Unit 4 The Forces within Earth

1. explain the roles of evidence, theories, and paradigms in the development of scientific knowledge

(114-2)

a. describe the theory of continental drift

b. describe the evidence to support the theory of continental drift. Include:

(i) fit of continents

(ii) fossil evidence (fossil correlation)

(iii) rock types (shields)

(iv) structural similarities (e.g., folded mountains)

(v) paleoclimatic (e.g., striations, coal deposits, glacial deposits)

2. explain how a major scientific milestone revolutionized thinking in the scientific communities

(115-3)

a. describe the evolution of plate tectonic theory through the contributions of various

scientists. Include:

(i) Frank Taylor

(ii) Alfred Wegener

(iii) Alexander DuToit

(iv) Arthur Holmes

(v) Harry Hess and Robert Deitz

(vi) Fredrick Vine and Drummond Matthews

(vii) J. Tuzo Wilson

(viii) Xavier Le Pichon and Dan McKenzie

3. explain how scientific knowledge evolves as new evidence comes to light and as laws and

theories are tested and subsequently restricted, revised, or replaced (115-7)

a. contrast the explanations provided by Wegener and Holmes for the mechanism of

continental movement

b. describe the theory of plate tectonics

c. describe and give examples of convergent, divergent and transform plate boundaries

d. describe and give examples of the different types of convergent plate boundaries.

Include:

(i) oceanic-oceanic collisions

(ii) oceanic-continental collisions

(iii) continental-continental collisions

e. describe a rift valley and how it evolves into a divergent plate boundary

4. analyse evidence for plate tectonics theory (332-8)

a. describe the evidence which supports plate tectonic theory. Include:

(i) paleomagnetism

(ii) polar wandering

(iii) magnetic reversals

(iv) earthquakes (Wadati- Benioff zone)

(v) deep-ocean drilling

(vi) hot spots

5. describe examples of Canadian contributions to science and technology (117-10)

6. analyse examples of Canadian contributions to science (117-11)

7. use appropriate evidence to describe the geologic history of an area (330-12)

a. describe the geology of the island of Newfoundland

8. provide examples of how science and technology are an integral part of their lives and their

community (117-5)

9. identify new questions or problems that arise from what was learned (214-17)

10. describe geological evidence that suggests life forms, climate, continental positions, and Earth’s

crust have changed over time (332-7)

a. define crustal deformation

b. define force

c. define stress

d. describe the types of forces/ stresses that produce crustal deformation. Include:

(i) compressional

(ii) tensional

(iii) shear

e. describe the types of deformation. Include:

(i) elastic

(ii) brittle

(iii) ductile

f. describe the factors that affect deformation. Include:

(i) temperature

(ii) confining pressure

(iii) rock type

(iv) time

g. define faulting as the breaking of rock layers and their subsequent motion

h. relate faulting to the factors that affect deformation

i. describe the two major types of faults and associated forces/stresses. Include:

(i) dip-slip

- normal (tensional)

- horst and graben (tensional)

- reverse (compressional)

- thrust (compressional)

(ii) strike-slip (transform)

- left-lateral (shear)

- right-lateral (shear)

j.define folding

k. relate folding to the factors that affect deformation

l. describe the two common types of folds. Include:

(i) anticline

(ii) syncline

11. describe methods of analyzing, monitoring and predicting earthquakes, volcanic eruptions, and

plate interactions (331-9)

a. define earthquake

b. describe the causes of an earthquake. Include:

(i) moving magma

(ii) elastic rebound

(iii) faulting

c. define earthquake terminology. Include:

(i) seismic wave

(ii) focus

(iii) epicentre

(iv) foreshock

(v) aftershock

d. identify the location of earthquakes and relate them to their plate boundary. Include:

(i) divergent (shallow)

(ii) transform (shallow)

(iii) convergent (shallow, intermediate, and deep)

e. describe properties of the different seismic waves. Include:

(i) surfaces waves (L waves)

(ii) primary waves (P waves)

(iii) secondary waves (S waves)

f. distinguish between earthquake scales. Include:

(i) Richter

(ii) Modified Mercalli

g. identify that the Richter scale increases in amplitude by a factor of ten for every

increment of one

h. identify in relation to the Richter scale, energy released increases by a factor of 30

(rounded down) for every increment of one

12. compile and display evidence and information, by hand or computer, in a variety of formats,

including diagrams, flow charts, tables, graphs, and scatter plots (214-3)

13. identify and explain sources of error and uncertainty in measurement and express results in a

form that acknowledges the degree of uncertainty (214-10)

14. identify multiple perspectives that influence a science-related decision or issue (215-4)

15. work cooperatively with team members to develop and carry our a plan, and troubleshoot

problems as they arise (215-6)

16. describe methods of analysing, monitoring and predicting earthquakes, volcanic eruptions, and

plate interactions (331-9)

a. describe how seismographs and resulting seismograms are used to measure seismic

waves

b. describe factors affecting the nature of volcanic eruptions. Include:

(i) magma temperature

(ii) magma viscosity

(iii) magma composition

c. define volcano

d. describe the three types of volcanoes:

(i) shield

(ii) ash and cinder

(iii) composite cone

e. describe the type of eruption for each volcano type in relation to the different plate

boundaries

f. identify the rocks that form in relation to each type of volcano. Include:

(i) shield – basalt

(ii) ash and cinder – basalt and scoria

(iii) composite – andesite, basalt, rhyolite

g. distinguish between the types of lava. Include:

(i) pahoehoe (ropy)

(ii) aa (jagged, angular)

h. describe intra-plate volcanism as it relates to hotspots

i. describe the formation of a lava plateau

17. describe major interactions among the hydrosphere, lithosphere, and atmosphere (332-3)

a. explain the global effects of volcanic activity

18. identify and describe science and technology-based careers related to the science they are

studying (117-7)

a. identify careers related to plate tectonics, earthquakes, and volcanoes. Include:

(i) structural geologist

(ii) volcanologist

(iii) seismologist

(iv) geomorphologist

(v) geochemist

(vi) geophysicist

(vii) petrologist

(viii) sedimentologist

Theory of Continental Drift

A German scientist, named Alfred Wegener was one of the first scientists to propose that continents had

once been closer together. This idea was presented in a publication called, “The Origin of Continents and

Oceans.”

