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Plate Tectonics and Earthquake Notes
Continental Drift
Maps were made starting 400 years ago.
As new and reliable maps were made, the similarity of the continental shoreline on either side of the Atlantic made
some scientist suspicious that continents had actually been together at one time.
The belief that continents have not always been fixed in their present positions was suspected long before the 20th
century (1596 Dutch map maker Abraham Ortelius).
However, it was not until 1912 that the idea of moving continents was seriously considered as a full-blown scientific
theory called Continental Drift.
Alfred Wegener
Introduced in two articles published by a 32-year-old German meteorologist named Alfred Wegener.
He contended that, around 200 million years ago, the supercontinent Pangaea began to split apart.
Wegener’s Controversal Theories
Wegener writes The Origin of Continents and Oceans, (1915) as a way to outline his beliefs.
(1) Demolishes sunken land bridge theory: It was suggested that all continents were still connected as one complete land
mass.
(2) Wegener proposed that the continents floated somewhat like icebergs in water (isostasy).
As proof he pointed out that the continents are made of a different, less dense rock (granite) than the volcanic basalt
that makes up the deep-sea floor.
(3) Wegener also offered a better explanation for mountain ranges.
According to the cooling, contracting-Earth theory, they formed on the Earth's crust just as wrinkles form on the skin of
a drying apple.
Under Wegener’s theory, since continents were drifting: collision between continents created crumpled and folded rock
forms known as mountains.
(4) He also would try and solve the climatic puzzle: Evidence of the Permo-Carboniferous ice-age era that peaked some
280 million years ago.
Scientist could not create a plausible theory as to why evidence was scattered over almost half the Earth, including the
hottest deserts.
To explain this, he creates the concept of Pangaea (all lands) – He believes that at one time there was a supercontinent
(Pangaea), which broke up, and the pieces drifted to their present location.
If you clustered Africa, Antarctica, Australia, and India into the Southern Hemisphere supercontinent (Gondwanaland)
then the Pangaea model would explain the remnants of the 280-million-year old ice age.
Alfred Wegener’s Problem
His hypothesis was strongly supported by the physical evidence:
(1) Rock type, (2) Geological structures, (3) Fossils.
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Rock Evidence- Geologist have found the ages and types of rocks in the coastal regions of widely separated areas
matched closely across the Atlantic Ocean.
Geological Evidence- Mountains like the Appalachian’s became an obvious fit with the Caledonian on the other side of
the Atlantic.
Fossil Evidence- Fossils of the same plants and animals could be found in areas of continents that had once been
connected.
Fossils of small land reptiles, mites and even earthworms have been found on South Africa and South America,
reinforcing Wegener's theory of continental drift.
Wegener’s Death
In his work, Wegener presented a large amount of very strong evidence in support of continental drift, but
the mechanism remained elusive.
He eventually proposed that it was the rotation of the earth that moved the tectonic plates.
He would spend the rest of his life searching for a mechanism and in 1930 Greenland – his expedition would not survive
getting lost in a blizzard.
Sea Floor Spreading
Seeking to improve our knowledge of Earth, scientists used new war tested technologies like sonar and magnetometers.
Hess Introduces Sea-Floor Spreading
Harry Hess, was a professor of geology at Princeton University and a Captain of the Navy.
During the wartime Hess collected ocean floor profiles across the North Pacific Ocean, resulting in the
(1) discovery of flat-topped submarine volcanoes.
It was widely assumed that the ocean was flat so Hess did more research and in the Atlantic Ocean he found that:
(2) The sediment that covers the sea floor is thinner closer to a ridge than it is farther from the ridge.
Atlantic Ridge
Using core samples Hess discovered the ocean floor is very young compared to continental rock.
(3) He also noticed rock that was closer to the ridge was younger then rock away from the ridge.
Mid-Ocean RidgesUsing this data, he published a paper titled "History of Ocean Basins”, in 1962.
In it, he introduces the hypothesis that later would be called ”Seafloor Spreading”.
He was convinced that the convection of the Earth’s mantle was creating the mountains he found and creating the
driving force to tectonic plates.
Seafloor Spreading
Sea Floor spreading is the process by which new oceanic lithosphere forms as magma rises to Earth’s surface and
solidifies at a mid-ocean ridge (Mountain Chain).
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As this process continues the rock closest to the ridge slowly pushes the older rock further away from the ridge.
A lot like old toothpaste or a mustard bottle.
Sea-Floor Spreading Lacking Evidence
Hess was well aware that solid evidence was still lacking to convince a skeptical scientific community and the evidence
he needed would come only one year later (‘63).
Paleomagnetism
Using magnetometers, scientists in 1963 (Paleomagnetism) began recognizing odd magnetic variations across the ocean
floor.
