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of the planet. Yet, observe the intricate network of stream

valleys that dissect the plateau around the canyon.

Stream valleys, in fact, are the most abundant landform

on the continents. As water flows in them, particles of

sand and other materials carried in the current act as

sandpaper on the stream bed, abrading it and deepen-

ing the valley. At the same time, water moving down the

valley sides in sheets and rivulets erodes the valley walls,

causing them to recede. Valleys are lengthened by a

process called headward erosion, in which erosion is

focused at the head of a valley by convergence of runoff

from several directions (see Figs. 34.3 and 34.4).

Groundwater

Runoff

3.6

Evaporation

32.0

Precipitation

28.4

Precipitation

9.6Evaporation

6.0

H O and CO2 2

Runoff

Evaporation

Precipitation

Evaporation fromlakes, streams, and

soil

Transpiration

Groundwater

H O and CO2 2H O and CO2 2

*

Mostly as CaCO usedby marine organisms

to build shells.

*3

Figure 34.1. The water-flow budget of the earth. Figures are in units of 104 km3, or tens of thousands of cubic kilo-

meters, per year. (Each cubic kilometer is 264 billion gallons!)

Figure 34.2. The hydrologic cycle, consisting largely of water moving in the hydrologic system (essentially a surfaceor near-surface system), and also in limited parts of the plate tectonic system (essentially a subsurface system).


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Figure 34.3. Headward erosion occurs when runoff

converges on the head of a valley from several direc-

tions.

Figure 34.4. A section of the Colorado Plateau, show-

ing a through-flowing stream with several tributary

canyons occupied by streams during periods of rain.

Headward erosion slowly extends the canyons into the

plateau.

Many stream-generated landforms are depositional

rather than erosional. As streams empty into the ocean

or lakes, they deposit sediment in deltas, fan-shaped or

fingerlike deposits that record, in part, a history of the

stream. In some areas streams do not flow to the ocean

or to lakes but end in dry basins that have no outlets. In

such places, the streams that intermittently flow from

canyons deposit their sediment on the valley floors in

alluvial fans, the subaerial analogs of the subaqueous

deltas. Streams tend to be efficient at sorting sediment

by size, so that coarser particles are deposited near the

source of the sediment while finer particles are carried

farther. When the sediment eventually becomes sedi-

mentary rock, its grain size provides insight into the dis-

tance of transportation of the sediment.

A little thought will convince you that there must

be a limit to the depth of stream erosion. For streams

that empty into the ocean, that limit is essentially sea

level, because a stream would have to flow uphill if it

cut more deeply than that. In order to erode at all, astream must flow fast enough to carry abrasive sedi-

ment, so this maximum depth of erosion must decrease

away from the sea in order that there be some slope to

the stream channel. (That is, the erosional limit must be

at higher and higher elevations further and further from

the sea.) The maximum possible depth of downcutting

is called base level, and it is the level toward which all

streams strive. They rarely attain base level, however,

because tectonic movement of the land intercedes and

changes it. Thus there is a constant competition

between the hydrologic system and the tectonic system,

the former striving to lower the land and the latter gen-

erally attempting to raise it.

Glaciers

Glaciers are masses of ice that form either at high

elevations (in mountains) or at high latitudes (far north

or far south) where temperatures are perennially low.

The process of forming a glacier requires that snow

accumulation in the winter exceed loss in the summer,

so that the deposit becomes deeper with time.

Eventually, the snow deep in the snow field is compact-

ed into a tough, granular form, similar texturally to the

old, grainy snow that accumulates at the side of the roadduring a long winter cold spell. The final stage is the

recrystallization of the granular snow into glacial ice, a

mass of intergrown ice crystals many meters below the

surface. As more and more ice forms, the mass

becomes heavy enough to move downhill as the ice at

the bottom slowly deforms under pressure.

Glaciers that form in valleys originally carved by

streams are called valley glaciers. They move slowly

downhill both by sliding (basal slip) and by deformation

of the ice near the base (plastic flow). Continental

glaciers cover very large areas and are not confined to

valleys. Color Plate 16 shows the Vatnajkull conti-

nental glacier in Iceland with outlet glaciers that aresimilar to valley glaciers near the fringe. Such glaciers

spread out from a central area in all directions, much

like pancake batter from the center of a griddle. Figures

34.5 and 34.6 are photographs of a valley glacier and a

continental glacier, respectively.


