemicals, called hormones, that regulate certain body functions. The movement ofthese chemicals enables one organ to control the function of another even thoughthe two organs may be located far apart. #n this way, the blood acts not ust as ameans of transportation but also as a communications system.

The blood is more than a pipeline for nutrients and information1 it is also responsiblefor the activities of the immune system, helping fend off infection and fight disease.#n addition, blood carries the means for stopping itself from leaking out of the bodyafter an inury. The blood does this by carrying special cells and proteins, known asthe coagulation system, that start to form clots within a matter of seconds afterinury.

%lood is vital to maintaining a stable body temperature1 in humans, bodytemperature normally fluctuates within a degree of 8.J 7 (@>.J +). 3eatproduction and heat loss in various parts of the body are balanced out by heattransfer via the bloodstream. This is accomplished by varying the diameter of bloodvessels in the skin. *hen a person becomes overheated, the vessels dilate and anincreased volume of blood flows through the skin. 3eat dissipates through the skin,effectively lowering the body temperature. The increased flow of blood in the skinmakes the skin appear pink or flushed. *hen a person is cold, the skin may becomepale as the vessels narrow, diverting blood from the skin and reducing heat loss.

III -COMPOSITION OF B+OOD

$bout !! percent of the blood is composed of a li&uid known as plasma. The rest ofthe blood is made of three maor types of cellsG red blood cells (also known aserythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

A P*a,ma

Blasma consists predominantly of water and salts. The kidneys carefully maintain thesalt concentration in plasma because small changes in its concentration will causecells in the body to function improperly. #n extreme conditions this can result insei'ures, coma, or even death. The p3 of plasma, the common measurement of theplasma9s acidity, is also carefully controlled by the kidneys within the neutral range of.> to 8.8. Blasma also contains other small molecules, including vitamins, minerals,

nutrients, and waste products. The concentrations of all of these molecules must becarefully regulated.

Blasma is usually yellow in color due to proteins dissolved in it. 3owever, after aperson eats a fatty meal, that person9s plasma temporarily develops a milky color asthe blood carries the ingested fats from the intestines to other organs of the body.

Blasma carries a large number of important proteins, including albumin, gammaglobulin, and clotting factors. $lbumin is the main protein in blood. #t helps regulate


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the water content of tissues and blood. Camma globulin is composed of tens ofthousands of uni&ue antibody molecules. $ntibodies neutrali'e or help destroyinfectious organisms. Each antibody is designed to target one specific invadingorganism. +or example, chicken pox antibody will target chicken pox virus, but willleave an influen'a virus unharmed. 7lotting factors, such as fibrinogen, are involvedin forming blood clots that seal leaks after an inury. Blasma that has had the clotting

factors removed is called serum. %oth serum and plasma are easy to store and havemany medical uses.

B -Re% B*oo% Ce**,

;ed blood cells make up almost /! percent of the blood volume. Their primaryfunction is to carry oxygen from the lungs to every cell in the body. ;ed blood cellsare composed predominantly of a protein and iron compound, called hemoglobin,that captures oxygen molecules as the blood moves through the lungs, giving bloodits red color. $s blood passes through body tissues, hemoglobin then releases theoxygen to cells throughout the body. ;ed blood cells are so packed with hemoglobinthat they lack many components, including a nucleus, found in other cells.

The membrane, or outer layer, of the red blood cell is flexible, like a soap bubble,and is able to bend in many directions without breaking. This is important becausethe red blood cells must be able to pass through the tiniest blood vessels, thecapillaries, to deliver oxygen wherever it is needed. The capillaries are so narrow thatthe red blood cells, normally shaped like a disk with a concave top and bottom, mustbend and twist to maneuver single file through them.

C -B*oo% Ty#e

There are several types of red blood cells and each person has red blood cells of ustone type. %lood type is determined by the occurrence or absence of substances,known as recognition markers or antigens, on the surface of the red blood cell. Type$ blood has ust marker $ on its red blood cells while type % has only marker %. #fneither $ nor % markers are present, the blood is type 5. #f both the $ and %

markers are present, the blood is type $%. $nother marker, the ;h antigen (alsoknown as the ;h factor), is present or absent regardless of the presence of $ and %markers. #f the ;h marker is present, the blood is said to be ;h positive, and if it isabsent, the blood is ;h negative. The most common blood type is $ positive:that is,blood that has an $ marker and also an ;h marker. 6ore than additional red bloodcell types have been discovered.

%lood typing is important for many medical reasons. #f a person loses a lot of blood,that person may need a blood transfusion to replace some of the lost red blood cells.0ince everyone makes antibodies against substances that are foreign, or not of theirown body, transfused blood must be matched so as not to contain these substances.+or example, a person who is blood type $ positive will not make antibodies againstthe $ or ;h markers, but will make antibodies against the % marker, which is not on

that person9s own red blood cells. #f blood containing the % marker (from types %positive, % negative, $% positive, or $% negative) is transfused into this person, thenthe transfused red blood cells will be rapidly destroyed by the patient9s anti-%antibodies.

#n this case, the transfusion will do the patient no good and may even result inserious harm. +or a successful blood transfusion into an $ positive blood typeindividual, blood that is type 5 negative, 5 positive, $ negative, or $ positive isneeded because these blood types will not be attacked by the patient9s anti-%


13/46

antibodies.

D -.te B*oo% Ce**,

*hite blood cells only make up about " percent of blood, but their small numberbelies their immense importance. They play a vital role in the body9s immune system:the primary defense mechanism against invading bacteria, viruses, fungi, and

parasites. They often accomplish this goal through direct attack, which usuallyinvolves identifying the invading organism as foreign, attaching to it, and thendestroying it. This process is referred to as phagocytosis.

*hite blood cells also produce antibodies, which are released into the circulatingblood to target and attach to foreign organisms. $fter attachment, the antibody mayneutrali'e the organism, or it may elicit help from other immune system cells todestroy the foreign substance. There are several varieties of white blood cells,including neutrophils, monocytes, and lymphocytes, all of which interact with oneanother and with plasma proteins and other cell types to form the complex andhighly effective immune system.

E -P*ate*et, an% C*ottn!