This idea that Wegener called, “Continental Drift”, was not widely accepted because scientist believed

that the continents and oceans were permanent features fixed in their position and did not move

Wegener’s idea was; Hundreds of millions of years ago, a supercontinent called, “Pangaea” once existed.

At approximately 200 million years ago, this landmass split into smaller pieces and drifted apart reaching

their present positions.

Wegener’s main criticism centered around why the continents move.

A weakness in Wegener’s theory was it could not satisfactorily answer the question asked by critics:

What kind of forces could be strong enough to move such large masses of solid rock over such great

distances?

Wegener suggested that the continents plow through the oceans as a snow plow would trough snow. This

reasoning for the movement of continents was not widely accepted. In 1915, Alfred Wegener believed the

Earth’s rotation and lunar gravitational forces were responsible for the drifting of continents.

After Wegener’s death in 1930, much evidence was discovered to support the Continental Drift Theory.

Majority of this evidence was discovered during ocean floor exploration and the question Wegener failed

to answer was explained by a scientist named Arthur Holmes. He suggested that continents and the ocean

floor move primarily due to forces in the asthenosphere (upper mantle) which causes material to move as

convection cells.

Mantle material moves up at ridges and move away in opposite directions moving the continents. In

cooler areas beneath trenches, mantle material sinks causing oceanic crust to descend into the mantle and

be recycled.

Main Points of Continental Drift Theory Include:

225 Million years ago, supercontinent called Pangaea.

200 Million years ago, supercontinent split to form two main land masses. Laurasia in the north and

Gondwanaland in the south.

Laurasia consisted of; Asia, Europe, and North America.

Gondwanaland consisted of; Africa, Australia, Antarctica, South America, and India.

Over the past 150 million years, these land masses split and drifted to their present positions.

Continental Drift – Evidence

1. Fit of the Continents:

Continental coastlines appear to fit closely together, for example, South America and Africa. But with

further investigation Alexander DuToit suggested that the continental shelves would fit better because of

the absence of erosion beneath the oceans

2. Fossil Correlation:

Wegener and other scientist had proof of similar organisms that existed in both South America and

Africa. Fossil evidence of a fern plant (Glossopteris) and an aquatic reptile (Mesosaurus) provided the

best evidence that the continents were once together

3. Rock Types( Sheilds) and Structures (Folded Mountains)

Scientist also noted that even though the continents appear to fit together, the overall picture has to be

continuous from one continent to another. This picture included the type of rock on neighboring

continents and structural similarities such as mountains. For example, the Appalachian mountains

4. Ancient Climates:

Glacial deposits were found in South America, Africa, India, and Australia. These continents are

presently not in cold climates, therefore must have been in a colder climate in the past and the continents

later moved to the positions they are presently in today.

During the past 50 years these ideas have brought about a scientific revolution in which new evidence and

data support a slowly but continually moving planet. It was not until 1968 that a theory called Plate

Tectonics provided reliable evidence supporting a mobile Earth.

Important people of the past and the contribution they made to support a moving Earth.

Frank Taylor

1910 – Explained the formation of the

Himalayan Mountains by moving continents (no

evidence given).

Alfred Wegener

1915 – Proposed the theory of continental drift

(evidence given, but no mechanism provided).

Alexander DuToit

1937 – Proposed that Earth’s continents would fit

more closely together at the continental margins.

Arthur Holmes

1950s – Proposed the existence of a mechanism for

movement; mantle convection.

Harry Hess and Robert Deitz 1960s – Proposed the theory of seafloor spreading.

Fredrick Vine and Drummond Matthews

1963 – Proposed the idea of magnetic reversals as

evidence to support the theory of seafloor

spreading.

J. Tuzo Wilson

1965 - Proposed the existence of “plates” on

Earth’s surface as a result of mapping the world’s

volcanoes and earthquakes. He also proposed the

existence of transform faults along plate

boundaries; and that stationary hotspots in Earth’s

mantle caused volcanism within plates.

Xavier Le Pichon and Dan McKenzie 1970s – Proposed the modern theory of plate

tectonics

Plate Tectonic Theory

The theory of plate tectonics is one of the great advances in the twentieth century. In the 1960's, scientist

such as Alfred Wegener proposed the “continental drift theory”, and Tuzo Wilson put forth the idea that,

“Earth consisted of several different fragments called plates, instead of being made up of one static, rigid,

solid layer.” This revolutionized the way scientist think of Earth today.

Theory of Plate Tectonics States:

Earth’s crust is divided into Approximately twenty (20) rigid slabs called tectonic plates.”These tectonic

plates are in continuous slow motion relative to each other which occurs along one of three types of

boundaries bordering each plate.”

A Tectonic Plate is a massive, irregularly shaped slab of solid rock, generally composed of both

continental and oceanic lithosphere. Plate size varies from a few hundred to thousands of kilometers

across, the Pacific and Antarctic are among the largest.

These massive slabs seem to float because of their composition. Continental crust is composed of

Granitic rocks which are made of lighter minerals and are less dense than the oceanic crust which is

composed of denser and heavier basaltic rocks. Thickness under continental crust ranges up to 100 km

and under ocean crust 5 km.

In the late 1960’s scientific studies of the ocean floor led to the development of a theory that better

explained the idea of a mobile Earth, This theory was called the Plate Tectonic Theory.

Plate Boundaries

According to the Plate tectonic theory, three boundaries exist at the edges of each tectonic plate.

1. Divergent Boundary (Ridge)

2. Convergent Boundary (Trench)

3. Transform Boundary

Divergent Boundary

Plates move apart, resulting in upwelling of molten material from the mantle to create new ocean

floor.Features on the ocean floor called Ridges, show this form of plate movement.

Tensional forces cause the plates to move apart.

Ocean Ocean Crust

Moho Magma Continental Crust

Convergent Plate Boundaries:

Three Types

Ocean – Ocean Convergent Boundary

Compressional forces cause plates to move together, causing one slab of lithosphere to be

consumed into the mantle initiating volcanic activity which creates volcanoes to form on the

ocean floor. Features called ocean trenches are formed at these boundaries. Lithosphere is

destroyed as one oceanic slab descends beneath another Fluid, basaltic magmas feed the volcanic

islands and form shield volcanoes. Example include the Japan island arc and the Japan trench.