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Figure 34.5. A typical valley glacier.

Figure 34.6. Part of the continental glacier that covers

Greenland.

Like running water, glaciers generate both erosion-

al and depositional landforms. Unlike running water,

however, even the slowest glaciers can carry huge boul-

ders as well as smaller material. Not only does the ice

itself abrade the land but it also carries embedded rocks

of all sizes, and thus acts very much like sandpaper.

Valley glaciers gouge and scoop out the valleys they

occupy, modifying them into U-shaped troughs. The

topography between adjacent glaciated valleys is often

angular and sharp. Continental glaciers scrape and

scour the rocks over which they flow, largely obliterat-ing previously developed stream drainage systems.

Glaciers do not flow uphill, of course, but they may

melt faster at their lower ends than they advance. When

this occurs, we say that a glacier is receding. Because

they can carry sediment of all sizes, but are incapable of

sorting it by size as is running water, glaciers leave

deposits of rough, unsorted debris called moraines

when they melt. Moraines deposited by ancient glaci-

ers are common landforms in some areas of the north-

eastern and midwestern parts of the United States.

More glaciers seem to be receding than advancing

today, but there have been times in the past when glacia-

tion was a much more active process than it is now. There

have been several ice ages during geologic time, as far

back as the Precambrian and as recently as the Great Ice

Age of the Pleistocene Epoch in the Cenozoic Era. The lat-

ter involved recurring glacial and interglacial stages and

was so recent that we cannot be sure that we are not sim-

ply in another interglacial stage of the same ice age now.

Moreover, there is no general agreement about the cause of

ice ages, although it may have much to do with the posi-

tioning of continent-size landmasses at high north or south

latitudes by plate motion, and their interference with ocean

currents that facilitate the worldwide transfer of heat.

Groundwater

Considerably less than a percent of the total water

in the hydrologic system resides underground, but it is

of critical importance20 percent of the freshwater

requirements of the United States is met by it. Nearly

all groundwater comes from precipitation that has

seeped into the ground. Some precipitation remains in

the upper soil layer as a film around soil particles, but

most descends to a depth at which all of the pore spaces

in the rock are filled with waterthe region ofground-

water. The upper surface of this region is the water

table, and its shape tends to mimic the topographyabove (see Fig. 34.7). The zone of pore saturation exists

under both humid and arid areas of the earth, but it is

deeper in arid areas. In humid areas, the water table is

shallow and groundwater contributes to stream flow, so

that even during dry spells there is water in the streams.

In arid areas, the water table is deep and does not con-

tribute to stream flow; rather, the streams leak and

provide water to the subsurface, so that most

streambeds in such areas contain no water during dry


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periods.

Like surface water and glaciers, groundwater also

flows in response to gravity. It moves generally from

areas of high elevation of the water table to areas of

lower elevation (streams, springs, or lakes), but may

move locally upward in order to reach regions of lesser

hydrostatic pressure caused by topography on the

water table itself. The flow of groundwater is slower

than water flow on the surface, averaging only centime-ters per daya fortunate circumstance, for were it not

so, wells drilled into groundwater would rapidly go dry

because supply by rainfall could not keep up with deple-

tion from pumping.

The erosional and depositional work of groundwa-

ter produces some spectacular results, as anyone who

has visited an underground cavern can attest. Rainwater

absorbs small amounts of carbon dioxide from the

atmosphere as it falls, creating a weak carbonic acid

(H2CO3) that is very effective in slowly dissolving lime-

stone. Consequently, in areas where limestone consti-

tutes a major part of the near-surface rock, large caverns

may be dissolved near or below the water table. Later,

when the water table has been lowered by deepening of

nearby stream channels, carbonate-rich waters percolat-

ing into these caverns will deposit calcium carbonate

(CaCO3) to make beautiful stalactites, stalagmites, and

other cavern formations.

Often, where limestone beds with dissolved voids

exist near the surface, the roofs of the voids are too thin

to support themselves and they collapse, forming sink-

holes (Fig. 34.8). In populated areas these cause greatdamage when they collapse beneath and engulf houses,

cars, and so forth.

