Intoduc.to Geol

8/13/2019 Intoduc.to Geol.

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Chapter One

Introduction to Geology

Geologyliterally means "study of the Earth."

Physical geologyexamines the materials and processes of the Earth.

Historical geologyexamines the origin and evolution of our planetthrough time.


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Geology is an evolving science - the theory of plate tectonics was just

accepted in the 1960's.

Plate tectonics is theunifying theoryin geology.

Although geologists treat it as a law - plate tectonics is still and will

likely remain a theory


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Geology is an extremely

controversial science - thetheory of evolution

(paleontology) is centralto

geology.

Geology seeks to understand theorigin of our planet and our

place in the Universe - answers

to these questions are also posed

outside of the realm of science.


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Although catastrophism was

abandoned, there is certainly evidencethat sudden events dooccur.


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Relative Dating:Putting geologic events into proper order (oldest

to youngest), but without absolute ages. We use a number ofprinciples and laws to do this:

Law of Original Horzontality- Sedimentary units and lava flows are

deposited horizontally.

Law of Superposition- the layer below is older than the layer above.

Principle of fossil succession- life forms succeed one another in a

definite and determinable order and therefor a time period can be

determined by its fossils.

Law of Cross-cutting Relationships- A rockis younger than any

rock across which it cuts.

Geologic Time


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The concept of geologic time is new

(staggering) to many nongeologists.

The current estimate is that the Earth is~4,600,000,000 (4.6 billion) years old.

As humans we have a hard time

understanding the amount of time required

for geologic events.

We have a good idea of how long a

century is. One thousand centuries is only

100,000 years. That huge amount of time

is only 0.002% of the age of the Earth!

An appreciation for the magnitude of

geologic time is important because many

processes are very gradual.

Geologic Time


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Our generation is unique in its perspective of our planet. From

space, Earth looks small, finite and fragile.

What's the first thing thatyou notice about our

planet when you see this

image?

The Earth is composed of

several integrated parts

(spheres) that interact with

one another: atmosphere

hydrosphere

solid earth (lithosphere)

biosphere (cryosphere)


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The Earth System

Hydrosphere: the global ocean is

the most prominent feature of our(blue) planet. The oceans cover

~71% of our planet and represent

97% of all the water on our

planet.

Atmosphere: the swirling clouds

of the atmosphere represent thevery thin blanket of air that

covers our planet. It is not only

the air we breathe, but protects us

from harmful radiation from thesun.


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The Earth System

Biosphere: includes all life on

Earth - concentrated at thesurface. Plants and animals don't

only respond the their

environment but also exercise a

very strong control over the other

parts of the planet.

Solid Earth: represents the

majority of the Earth system.

Most of the Earth lies atinaccessible depths. However,

the solid Earth exerts a strong

influence on all other parts (ex.

magnetic field).


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The Earth System

This figure shows the dynamic

interaction between the majorspheres.

As humans, we desire to divide

the natural world into artificial

portions to make it easier. It

should be stressed that these

divisions are artificial.

What are some of theinteractions between these

spheres?


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The Rock Cycle

Three basic rock types:

igneous - form frommagma/lava

sedimentary- form from

sediment and chemical

precipitation from seawater

metamorphic- form from

other rocks that recrystallize

under higher pressures and/or

temperatures.

A number of geological

processes can transform one

rock type into another.


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The Rock Cycle


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The Earth and the other 8 planets and the Sun

accreted at about the same time from a vast cloud

of dust and gas (nebula).

About 5 billion years ago, the nebula began to

gravitationally contract, began to rotate and

flattened. Eventually, the Sun ignited (fusion)and the newly formed planets began to

differentiate - heavier elements and chemical

components sank to the center and rocky material

formed the crust. The newly formed planets andmoons released gas forming early atmospheres.

We will spend more time talking about the

Earth's place in our solar system later in this

course.

The Origin of the Earth


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The Earth's interior is

characterized by a gradual

increase in temperature,pressure and density with

depth.

At only 100 km depth, the

temp is ~1300C.

At the Earth's center, the

temperature is >6700C.

The pressure in the crust

increases ~280 bars for every

kilometer depth.

Earth's Internal Structure


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Earth's Internal StructureThe Earth consists of 3

major regions markedby differences in

chemical composition.

