Plate Tectonics Unit: Tectonic Plates, Continental
Drift and Earthquakes
Text: Chapters 1, 10, 12
Lab: Laboratory 2
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Geology 12: Plate Tectonics
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Plate Tectonics 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.
Geology 12: Plate Tectonics
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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 Tectonics
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Geology 12: Plate Tectonics
Part A: Continental Drift
The 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:
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b) Expanding Earth Hypothesis:
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How was Wegener’s hypothesis different from the hypothesis above?
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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:
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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.
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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 mid1800s).
What is the significance of the MidAtlantic Ridge?
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What is the significance of ocean trenches?
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If the above is true, what supporting pieces of evidence should geologists have discovered from the
ocean’s crust?
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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: ________________________________________________________________________
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ii) Ridge Sliding: ______________________________________________________________________
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iii) Slab Pull: _________________________________________________________________________
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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
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ii) Ocean Drilling
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iii) Hot Spots
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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
Oceanic-Oceanic
Example and Diagram
Mid-Atlantic Ridge
Features
Underwater
Volcanism
Basalt (Fissure
eruption)
Mountain Ranges and
Rift Valleys
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ii) Convergent Plate Boundaries
Interacting Plates
Example and Diagram
Features
Volcanism
Features
Volcanism
iii) Transform Plate Boundaries
Interacting Plates
Example and Diagram
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d) Types of Faults (Lab Book 32-34)
Investigate the three types of faults by completing the following chart:
Stress Applied
Geology 12: Plate Tectonics
Type of Fault and Diagram
Plate Boundary
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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?
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Elastic Rebound is the mechanism that produces earthquakes. Use
the diagram right to explain how this process works:
a) Original Position
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b) Buildup of Strain
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c) Slippage
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d) Strain Release
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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).
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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.
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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.
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