Basalt is the iron-rich, volcanic rock making up the ocean floor-- contains a strongly magnetic mineral known as
magnetite.
When magma solidifies, the magnetite in the magma with Earth’s magnetic field.
Magnetic Reversals Within Rock
Geologist begin to date the basalt rocks and they notice something odd.
(4) Some rocks have magnetic fields that point north (normal polarity) while other rocks had magnetic fields that
pointed south (reversed polarity).
Magnetic Reversals
When scientists dated these rocks they realized that all rocks placed these periods of normal and reversed polarity in
chronological order or a pattern.
The current time scale contains 184 polarity intervals in the last 83 million years.
Magnetic Reversal Time Scale
Scientists used this pattern to create the geomagnetic reversal time scale.
The last recorded change in polarity happened
780,000 years ago.
Atlantic Ridge Symmetry
Further research of the Atlantic Ocean rock would discover (5) symmetrical magnetic pattern on the ocean floor on each
side of a mid-ocean ridge.
This mirror image or pattern matched the geomagnetic reversal time scale.
Continental Reversal Patterns
Reversal patterns on the sea floor could also be found on land.
The reversals in land rocks also matched the geomagnetic reversal time scale.
Atlantic Ridge
Finally, (6) improved seismic data confirmed that oceanic crust was indeed sinking into the trenches, fully proving Hess'
hypothesis, which was based largely on intuitive geologic reasoning.
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This would also rescue Alfred Wegener’s namesake.
How Continents Move
Plate tectonics the theory that explains how large pieces of the lithosphere, called plates, move and change shape.
The two layers of Earth that determine our plate tectonics are the
(1) Lithosphere- outer layer
(2) Asthenosphere the solid, plastic layer of the mantle beneath the lithosphere.
The lithosphere forms the thin outer shell of Earth and is broken into several blocks or tectonic plates.
The tectonic plates ride on the asthenosphere in much the same way that blocks of wood float on water.
How Tectonic Plates Move
Tectonic plates can include continental crust, or oceanic crust.
Continents and oceans are carried along on the moving tectonic plates in the same way that passengers are carried by a
bus.
Tectonic Plate Boundaries
Scientists have identified 15 major tectonic plates.
These plate boundaries are defined by studying data from earthquakes.
Tectonic plate boundaries may be in the middle of the ocean floor, around the edges of continents, or even within
continents
3 Major Types of Plate Boundaries
(1) Divergent Boundaries - In divergent boundaries plates move away from each other forming a rift in Earth’s crust.
Divergent boundaries occur on both sea and land.
(2) Convergent Boundaries - In convergent boundaries the plates collide into each other, typically forming mountains
and a subduction zone.
Convergent boundaries can occur between two oceanic plates, two continental plates or occur between one oceanic
plate and one continental plate.
(3) Transform Boundaries - In transform boundaries the plates slide past each other. Like our San Andreas Fault
Cause of Plate Motion
Plates at our planet’s surface move because of the intense heat in the Earth’s core that causes molten rock in the mantle
layer to move
Causes of Plate Motion
It moves in a pattern called a convection cell that forms when warm material rises, cools, and eventually sink down.
As the cooled material sinks down, it is warmed and rises again (Like a lava lamp).
Scientists once thought that Earth’s plates just surfed on top of the mantle’s giant convection cells.
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Now believed that plates help themselves move instead of just surfing along.
Plate Density
One way plates help move themselves is by their own densities.
Just like convection cells, plates have warmer, thinner parts that are more likely to rise, and colder, denser parts that are
more likely to sink.
Earth’s Convecting System
There are also three mechanisms to Earth’s convecting system.
(1) Drag on tectonic plates
(2) Ridge Push
(3) Slab Pull
(1) Drag of Tectonic Plates
The drag of tectonic plates refers to the resistance or dragging force associated with the interface between the upper
mantle and the lithosphere.
(2) Ridge Push
New parts of a plate rise because they are warm and the plate is thin.
As hot magma rises to the surface at spreading ridges and forms new crust, the new crust pushes the rest of a plate out
of its way and this is called ridge push.
This occurs because newer warm rock cools and becomes denser allowing it to sink into the asthenosphere and push
away from the ridge.
The largest ridge on our planet is located in the Atlantic Ocean and is known as the Mid-Atlantic Ridge
(3) Slab Pull
Old parts of a plate will sink down into the mantle at subduction zones because they are colder, denser and thicker than
the warm mantle material underneath them.
This force is called slab pull.
Ultimately it is the plate densities that determines which of the two lithospheric plates will ride over the other.
The plates density along with the Slab Pull helps move the plate along.
The largest subduction zone in the world runs along the entire Pacific Ocean and is known as the ‘Ring of Fire’.