Crust: rigid outermost

layer of the Earth.

Consists of two types:

1. oceanic- 3-15 km thick and

is composed of basalt

(igneous). Young (3.8billion years old).

h' l S


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Earth's Internal Structure

Mantle: comprises ~82% of the

Earth by volume and is ~2900

km thick.

The mantle is characterized by

a change in composition from

the crust.

The mantle is able to flow

(plastically) at very slow rates.

Core: composed of iron, nickel

and other minor elements.

The outer core is liquid

capable of flow and source of

the Earth's magnetic field.The inner core is solid Fe-Ni.

There is no major chemical

difference between the outer and

inner core.


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Asthenosphere (~100 to 660 km)

It's hot and flows like molasses

Radioactive dacay causes the Earth to heat up on time scales of millionsof years. In the course of tens/hundreds of millions of years, this heat

production is enough to warm the interior by hundreds of C.

This heat is carried away by the convective circulation of the earth's

interior. The convection delivers heat to the surface, so it can eventually

be lost into space.

Most of the earth's interior is heated to a temperature (> 300C) which

makes it ductile, so that it is soft, and can flow like a viscous liquid. Youhave seen this behavior as glass is heated to near its melting point. The

soft region (just below the lithospheric plates) is called the asthenosphere


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Mesosphere / Lower Mantle (660 to 2900 km)

Rock in the lower mantle gradually strengthens with depth, but it is still

capable of flow.

Outer (2900 to 5170 km) and Inner Core (5170 to 6386 km)

Outer core is liquid and composed of an iron-nickel alloy. Convectiveflow of this fluid generates much of the Earths magnetic field.

Inner core is solid iron-nickel alloy. It is hotter than the outer core, but

the intense pressure keeps it solid.

Pl T i


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A relatively recent theory that the

Earth's crust is composed of rigid

plates that move relative to oneanother.

Plate movements are on the order

of a few centimeters/year - aboutthe same rate as your fingernails

grow!

Plate Tectonics

There are 3 types of plate

boundaries:

1. divergent

2. convergent

3. transform


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Pl t T t i


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Plate TectonicsConvergent boundaries- plates move together forming a subduction zone and

mountain chains.

Divergent boundaries- plates move apart forming the mid-ocean ridge and seafloor

spreading.Transform boundaries- plates grind past one another. These boundaries subdivide

the mid-ocean ridge and also form the San Andreas fault system.


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A simplifed model of tectonic

plates and the location and

nature of earthquakes.

Pl t B d i h h l i


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Plate Boundaries: where the real action occurs.

The plates are all moving relative to each other. At the boundary

between two plates, there must be some motion of one relative to the

other. You get three possibilities:

Spreading center: Divergent boundary

At the top of a rising convection limb. Heat is being brought up.

Volcanism. Usually under-ocean. Often associated with a rift valley.

Collision zone: Convergent boundary

Cold lithosphere bends downward and begins sinking into the mantle

(subduction). Mountains are squeezed up here by the collision. Most

earthquakes occur here.

Parallel plate motion: Transform / Transcurrent / Strike Slip faulting

The San Andreas Fault is the most famous transform fault system.

Pl M i


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Plate Margins


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Oceanic Continent Convergence Example: Andes Cascades


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Oceanic - Continent Convergence - Example: Andes, Cascades

At an ocean-continent collision, the ocean subducts, and the

continent rides high. Volcanoes are built on the continental side dueto melt which comes off the subducting plate. Nazca-South America

is an excellent example.

Continent Continent Convergence Example: Himalayas


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Continent - Continent Convergence - Example: Himalayas

A continent-continent collision is like a train wreck - both sides end

up taking severe damage. Neither side wants to subduct. The entire

Alpine-Himalayan mountain system from Spain to Thailand is

behaving this way. Mountain belts are stacked range upon range

across the landscape for 1000's of km. These mountains are

permeated with thrust faults, which carry slices of crust many

dozens or 100's of km over other slices.


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O i Di B d


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Oceanic Divergent Boundary

Example: Mid-Atlantic Ridge

C i l Di B d


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Continental Divergent Boundary

Example: Red Sea / E. African Rift

Thi i f h Si i i l h h h R d S di


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This image of the Sinai peninsula shows where the Red Sea spreading

center forks into two branches which can be seen as forming a brand-

new oceanic rift in the land.