Wind

Wind is incapable of carrying the heavy particles

that denser agents like running water or ice can carry,

but anyone who has been caught in a dust storm or sand-

storm recognizes that wind can lift and transport very

large amounts of smaller particles. Wind plays its most

important role in shaping the face of the land in deserts

and near-desert regions, and these areas constitute about

one-fifth of the land surface of the earth. Even in most

of these, water is a very important, though infrequent,

agent of erosion.

Sand dunes are probably the best known of wind-

generated landforms. Color Plate 17 shows large dunes

on the Arabian Peninsula; some of them are 100 meters

high (higher than a football field on end) and up to 200

kilometers long. Depending upon the abundance of

sand, the density of plant cover, and the constancy and

strength of winds, other types of dunes with different

shapes may develop in other arid areas.

Watertable

Zone

ofsaturation

Figure 34.8. Sinkholes produced by the collapse of

underground voids in limestone. The voids develop as

weak carbonic acid, formed when rain combines with

atmospheric carbon dioxide, dissolves the limestone.Figure 34.9. A typical sea cliff, a feature created by

marine erosion.

Figure 34.7. The water table.


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Oceans

The crashing breakers during a storm at the

seashore demonstrate the power of the ocean to modify

the land, but of course the effects are limited to that nar-

row band where sea and land meetthe shoreline.

Wave action can be very effective in modifying the

shoreline, as seen in Figure 34.9, which shows a sea

cliff carved by marine erosion. Many beach cottages

that have been built well back from sea cliffs have even-

tually become unsafe because of receding shorelines.

The sea can create depositional landforms as well

as erosional ones. As sediment is carried by waves ontoa beach at an angle oblique to the shoreline, it washes

back toward the sea directly downslope, perpendicular

to the shoreline (Fig. 34.10). Thus sediment migrates

slowly down a coast. Such sediments are often spread

into elongate landforms, as shown in Figure 34.11.

Some of these become large enough to support build-

ings or communities.

The various facets of the hydrologic system do not

always work independently. Where a stream empties

into the sea, it builds a delta, but the waves and currents

of the sea may modify the form of that deposit. Within

many areas in the world, one may see the combined

effects of waves, wind, and running water; glaciers andwaves; or running water and groundwater.

The Face of the Earth Through Time

While the interaction of the tectonic system and the

hydrologic system explains the appearance of the earth

today, our planet has not always appeared as it does

noweven vaguely. To conclude this brief study of the

earth, we summarize the various major stages through

which it has passed since its formation. Many of the

details of this long history have yet to be worked out by

geologists, but the general outline is probably correct.

Accretion Stage

Recall from Chapter 28 that the early history of the

solar system involved the gravitational collapse of the

condensing solar nebula into countless small clumps

and that larger clumps gradually swept up smaller ones

and grew into planetesimals. The process continued

until there emerged from the mass of gas, dust, and

Wavedirection

Motion ofsandgrains

Sand migration

Figure 34.10. When waves impinge obliquely on the beach, sediment is carried in from an angle, but recedes directly

downslope, resulting in migration of the sediment down a coastline.

Figure 34.11. An elongate landform created by the

spreading of sediment transported in the way shown in

Figure 34.10.


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chunks a star, nine planets, their assorted moons, and

various swarms of material like meteors. One of the

planets was ours, of course, but it bore little resem-

blance to todays earth. There were neither continents

nor oceans, and there was no internal structurethat is,

it was homogeneous, the same all the way from surface

to center. The earth was essentially age zero, about 4.6

billion years ago, and its temperature was around 1000

C in the interior.

Bombardment and Heating

After the initial formation of the planets there were

countless small bits of matter in the nebular disk yet to

be gravitationally swept up. Bombardment of the earth

by meteors was intense and, as each meteor hit, its

kinetic energy was transformed into internal energy

heat. (The record of the bombardment stage is largely

absent from the earth now, but it is present on other bod-

ies such as the moon, Mercury, Mars, and some satel-

lites of the outer planets, none of which have had vigor-

ous tectonic or hydrologic systems capable of obliterat-

ing it.) As gravity continued to contract the new planet,

gravitational energy was likewise transformed into heat

faster than it could be dissipated. In addition, radioac-

tive elements (mostly uranium, thorium, and potassi-

um), all of which must have been more abundant in the

early earth than they are now, decayeda third source

of heat. The result of all this heating was that, by

around 4.2 to 4.5 billion years ago, the temperature rose

to the melting point of iron in a shell beginning at 400

kilometers deep and extending to 800 kilometers. (The

temperature must have been hotter deeper than this, but

the melting temperature of iron also increases withdepth. The situation is analogous to the one for the ori-

gin of the asthenosphere, and the general geometry

shown in Figure 30.9 applieswith different actual

temperatures and depths, of course.)