The smallest cells in the blood are the platelets, which are designed for a singlepurpose:to begin the process of coagulation, or forming a clot, whenever a bloodvessel is broken. $s soon as an artery or vein is inured, the platelets in the area ofthe inury begin to clump together and stick to the edges of the cut. They alsorelease messengers into the blood that perform a variety of functionsG constrictingthe blood vessels to reduce bleeding, attracting more platelets to the area to enlargethe platelet plug, and initiating the work of plasma-based clotting factors, such asfibrinogen. Through a complex mechanism involving many steps and many clottingfactors, the plasma protein fibrinogen is transformed into long, sticky threads offibrin. Together, the platelets and the fibrin create an intertwined meshwork thatforms a stable clot. This self-sealing aspect of the blood is crucial to survival.

IV -PRODUCTION AND E+IMINATION OF B+OOD CE++S%lood is produced in the bone marrow, a tissue in the central cavity inside almost allof the bones in the body. #n infants, the marrow in most of the bones is activelyinvolved in blood cell formation. %y later adult life, active blood cell formationgradually ceases in the bones of the arms and legs and concentrates in the skull,spine, ribs, and pelvis.

;ed blood cells, white blood cells, and platelets grow from a single precursor cell,known as a hematopoietic stem cell. ;emarkably, experiments have suggested thatas few as " stem cells can, in four weeks, multiply into trillion red blood cells, billion white blood cells, and ". trillion platelets:enough to replace every blood cellin the body.

;ed blood cells have the longest average life span of any of the cellular elements ofblood. $ red blood cell lives " to " days after being released from the marrowinto the blood. 5ver that period of time, red blood cells gradually age. 0pent cells areremoved by the spleen and, to a lesser extent, by the liver. The spleen and the liveralso remove any red blood cells that become damaged, regardless of their age. Thebody efficiently recycles many components of the damaged cells, including parts ofthe hemoglobin molecule, especially the iron contained within it.

The maority of white blood cells have a relatively short life span. They may survive


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only "> to hours after being released from the marrow. 3owever, some of thewhite blood cells are responsible for maintaining what is called immunologic memory.These memory cells retain knowledge of what infectious organisms the body haspreviously been exposed to. #f one of those organisms returns, the memory cellsinitiate an extremely rapid response designed to kill the foreign invader. 6emory cellsmay live for years or even decades before dying.

6emory cells make immuni'ations possible. $n immuni'ation, also called avaccination or an inoculation, is a method of using a vaccine to make the humanbody immune to certain diseases. $ vaccine consists of an infectious agent that hasbeen weakened or killed in the laboratory so that it cannot produce disease wheninected into a person, but can spark the immune system to generate memory cellsand antibodies specific for the infectious agent. #f the infectious agent should everinvade that vaccinated person in the future, these memory cells will direct the cellsof the immune system to target the invader before it has the opportunity to causeharm.

Blatelets have a life span of seven to ten days in the blood. They either participate inclot formation during that time or, when they have reached the end of their lifetime,are eliminated by the spleen and, to a lesser extent, by the liver.

V -B+OOD DISEASES

6any diseases are caused by abnormalities in the blood. These diseases arecategori'ed by which component of the blood is affected.

A -Re% B*oo% Ce** D,ea,e,

5ne of the most common blood diseases worldwide is anemia, which is characteri'edby an abnormally low number of red blood cells or low levels of hemoglobin. 5ne ofthe maor symptoms of anemia is fatigue, due to the failure of the blood to carryenough oxygen to all of the tissues.

The most common type of anemia, iron-deficiency anemia, occurs because themarrow fails to produce sufficient red blood cells. *hen insufficient iron is availableto the bone marrow, it slows down its production of hemoglobin and red blood cells.#n the Dnited 0tates, iron deficiency occurs most commonly due to poor nutrition. #nother areas of the world, however, the most common causes of iron-deficiencyanemia are certain infections that result in gastrointestinal blood loss and theconse&uent chronic loss of iron. $dding supplemental iron to the diet is oftensufficient to cure iron-deficiency anemia.

0ome anemias are the result of increased destruction of red blood cells, as in thecase of sickle-cell anemia, a genetic disease most common in persons of $fricanancestry. The red blood cells of sickle-cell patients assume an unusual crescentshape, causing them to become trapped in some blood vessels, blocking the flow of

other blood cells to tissues and depriving them of oxygen.

B -.te B*oo% Ce** D,ea,e,0ome white blood cell diseases are characteri'ed by an insufficient number of whiteblood cells. This can be caused by the failure of the bone marrow to produceade&uate numbers of normal white blood cells, or by diseases that lead to thedestruction of crucial white blood cells. These conditions result in severe immunedeficiencies characteri'ed by recurrent infections.


15/46


16/46

there are also some significant differences, typically at the cellular level. +orexample, fish, amphibians, and reptiles possess red blood cells that have a nucleus,unlike the red blood cells of mammals. The immune system of invertebrates is moreprimitive than that of vertebrates, lacking the functionality associated with the whiteblood cell and antibody system found in mammals. 0ome arctic fish species produceproteins in their blood that act as a type of antifree'e, enabling them to survive in

environments where the blood of other animals would free'e. 4onetheless, theessential transportation, communication, and protection functions that make bloodessential to the continuation of life occur throughout much of the animal kingdom.AAAAAAAAAAAAAAAAAAThe difference between a successful person and others is not a lack of strength,notlack of knowledge, but rather in a lack of will.

#magination is more important than knowledge. +or knowledge is limited to all wenow know and understand, while imagination embraces the entire world, and allthere ever will be to know and understand.$lbert EinsteinEn/"onmenta* E$$et, o$ t.e Fo,,* Fe* A!e

5ver the last two centuries, human activity has transformed the chemistry of Earth9swater and air, altered the face of Earth itself, and rewoven the web of life. *hy hasthis time period, more than any other, brought so much widespread environmentalchangeK The reasons are many and complex. %ut a maor influence surely is the useof fossil fuels, which has made far more energy available to more people than hadever been available before.