Ring of Fire
The Ring of Fire has 452 volcanoes and is home to over 75% of the world's active and dormant volcanoes.
About 90% of the world's earthquakes occur along the Ring of Fire.
Plate Tectonics Effect on Climate
Modern climates are a result of past movements of tectonic plates.
The three factors that effect the continents climate are
(1) location in relation to the equator and poles
(2) location in relation to ocean and other continents
(3) the continents mountain ranges
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During the past billion years, the Earth's climate has fluctuated between warm periods – and cold periods (ice age).
Cold periods included large glaciers that covered the continents.
Although the exact causes for ice ages, and the glacial cycles within them, have not been proven, they are most likely the
result of a complicated dynamic interactions.
The major dynamic being the position and height of the continents along with solar output, ocean circulation, and the
composition of the atmosphere.
Law of Superposition
Using a few basic principles, scientists have developed laws to determine the order and age in which rock layers formed.
The Law of Superposition states that a sedimentary rock layer is older than the layers above it and younger than the
layers below it if the layer are not disturbed.
Law of Crosscutting Relationships
When rock layers have been disturbed by faults or intrusions, determining relative age may be difficult.
In such cases scientists apply the Law of Cross-Cutting Relationships which states a fault or igneous intrusion is always
younger than the rock layers it cuts though.
Folds
- When a rock is compressed or squeezed inward it will fold.
- Although a fold commonly results from compression, it can also from as a result of shear stress.
3 Types of Folds (1) Syncline
They occur at a convergent boundary.
Compressed Rock fold the surrounding rock upward into a bowl shape.
The youngest layer is in the center of this fold and if the softer rocks are in the core of the syncline, they will erode to
form a valley, but if the inner layers are harder, erosion carves them into a peak instead.
Think of it as a smile
(2) Anticline
Occurs at a convergent boundary.
An anticline is a fold that arches upward and looks like a bowl that is turned over.
Think of it as a frown
The oldest rocks are found at the center of an anticline and the youngest ones are draped over them at the top of the
structure.
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Sizes of Folds
Folds vary greatly in size. Some folds are small enough to be contained in a hand-held rock specimen.
Other folds cover thousands of square miles can be seen only from the air.
Faults
Stress on rock can also cause rock to break.
Breaks in rock along which there is no movement of the
surrounding rock is called a fracture.
Strike-Slip
A break along which the surrounding rock moves is called a
fault.
A fault is a break in a body of rock along which one block slides
relative to another.
Anatomy of a Fault
The hanging wall is the rock above the fault plane.
The footwall is the rock below the fault plane.
Three Types of Faults There are three types of faults
(1) Normal Faults
(2) Reverse (Thrust) Fault
(3) Strike-Slip Faults
1) Normal Faults
A normal fault is a dip-slip fault in which the hanging wall moves downward relative to the footwall (Think of it at an
extensional fault).
Normal faults commonly form at divergent boundaries, where the crust is being pulled apart by tension.
2) Reverse Faults & Thrust Faults
When compression causes the hanging wall to move upward relative to the footwall, a reverse fault forms.
A thrust fault is a special type of reverse fault in which the fault plane is at a low angle or is nearly horizontal.
Reverse faults & thrust faults are common in convergent boundaries and responsible for steep mountain ranges, such as
the Himalayas and the Alps.
Stresses that raised up the Rocky Mountains caused a block of ancient Precambrian crust to be thrust more than 50
miles over much younger Cretaceous rocks.
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Result - upper rocks are more than 1 billion years older than the lower rocks.
3) Strike-Slip Faults
In a strike-slip fault, the rock on either side of the fault plane slides horizontally (or by each other) in response to shear
stress.
Strike-slip faults got their name because they slide, or slip, parallel to the direction of the length, or strike, of the fault.
Strike-slip faults commonly occur at transform boundaries like our own San Andreas Fault.
Sizes of Faults
Like folds, faults vary greatly in size. Some faults are so small that they affect only a few layers of rock in a small region.
Other faults are thousands of miles long and may extend several miles below Earth’s surface.
The San Andreas fault is an example of a large fault system.
Why Earthquakes Happen
Earthquakes are a movement or trembling of the ground that is caused by a sudden release of energy when rocks along
a fault move.
Earthquakes usually occur when rocks under stress suddenly shift along a fault.
The shift is a response to the stress that has built up at a fault, deforming the locked up crust.
Geologists believe earthquakes occur as a result of elastic rebound.
Elastic Rebound
Elastic rebound is the sudden return of elastically deformed rock to its un-deformed shape.
Just like a rubber-band.
This occurs when rocks are stressed past the point at which they can maintain their integrity.