The Iron Catastrophe and Differentiation

As iron in the 400-800 kilometer depth range began

to melt, it formed droplets that migrated gravitationally

toward the center of the earth, displacing the less dense

material. The process was self-accelerating: As iron

sank, it raised the temperature through friction, and

more iron melted and sank. As the temperatureincreased, minerals of all sorts melted as their melting

points were reached. Most silicate minerals with low

melting points also have relatively low densities, so

they floated to the surface. The process finally elevated

the earths temperature to around 2000 C, and so a

large fraction of the planet melted. This event, called

the iron catastrophe, resulted in the complete reorgani-

zation of the interior. When it was over, the metallic

iron was mostly in the center of the earth, surrounded by

an oxide-and-silicate mantle of mostly iron-magne-

sium-oxygen compounds, which in turn was surround-

ed by a basaltlike crust. The process of forming layers

based on density is called planetary differentiation.

Because the radioactive elements were chiefly involved

in compounds with oxygen, they tended to concentrate

in the outer layers of the planet where radiogenic heat

was dissipated rapidly, and the heating therefore slowed

down. By this point 400 or 500 million years hadpassed since the formation of the solar system, and the

time was about 4.2 billion years ago. The face of the

earth was still not recognizable.

Onset of the Tectonic System

The heat generated by the iron catastrophe must

have repeatedly melted silicate minerals surrounding

the developing core, including those constituting the

thin primitive crust that was forming. Eventually, as the

distillation process of planetary differentiation pro-

ceeded, the least dense materials must have accumulat-

ed on top and formed the earliest continents, perhaps

around 4.2 billion years ago. It is difficult to know just

when the tectonic system, as we now define it, had its

beginning, but by 3.9 billion years ago there were prob-

ably thin, rather fragile plates moving comparatively

rapidly over an asthenosphere, being subducted and

recycled. These would gradually thicken, support vol-

canic activity and igneous intrusion, weather and erode,

be metamorphosed, and become the shields of todays

continents. Not yet wholly familiar, the face of the earth

at least had the beginnings of familiar features.

Origin of the Atmosphere and Oceans

The original atmosphere of the earth was probably

very unlike the one we know today and was swept

away by the vigorous solar wind of the young sun. The

atmosphere we breathe is probably mostly of volcanic

origin and was produced by release of gases from the

interior. If you have seen pictures of volcanic erup-

tions, you might have noticed that large amounts of

gases are emitted along with lava, ash, or other vol-

canic emanations. Most of this gas is water vapor, with

some carbon dioxide and nitrogen, and was no doubt

produced by the early volcanism of the heated earth

just as it is today. Even at the present rate of volcanism(and the rate in the hot, differentiating earth would

have been significantly higher), enough water would

have been produced in this way over the span of geo-

logic time to fill the oceans, along with enough nitro-

gen, carbon dioxide, and other gases to create nearly all

of the atmosphere. The part of the atmosphere not pro-

duced by this processapproximately 20 percent of

itis the oxygen on which life depends, but which is

essentially absent in volcanic gases.


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Sunlight striking the upper layers of this oxygen-

poor atmosphere dissociated some of the water mole-

cules into hydrogen and oxygen. Hydrogen is so light

that it escaped the earths gravity, but oxygen was car-

ried by atmospheric turbulence downward toward the

surface of the planet, to become part of the permanent

gaseous envelope of the earth. Most of the oxygen must

eventually have been produced by photosynthesis, how-

ever, and this required plant life. The first primitivecells could have developed in only a small fraction (per-

haps as little as one five-thousandth) of the present oxy-

gen level in the atmosphere and begun the slow, contin-

ual process of combining carbon dioxide and water to

produce carbohydrates and oxygen.