%y "@@, humans were using about > times as much energy as was being used in">. The great maority of this energy was derived from fossil fuels. The availabilityand use of this new energy source has allowed people to produce more and consumemore. #ndirectly, this energy source caused a rapid increase in population as peopledeveloped much more efficient means of agriculture:such as mechani'ed farming:

that re&uired the use of fossil fuels. #mproved farming techni&ues brought about anincrease in food supply, which fostered the population growth. %y the end of the"@@s, the human population was about six times what it was in ">. *idespreadchanges to the environment resulted from other factors as well. The breakneck paceof urbani'ation is a factor, as is the e&ually di''ying speed of technological change.4o less important a factor in environmental change is the heightened emphasis ofmodern governments on economic growth. $ll of these trends are interrelated, eachone helping to advance the others. Together, they have shaped the evolution ofhuman society in modern times. These growth trends have recast the relationshipsbetween humanity and other inhabitants of Earth.

+or hundreds of thousands of years, human beings and their predecessors have bothdeliberately and accidentally altered their environments. %ut only recently, with the

harnessing of fossil fuels, has humankind ac&uired the power to effect thoroughchanges on air, water, soils, plants, and animals. $rmed with fossil fuels, people havechanged the environment in ways they never had in pre-modern times:for example,devastating natural habitats and wildlife with oil spills. Beople have also been able tobring about environmental change much more rapidly, through acceleration of oldactivities such as deforestation.

O"!n, o$ Fo,,* Fe*,

+ossil fuels include coal, natural gas, and petroleum (also known as oil or crude oil),


17/46

which are the petrified and li&uefied remains of millions of years9 accumulation ofdecayed plant life. *hen fossil fuels are burned, their chemical energy becomes heatenergy, which, by means of machines such as engines and turbines, is converted intomechanical or electrical energy.7oal first became an important industrial fuel during the ""th and "th centuries in7hina, where iron manufacturing consumed great &uantities of this resource. The

first maor usage of coal as a domestic fuel began in "th-century !@, and the firstlarge oil fields were tapped in southeastern Texas in "@". The world9s biggest oilfields were accessed in the "@/s in 0audi $rabia and in the "@s in 0iberia. *hydid oil overshadow coal as the fuel of choiceK 5il has certain advantages over coal. #tis more efficient than coal, providing more energy per unit of weight than coal does.5il also causes less pollution and works better in small engines. 5il is less plentifulthan coal, however. *hen the world runs low on oil, copious supplies of coal willremain available.

Mo%e"n A" Po**ton

The outermost layer of the Earth9s living environment is the atmosphere, a mixtureof gases surrounding the planet. The atmosphere contains a thin layer called o'one,which protects all life on Earth from harmful ultraviolet radiation from the 0un. +ormost of human history, people had very little effect on the atmosphere. +or manythousands of years, humans routinely burned vegetation, causing some intermittentair pollution. #n ancient times, the smelting of ores, such as copper ore, releasedmetals that traveled in the atmosphere from the shores of the 6editerranean 0ea asfar as Creenland. *ith the development of fossil fuels, however, much more intenseair pollution began to trouble humanity.

%efore widespread use of fossil fuels, air pollution typically affected cities more thanit did rural areas because of the concentration of combustion in cities. Beople in cold-climate urban areas kept warm by burning wood, but local wood supplies were soonexhausted. $s a result of the limited supply, wood became expensive. Beople thenburned comparatively little amounts of wood and heated their homes less. The firstcity to resolve this problem was th century introduced coal to industry.The resultant growth from the #ndustrial ;evolution meant more steam engines,more factory chimneys, and, thus, more air pollution. 0kies darkened in the

industrial heartlands of %ritain, %elgium, Cermany, and the Dnited 0tates. 7ities thatcombined energy-intensive industries such as iron and steel manufacturing, andcoal-heated buildings, were routinely shrouded in smoke and bathed in sulfurdioxide. Bittsburgh, Bennsylvania, one of the Dnited 0tates9 maor industrial cities atthe time, was sometimes referred to as H3ell with the lid taken off.I The coalconsumption of some industries was so great that it could pollute the skies overentire regions, as was the case in the ;uhr region in Cermany and around 3anshin,the area near Lsaka, =apan.


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Ea"*y A" Po**ton Cont"o*

Efforts at smoke abatement were largely ineffective until about "@/, so residents ofindustrial cities and regions suffered the conse&uences of life with polluted air. uringthe 2ictorian $ge in England, dusting household surfaces twice a day to keep up withthe dustfall was not uncommon. ;esidents of industrial cities witnessed the loss ofpine trees and some wildlife, due to the high levels of sulfur dioxide. These people

suffered rates of pneumonia and bronchitis far higher than those of their ancestors,their relatives living elsewhere, or their descendants.

$fter "@/, leaders of industrial cities and regions managed to reduce the severity ofcoal-based air pollution. 0t.


19/46

saw the construction of still larger dams in these countries, as well as in 7entral $sia,7hina, and elsewhere. $fter the "@s, dams built for irrigation also served togenerate hydroelectric power. %etween "@/! and "@>, most of the worldMs riversthat had met engineers9 criteria for suitability had ac&uired dams.

%ecause they provided electric power as well as irrigation water, dams made life

easier for millions of people. 7onvenience came at a price, however, as damschanged established water ecosystems that had developed over the course ofcenturies. #n the 7olumbia ;iver in western 4orth $merica, for example, salmonpopulations suffered because dams blocked the annual migrations of the salmon. #nEgypt, where a large dam spanned the 4ile at$swan after "@8", many humans and animals paid the price. 6editerranean sardinesdied and the fisherman who caught these fish lost their business. +armers had toresort to chemical fertili'ers because the dam prevented the 4ile9s spring floodingand the resultant annual coating of fertile silt on land along the river. #n addition,many Egyptians who drank 4ile water, which carried increasing amounts of fertili'errunoff, experienced negative health effects. #n 7entral $sia, the $ral 0ea paid theprice. $fter "@ this sea shrank because the waters that fed into it were diverted toirrigate cotton fields.

