Plate Tectonics Unit: Tectonic Plates, Continental Drift and Earthquakes

Text: Chapters 1, 10, 12

Lab: Laboratory 2

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Plate Tectponics Unit

Purpose: To explain the development and significance of plate tectonics, recognize and interpret geological structures and identify applications of seismology.

-By the end of this unit, students are expected to be able to:

1. Analyse and evaluate applications of seismology.

a) Describe fault creep and elastic rebound as they relate to seismic activity.

b) Describe the generation and propagation of body waves and surface waves.

c) Distinguish between magnitude and intensity.

d) Compare and contrast the Richter and Mercalli scales.

e) Use seismograms to determine the distance and location of an earthquake.

f) Assess the seismic risks for a particular area using: Geographic location, topography, ground strength, rock types, proximity to faults, construction design.

g) Evaluate methods of earthquake prediction.

2. Demonstrate knowledge of Earth’s layers.

a) Give evidence to support the conclusion that Earth is layered.

b) Describe the characteristics of the various layers of Earth.

3. Relate rock formations and structures to the forces that create them

a) Distinguish between faults and joints.

b) Distinguish between dip-slip, strike-slip and transform faults from maps, cross sections or photographs.

c) Relate compressional, tensional and shear forces to the various types of faults and folds.

d) Interpret the dip and strike of an outcrop to determine subsurface structures.

e) Diagram domes, basins, anticlines, synclines, over-turned folds and unconformities and identify these structures in maps, cross-sections or photographs.

4. Analyse structures, processes and evidence that support plate tectonic theory.

a) Outline evidence for continental drift theory.

b) Explain seafloor spreading and outline evidence to support it.

c) Relate plate motion to mantle convection and slab pull.

d) Describe the origin of magma formed during plate tectonic processes.

e) Relate volcanic activities and features to plate tectonic theory.

f) Describe the geological activities that occur at lithospheric plate boundaries (earthquakes).

Geology 12: Plate TectonicsPart A: Continental DriftThe origins of Earth’s continents, mountain belts, ocean basins, rifts and trenches had been theorized about for hundreds of years before Alfred Wegener proposed his Continental Drift Hypothesis.

Wegener’s hypothesis was not immediately accepted and, in fact, there were a number of other theories competing with his in the early 1900’s. Here are a few of those other theories:

a) Shrinking Earth Hypothesis:

-Earth is cooling from a molten state and so is shrinking.

-Continents are moving together, with ocean basins squeezed and folded between continents to form mountains.

b) Expanding Earth Hypothesis:

-Earth was once much smaller and covered by a granite crust, which has split apart as Earth has expanded. This has created the shape of the modern continents and explains why Africa and South America are complementary.

How was Wegener’s hypothesis different from the hypothesis above?

-All continents were once part of a supercontinent (Pangea), parts of which have drifted apart to form modern continents.

Wegener’s Continental Drift Hypothesis encountered resistance from the scientific community for over 50 years because he could not explain the mechanism behind the continents’ movements. However, he did have a few pieces of key evidence that others would expand on:

-Matching continental coastlines/shorelines

-Matching mountain ranges extending across separated continents (Rock types too)

-Fossils

-Glacial features (Paleoclimate) found on separate continents.

Wegener died tragically on the Greenland Ice Sheet in 1930, but in the 1950s and 1960s more evidence was added to lend support to his idea:

a) Paleomagnetism

Earth produces a magnetic field in much the same way that a bar magnet does, with its magnetic poles roughly corresponding to its geographic poles.

Paleomagnetism is the record of past magnetic fields preserved in rocks created by iron-rich minerals in cooling magma aligning themselves to Earth’s magnetic field. This record will be retained as long as the rock is not re-heated above a threshold temperature (Curie Point).

Geologists found that rocks from different time periods exhibited different paleomagnetisms. There were two possible explanations for this:

i) Polar Wandering: The movement of Earth’s poles around the geographic poles.