For a long time the atmosphere was too poor in oxy-

gen to support complex forms of life; hence, the

Precambrian is represented by few fossils, virtually all

soft-bodied organisms. However, as simple plants

(chiefly algae) built the oxygen supply, increasingly

complex organisms developed, and the attainment of

some critical level of oxygen concentration may have

led to the almost explosive flowering of shelled forms

(and hence fossils) at the beginning of the Paleozoic Era.

That the chemistry of our atmosphere is highly

dynamic and has evolved significantly over time is a

fascinatingly troubling concept. To be sure, we are the

beneficiaries of that long course of development, but we

are also now its modifiersand it is clear that it can be

modified. By our industrial and cultural activities, we

are adding to the levels of some noxious compounds in

the air (such as carbon monoxide and sulfur dioxide),

and we appear to be decreasing the abundance of at least

one crucially necessary substance, ozone. Such tam-

pering is not without consequences, and hence the cur-rent concern with control of atmospheric pollution. One

may argue over the methods and priorities suggested by

those on various sides of the issue, but its importance

and urgency cannot be in doubt.

Toward Pangaea

In Chapter 31 we examined some of the evidence for

the existence of the supercontinent Pangaea. We recog-

nized that Pangaea was not the initial event in the history

of plate motion, but simply a part of the continuum of

plate tectonic history that spans nearly four billion years.

The existence of two or more continental masses makesit virtually inevitable that there will be eventual conti-

nental collisions, and these produce large ranges of fold

mountains. Even when such mountains have been com-

pletely eroded, their roots are still discernible through

careful geologic mapping, and the large amounts of sedi-

ment shed from them show up as thick sequences of sed-

imentary rock layers that become thinner away from the

sites of the former mountains. By understanding the sig-

nificance of such geologic features, it is possible to

reconstruct some of the plate interactions that occurred

before Pangaea, although the details become less and less

distinct as we look further back in time. For example, the

Ural Mountains, which constitute the traditional geo-

graphic separation between Europe and Asia, testify of an

ancient collision between those two continents that weld-

ed them into a single large landmass.

It appears that there have been six periods of very

intense and widespread mountain-building activity dur-ing the earths history. Some of these have been longer

and more intense than others, but they may each repre-

sent a time when continents were converging to form a

supercontinent. The times are about 2600, 2100, 1700,

1100, 650, and 250 million years ago. The 400- to 600-

million-year periodicity in these events has led some to

suggest that there is a plate-tectonic cycle consisting of

the repeated assembly and fragmentation of superconti-

nents, caused largely by the way in which heat builds up

under large land masses. Regardless of whether this is

the case, it is clear that mountain ranges have been built

during several eventsnot only those mentioned above

but also numerous more local events at other times

only to be leveled by the hydrologic system.

Since Pangaea

The division of the geologic account into periods

before and after the break-up of Pangaea is somewhat arti-

ficial, and post-Pangaea history was included in the sec-

tion on general continental evolution in Chapter 32. Here

we mention only a major Cenozoic event that changed the

face of much of the landthe Great Ice Age of the

Pleistocene Epoch. For nearly two-million years, great

sheets of ice up to two kilometers thick episodicallyadvanced over northern North America and northern

Europe, and then retreated during interglacial periods,

exposing the scraped and scoured topography typical of

continental glaciation. The ice obliterated stream

drainage systems and left deposits of coarse, unsorted sed-

iment. It created myriad lakes and clusters of low, stream-

lined hills (one of them Hill Cumorah near Palmyra, New

York). It gouged out stream valleys to make the magnifi-

cent fjords of Scandinavia and lowered sea level to facili-

tate the carving of some immense canyons on the now-

submerged continental shelves. While intense glaciation

is by no means unique in the history of the earth, the

Pleistocene Ice Age was recent enough to have clearly leftits mark on the world we inhabit. In fact, it is quite possi-

ble that we are now living during an interglacial period,

merely awaiting the return of the ice.

Summary

The hydrologic system consists of all fluids

that move near the surface of the earth. Its various

agents (running water, groundwater, wind, waves, and


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ice) have eroded and deposited material to shape the

face of the earth for many millions of years, but always

in concert with the tectonic system. As plate motion has

raised mountains, the hydrologic system has worked to

wear them down. The conflict between the two sys-

tems, one driven by heat from radioactive decay within

the earth and the other by the heat of solar radiation

from outside the earth, has yielded the variety and beau-

ty of our planet. The application of physical and chem-ical principles we have learned previously to the pre-

ferred model of the origin of the solar system leads us

through a series of predictable stages for a developing

earth, ending with a planet whose face is now familiar

to us but which we realize is still undergoing slow and

inexorable change.