;iver water alone did not suffice to meet the water needs of agriculture and cities.Croundwater in many parts of the world became an essential source of water. Thissource was available at low cost, because fossil fuels made pumping much easier. +orexample, after "@ an economy based on grain and livestock emerged on the 3ighBlains, from Texas to the akotas. This economy drew water from the 5gallala$&uifer, a vast underground reservoir. To meet the drinking, washing, and industrialneeds of their growing populations, cities such as %arcelona, 0pain1 %eiing, 7hina1and 6exico 7ity, pumped up groundwater. %eiing and 6exico 7ity began sinkingslowly into the ground as they pumped out much of their underlying water. $sgroundwater supplies dwindled, both cities found they needed to bring water in fromgreat distances. %y "@@@ humanity was using about times as much fresh water as

was used in ">.

4ot only was the water use increasing, but more of it was becoming polluted byhuman use. *hile water pollution had long existed in river water that flowed throughcities, such as the 0eine in Baris, +rance, the fossil fuel age changed the scope andcharacter of water pollution. *ater usage increased throughout this era, and a farwider variety of pollutants contaminated the world9s water supplies. +or most ofhuman history, water pollution was largely biological, caused mainly by human andanimal wastes. 3owever, industriali'ation introduced countless chemicals into thewaters of the world, complicating pollution problems.

E$$o"t, to Cont"o* ate" Po**tonDntil the early th century, biological pollution of the worldMs lakes and rivers

remained a baffling problem. Then experiments in filtration and chemical treatmentof water proved fruitful. #n Europe and 4orth $merica, sewage treatment and waterfiltration assured a cleaner and healthier water supply. $s late as the ">>s in7hicago, #llinois, thousands of people died each year from waterborne diseases, suchas typhoid fever. %y "@, though, 7hicagoMs water no longer carried fatal illnesses.6any communities around the world, especially in poor countries such as #ndia and4igeria, could not afford to invest in sewage treatment and water filtration plants,however.


20/46

$s was the case with air pollution, the industriali'ation and technological advances ofthe th century brought increasing varieties of water pollution. 0cientists inventednew chemicals that did not exist in nature, and a few of these chemicals turned outto be very useful in manufacturing and in agriculture. Dnfortunately, a few of thesealso turned out to be harmful pollutants. $fter "@ chemicals called polychlorinatedbiphenyls (B7%s) turned up in dangerous &uantities in 4orth $merican waters, killing

and damaging a&uatic life and the creatures that eat these plants and animals. $fter"@8, legislation in 4orth $merica and Europe substantially reduced point pollution,or water pollution derived from single sources. %ut nonpoint pollution, such aspesticide-laced runoff from farms, proved much harder to control. The worst waterpollution prevailed in poorer countries where biological pollution continued unabated,while chemical pollution from industry or agriculture emerged to complement thebiological pollution. #n the late "@s, 7hina probably suffered the most from thewidest variety of water pollution problems.

So* Po**ton

uring the era of fossil fuels, the surface of Earth also has undergone remarkablechange. The same substances that have polluted the air and water often lodge in thesoil, occasionally in dangerous concentrations that threaten human health. *hile thissituation normally happened only in the vicinity of industries that generated toxicwastes, the problem of salini'ation, or salting, which was associated with irrigation,was more widespread.

$lthough irrigation has always brought the risk of destroying soils by waterloggingand salini'ation:the ancient middle-eastern civili'ation of 6esopotamia probablyundermined its agricultural base this way:the modern scale of irrigation hasintensified this problem around the world. %y the "@@s, fields ruined by salini'ationwere being abandoned as fast as engineers could irrigate new fields. 0alini'ation hasbeen the most severe in dry lands where evaporation occurs the fastest, such as in6exico, $ustralia, 7entral $sia, and the 0outhwestern Dnited 0tates.

0oil erosion due to human activity was a problem long before salini'ation was.6odern soil erosion diminished the productivity of agriculture. This problem wasworst in the ">s in the frontier lands newly opened to pioneer settlement in theDnited 0tates, 7anada, $ustralia, 4ew Nealand, $rgentina, and elsewhere.Crasslands that had never been plowed before became vulnerable to wind erosion,which reached disastrous proportions during droughts, such as those during the"@s in the ust %owl of Oansas and 5klahoma. The last maor clearing of virgingrassland took place in the Dnion of 0oviet 0ocialist ;epublics (D00;) in the "@!s,when Bremier 4ikita Ohrushchev decided to convert northern Oa'akhstan into awheat belt. +ossil fuels also played a crucial role at this time, because railroads andsteamships carried the grain and beef raised in these frontiers to distant markets.

%y the late th century, pioneer settlement had shifted away from the worldMs

grasslands into tropical and mountain forest regions. $fter "@!, farmers in $sia,$frica, and


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The impact of soil erosion does not stop with the loss of soil. Eroded soil does notsimply disappear. ;ather, it flows downhill and downstream, only to rest somewhereelse. 5ften this soil has lodged in inconvenient places, silting up dam reservoirs orcovering roads. *ithin only a few years of being built, some dams in $lgeria and7hina became useless because they were clogged by soil erosion originatingupstream.

Anma* an% P*ant +$e

3uman activity has affected the worldMs plants and animals no less than it has the air,water, and soil. +or millions of years, life evolved without much impact from humanbeings. 3owever, as early as the first settlements of $ustralia and 4orth $merica,human beings probably caused mass extinctions, either through hunting or throughthe use of fire. *ith the domestication of animals, which began perhaps ",years ago, humanity came to play a more active role in biological evolution. %y the">s and "@s, the role that human beings played in species survival hadexpanded to the extent that many species survive only because human beings allowit.

0ome animal species survive in great numbers thanks to us. +or example, todaythere are about " billion chickens on Earth:about thirteen to fifteen times as manyas there were a century ago. This is because people like to eat chickens, so they areraised for this purpose. 0imilarly, we protect cattle, sheep, goats, and a few otherdomesticated animals in order to make use of them. #nadvertently, moderncivili'ations have ensured the survival of certain other animals. ;at populationspropagate because of all of the food available to them, since humans store so muchfood and generate so much garbage. 0&uirrels prosper in large part because we havecreated suburban landscapes with few predators.