Ocean – Continent Convergent Boundary

Compressional forces cause an ocean plate and a continent plate to move together, causing the

more dense ocean plate to sink into the asthenosphere. This region where the ocean plate sinks is

called a subduction zone.

Deep ocean trenches form adjacent to the zone of subduction. These trenches can range up to

thousands of kilometers long and 8 – 10 km deep. Lithosphere is destroyed as one oceanic slab

descends beneath another.

At depths of about 100 km the oceanic plate and parts of the mantle partially melt producing

viscous magmas. This molten rock rises slowly where it cools and solidifies at depths producing

plutons. However, some magma may reach the surface and erupt through composite volcanoes as

violent volcanic eruptions. If the subduction occurs beneath continental crust, a continental

volcanic arc is produced (such as the Cascades of the western U.S or the Andes mountains of the

South America).

Continent – Continent Convergent Boundary

Compressional forces cause two continental plates to move together. Because of the low density

of continental crust neither plate will subduct and the two plates ram into one another forming

mountains.Such a collision occurred when India collided with Asia forming the Himalayas.

During these collisions the continental crust is buckled and fractured pushing rock up to very high

elevations.

Transform Boundary

Where lithospheric plates slide past one another in a horizontal manner, a transform fault is created.

Earthquakes along such transform faults are shallow focus earthquakes. Lithosphere is not created or

destroyed at these boundaries.

One of the largest such transform boundaries occurs along the boundary of the North American and

Pacific plates and is known as the San Andreas Fault. Here the transform fault cuts through continental

lithosphere

Most transform faults occur where oceanic ridges are offset on the sea floor.

Plate Tectonic Theory - Evidence:

1) Earthquakes and Volcanoes

Earthquakes and volcanoes do not occur randomly throughout the world, but occur in rather limited belts.

These belts mark the location of Plate Boundaries. The largest active belt in the world surrounds the

Pacific Ocean and is referred to as “The Pacific Ring of Fire”. 90% of all the world’s earthquakes

occur there. Some of the more famous volcanoes are found surrounding the Pacific

These boundaries are areas where compressional forces cause tectonic plates to move toward one another

and stress builds up. When the stress is to great, fractures (faulting) may occur within the tectonic plates

or the plates may slip abruptly and earthquakes result. The boundaries are also places of high heat flow,

where molten rock rises to the surface and forms volcanoes. Example: Mount Saint Helens in USA.

The Wadati-Benioff zone.

A dipping planar (flat) zone of earthquakes that is produced by

the interaction of a down-going oceanic crustal plate with a

continental plate. These earthquakes can be produced by slip

along the subduction thrust fault or by slip on faults within the

down-going plate as a result of bending and extension as the

plate is pulled into the mantle.

2) Polar wandering

Is the apparent movement of the magnetic poles as outlined from studying the magnetism fossilized in

successive basaltic lava flows ranging in age over millions of years. The permanent magnetism in rocks

indicates the direction of the magnetic field when the minerals became magnetized.

The most persuasive evidence to support the Plate Tectonic theory comes from the study of Earth’s

magnetic field. Polar wandering and magnetic reversals in the ocean floor provide this evidence.

Basaltic rocks contain iron-rich minerals which become magnetized in the direction of the magnetic field

at the time when the rock solidified. If the rocks move or if the magnetic poles change the magnetism in

Transform Fault

Fracture Zone Oceanic Ridge

(spreading center)

the rocks retain its original magnetic alignment. Rocks that formed millions of years ago “remember” the

location of the magnetic poles at that time.

A plot of this magnetism showed that the magnetic pole appeared to change position considerably over

the past 500 million years. This was clear that either the magnetic pole had moved with time, an idea

known as polar wandering, or the lava basaltic lava flows had moved, explained by continental drift.

Plate Tectonic theory is believed to be the best explanation for polar wandering. If the magnetic poles

remain stationary, then their apparent movement was caused by the drifting of continents.

3) Magnetic Reversals and Seafloor Spreading

Paleomagnetism also provided evidence for the Plate Tectonic theory when scientist discovered that the

magnetic field reverses polarity. Basaltic lavas solidifying during a time of reverse polarity would display

opposite magnetism as rocks forming today. Rocks with magnetism the same as our present magnetic

field is said to have normal polarity, while rocks with opposite polarity is said to have reverse polarity.

This alternating magnetic polarity can be seen in;

1) successive lava flows making up a volcano and

2) the basaltic rock making up the ocean floor.

At oceanic ridges the plates move apart and new basaltic rock is added to each plate. The magnetism of

these basaltic rocks appears to alternate to produce identical magnetic patterns on both sides of oceanic

ridges. This proved to be the strongest evidence to support seafloor spreading and therefore Plate

Tectonics.

4) Ocean Drilling and Heat Flow

From 1968 to 1983, the Deep Sea Drilling Project collected convincing evidence confirming the seafloor

spreading idea and the Plate Tectonic theory. Drill core samples of the ocean floor and sediments on the

ocean floor were collected with increasing distance from ocean ridges.

When the oldest sediment from each drill site was plotted against the distance from the ocean ridge, it was

noted that the age of the sediment increased with increasing distance from the ridge.This evidence also

confirmed the idea that the ocean basins are relatively young, because no sediment older than 160 million

years was found. Continents were dated to be 4.6 billion years.

5) Hot Spots

Mapping of the seafloor in the Pacific revealed a chain of volcanoes and seamounts that extend from the

Hawaiian Islands to the Midway Islands and continue north to the Aleutian trench of the coast of Alaska.

Scientist proposed that a plume of magma presently exist beneath Hawaii and the Pacific plate moved

over this stationary magma chamber. This confirmed that the tectonic plates do move in relation to

earth’s interior thereby supporting the theory of Plate Tectonics.

Radioactive age dates of the seamounts and volcanic islands confirm that the age increases the farther

away you go from Hawaii, and the hot spot.

