Plate Tectonics Study Guide

PLATE TECTONICS STUDY GUIDE

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According to the plate tectonic model, the surface of the Earth consists of a series of relatively thin, but rigid, plates which are in constant motion. The surface layer of each plate is composed of oceanic crust, continental crust, or a combination of both. The lower part

consists of the rigid upper layer of the Earth's mantle. The rigid plates pass gradually downwards into the plastic (soft) layer of the mantle, the asthenosphere. The plates may be up to 15 km thick if composed of oceanic crust or 40 - 70 km

incorporating continental crust.

Plates move at different velocities, The African plate moves about 3-5 cm per year, whereas the Australian plate moves about 6-7 cm per year. Most of the Earth's tectonic, seismic and volcanic activity occurs at the boundaries of neighboring plates.

PLATE BOUNDARIES - Tectonic plates interact in various ways as they move across the asthenosphere, producing volcanoes, earthquakes, and mountain systems.

Types of plate tectonic boundaries include:

 CONVERGENT - Plates move toward one another, creating a compressional environment. Characterized by deformation, volcanism, metamorphism, mountain building, seismicity, and important mineral deposits.

Three possible kinds of convergent boundaries:

1. Oceanic-Oceanic Boundary - One plate is subducted, initiating andesitic ocean floor volcanism on the other. Can eventually form an island arc volcanic island chain with an adjacent deep ocean trench. Characterized by a progression from shallow to deep focus

earthquakes from the trench toward the island arc.

2. Oceanic-Continental Boundary - Oceanic plate is dense and subducts under the lighter continental plate. Produces deep ocean trench at the edge of the continent. About half the oceanic sediment descends with the subducting plate; the other half is piled up against the continent. Subducting plate and sediments partially melt, producing andesitic or granitic magma. Produces volcanic

mountain chains on continents called volcanic arcs and batholiths. Part of the oceanic plate can be broken off and thrust up onto the continent during subduction. Characterized by shallow, intermediate, and deep focus earthquakes.

3. Continental-Continental Boundary - Continental crust cannot subduct, so continental rocks are piled up, folded, and fractured into very high complex mountain systems. Characterized by shallow-focus earthquakes, rare intermediate-focus earthquakes. Practically no volcanism.

 TRANSFORM - Plates move laterally past one another. Largely shear stress with

lithosphere being neither created nor destroyed. Characterized by faults that parallel the direction of plate movement, shallow-focus earthquakes, intensely shattered rock, and no volcanic activity. Shearing motion can produce both compressional stress and

tensional stress where a fault bends. Transform faults occur on land, connect segments of the oceanic ridge, and provide the mechanism by which crust can be carried to

subduction zones.

The mechanisms responsible for plate movement are thought to be:

 Convection Cells - Thought to be primary driving force for plate motion. Unequal

heat distribution in the mantle may produce convection cells below the lithosphere. Hot material rises (correlates to spreading center), spreads laterally, cools and sinks deeper into the mantle to be reheated.

 Ridge Push - the force applied to plates at spreading centers. As plates separate, new, hotter material is extruded into the gap between plates. The elevated temperatures lower the density of the recently deposited material, causing it to float higher in the mantle; hence the formation of ridges. Gravity then takes over, drawing the ridge material down and away from the spreading center, thereby widening the gap for hotter mantle material to upwell

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 Be able to list the evidence for plate tectonics including the early evidence related to continental drift, palaeomagnetism, and sea floor spreading.

 Know the concept of sea floor spreading and its relationship to the Plate Tectonic Theory.

 Be able to describe the basic principles of Plate Tectonics.

 Be able to describe the basic plate tectonic boundaries and the forces at work there including the presence of earthquake and volcanic activity.

 Understand the basic driving forces or mechanisms for Plate Tectonics.

 Understand the difference of hot spot volcanism relative to other volcanic activity such as sea floor spreading or subduction.

Be able to answer the following questions:

1. What kinds of evidence did Alfred Wegener use in support of the idea of "continental drift"? 2. The ages of sea floor rocks show a regular pattern around an oceanic ridge. Describe this pattern

3. The magnetic properties of sea floor rocks also show an interesting pattern. How does this pattern compare to the age pattern and how is the pattern useful in understanding plate tectonic motion?