Even as modern human beings intentionally or unintentionally encourage the survivalof a few species, humans threaten many more. 6odern technology and fuels havemade hunting vastly more efficient, bringing animals such as the blue whale and the

4orth $merican bison to the edge of extinction. 6any other animals, most notablytropical forest species, suffer from destruction of their preferred habitats. Puiteinadvertently, and almost unconsciously, humankind has assumed a central role indetermining the fate of many species and the health of Earth9s water, air, and soil.3umans have therefore assumed a central role in biological evolution.

The environmental history of the last two centuries has been one of enormouschange. #n a mere years, humanity has altered Earth more drastically than sincethe dawn of agriculture about ", years ago. 5ur vital air, water, and soil havebeen eopardi'ed1 the very web of life hangs on our whims. +or the most part,human beings have never been more successful nor led easier lives. The age of fossilfuels is changing the human condition in ways previously unimaginable. %ut whetherwe understand the impact:and are willing to accept it:remains an unanswered

&uestion.$bout the authorG =ohn ;. 6c4eill is a professor of history at Ceorgetown Dniversity.3e is the author of Clobal Environmental 3istory of the Twentieth 7entury amongnumerous other publications.AAAAAAAAAAAAAAAAAAThe difference between a successful person and others is not a lack of strength,notlack of knowledge, but rather in a lack of will.

#magination is more important than knowledge. +or knowledge is limited to all we


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now know and understand, while imagination embraces the entire world, and allthere ever will be to know and understand.$lbert EinsteinDa"0n1 C.a"*e, Ro'e"t

I -INTRODUCTION

arwin, 7harles ;obert (">@-">>), %ritish scientist, who laid the foundation ofmodern evolutionary theory with his concept of the development of all forms of lifethrough the slow-working process of natural selection. 3is work was of maorinfluence on the life and earth sciences and on modern thought in general.

%orn in 0hrewsbury, 0hropshire, England, on +ebruary ", ">@, arwin was the fifthchild of a wealthy and sophisticated English family. 3is maternal grandfather was thesuccessful china and pottery entrepreneur =osiah *edgwood1 his paternalgrandfather was the well-known ">th-century physician and savant Erasmus arwin.$fter graduating from the elite school at 0hrewsbury in ">!, young arwin went tothe Dniversity of Edinburgh to study medicine. #n ">8 he dropped out of medicalschool and entered the Dniversity of 7ambridge, in preparation for becoming aclergyman of the 7hurch of England. There he met two stellar figuresG $dam0edgwick, a geologist, and =ohn 0tevens 3enslow, a naturalist. 3enslow not onlyhelped build arwin9s self-confidence but also taught his student to be a meticulousand painstaking observer of natural phenomena and collector of specimens. $ftergraduating from 7ambridge in ">", the -year-old arwin was taken aboard theEnglish survey ship 360 %eagle, largely on 3enslow9s recommendation, as an unpaidnaturalist on a scientific expedition around the world.

II -VOYAGE OF THE BEAG+E

arwin9s ob as naturalist aboard the %eagle gave him the opportunity to observe thevarious geological formations found on different continents and islands along theway, as well as a huge variety of fossils and living organisms. #n his geologicalobservations, arwin was most impressed with the effect that natural forces had on

shaping the earth9s surface.$t the time, most geologists adhered to the so-called catastrophist theory that theearth had experienced a succession of creations of animal and plant life, and thateach creation had been destroyed by a sudden catastrophe, such as an upheaval orconvulsion of the earth9s surface (see CeologyG 3istory of CeologyG Ceology in the">th and "@th 7enturies). $ccording to this theory, the most recent catastrophe,4oah9s flood, wiped away all life except those forms taken into the ark. The rest werevisible only in the form of fossils. #n the view of the catastrophists, species wereindividually created and immutable, that is, unchangeable for all time.

The catastrophist viewpoint (but not the immutability of species) was challenged bythe English geologist 0ir 7harles ).


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the various forms were closely related but differed in structure and eating habitsfrom island to island. %oth observations raised the &uestion, for arwin, of possiblelinks between distinct but similar species.

III -THEORY OF NATURA+ SE+ECTION

$fter returning to England in ">, arwin began recording his ideas about

changeability of species in his 4otebooks on the Transmutation of 0pecies. arwin9sexplanation for how organisms evolved was brought into sharp focus after he read$n Essay on the Brinciple of Bopulation ("8@>), by the %ritish economist Thomas;obert 6althus, who explained how human populations remain in balance. 6althusargued that any increase in the availability of food for basic human survival could notmatch the geometrical rate of population growth. The latter, therefore, had to bechecked by natural limitations such as famine and disease, or by social actions suchas war.

arwin immediately applied 6althus9s argument to animals and plants, and by ">>he had arrived at a sketch of a theory of evolution through natural selection (see0pecies and 0peciation). +or the next two decades he worked on his theory andother natural history proects. (arwin was independently wealthy and never had toearn an income.) #n ">@ he married his first cousin, Emma *edgwood, and soonafter, moved to a small estate, own 3ouse, outside !@, in5n the 5rigin of 0pecies. 5ften referred to as the Hbook that shook the world,I the5rigin sold out on the first day of publication and subse&uently went through sixeditions.

arwin9s theory of evolution by natural selection is essentially that, because of the

food-supply problem described by 6althus, the young born to any species intenselycompete for survival. Those young that survive to produce the next generation tendto embody favorable natural variations (however slight the advantage may be):theprocess of natural selection:and these variations are passed on by heredity.Therefore, each generation will improve adaptively over the preceding generations,and this gradual and continuous process is the source of the evolution of species.4atural selection is only part of arwin9s vast conceptual scheme1 he also introducedthe concept that all related organisms are descended from common ancestors.6oreover, he provided additional support for the older concept that the earth itself isnot static but evolving.

IV -REACTIONS TO THE THEORYThe reaction to the 5rigin was immediate. 0ome biologists argued that arwin could

not prove his hypothesis. 5thers critici'ed arwin9s concept of variation, arguing thathe could explain neither the origin of variations nor how they were passed tosucceeding generations. This particular scientific obection was not answered untilthe birth of modern genetics in the early th century (see 3eredity1 6endel9s years.The most publici'ed attacks on arwin9s ideas, however, came not from scientists butfrom religious opponents. The thought that living things had evolved by naturalprocesses denied the special creation of humankind and seemed to place humanityon a plane with the animals1 both of these ideas were serious contradictions to


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orthodox theological opinion.