Rift Valleys

If tensional forces, due to the motion of convection currents

in the upper mantle, begin beneath continents, it can cause

continents to split into two or more smaller segments. A

spreading center is formed and a rift valley is formed as a

result of the associated faulting (normal) and thinning of the

crust. Eventually the thinning becomes significant enough for

ocean water to move into the rift valley. Ridge volcanism and

seafloor spreading begin and a divergent plate boundary is

produced. As both continue, the segments (i.e., landmasses)

on both sides of the ridge continue to move apart from one

another. A global example of a current rift valley is the East

African rift valley.

Canadian Contributions

In 1965, J. Tuzo Wilson proposed the existence of transform faults to explain the numerous narrow

fracture zones and earthquakes found in the crust. He realized that ridges at divergent plate boundaries

were not perfectly linear and came to understand that transform faults exist where segments of ridges are

offset.

Figure 19.24 on page 539 of the student text shows a transform fault joining offset segments of a ridge. In

making this discovery, Wilson was able to solidify his idea that Earth is covered by rigid plates. Wilson

suggested that transform faults connect with divergent and convergent boundaries, and other transform

faults, resulting in a network of plates that cover Earth’s surface.

Wilson, along with other scientists, also proposed that there are places on Earth’s surface, not on plate

boundaries, where molten up-wells creating volcanism. Wilson referred to these local, stationary plumes

of upwelling molten magma as hotspots. Hotspot volcanism (also called intra-plate volcanism), which

explains the volcanoes in Hawaii and Yellowstone National Park,

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Convection and Seafloor Spreading

To fully understand the theories of Continental Drift and Plate Tectonics, you must first have an

understanding of the internal processes within Earth.

Convection currents in the asthenosphere, proposed by Arthur Holmes, and the seafloor spreading idea,

propose by Harry Hess, provides evidence for a mobile Earth. By combining the seafloor spreading

theory with continental drift and earthquake information the new theory of Plate Tectonics became a

coherent theory to explain crustal movements. The many plates that make up the earth’s crust sit directly

on a plastic like layer within the mantle called the Asthenosphere. A scientist named Arthur Holmes

provided evidence to prove that tectonic plates moved on what he referred to as convection currents.

Convection can not take place without a source of heat. Heat within Earth comes from two main sources;

1) radioactive decay

2) residual heat.

If the asthenosphere is in fact moving as a result of convection, then convection could be the mechanism

responsible for plate tectonics.

Harry Hess influenced by Holmes’ ideas, suggested that deep within the asthenosphere, heated material

expands, becomes less dense, rises, and pushes it way up through ridges. It then moves along the base of

oceanic plates, pulling the plates in opposite directions. This concept we call Seafloor Spreading.

When this slowly moving material reaches cooler areas it contracts and sinks causing one plate to move

downward (subducting plate) beneath another (over-riding plate). This material is then recycled back into

the mantle.

During World War II, geologists employed by the military carried out studies of the sea floor, a part of

the Earth that had received little scientific study.The topographic studies involved measuring the depth to

the sea floor. These studies revealed the presence of two important topographic features of the ocean

floor:

Oceanic Ridges - long sinuous ridges that occupy the middle of the Atlantic Ocean and the eastern part of

the Pacific Ocean.

Oceanic Trenches - deep trenches along the margins of continents, particularly surrounding the Pacific

Ocean.

Studies also noted that as oceanic lithosphere moves away from the ridge, it cools and sinks deeper into

the asthenosphere. Thus, the depth to the sea floor increases with increasing age away from the ridge.

Seafloor Sediment and Age

Because the oceanic ridges are areas of young crust, there is very little sediment accumulation on the

ridges. Sediment thickness increases in both directions away from the ridge, and is thickest where the

oceanic crust is the oldest.As new oceanic crust is created it is pushed aside in two directions. Thus, the

age of the oceanic crust becomes progressively greater in both directions away from the ridge.

Because oceanic lithosphere is created at ridges and destroyed at subduction zones (trenches), scientist

noted that the oceanic basins is continuously being recycled and are relatively young. The oldest oceanic

crust occurs farthest away from a ridge. In the Atlantic Ocean, the oldest oceanic crust is about 180

million years old (Jurassic in age

Mountain Building

The name given to the processes that collectively produce a mountain system is Orogenesis. Mountain

systems show evidence of great forces that have bent (folded), broken (faulted), and deformed large

sections of Earth’s crust.

The building up of mountains is directly related to plate tectonics and in particular Convergent Plate

Boundaries, where two plates collide. The two plates could be an ocean-continent boundary, or

continent-continent boundary. Regardless of the type of collision, when two plates collide the over-riding

plate becomes greatly deformed. This deformation occurs in the form of faulting, folding and volcanoes.

At an Ocean – Continent Convergent Boundary

Compressional forces cause an ocean plate and a continent plate to move together, causing the more

dense ocean plate to sink into the asthenosphere. This region where the ocean plate sinks is called a

subduction zone. The forces generated during this collision causes the over-riding continental crust to be

deformed and folding along with faulting pushes the crust upward forming a mountain chain along the

entire length of the subduction zone. The Andes Mountains in South America and Rocky Mountains in

Western Canada formed in this way.

Continent – Continent Convergent Boundary

Compressional forces cause two continental plates to move together. Because of the low density of

continental crust neither plate will subduct and the two plates ram into one another forming mountains.

Such a collision occurred when India collided with Asia forming the Himalayas. During these collisions

the continental crust is buckled and fractured pushing rock up to very high elevations.

The 6,000-km-plus journey of the India landmass (Indian Plate) before its collision with Asia (Eurasian

Plate) about 40 to 50 million years ago (see text). India was once situated well south of the Equator, near

the continent of Australia.

The collision between the Indian and Eurasian plates has pushed up the Himalayas and the Tibetan

Plateau.

Crustal Deformation: Folding and Faulting

Deformation is a general term that refers to all changes in the original form and/or size of a rock body. It

may also produce changes in the location and orientation of rocks. Most crustal deformation occurs along

plate tectonic margins.

Force: that which tends to put stationary objects in motion or change the motion of moving bodies.

Stress: the amount of force applied to a given area.