C ti t l Di t B d


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Continental Divergent Boundary

Example: Baja California

Continental Transform Boundary Example: San Andreas


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Continental Transform Boundary - Example: San Andreas


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The major fields in GEOLOGY are mineralogy, petrology, sedimentology, geochemistry,

geomathematics, stratigraphy, palaeontology, structural geology, economic geology, petroleum

geology, mining geology, structural geology, marine geology, engineering geology,geomorphology, hydrogeology, environmental geology and geoscience education. GEOPHYSICS

is often regarded as a separate Earth Science discipline. Check out the links at left to find out

more about each of these Earth Science Disciplines.GEOLOGYThe primary objective of the science of geology is to understand the processes by which the

planet earth was formed, the evolution of the continents and seas, and the origins of the materials

within the earthscrustthe igneous, sedimentary and metamorphic rocks and their minerals.

PETROLOGYPetrology is the study of rocksthe minerals that they are composed of, and the textures and

other features that provide clues about how the rocks formed. It is subdivided into sedimentary,

igneous and metamorphic petrology. This is because the processes under which sedimentary,

igneous and metamorphic rocks form are quite different and require different skills in their study.

Sedimentary petrology is the study of the mineral composition of sedimentary rocks, mainly as a

guide of where on the earthscrust the rocks originated. Igneous petrology involves the study ofmagmas and the processes which give rise to varying compositions and textures of intrusive and

extrusive igneous rocks. Metamorphic petrology concentrates on how rocks of all kinds can be

changed by heat and pressure within the Earthscrust into metamorphic rocks. MINERALOGY is

the study of the minerals themselvestheir chemical composition and crystal formsfor which a

background in chemistry is desirable


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SEDIMENTOLOGY AND VOLCANOLOGYSedimentology and volcanology are closely allied fields that examine rocks at

outcrop and larger scales, with the aim to unravel the geological history of a

reasonably large areaancient volcanoes, or river valleys, or entiresedimentary basins. Sedimentology is devoted to the study of rock sequences

laid down as sedimentary rocks by water, wind or ice, whereas Volcanology

studies the results of eruptions of igneous rocks. Volcanic eruptions involve

the passage of large volumes of hot fluids rich in metallic minerals through the

earthscrust so that the study of volcanology is an important component of

Economic Geology.

STRATIGRAPHYStratigraphy is the study of the composition, ordering and relationships of rock

strata in order to determine their geological history. It was one of the firstdisciplines in Geology and remains one of the most important skills. Principles

such as the law of superposition, recognition of erosional breaks

(unconformities) and cross-cutting relationships are peculiar to geology. The

stratigrapher must understand the individual events that have resulted in the

rock formations as they occur today. Many of the other disciplines

(Palaeontology, Sedimentology, Volcanology, Structural geology,

Geochronology) are used by the stratigrapher.


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PALAEONTOLOGYPalaeontology is the systematic study of animal/plant fossil remains. At its

core is the principle that organisms evolve, and that the changes wrought by

evolution can be used to determine the age of the fossil and its host rock. The

last 500 million years of earthshistory (known as the Phanerozoic) has been

divided into very fine subdivision on the basis of this principle, with the fine

subdivisions named after the fossils that are found in it. Age determination

and correlation (using the same fossil to date rocks in different places, often at

a global scale) remain one of the most important services that

palaeontologists provide. Palaeontology is also used to help identify where

sedimentary rocks have been laid down (e.g. in a river, or near a seashore)

and can help determine the nature of biological provinces that result from themigration of continents. A knowledge of biology is a good start for budding

palaeontologists. Specialists studying plant fossils are called

PALAEOBOTANISTS. MICROPALAEONTOLOGISTS and PALYNOLOGISTS

study fossilised microscopic animal remains, spores, pollens and certain other

microfossils. Both fields are particularly useful in petroleum exploration.

Palaeontologists need to use their studies of fossils to interpret the

PALAEOECOLOGY and PALAEOCLIMATOLOGY i.e. the environmental

conditions in which the fossils were laid down. Most palaeontologists are

employed by museums, mining companies specialising in oil, coal and

limestone and universities.