V -+ATER YEARS

arwin spent the rest of his life expanding on different aspects of problems raised inthe 5rigin. 3is later books:including The 2ariation of $nimals and Blants Dnderomestication (">>), The escent of 6an (">8"), and The Expression of the

Emotions in 6an and $nimals (">8):were detailed expositions of topics that hadbeen confined to small sections of the 5rigin. The importance of his work was wellrecogni'ed by his contemporaries1 arwin was elected to the ;oyal 0ociety (">@)and the +rench $cademy of 0ciences (">8>). 3e was also honored by burial in*estminster $bbey after he died in owne, Oent, on $pril "@, ">>.AAAAAAAAAAAAAAAAAAThe difference between a successful person and others is not a lack of strength,notlack of knowledge, but rather in a lack of will.

#magination is more important than knowledge. +or knowledge is limited to all wenow know and understand, while imagination embraces the entire world, and allthere ever will be to know and understand.$lbert EinsteinA%a#taton

I -INTRODUCTION$daptation word used by biologists in two different senses, bothof which imply the accommodation of a living organism to its environment. 5ne formof adaptation, called physiological adaptation, involves the acclimati'ation of anindividual organism to a sudden change in environment. The other kind ofadaptation, discussed here, occurs during the slow course of evolution and hence iscalled evolutionary adaptation.

II -MECHANISMS OF ADAPTATION

Evolutionary adaptations are the result of the competition among individuals of aparticular species over many generations in response to an ever-changing

environment, including other animals and plants. 7ertain traits are culled by naturalselection (see Evolution), favoring those individual organisms that produce the mostoffspring. This is such a broad concept that, theoretically, all the features of anyanimal or plant could be considered adaptive. +or example, the leaves, trunk, androots of a tree all arose by selection and help the individual tree in its competition forspace, soil, and sunlight.

%iologists have been accused of assuming adaptive ness for all such features of aspecies, but few cases have actually been demonstrated. #ndeed, biologists find itdifficult to be certain whether any particular structure of an organism arose byselection and hence can be called adaptive or whether it arose by chance and isselectively neutral.

The best example of an evolutionary development with evidence for adaptation ismimicry. %iologists can show experimentally that some organisms escape predatorsby trying to be inconspicuous and blend into their environment and that otherorganisms imitate the coloration of species distasteful to predators. These testedcases are only a handful, however, and many supposed cases of adaptation aresimply assumed.

5n the contrary, it is possible that some features of an organism may be retainedbecause they are adaptive for special, limited reasons, even though they may be


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maladaptive on the whole. The large antlers of an elk or moose, for example, may beeffective in sexual selection for mating but could well be maladaptive at all othertimes of the year. #n addition, a species feature that now has one adaptivesignificance may have been produced as an adaptation to &uite differentcircumstances. +or example, lungs probably evolved in adaptation to life in waterthat sometimes ran low on oxygen. +ish with lungs were then HpreadaptedI in a way

that accidentally allowed their descendants to become terrestrial.

III -ADAPTIVE RADIATION

%ecause the environment exerts such control over the adaptations that arise bynatural selection:including the coadaptations of different species evolving together,such as flowers and pollinators:the kind of organism that would fill a particularenvironmental niche ought to be predictable in general terms. $n example of thisprocess of adaptative radiation, or filling out of environmental niches by thedevelopment of new species, is provided by $ustralia.

*hen $ustralia became a separate continent some million years ago, onlymonotremes and marsupials lived there, with no competition from the placentalmammals that were emerging on other continents. $lthough only two livingmonotremes are found in $ustralia today, the marsupials have filled most of theniches open to terrestrial mammals on that continent. %ecause $ustralian habitatsresemble those in other parts of the world, marsupial e&uivalents can be found to themaor placental herbivores, carnivores, and even rodents and moles.

This pattern can be observed on a restricted scale as well. #n some sparselypopulated islands, for example, one species of bird might enter the region, find littleor no competition, and evolve rapidly into a number of species adapted to theavailable niches. $ well-known instance of such adaptive radiation was discovered by7harles arwin in the CalQpagos #slands. 3e presumed, probably correctly, that onespecies of finch coloni'ed the islands thousands of years ago and gave rise to the "/species of finchlike birds that exist there now. Thus, one finch behaves like a warbler,

another like a woodpecker, and so on. The greatest differences in their appearancelie in the shapes of the bills, adapted to the types of food each species eats.

IV -ANA+OGY AND HOMO+OGY

*hen different species are compared, some adaptive features can be described asanalogous or homologous. +or example, flight re&uires certain rigid aeronauticalprinciples of design1 yet birds, bats, and insects have all con&uered the air. #n thiscase the flight structures are said to be analogous:that is, they have differentembryological origins but perform the same function. %y contrast, structures thatarise from the same structures in the embryo but are used in entirely different kindsof functions, such as the forelimb of a rat and the wing of a bat, are said to behomologous.AAAAAAAAAAAAAAAAAA

The difference between a successful person and others is not a lack of strength,notlack of knowledge, but rather in a lack of will.

#magination is more important than knowledge. +or knowledge is limited to all wenow know and understand, while imagination embraces the entire world, and allthere ever will be to know and understand.$lbert EinsteinP*ate Teton,


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I -INTRODUCTION

Blate Tectonics, theory that the outer shell of the earth is made up of thin, rigidplates that move relative to each other. The theory of plate tectonics was formulatedduring the early "@s, and it revolutioni'ed the field of geology. 0cientists havesuccessfully used it to explain many geological events, such as earth&uakes andvolcanic eruptions as well as mountain building and the formation of the oceans and

continents.Blate tectonics arose from an earlier theory proposed by Cerman scientist $lfred*egener in "@".