Includes:

(i) compressional

(ii) tensional

(iii) shear

(i)Stresses that shorten a rock body are compressional in nature

(ii)Those that elongate a rock body are tensional in nature.

(iii)Stresses that cause sections of a rock body or two separate rock bodies to slide past one another are

referred to as shear.

There are 3 types of deformation. Including :

(i) elastic: recoverable after stressing

(ii) brittle: results in fracturing and is permanent.

(iii) ductile: a solid state flow and is permanent.

Factors that affect deformation.

(i) Temperature: temperature affects the type of deformation that occurs. The colder the Earth materials,

the more brittle the deformation will be. The warmer the Earth materials, the more elastic or ductile the

deformation will be.

(ii) Confining pressure: confining pressure will only result in elastic or ductile deformation. Brittle

deformation typically occurs on the surface when there is no pressure from above.

(iii) Rock type: all rock types can be deformed; however, sedimentary rocks could be more easily

deformed since they are softer.

(iv) Time: rapid deformation tends to result in brittle deformation whereas deformation over longer

periods of time tends to result in elastic or ductile deformation.

Folding and faulting are features that occur when stresses are greater than the strength of the rock and

the rock it deforms.

Scientists note that when stress is applied to rocks they first respond by deforming elastically (bending).

Once the elastic limit is reached then one of two things happen depending if the rock is deep in the earth

environment, or a surface environment.

Rapid, continual, compressional (or tensional) forces, usually at shallower depths, result in brittle

deformation (faulting).

Deep earth – elastic deformation – resulting in folding or flow

Faults: a crack or break in the earth’s crust along which movement has occurred.

3 parts of a fault include:

Hanging wall: the top part of the rock above the fault plane

Foot wall: the bottom part of the rock below the fault plane

Fault plane: the surface that separates the two moving pieces.

Different Types of Faults:

1. Dip-Slip

Normal Fault (dip-slip): Caused by tensional forces. Hanging wall drops in comparison to foot wall

Horst and Graben Fault

Pieces of land are either pushed up or down in relation to surrounding rock

Horst: an uplifted block of crust bounded by 2 normal faults.

Caused by tensional forces

Graben: A valley formed by the downward displacement of a block of crust

bounded by two faults.

Caused by tensional forces

Reverse Fault (dip-slip): Caused by compressional forces. Hanging wall moves upward in comparison

to foot wall

Thrust Fault (dip-slip): Caused by compressional forces Hanging wall moves up over foot wall.

Identical to a Reverse fault but occurs at a lower angle. Known as a low angle reverse fault (<45˚)

2. Strike-slip

Transform Fault Caused by shearing forces can be left-lateral (movement to the left ) or right-lateral

(movement to the right)

o Two plates slide past each other in opposite directions

o No vertical movement.

Folds Often occurs in deep earth environments.

Slow, continual, compressional forces, usually at

greater depth results in ductile deformation. (folding).

Main parts of a fold include:

Anticline

Caused by compressional forces.

Crust moves upward forming a hill.

Referred to as an up-fold.

Syncline Caused by compressional forces.

Crust moves downward forming a valley.

Referred to as a down-fold.

Earthquakes and Seismic Waves What is an Earthquake? An earthquake is the vibration of Earth produced by the sudden, rapid release of

energy.Ex: moving magma, elastic rebound and faulting. Earthquakes can be generated by bomb blasts,

volcanic eruptions, and sudden slippage along faults

Anatomy of an earthquake!

Movement in areas along the fault plane stops (fault sticks).

Elastic energy is stored in the rock as the rock becomes deformed and bends, much like a bent stick.

When the elastic strain built up along the fault exceeds the elastic limit, the rock will break or slip at its

weakest point which we call the focus.

This slippage along the fault allows the rock to “snap back” and the vibrations send out waves of energy

in all directions called seismic waves, or earthquake waves.

The springing back of the rock is call “elastic rebound”.

Locations of Earthquakes:

Depending on the plate boundary, the location of an earthquake within the crust varies.

1. divergent (shallow)

2. transform (shallow)

3. convergent (shallow, intermediate, and deep) ( SEE WADATI-BENIOFF ZONE)

Seismology

When an earthquake occurs, the elastic energy is released and sends out vibrations that travel throughout

the Earth. These vibrations are called seismic waves. The study of how seismic waves behave within

Earth is called seismology.

Focus and Epicenter

Focus: The exact location within Earth where seismic waves are generated

by sudden release of stored energy. Most often located on a pre-existing

fault.

Epicenter: The point on the surface of Earth directly above the focus

Seismic waves originate at the focus and travel outward in all directions.

Two types of seismic waves

1. Surface Waves (travel along the surface of Earth)

Love Wave

Rayleigh Wave

Love Wave

Surface waves that cause horizontal shearing of the ground. They move

in much the same way as a snake slithering across the ground.

Vibrate in a perpendicular direction compared to that of wave motion.

Rayleigh Wave

Surface waves that cause both horizontal (side-to-side) and vertical (up

and down) movement within the ground.

Vibrate in a rolling motion in the same direction as wave motion.

Most of the shaking felt from an earthquake is due to these waves and

these waves are the most destructive and cause the most damage.

2. Body Waves (originate from the focus and travel in all directions through the body of the Earth)

Primary Wave

Secondary Wave

Primary Wave (P-Wave)

P-waves move by compressing and expanding

(push-pull motion) the material as it travels. Much

like sound waves.

These waves can pass through solids, liquids, and

gases.

Vibrate in the same direction as wave motion.

These waves have the greatest velocity (6 km/sec)

and are the first to reach the seismograph stations.

Secondary Wave (S-Wave)

S-waves travel through material by shearing it or

changing its shape in the direction perpendicular to

the direction of travel.

Because liquids and gases have no shape, these

waves only pass through solids.

These waves are much like the waves on the ocean.

These waves travel through Earth slower (3.5

km/sec) and are the second to reach seismograph stations.