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GEOCHRONOLOGYThe study of the radioactive decay of isotopes that occur naturally in some

minerals, with the aim to determine the absolute age of a rock.

Geochronology also includes other methods of age determination, such as

fission track studies and solar irradiation studies.

MINING GEOLOGYInvolves working in an operating mine or quarry to accurately survey the

progress of operations in respect of geological structure, sample mineral or

rock to obtain assays to determine whether economic grade or yield is being

maintained; design and supervise exploration programs ahead of production

to maintain reserves. Mine geologists need to have a broad knowledge ofearth sciences as well as general knowledge of mining engineering,

metallurgy and mineral economics.

MARINE GEOLOGYMarine geology is the application of earth science studies to modern marine

environments. Specialised ships are used as a platform for drilling the sea

bed and for undertaking seismic studies. Marine Geology is used in the study

of PLATE TECTONICS and in oil and gas exploration. In recent years Marine

Geology has been applied to the exploration for rich sea-floor massive

sulphide deposits of gold, silver, copper, zinc and lead. Mining of these

deposits remains some years off, however. Marine Geology expeditions often

utilise specialists in Sedimentology, Palaeontology, Geophysics, Economic

Geology and Petrology.

ENGINEERING GEOLOGY


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ENGINEERING GEOLOGYEngineering geology involves the study of the stability and structure of the materials of the Earths

crust in particular reference to the foundations of man-made structures such as roads, bridges,

tunnels, freeways, dams, power stations, and large buildings. A good knowledge of the stratigraphy

and geological structure of the local area is necessary, as well as the physical and chemical

characteristics of the foundation materials. The nature and movement of underground waters is alsonecessary as an essential component of such studies. The continual expansion of cities requires

prior studies of landforms by geologists to assess whether land is susceptible to landslides, unstable

foundation materials or whether development will lead to pollution of valuable sources of surface or

underground water. Coastal areas are particularly susceptible to ill-considered developments.

GEOTECHNICAL ENGINEERING makes such studies available to architects and engineers.

GEOMORPHOLOGYGeomorphology is the study of landforms from the interplay of constructional and destructional forces

acting on the Earthssurface. It involves study of the erosional and depositional work of water, wind, ice

and gravity as well as the constructional influences of earth movements and volcanoes. It is an

important aspect in the study of the geological history of a region, especially in Australia whereerosional forces have been a dominant influence for a long time. A Geomorphologist needs to

understand Stratigraphy, Petrology, Geochemistry and Palaeoclimatology. An important skill gained by

those trained in Geomorphology is aerial photo interpretation, the ability to identify the rock types and

their history from aerial photographs. Geomorphology is needed for tourism in national parks, and in

the study of physical geography and PLANETARY GEOLOGY.

HYDROGEOLOGY


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HYDROGEOLOGYHydrogeology is an increasingly important area of Earth Science. It is the study of groundwaters

including the location and nature of water-bearing layers (aquifers) and structures in geological

formations. The role of Hydrogeologists is to plan drilling programs and use geophysical techniques to

locate water supplies and assess their quality and yield for towns and cities, mining and agriculture

projects. Another important function is to protect water resources from overdevelopment and pollution

from industrial and domestic disposal of waste materials. The issue of groundwater salinity is set to

become an increasingly serious problem throughout inland Australia. Hydrogeologists will play a vital

part in understanding and addressing this problem. By identifying and mapping salt-water reservoirs

before they erupt at the surface, hydrogeologists will provide catchment managers with the raw data

required to manage groundwater aquifers

GEOPHYSICSGeophysics is often regarded as a distinct field of study different from Geology. It combines a

knowledge of physics and geology in analysing the physical characteristics of the materials in the

Earthscrust. Highly sophisticated instruments and techniques are used to measure a wide range of

properties. These include magnetism, radioactivity, electrical conductivity, rock density, seismicvariations, heat flow, spectral properties (light wavelengths reflected by different minerals), radar

reflectivity and others. Many of these are important in various branches of economic geology.

Earthquake hazards provide an important impetus to seismic studies. A sound training in physics and

mathematics is essential for those wanting to tackle this rewarding discipline.

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Intoduc.to Geol