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C -P*ate S2e,

7urrently, there are seven large and several small plates. The largest plates includethe Bacific plate, the 4orth $merican plate, the Eurasian plate, the $ntarctic plate,and the $frican plate. 0maller plates include the 7ocos plate, the 4a'ca plate, the7aribbean plate, and the Corda plate. Blate si'es vary a great deal. The 7ocos plateis km ("/ mi) wide, while the Bacific plate is the largest plate at nearly

"/, km (nearly @ mi) wide.

III -P+ATE MOVEMENT

Ceologists study how tectonic plates move relative to a fixed spot in the earth9smantle and how they move relative to each other. The first type of motion is calledabsolute motion, and it can lead to strings of volcanoes. The second kind of motion,called relative motion, leads to different types of boundaries between platesG platesmoving apart from one another form a divergent boundary, plates moving towardone another form a convergent boundary, and plates that slide along one anotherform a transform plate boundary. #n rare instances, three plates may meet in oneplace, forming a triple unction. 7urrent plate movement is making the Bacific 5ceansmaller, the $tlantic 5cean larger, and the 3imalayan mountains taller.

A -Mea,"n! P*ate Mo/ementCeologists discovered absolute plate motion when they found chains of extinctsubmarine volcanoes. $ chain of dead volcanoes forms as a plate moves over aplume, a source of magma, or molten rock, deep within the mantle. These plumesstay in one spot, and each one creates a hot spot in the plate above the plume.These hot spots can form into a volcano on the surface of the earth. $n activevolcano indicates a hot spot as well as the youngest region of a volcanic chain. $sthe plate moves, a new volcano forms in the plate over the place where the hot spotoccurs. The volcanoes in the chain get progressively older and become extinct asthey move away from the hot spot (see 3awaiiG +ormation of the #slands and2olcanoes).

0cientists use hot spots to measure the speed of tectonic plates relative to a fixedpoint. To do this, they determine the age of extinct volcanoes and their distance froma hot spot. They then use these numbers to calculate how far the plate has moved inthe time since each volcano formed. Today, the plates move at velocities up to ">.!cm per year (8. in per year). 5n average, they move nearly / to 8 cm per year (to in per year).

B -D/e"!ent P*ate Bon%a"e,ivergent plate boundaries occur where two plates are moving apart from eachother. *hen plates break apart, the lithosphere thins and ruptures to form adivergent plate boundary. #n the oceanic crust, this process is called seafloorspreading, because the splitting plates are spreading apart from each other. 5n land,divergent plate boundaries create rift valleys:deep valley depressions formed as the

land slowly splits apart.

*hen seafloor spreading occurs, magma, or molten rock material, rises to the seafloor surface along the rupture. $s the magma cools, it forms new oceanic crust andlithosphere. The new lithosphere is less dense, so it rises, or floats, higher aboveolder lithosphere, producing long submarine mountain chains known as mid-oceanridges. The 6id-$tlantic ;idge is an underwater mountain range created at adivergent plate boundary in the middle of the $tlantic 5cean. #t is part of aworldwide system of ridges made by seafloor spreading. The 6id-$tlantic ;idge is


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currently spreading at a rate of .! cm per year (" in per year). The mid-oceanridges today are , km (about /, mi) long, forming the largest continuousmountain chain on earth. Earth&uakes, faults, underwater volcanic eruptions, andvents, or openings, along the mountain crests produce rugged seafloor features, ortopography.ivergent boundaries on land cause rifting, in which broad areas of land are uplifted,

or moved upward. These uplifts and faulting along the rift result in rift valleys.Examples of rift valleys are found at the Orafla 2olcano rift area in #celand as well asat the East $frican ;ift None:part of the Creat ;ift 2alley that extends from 0yria to6o'ambi&ue and out to the ;ed 0ea. #n these areas, volcanic eruptions and shallowearth&uakes are common.

C -Con/e"!ent P*ate Bon%a"e,7onvergent plate boundaries occur where plates are consumed, or recycled back intothe earth9s mantle. There are three types of convergent plate boundariesG betweentwo oceanic plates, between an oceanic plate and a continental plate, and betweentwo continental plates. 0ubduction 'ones are convergent regions where oceanic crustis thrust below either oceanic crust or continental crust. 6any earth&uakes occur atsubduction 'ones, and volcanic ridges and oceanic trenches form in these areas.

#n the ocean, convergent plate boundaries occur where an oceanic plate descendsbeneath another oceanic plate. 7hains of active volcanoes develop " to "! km( to @ mi) above the descending slab as magma rises from under the plate. $lso,where the crust slides down into the earth, a trench forms. Together, the volcanoesand trench form an intra-oceanic island arc and trench system. $ good example ofsuch a system is the 6ariana Trench system in the western Bacific 5cean, where theBacific plate is descending under the Bhilippine plate. #n these areas, earth&uakes arefre&uent but not large. 0tress in and behind the arc often causes the arc and trenchsystem to move toward the incoming plate, which opens small ocean basins behindthe arc. This process is called back-arc seafloor spreading.

7onvergent boundaries that occur between the ocean and land create continentalmargin arc and trench systems near the margins, or edges, of continents. 2olcanoesalso form here. 0tress can develop in these areas and cause the rock layers to fold,leading to earth&uake faults, or breaks in the earth9s crust called thrust faults. Thefolding and thrust faulting thicken the continental crust, producing high mountains.6any of the world9s large destructive earth&uakes and maor mountain chains, suchas the $ndes 6ountains of western 0outh $merica, occur along these convergentplate boundaries.

*hen two continental plates converge, the incoming plate drives against and underthe opposing continent. This often affects hundreds of miles of each continent and, attimes, doubles the normal thickness of continental crust. 7olliding continents causeearth&uakes and form mountains and plateaus. The collision of #ndia with $sia has

produced the 3imalayan 6ountains and Tibetan Blateau.