Other

Names

Movement

Through

Speed

Particle Motion

States of

Matter

Surface

Wave

(L wave)

long wave

Solid

Slowest (10%

slower than S

wave)

Shearing motion

(horizontal only)

Primary

Wave

(P wave)

push-pull

wave

Solid

Liquid

Gas

6 km/s

compressional

(expansion &

contraction)

Secondary

Wave

(S wave)

shear wave

Solid

3.6 km/s

Shearing motion

(horizontal only or

vertical only)

Richter and Mercalli Scales

An earthquake is the vibration of earth caused by a rapid release of energy. Slippage along pre-existing

faults and sudden movement within a subduction zone are commonly the cause of earthquakes.

Seismologist use two scales when classifying earthquakes.

1) Modified Mercalli Scale

Measures the Intensity of an earthquake on a twelve point scale.

In 1902 G. Mercalli developed a fairly reliable intensity scale which assesses the damage to various types

of structures at a specific location. Note that earthquake intensity is determined by several factors

including:

1) Strength of earthquake

2) Distance from epicenter

3) Nature of surface materials

4) Building design

The Mercalli scale does not give a true indication of the actual strength of an earthquake because the

amount of damage done to different places will largely depend on, the type of materials used and the

degree of construction of buildings and structures.

2) Richter Scale

Measures the Magnitude of an earthquake on a ten (10) point scale.

In 1935, Charles Richter introduced the concept of earthquake magnitude. Richter magnitude is

determined by measuring the largest amplitude (wave height) recorded on the seismogram.

Richter magnitude can be expressed in two ways:

1) wave amplitude

2) energy released

Wave amplitude increases tenfold (10X) with each increase in Richter magnitude.

While it is correct to say that for each increase in 1 in the Richter Magnitude, there is a tenfold (10X)

increase in amplitude (height) of the wave; It is incorrect to say that each increase of 1 in Richter

Magnitude represents a tenfold (10X) increase in the size of the Earthquake. A better measure of the size

of an earthquake is the amount of energy released by the earthquake.

Each increase in 1 in Richter Magnitude represents a 10 fold increase in the wave amplitude (height).

Thus, a magnitude 7 earthquake measures 10 times more amplitude than a magnitude 6 earthquake. A

magnitude 8 earthquake measures 10 x 10 (or 100 times) more amplitude than a magnitude 6 earthquake.

And so on.

Energy released increases thirtyfold (30X) with each increase in Richter magnitude. Each increase in 1

in Richter Magnitude represents a 30 fold increase in the energy released (size). Thus, a magnitude 7

earthquake releases 30 times more energy than a magnitude 6 earthquake A magnitude 8 earthquake

releases 30 x 30 or 900 times more energy than a magnitude 6 earthquake. And so on..

Richter

Magnitude (energy

released)

Uses instruments

(seismographs)

uses Arabic

values

open-ended

scale

Modified

Mercalli

Intensity (amount

of destruction)

Uses human

observations

uses Roman

numerals

closed scale

Locating the Epicenter

The source of an earthquake is called the focus, which is an exact location within Earth were seismic

waves are generated by sudden release of stored elastic energy. The epicenter is the point on the surface

of Earth directly above the focus. This is the location that scientist calculate. The focus is directly below,

however, scientist can not determine its depth.

To locate the position of an earthquakes epicenter, we need a seismogram reading from at least three

different seismograph stations and a travel - time graph which shows the speed of both P- and S-waves

Step 1: 3 Seismogram Records

Find the difference in arrival time between the P-wave and the S-wave.

To do this, refer to each of the three seismogram readings and record arrival times for P-wave and S-

wave.

Step 2: Record Difference in Arrival Times

Subtract the arrival time of

the S-wave from the arrival

time of the P-wave.

27 minutes – 23 minutes = 4

minutes

Thus, the S-wave arrived 4

minutes after the P-wave

Next you plot 4 minutes on

the travel-time graph

Slide the yellow line

representing 4 minutes up

between the P-wave and S-

wave lines until it fits

between the two lines.

Read off the distance below.

Distance = 3000 km.

Step 3: Record Distance to the Epicenter:

P-

Wave

s

S-

Wave

s A

r

r

i

v

a

l

t

i

m

e

o

f

P

-

w

a

v

e

s

(

2

3

M

i

n

u

t

e

s

)

Arrival time

of P-waves

(23 Minutes)

Arrival

time of S-

waves (27

Minutes)

The S-P interval tells us the distance to the epicenter from each seismograph station where the earthquake

was recorded. The epicenter of the earthquake is located 2500 kilometers from the first seismograph

station. You need to do the same procedure for two other seismograph stations

Seismograph station 1 = 3000 km

Assume that the other readings are:

Seismograph station 2 = 3500 km

Seismograph station 3 = 4500km

Step 4: Use Triangulation to Pin Point the Epicenter:

The distance to the epicenter from each seismograph station is;

Station #1 -- d1 = 2500km

Station #2 -- d2 = 3500km

Station #3 -- d3 = 4500km

At each station we can draw a circle on a map that has a radius equal to the distance to the epicenter from

each seismograph station. Three such circles will intersect in a point that locates the epicenter of the

earthquake

P-wave Shadow Zone

P-waves are bent (refracted) at the mantle-outer core boundary and bend deeper into the outer core. This

causes these P-waves to arrive at Earth’s surface some distance away from P-waves that do not travel

through the Outer core.

This resulted in an area from 105 degrees to 140 degrees from the focus of an earthquake where no P-

waves were detected. This is called a P-wave shadow zone

S-wave Shadow Zone

The S-wave shadow zone occurs because no S-

waves were detected after 105 degrees from the

focus. Since no direct S-waves arrive in this zone, it

implies that no S-waves pass through the core.

Therefore, the core or at least part of the core is in

the liquid state, since no S-waves are transmitted

through liquids.

Thus, the S-wave shadow zone is best explained by

a liquid outer core.

The Nature and Products of Volcanic Eruptions

Volcano: An opening in Earth’s crust through which igneous matter (lava, ash, cinder, and

gases) are erupted.