4. Explain the concept of sea floor spreading as proposed by Harry Hess.

5. What is the distinction between "lithosphere" and "asthenosphere"? How are these terms different from the terms "crust" and "mantle" which are also used to describe the Earth?

6. Name the three basic types of plate tectonic boundaries. Include a description of basic plate motion as well as the type of force producing the interaction along the plate boundary.

7. What tectonic process produces mid-ocean ridges?

8. Convergent boundaries are the most complex of the plate tectonic boundaries. Describe the three different kinds of convergent boundaries.

9. What are believed to be the basic mechanisms driving plate tectonic motion?

10. How is hot spot volcanism different from volcanism associated with sea floor spreading or subduction? 11. What kinds of information have hot spots provided in the understanding of plate tectonic motion?

EARTHQUAKES

releasing the elastic energy as seismic waves radiating outward from the break. The greater the stored strain, the greater the release of energy.

 Seismic and Volcanic Activity - The coincidence of many active volcanic belts with major belts of earthquake activity indicates that volcanoes and

earthquakes may have a common cause. Plate interactions commonly cause both earthquakes and volcanoes.

EARTHQUAKE FOCUS AND EPICENTER

 Focus - Point within the Earth from which energy is released. The size of the earthquake source cannot be accurately determined. Deepest earthquakes are at the bottoms of active subduction zones.

 Epicenter - The point on the Earth's surface directly above the focus. Given a geographic location.

About 90% of all earthquakes have depths < 100 km. Earthquakes can be grouped into three categories based on the depth of their foci:

 Shallow focus - Foci are less than 70 km depth. Most destructive earthquakes.

 Intermediate focus - Foci are between 70 and 300 km depth.

 Deep focus - Foci are greater than 300 km.

FREQUENCY AND DISTRIBUTION OF EARTHQUAKES

 Most earthquakes (about 95%) occur in seismic belts along the margins of tectonic plates. Enormous pressures are built up at plate boundaries, particularly convergent and transform boundaries. The pressure is ultimately released as earthquakes.

 Frequency - More than 150,000 earthquakes strong enough to be felt by someone are recorded

annually worldwide. An additional 900,000 earthquakes occur annually that are too small to be felt or recorded as separate events.

 Earthquake Distribution - Occur at convergent (primarily), divergent, and transform plate boundaries (may occur in clusters as plates shift position).

SEISMIC BELTS - There are two major earthquake belts on the surface of the Earth:

1. Ring of Fire- About 80% of all earthquakes occur in this belt, including the world's most devastating in terms of life and property loss. 2. Mediterranean – Asiatic Belt - Accounts for

about 15% of all earthquakes.

SEISMIC WAVES

There are two general types of seismic earth waves:

Body Waves - Speed decreases with increasing density of rock and increases with increasing rock elasticity. Rock elasticity increases faster than density with depth. Two kinds of body waves:

1. Primary (P) Waves - Compressional waves that vibrate parallel to the direction of wave movement and travel through solids, liquids, and gases. Fastest seismic wave.

2. Secondary (S) Waves (also known as Shear Waves) - Vibrate perpendicular to the direction of wave movement. Only travel through solids (liquids have no shear strength). Slower than p-waves.

Surface Waves - Travel along the outer layer of Earth. Rolling, shaking motion causes most of an earthquake's damage. They are the slowest seismic waves. Two major types:

1. Rayleigh (R) Waves - Behave like water waves with elliptical motion of material in wave. Generally slower than Love waves. Most destructive kind of seismic wave.

2. Love (L) Waves - Involve shear motion in a horizontal plane.

Locating an Earthquake

Seismograms show that P waves always arrive at a recording station first, followed by S waves, and finally surface waves (Love before Raleigh).

Time-Distance Graphs (Diagram to the right)

Plots of average P- and S-wave travel times against distance from the focus. Allow the distance to the earthquake source to be calculated. The farther the distance between the seismograph station and the earthquake source, the greater the difference in arrival times between the P and S waves.