D -T"an,$o"m P*ate Bon%a"e,$ transform plate boundary, also known as a transform fault system, forms as platesslide past one another in opposite directions without converging or diverging. Early inthe plate tectonic revolution, geologists proposed that transform faults were a newclass of fault because they HtransformedI plate motions from one plate boundary toanother. 7anadian geophysicist =. Tu'lo *ilson studied the direction of faulting alongfracture 'ones that divide the mid-ocean ridge system and confirmed that transform


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plate boundaries were different than convergent and divergent boundaries. *ithinthe ocean, transform faults are usually simple, straight fault lines that form at a rightangle to ocean ridge spreading centers. $s plates slide past each other, the transformfaults can divide the centers of ocean ridge spreading. %y cutting across the ridges ofthe undersea mountain chains, they create steep cliff slopes. Transform fault systemscan also connect spreading centers to subduction 'ones or other transform fault

systems within the continental crust. $s a transform plate boundary cutsperpendicularly across the edges of the continental crust near the borders of thecontinental and oceanic crust, the result is a system such as the 0an $ndreastransform fault system in 7alifornia.

E -T"#*e 3nton,

;arely, a group of three plates, or a combination of plates, faults, and trenches, meetat a point called a triple unction. The East $frican ;ift None is a good example of atriple plate unction. The $frican plate is splitting into two plates and moving awayfrom the $rabian plate as the ;ed 0ea meets the Culf of $den. $nother example isthe 6endocino Triple =unction, which occurs at the intersection of two transformfaults (the 0an $ndreas and 6endocino faults) and the plate boundary between theBacific and Corda plates.

F -C""ent P*ate Mo/ementBlate movement is changing the si'es of our oceans and the shapes of ourcontinents. The Bacific plate moves at an absolute motion rate of @ cm per year (/ inper year) away from the East Bacific ;ise spreading center, the undersea volcanicregion in the eastern Bacific 5cean that runs parallel to the western coast of 0outh$merica. 5n the other side of the Bacific 5cean, near =apan, the Bacific plate is beingsubducted, or consumed under, the oceanic arc systems found there. The Bacific5cean is getting smaller as the 4orth and 0outh $merican plates move west. The$tlantic 5cean is getting larger as plate movement causes 4orth and 0outh $mericato move away from Europe and $frica. 0ince the Eurasian and $ntarctic plates arenearly stationary, the #ndian 5cean at present is not significantly expanding or

shrinking. The plate that includes $ustralia is ust beginning to collide with the platethat forms 0outheast $sia, while #ndia9s plate is still colliding with $sia. #ndia movesnorth at ! cm per year ( in per year) as it crashes into $sia, while $ustralia movesslightly farther away from $ntarctica each year.

IV -CAUSES OF P+ATE MOTION

$lthough plate tectonics has explained most of the surface features of the earth, thedriving force of plate tectonics is still unclear. $ccording to geologists, a model thatexplains plate movement should include three forces. Those three forces are the pullof gravity1 convection currents, or the circulating movement of fluid rocky material inthe mantle1 and thermal plumes, or vertical columns of molten rocky material in themantle.

A -P*ate Mo/ement Ca,e% 'y G"a/tyCeologists believe that tectonic plates move primarily as a result of their own weight,or the force of gravity acting on them. 0ince the plates are slightly denser than theunderlying asthenosphere, they tend to sink. Their weight causes them to slide downgentle gradients, such as those formed by the higher ocean ridge crests, to the lowersubduction 'ones. 5nce the plate9s leading edge has entered a subduction 'one andpenetrated the mantle, the weight of the slab itself will tend to pull the rest of theplate toward the trench. This sinking action is known as slab-pull because the sinkingplate edge pulls the remainder of the plate behind it. $nother kind of action, called


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ridge-push, is the opposite of slab-pull, in that gravity also causes plates to slideaway from mid-ocean ridges. 0cientists believe that plates pushing against oneanother also causes plate movement.

B -Con/eton C""ent,

#n "@@ %ritish geologist $rthur 3olmes proposed the concept of convection currents

:the movement of molten material circulating deep within the earth:and theconcept was modified to explain plate movement. $ convection current occurs whenhot, molten, rocky material floats up within the asthenosphere, then cools as itapproaches the surface. $s it cools, the material becomes denser and begins to sinkagain, moving in a circular pattern. Ceologists once thought that convection currentswere the primary driving force of plate movement. They now believe that convectioncurrents are not the primary cause, but are an effect of sinking plates thatcontributes to the overall movement of the plates.

C -T.e"ma* P*me,

0ome scientists have proposed the concept of thermal plumes, vertical columns ofmolten material, as an additional force of plate movement. Thermal plumes do notcirculate like convection currents. ;ather, they are columns of material that rise upthrough the asthenosphere and appear on the surface of the earth as hot spots.0cientists estimate thermal plumes to be between " and ! km ( and " mi)in diameter. They may originate within the asthenosphere or even deeper within theearth at the boundary between the mantle and the core.

V -E4TRATERRESTRIA+ P+ATE TECTONICS

0cientists have also observed tectonic activity and fracturing on several moons ofother planets in our solar system. 0tarting in "@>!, images from the 2oyager probesindicated that 0aturn9s satellite Enceladus and Dranus9 moon 6iranda also show signsof being tectonically active. #n "@>@ the 2oyager probes sent photographs and datato Earth of volcanic activity on 4eptune9s satellite Triton. #n "@@! the Calileo probebegan to send data and images of tectonic activity on three of =upiter9s four Calilean

satellites. The information that scientists gather from space missions such as thesehelps increase their understanding of the solar system and our planet. They canapply this knowledge to better understand the forces that created the earth and thatcontinue to act upon it.

0cientists believe that Enceladus has a very tectonically active surface. #t has severaldifferent terrain types, including craters, plains, and many faults that cross thesurface. 6iranda has fault canyons and terraced land formations that indicate adiverse tectonic environment. 0cientists studying the 2oyager images of Tritonfound evidence of an active geologic past as well as ongoing eruptions of icevolcanoes.

0cientists are still gathering information from the Calileo probe of the =upiter moon

system. Three of =upiter9s four Calilean satellites show signs of being tectonicallyactive. Europa, Canymede, and #o all exhibit various features that indicate tectonicmotion or volcanism. Europa9s surface is broken apart into large plates similar to theplates found on Earth. The plate movement indicates that the crust is brittle and thatthe plates move over the top of a softer, more fluid layer. Canymede probably has ametallic inner core and at least two outer layers

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