This theory was largely discredited as the magnetic poles do not move far enough away from the geographic poles to cause the paleomagnetisms discovered. Additionally, polar wandering curves in North American and Europe are identical. This means that either two North Poles existed in the past and followed the same path or that the continents were once joined.

ii) Continental Drift: Was becoming more accepted given the above evidence, but the theory was still missing the mechanism for continent movements.

b) Seafloor Spreading

The ocean floor was mapped in the 1950s and 1960s and the Mid-Atlantic Ridge was re-discovered (scientists had known about it since the mid-1800s).

What is the significance of the Mid-Atlantic Ridge?

-Ridges are sites of ocean crust creation as magma upwelling from the mantle breaks through the crust and cools.

What is the significance of ocean trenches?

-Areas of subduction where ocean plates are re-cycled back into the mantle.

If the above is true, what supporting pieces of evidence should geologists have discovered from the ocean’s crust?

-Youngest rocks should be closer to the ridges.

-Ocean crust on either side of a ridge should be symmetrical in age, etc.

-Rocks further from the ridges should have more sediments deposited on top of them.

-Oceanic crust should be younger than continental crust, on average.

c) Geomagnetic Reversals

Paleomagnetisms reveal that Earth’s poles periodically reverse their locations and evidence of this phenomenon was crucial for demonstrating that seafloor spreading does occur.

d) Tectonic Plates

The last idea to be developed that would provide Wegener right was the concept that Earth is made up of rigid lithospheric plates that move in relation to one another. With this, all of the independent observations discussed above made sense in relation to one another and the modern Plate Tectonics Hypothesis was created.

Part B: Tectonic Plates

a) Mechanism of Tectonic Plate Movement

The method by which tectonic plates move remains controversial, with three main mechanisms being proposed:

i) Mantle Drag: Friction between the top of a convection current and bottom of the oceanic lithosphere. Amount of

friction and exact location of convection cells in Earth’s interior is unknown.

ii) Ridge Sliding: Ocean plates move down and away from rifts due to gravity.

iii) Slab Pull: Subduction of old, dense oceanic lithosphere pulls the rest of the plate along. Currently favoured mechanism, but may occur in conjunction with the other two.

b) Evidence of Tectonic Plate Movement

Other than paleomagentism evidence of geomagnetic reversals, we know that tectonic plates do in fact move based on evidence provided by:

i) Earthquakes

-Subduction zones are associated with earthquakes as the descending oceanic plate interacts with the continental.

-The depth of the oceanic plate’s subduction is revealed by the focus of an earthquake.

ii) Ocean Drilling

-The oldest ocean floor sediments sitting on top of the oceanic plate are found furthest from ocean rifts, which is consistent with the sea-floor spreading hypothesis. These sediments are also become thicker the farther they are located from the rift.

-Analysis of sediments revealed that ocean basins are relatively young features (180 myo max).

iii) Hot Spots

-The Hawaiian Island-Emperor Seamount Chain chart the path of the Pacific Plate over a stationary hot spot/mantle plume.

c) Types of Plate Boundaries (Text 288-295)

Investigate the three types of plate interactions that occur on Earth and the features they form by completing the following chart. Use the following link as a resource:

The Geological Society: Plate Tectonics

http://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Convergent/Continental-Collision

i) Divergent Plate Boundaries

Interacting Plates Example and Diagram Features Volcanism

Oceanic-Oceanic Mid-Atlantic Ridge Underwater

Mountain Ranges and Rift Valleys

Basalt (Fissure eruption)

ii) Convergent Plate Boundaries

Interacting Plates Example and Diagram Features Volcanism

iii) Transform Plate Boundaries

Interacting Plates Example and Diagram Features Volcanism

d) Types of Faults (Lab Book 32-34)

Investigate the three types of faults by completing the following chart:

Stress Applied Type of Fault and Diagram Plate Boundary

Geology 12: Earthquakes

Part A: What is an Earthquake?

Earthquakes are vibrations (seismic waves) in the Earth caused by the rapid release of energy as a result of slippage along a fault.

What is the difference between an earthquake’s focus and epicentre?

-Focus is where the displacement of earth’s crust occurs. Seismic waves radiate out in all directions from here.

-Epicentre is the surface location directly above the focus.

Elastic Rebound is the mechanism that produces earthquakes. Use the diagram right to explain how this process works:

a) Original Position

-Existence of a fault (or convergent boundary)

b) Buildup of Strain

-Movement of tectonic plates build up strain on both sides of the fault. Demonstrated by deformation of rocks on either side of the fault.

c) Slippage

-Rocks store elastic energy as they bend and eventually this overcomes the frictional force holding the rocks together.