Travel Times

If the Earth were composed of homogeneous material, seismic waves should travel in straight line paths. Variations can give clues to the Earth's interior layering.

 Experiments have shown that P waves are slowed down by liquids, and S waves cannot pass through liquids. Strange behavior of seismic waves could be explained if the Earth's interior contained a liquid layer (outer core).

Triangulation Method

Time-distance graphs allow the source of an earthquake to be calculated. The epicenter can be determined from the time elapsing between the arrival of P and S waves at a minimum of three

different seismograph stations. Time of the earthquake can be determined by knowing the speed of P waves. Intersection of three circles representing distance from seismic station locates epicenter.

Measuring Earthquake Intensity and Magnitude

 Earthquake Intensity - Measure of an earthquake's destructive power (size and strength). Measured by the Modified Mercalli Intensity Scale which qualitatively links earthquake power to its effect on buildings. Somewhat subjective.

Intensity depends on:

1. Amount of energy released by the earthquake. 2. Duration of shaking.

3. Distance from the epicenter. 4. Focal depth of earthquake.

Earthquake Magnitude – is a quantitative measure of the energy released by an earthquake at its source. Measured by the Richter scale magnitude (each step = 32 times increase in energy = 10 times increase in

earthquake wave amplitude). Magnitude 6 earthquake = Megaton nuclear bomb. Largest recorded earthquake was magnitude 9.5 (1960 Chile). Most earthquakes have a magnitude of < 2.5. The largest earthquakes (> 8.0

magnitude) occur on average every 5 years. Magnitude determined by:

 Measurement of size (amplitude) of seismic waves.

 Distance from epicenter - Magnitude decreases with distance from source.

Destructive Effects of Earthquakes

Of the hundreds of thousands of earthquakes that occur every year, only one or two are likely to cause severe results. Destructive effects depend on earthquake magnitude, distance from epicenter, time of day, geology of area, type of building construction, and duration of shaking. The most destructive earthquakes occur during working and school hours in densely populated areas. Earthquake hazards include:

 Ground Shaking - Causes the most damage and loss of life. Buildings on bedrock move as a unit with the ground and suffer the lower damage during an earthquake. Worst damage occurs to buildings on poorly consolidated material or water-saturated sediment. Structural damage depends on:

1. Earthquake magnitude - Determines the intensity and duration of vibrations.

2. Underlying geology - The nature of material supporting the foundation determines how much damage occurs. Poorly consolidated material is subjected to longer shaking and greater S-wave amplitudes. Water-saturated sediments tend to behave like a fluid and flow when shaken (liquefaction).

3. Design of the structure (materials and type of construction). Adobe and mud-walled structures are weakest. Un-reinforced brick and concrete buildings have no flexibility and tend to collapse.

 Fire - Often more dangerous that the earthquake itself. Severed gas and water lines can lead to devastating fires. Valves can be installed to cut off lines from breaks.

 Tsunami (seismic sea wave) - Most result from vertical displacement of the ocean floor or from submarine landslides during an earthquake. May also be generated from volcanic explosions (Krakatoa). Can reach speeds up to 800 km/hr and heights up to 65 m. A seismic sea wave warning system was established in 1948 for the Pacific basin after a 1946 tsunami killed

154 in Hawaii.

Earthquake Prediction

Successful prediction would include time frame, magnitude, and location. Remains an elusive goal, although measurement techniques have improved dramatically in the last 50 years:

 Seismic Risk Maps - Indicate the likelihood and potential severity of future earthquakes. Based on historical records of past earthquakes and the

distribution of known faults. Identify areas of seismic gaps and earthquake periodicity.

 Earthquake Precursors - Most earthquakes have precursors (short term and long-term changes in the Earth that take place prior to an earthquake).

Unfortunately no reliable precursors have been identified.

Be able to answer the following questions.

1. How do earthquakes relate to plate tectonics? 2. What does the term “elastic limit” refer to? 3. What causes earthquakes?

4. What are the different types of earthquake waves? What are their characteristics? 5. What is a seismograph and how does it work?