The Nature of Volcanic Eruptions

Factors determining the “violence” or explosiveness of a volcanic eruption

Composition of the magma

Temperature of the magma

Dissolved gases in the magma

The above three factors actually control the viscosity of a given magma which in turn controls

the nature of an eruption

Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity materials flow

with great difficulty)

Factors affecting viscosity

Temperature - Hotter magmas are less viscous

Composition - Silica (SiO2) content

Higher silica content = higher viscosity (e.g., felsic lava such as rhyolite)

10

5

10

5

Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as

basalt)

Dissolved Gases

o Gas content affects magma mobility

o Gases expand within a magma as it nears the Earth’s surface due to decreasing

pressure

o The violence of an eruption is related to how easily gases escape from magma

In Summary

Fluid basaltic lavas generally produce quiet eruptions

Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions

Lava Flows

Basaltic lavas are much more fluid

Types of basaltic flows

Pahoehoe lava (resembles a twisted or ropey texture)

Aa lava (rough, jagged blocky texture)

Dissolved Gases

One to six percent of a magma by weight

Mainly water vapor and carbon dioxide

Materials extruded from a volcano

Pyroclastic materials – “Fire fragments”

Types of pyroclastic debris

Ash and dust - fine, glassy fragments

Pumice - porous rock from “frothy” lava

Lapilli - walnut-sized material

Cinders - pea-sized material

Particles larger than lapilli

o Blocks - hardened or cooled lava

o Bombs - ejected as hot lava

Volcano General Features

Opening at the summit of a volcano

Crater - steep-walled depression at the summit, generally less than 1 km diameter

Caldera - a summit depression typically greater than 1 km diameter, produced by

collapse following a massive eruption

Vent – opening connected to the magma chamber via a pipe

Types of Volcanoes

Shield volcano

Broad, slightly domed-shaped

Composed primarily of basaltic lava

Generally cover large areas

Produced by mild eruptions of large volumes of lava

Mauna Loa on Hawaii is a good example

Ash and Cinder cone

Built from ejected lava fragments (cinders)

Steep slope angle

Form on the base of larger volcanoes and are the smallest of the three types of volcanoes.

Frequently occur in groups

Composite cone (Stratovolcano)

Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)

Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)

Composed of interbedded lava flows and layers of pyroclastic debris

Most violent type of activity (e.g., Mt. Vesuvius)

Often produce a nueé ardente

Fiery pyroclastic flow made of hot gases infused with ash and other debris

Move down the slopes of a

volcano at speeds up to

200 km per hour

May produce a lahar,

which is a volcanic

mudflow

A size comparison of the three types of volcanoes

Base

Slope Type of

Material

Rock

Types

Example

Shield

wide

< 5

degrees

lava flows (< 1%

pyroclastic)

basalt

Mauna Loa,

USA

Ash and

Cinder

narrow

30 - 40

degrees

ejected lava

and fragments

(pyroclastic)

scoria

basalt

Paricutin,

Mexico

Composite

intermediate

> 40

degrees

both lava and

pyroclastic

basalt

andesite

rhyolite

Mount St.

Helens, USA

Volcano

Type

Silica

Content

Viscosity Gas

Content

Eruption

Style

Plate

Boundary

Shield

least

(~50%)

least

least

(1-2%)

quiet, free flowing

lava

divergent

Ash and

Cinder

greatest

(~70%)

greatest

greatest

(4-6%)

violent and

explosive,

pyroclastic

convergent

and divergent

Composite

intermediate

(~60%)

intermediate

intermediate

(3-4%)

alternating

quiet, free flowing

lava and violent

and explosive,

pyroclastic

convergent

Volcanoes are associated with two of the three types of plate boundaries, these being convergent and

divergent boundaries. Very little volcanic activity is seen at transform fault boundaries. Volcanism

associated with plate tectonic activity are found in three areas on Earth;

1) Ridges (or spreading centers)

2) Subduction zones

3) Interior of tectonic plates.

1.Spreading Center (Rift) Volcanism

Rift eruptions generally flow out smoothly and fluidly because the basaltic lava generated in the upper

mantle contains few gases and has very little silica. Gases and a high percentage of silica cause lavas to

be viscous and explosive. Shield volcanoes are produced by these eruptions.

Rift eruptions produce the greatest volume of volcanic rock.

Rift volcanism can occur in two areas;

1) Beneath the oceans (Mid Atlantic Ridges)

2) Within continental plates (African Rift Zone)

Beneath Oceans

At ridges, ocean floor moves apart creating a rift (crack) through which basaltic magma moves up

through and forms new ocean floor. Basaltic magmas are produced at spreading centers.

The greatest volume of volcanic rock is produced within oceanic ridges where seafloor spreading is

active. Example; along the Mid Atlantic Ridge. Shield volcanoes are formed along ridges when basaltic

lava flows on the ocean floor. In some case these volcanoes can rise above sea level and form a volcanic

Island. Example; Surtsey near Iceland.

Within Continents

Some spreading centers located within continents, cause the land

mass to separate. African rift zone.

When land becomes separated, water moves in between and

forms a sea, for example, Mediterranean Sea. These seas may

eventually form an ocean.

2. Subduction Zone Volcanism

At trenches, ocean floor bends and moves downward into the upper mantle. At depths of 100 km partial

melting of the ocean crust and mantle takes place. Basaltic and andesitic magmas are produced at

subduction zones. After great quantities of magma are produced, the molten rock moves upward toward

the surface because it is less dense than the surrounding rock.

Rift

Valley

Ocean – Ocean convergent boundaries and Ocean – Continent convergent boundaries have this type of

volcanism.

Ocean – Ocean Volcanism

Ocean crust subducts beneath ocean crust. Basaltic magmas are produced and burns upward toward the

surface forming a chain of volcanoes called a “volcanic island arc” parallel to the trench.

Examples include; Islands of Japan.

Ocean – Continent Volcanism

Ocean crust subducts beneath continental crust. Andesitic and Granitic magmas burns upward into the

continental crust adding to mountain systems. This type of volcanism is very explosive and is mainly

found surrounding the Pacific Ocean.

Examples include; volcanoes in the Andes mountains

At a depths of 100 km, the subducting ocean crust starts to melt. This generates magmas that are thick

and contains large amounts of gases. As a result, subduction eruptions at ocean-continent boundaries are

very explosive and produce composite volcanic cones. Most of the world’s volcanoes are of this type and

border the Pacific Ocean, called the Pacific Ring of Fire.

3 Intraplate Volcanism

This type of volcanism is produced by rising hot mantle material called Hot Spots.