STUDY GUIDE

Chapter 34: The Changing Face of the Earth

A. FUNDAMENTAL PRINCIPLES: No new funda-

mental principles.

B. MODELS, IDEAS, QUESTIONS, AND APPLI-

CATIONS

1. What is the hydrologic system? What are the main

elements of the system and what influence do they

have on the features of the surface of the earth?

2. What has happened and what is now happening to

determine the main features observed on the sur-

face of the earth?

3. Sketch the geologic and biological history of the

earth with approximate dates as we now understand

it from interpretation of the physical data.

C. GLOSSARY

1. Alluvial Fan: A fan-shaped deposit formed when

a stream that flows intermittently from a canyon

deposits its sediments on the valley floor.

2. Base Level: The maximum possible depth to

which a stream can cut by erosion; the level to

which each stream strives.

3. Continental Glacier: Large dollar shaped glacier

covering an extended land mass (Greenland,

Antarctica). Ice accumulates near the center and

flows to the periphery.

4. Delta: A fan-shaped or fingerlike deposit formed

when a stream or river empties into an ocean orlake.

5. Evaporation: The changing of water from its liq-

uid state to its gaseous state.

6. Groundwater: Water which exists in the subsur-

face of the earths crust, usually in the pore spaces

of the rock.

7. Headward Erosion: A process by which valleys

are lengthened because erosion is focused at the

head of a valley by convergence of runoff from sev-

eral directions.

8. Hydrological Cycle: The cyclic movement of the

earths water supply as it moves among the oceans,

the atmosphere, and the land.

9. Hydrologic System: The methods by which the

water moves through the hydrological cycle, i.e.,

running water, glaciers, subsurface water, wind,

evaporation, etc.

10. Iron Catastrophe: An early event in the earthshistory (about 4.2 billion years ago) during which

the interior melted rather quickly and extensively

and the denser elements (iron and nickel) moved to

the center, displacing less dense oxides and sili-

cates to the mantle and crust.

11. Moraine: A deposit of rough, unsorted debris left

by a glacier when it melts.

12. Planetary Differentiation: The process by which

the materials of a forming planet organize them-

selves into layers, with the more dense materials on

the inside and the less dense materials on the out-

side. See Chapter 30.

13. Precipitation: Water which condenses into liquid

or solid form and falls toward the surface of the

earth.

14. Sand Dune: A wind-generated deposit of sand,

commonly found in arid areas.

15. Sinkhole: A hole or depression in the ground

formed by the collapse of the roof above an under-

ground void in limestone.

16. Water Table: The top surface of the underground

region where all of the pore spaces in the rock are

filled with water.

17. Valley Glacier: A glacier (mass of moving ice and

snow) that forms in a valley originally carved by astream.

D. FOCUS QUESTIONS

1. Describe three main elements of the hydrologic

system. Use examples to explain how individual

elements of the cycle influence the appearance of

the earth.

2. Describe the various stages in the history of the

earth from its formation as a planet to its present

stage according to current geologic thought.

Include at least five important dates in your outline.

E. EXERCISES34.1. Discuss the general features of the hydrolog-

ic cycle.

34.2. The hydrologic system causes the most

extensive changes in the surface of the earth through

(a) running water.

(b) glaciers.

(c) shoreline processes.

(d) wind.


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34.3. Why is the central region of a large conti-

nental mass never eroded all the way to sea level?

34.4. Ice, running water, and wind are all able to

transport sediment from a source area to an area of

deposition. List these three in order of their ability to

carry the largest particles.

(a) Wind, ice, running water

(b) Running water, wind, ice(c) Ice, wind, running water

(d) Ice, running water, wind

34.5. It is believed that volcanic eruptions were the

major source of most of the atmosphere of the earth.

What major component would not have been con-

tributed in this way, and where did it come from?

34.6. How do mountain ranges provide evidence

that much plate tectonic activity occurred prior to the

assembly of Pangaea?

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The Changing Face of the Earth