-Slippage occurs at the weakest area, exerting stress on other areas along the fault, causing additional slippage.

d) Strain Release

-Slippage releases the built up energy and the rocks elastically snap back to their original locations = earthquake vibrations.

Part B: Types of Seismic Waves

There are two groups of seismic waves produced by earthquakes.

a) Body Waves

These seismic waves travel through Earth’s interior and are divided into two types:

i) Primary Waves:

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ii) Secondary Waves:

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b) Surface Waves (Long or Rayleigh)

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What does the seismograph record indicate about the differences between seismic waves?

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Part C: Locating the Epicentre

Seismographs are also useful as the can be used to locate earthquake epicentres through recording the difference in arrival times of P and S waves

In the Time-Distance Graph below, a difference in 5 minutes between P and S waves means the epicentre was located 3800km away.

By using the Time-Distance Graphs for three of more seismic stations we can figure out exactly where the epicentre occurred on Earth. Each circle represents the epicentre distance from a station and where they intersect is epicentre (Triangulation).

Part D: Measuring Earthquake Size

There are two ways to measure an Earthquake:

i) Intensity

Measure of the degree of shaking based on the amount of damage caused. This is a qualitative scale and was the first type of earthquake measurement system developed.

Modified Mercalli Intensity Scale was developed in 1902 and is based on the scale below:

What might be some problems or shortfalls of this method?

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ii) Magnitude

Measures the amount of energy released at the focus. This is a quantitative scale that relies on data provided by seismic records.

There are several different types of magnitude scales:

1. Richter Scale

Based on the amplitude of the largest seismic wave recorded on a seismograph.

Seismic waves weaken as the distance between the epicentre and seismograph increase. Richter developed his scale to account for the decrease in wave amplitude with the increase in distance.

Richter scale is a magnitude scale, where an increase in one unit is actually a 10x increase in wave amplitude and 32x increase in energy released!

Problems with it include that it under-estimates the magnitude of large earthquakes as it only uses the largest amplitude recorded. This only reflects one moment of the earthquake and not the entire amount of energy released at the fault.

2.Moment Magnitude Scale

Derived from the amount of displacement along a fault zone rather than measuring ground motion.

Calculated using a combination of factors: Average amount of displacement around a fault, the area of the rupture surface and the shear strength of the faulted rock.

It is becoming accepted as it: a) Adequately estimates large earthquake magnitudes; b) Better reflects the total energy released during an earthquake; c) Can be verified through independent methods (field studies and seismic methods using long-period waves).

Part E: Destruction Caused by Earthquakes

Take your own notes on the following destruction caused by earthquakes using your text and lab book.

a) Destruction from Seismic Vibrations

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b) Tsunami

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c) Landslides

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d) Fire

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Part F: Earthquakes and Earth’s Interior

If Earth’s internal structure was homogenous, seismic waves would travel in all directions at an equal rate. However, scientists have discovered:

a) Seismic wave velocities increase with depth. This is a result of increased pressure, which enhances the elastic properties of buried rock.

b) Abrupt velocity changes also occur at particular depths, leading seismologists to conclude that earth is made up of distinct layers.

Other important observations included:

a) Crust-Mantle Boundary (Moho Discontinuity)

Seismic stations more than 200km away from the focus recorded faster average P-wave travel velocities than closer stations.

This data allowed Czech seismologist Mohotovivic to determine that a new Earth layer existed below 50km that had different properties than Earth’s crust, Earth’s mantle.

b)

Core-Mantle Boundary

P-waves are non-existent 105 to 140 degrees from an earthquake as a product of the core’s properties causing the waves to refract/bend around it. (P-Wave Shadow Zone)

S-waves do not travel through the core, indicating that it is a liquid (S-Wave Shadow Zone). This is also supported by P-wave velocities decreasing by 40% as they enter the core.

c) Inner Core

The inner core was discovered through the additional refraction/reflection it caused in P-waves.

The noticed increase in velocities the core added to P-waves allowed scientists to determine that it is solid.