Hotspots are stationary plumes of rising magma. Plates above hotspots move and the result are chains of

volcanoes on either the land or ocean floor.

Hot spots can be found under continents and the ocean floor. Intraplate volcanism occurs in two areas;

1) Beneath the oceans (Hawaiian Islands)

2) Within continental plates (Yellowstone National Park)

The Hawaiian island chain, in the middle of the Pacific Ocean plate, has been formed by a hotspot.

Yellowstone National Park and the surrounding area, in the middle of the North American plate, host a

chain of volcanoes due to the presence of a hotspot.

The Yellowstone National Park and Hawaiian island chain examples differ based on volcano type, molten

composition, and eruption style.

Beneath Oceans

Hotspots within the mantle cause magma to move upward and flow onto the ocean floor forming shield

volcanoes and volcanic islands. An example includes the Hawaiian Islands

Within Continents

Hotspots within the mantle cause magma to move upward and flow onto the continental surface forming

volcanoes and lava plateaus.

Yellowstone National Park has this type of volcanism.

Lava Plateaus:

A lava plateau is a geological landform that is created when large amounts of runny lava leak from large

cracks in the earth called fissures. The lava may run and spread evenly over large areas of land to create a

unique look. Two examples of lava plateaus in the United States are Lassen National Park in California,

and the Columbia River Plateau in Washington.

Lava plateaus forms from one or more fissures, which are fractures that extend to the depths of the

mantle.

These do not form from volcanic craters.

Volcanoes and climate

Long-term global effects of volcanic activity include:

1. Volcanoes release gases like carbon dioxide and water vapour, which in large amounts, could

contribute to global warming and climate change.

2. Volcanoes release sulphur dioxides and nitrogen oxides, which can mix with water vapour in the

atmosphere leading to increased, long term, acid precipitation.

3. Volcanoes create fertile soils which enhance agriculture.

4. Volcanoes, depending on number, frequency, and eruption size, could contribute to global cooling and

the origin of ice ages, due to the blocking out of the sun. Plants failing to photosynthesize could result in

total collapse of food webs and ecosystems.

Examples of volcanism affecting climate

• Mount Tambora, Indonesia – 1815

• Krakatau, Indonesia – 1883

• Mount Pinatubo, Philippines - 1991

Short term Effects of Volcanic Activity

Lava Flows could damage and destroy populated areas. Examples include villages and roads buried by

lava flows.

Volcanic sediment, such as ash and cinder, could flow when saturated with water and cause deadly

mudslides called lahars.

Tephra and hot poisonous gases could suffocate and bury people. Such an event happened when Mount

Vesuvius erupted and destroyed the Roman city of Pompeii.

Violent undersea or coastal eruptions could cause a tsunami. These gigantic water waves can flood

coastal areas.

Volcanic material can block out sunlight causing short-term cooling.

Volcanic material can disrupt air travel.

Erupting volcanic material can quickly create new land.

Volcanoes and associated material can be destructive (e.g., death to organisms, property damage, road

damage).

Volcanoes release sulphur dioxides and nitrogen oxides which can mix with water vapour in the

atmosphere leading to increased, short term, acid precipitation.

Careers related to plate tectonics, earthquakes, and volcanoes.

(i) structural geologist: Structural geology is the study of the three-dimensional distribution of rock units

with respect to their deformational histories. The primary goal of structural geology is to use

measurements of present-day rock geometries to uncover information about the history of deformation in

the rocks

(ii) volcanologist A volcanologist is a person who studies the formation of volcanoes, and their current

and historic eruptions. Volcanologists frequently visit volcanoes, especially active ones, to observe

volcanic eruptions, collect eruptive products including tephra (such as ash or pumice), rock and lava

samples

(iii) seismologist Seismologists are Earth scientists, specialized in geophysics, who study the genesis and

the propagation of seismic waves in geological materials. These geological materials can range from a

laboratory sample to the Earth as a whole, from its surface to its core.

(iv) A geomorphologist is a person who studies how the earth's surfaces were formed by rivers,

mountains, oceans, air, or ice and how these elements will change the landforms in the future. A

geomorphologist gathers organic materials from the earth like sediments from mountains, water from

streams or rivers, and pollen from flowers. They are trying to figure out if any these materials from the

earth had an effect on way that some lands are shaped.

(v) geochemist A wealth of information is buried in the liquids, gases, and mineral deposits of rock.

Geochemists must understand this information and use it to make decisions about a range of industrial

and scientific research applications. Understanding the chemical composition of rocks tells oil companies

where to drill for oil, enables scientists to put together broad-based theories about the way the earth is

changing, helps environmental management companies decide how to dispose of toxic or hazardous

substances, and steers mining companies toward using natural resources with a minimum environmental

impact.

(vi) A geophysicist is someone who studies the Earth using gravity, magnetic, electrical, and seismic

methods. Some geophysicists spend most of their time outdoors studying various features of the Earth,

and others spend most of their time indoors using computers for modeling and calculations. Some

geophysicists use these methods to find oil, iron, copper, and many other minerals. Some evaluate earth

properties for environmental hazards and evaluate areas for dams or construction sites. Research

geophysicists study the internal structure and evolution of the earth, earthquakes, the ocean and other

physical features using these methods.

(vii) Petrologists use different analyses of rocks to determine the root names of fine-grained igneous

rocks, quantify and understand differentiation trends, understand the origin and genesis of igneous

magmas, and correlate physical properties such as viscosity and density with chemical composition.

Constant advances in analytical technology make experimental, geochemical and isotopic data available

faster and cheaper to obtain. Petrologists investigate the composition, structure, and history of rock

masses forming the Earth's crust. They usually study the differences between igneous, sedimentary and

metamorphic rocks. They apply their findings to such fields of investigation as causes of formations,

breaking down and weathering, chemical composition and forms of deposition of sedimentary rocks,

methods of eruption, and origin and causes of metamorphosis.

(viii) Sedimentologist: Sedimentology encompasses the study of modern sediments such as sand, mud

(silt), and clay, and the processes that result in their deposition. Sedimentologists apply their

understanding of modern processes to interpret geologic history through observations of sedimentary

rocks and sedimentary structures.[5]