6. How do you interpret a seismogram?

7. How do you locate the epicenter of an earthquake? Know how to read a seismogram, calculate P-S travel time differences, calculate the distance to an earthquake, and triangulate an earthquake’s epicenter. 8. What is the difference between intensity and magnitude? What scales are used to measure each?

9. By how much more energy does an earthquake have with a change between one whole number increase of magnitude on the Richter scale?

10. What are hazards associated with earthquakes? 11. What are tsunamis? How do they form?

12. How does the type of ground material that you live on determine the intensity of the earthquake? 13. Where are the areas of major earthquake risk in the world?

14. Where are the areas of major earthquake risk in the United States?

STRUCTURE OF THE EARTH

Study of the travel of seismic waves through the Earth has given information about its interior. Body waves are refracted (bent) and reflected at boundaries between two different kind of materials (discontinuity) and travel more slowly through dense material than through less dense material. From the study of seismic waves, the Earth is divided into three major layers:

1. Crust - Thin, outer layer of the Earth. Composed of solid rock. Base of the crust is marked by the

Mohorovicic discontinuity (Moho), which is marked by increased body wave speed below the discontinuity and by partial refection of body waves at the boundary. Two basic kinds of crust are indicated by seismic waves:

a. Continental Crust - Averages about 35 km thick, but ranges from 20-90 km. Average composition consists of granite with low density.

b. Oceanic Crust - Ranges from 5-10 km thick. Composed primarily of basalt with higher density than continental crust.

2. Mantle - Thickest layer of the Earth (contains > 80% of the Earth's volume). Increase in body wave speed below the Moho indicates high density rock. The mantle is essentially solid, but portions of it are capable of slow flow. Bottom of mantle is about 2900 km deep.

3. Core - Composed of a liquid outer core (Shadow Zones - S waves disappear when traveling deeper than 2900 km and P waves are slowed down) and a solid inner core (P wave velocity increases again). Believed

to be composed primarily of iron mixed with nickel, cobalt, oxygen, sulfur, silica, and carbon. The Earth's magnetic field is caused by the circulation of the outer liquid core, but the exact mechanism is unknown. Core extends to the Earth's center at 6370 km depth.

Lithosphere

The lithosphere is the brittle uppermost shell of the earth, broken into a number of tectonic plates. The lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the mantle. The crust and mantle are separated by the Moho or Mohorovicic discontinuity. The thickness of the lithosphere varies from to around 1 mi (1.6 km) at the mid-ocean ridges to approximately 80 mi (130 km) beneath older oceanic crust. The thickness of the continental lithospheric plates is probably around 185 mi (300 km) but is uncertain due to the irregular presence of the Moho discontinuity. The lithosphere rests on a soft layer called the asthenosphere, over which the plates of the lithosphere glide.

Asthenosphere

The asthenosphere is solid even though it is at very hot temperatures of about 1600 C due to the high pressures from above. However, at this temperature, minerals are almost ready to melt and they become ductile and can be pushed and deformed like silly putty in response to the warmth of the Earth. These rocks actually flow, moving in response to the stresses placed upon them by the churning motions of the deep interior of the Earth. The flowing asthenosphere carries the lithosphere of the Earth, including the continents, on its back.

MohorovičićDiscontinuity

The Mohorovičić discontinuity, usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. The Moho serves to separate both oceanic crust and continental crust from underlying mantle. The Moho mostly lies entirely within the lithosphere; only beneath mid-ocean ridges does the Moho also define the

lithosphere-asthenosphere boundary. The Mohorovičić discontinuity was first identified in 1909 by Andrija Mohorovičić, a Croatian seismologist, when he observed the abrupt increase in the velocity of earthquake waves (specifically P-waves) at this point.

Be able to answer the following questions:

1. Think of a specific example of what seismic waves tell us about the interior of the earth.

2. Be able to draw a picture of Earth’s interior showing Inner Core, Outer Core, Mantle, and Crust, as well as the Lithosphere, Asthenosphere, “Moho”

3. Where does lava come from?

4. What does the term “convection” mean?