At this point they fracture then separate at their weakest point along the fault and rebound, or spring back to their
original shape.
Anatomy of an Earthquake
The point on Earth’s surface above an earthquake’s starting point is called the epicenter.
The focus is the starting point within Earth (underground) at which the first motion of an earthquake
occurs.
Although the focus depths of earthquakes vary, 90% of continental earthquakes have a shallow focus.
Earthquakes that cause the most damage usually have shallow foci (less than ~ 40 miles deep).
Seismic Waves
When an earthquake occurs it gives of energy in the form of seismic waves.
Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion.
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Each type of wave travels outward from the focus and their speed and direction is effected by Earth’s interior.
The energy released by an earthquake (seismic waves) are recorded on seismographs.
Earthquakes and Plate Boundaries
The diagram below shows the different tectonic boundaries where earthquakes typically occur.
The three main types of tectonic settings that earthquakes occur at are:
(1) Convergent oceanic environments,
(2) Divergent oceanic environments, and
(3) Continental environments.
(1) Convergent Oceanic Environments
Convergent oceanic boundaries can occur between two oceanic plates or between one oceanic plate and one
continental plate.
(2) Divergent Oceanic Environments
Earthquakes can occur along mid-ocean ridges because oceanic lithosphere is pulling away from both sides of the ridge.
(3) Continental Environments
Earthquakes also occur at locations where two continental plates converge, diverge, or move horizontally in opposite
directions at transform boundaries.
Two Types of Seismic Waves
(1) Body waves are seismic sound waves that travel through the interior of Earth.
(2) Surface waves are seismic sound waves that are formed by the interaction of P and S waves with Earth's surface.
Body Wave #1 - P waves (Primary Waves)
The fastest kind of body wave is the P-wave or primary wave.
P-waves are also referred to as compression waves because the rock it moves through it pushes and pulls just like sound
waves push and pull the air.
Example: thunder
Not only do you hear it but it also is able to rattle the windows at the same time because the sound waves were pushing
and pulling on the window glass much like P waves push and pull on rock.
The P-wave will be the first to 'arrive' at a seismic station.
It can travel through any type of material, including fluids, and can travel at nearly twice the speed of S waves.
In air, they take the form of sound waves, hence they travel at the speed of sound (768mph).
Sometimes animals can hear the P-waves of an earthquake.
Dogs, for instance, commonly begin barking hysterically just before an earthquake 'hits' (or more specifically, before the
surface waves arrive).
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Body Wave #2 - S waves (Secondary Waves)
The second type of body wave is the S-wave or secondary wave.
S-waves move rock particles up and down, or side-to-side--perpendicular to the direction that the wave is traveling in.
An S-wave can be up to 50% slower than a P-wave depending on the rock it is going through.
They can only move through solid rock, but not through any liquid medium which includes our liquid core.
It is this property of S-waves that led seismologists to conclude that the Earth's outer core has the consistency of liquid.
Seismic Waves and Earth’s Interior
By studying seismic waves, scientists have discovered Earth’s five structural layers:
The lithosphere,
The asthenosphere,
The mesosphere,
The outer core,
The inner core.
Shadow Zones
The properties of body waves also create a shadow zone.
A shadow zone is an area where no direct seismic waves from an earthquake can be detected.
Surface Waves
Events with magnitudes greater than 4.5 are strong enough to be recorded by a seismograph anywhere in the world, so
long as its sensors are not located in the earthquake's shadow.
The shadow zone results from the S-waves being stopped entirely by the liquid core and P-waves being bent (refracted)
by the liquid core.
Surface Waves
When a body wave gets to the surface it creates two distinct surface waves, which move slower and at a lower
frequency making them easily distinguishable on a seismogram.
Because of their long duration, and large amplitude, they can be the most destructive type of seismic wave.
Surface waves are a lot like water waves.
Studying Earthquakes
The study of earthquakes and seismic waves is called seismology.
Seismologist can detect and record vibrations in the ground using an instrument called a seismograph.
Seismograph
Seismographs record three types of ground motion—vertical, east-west, and north-south.
Because they are the fastest, P waves are the first seismic waves to be recorded by a seismograph.
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S waves are the second seismic waves to be recorded, and surface waves are the last to be recorded by a seismograph.
Locating an Earthquake
To determine the distance to an epicenter, scientists can use the simple formula of (distance = velocity x time (d=rt).
In order to save time a lag-time graph was created.
By analyzing the difference in the arrival times of the P & S waves seismologist can determine the distance to the
epicenter, using this graph.
Scientists use the triangulation method which means they need information from at least 3 seismograph stations to
locate the earthquake.
History of Earthquake Measurement
Intensity measures the strength of shaking produced by the earthquake at a certain location.
Earthquakes were originally measured based on intensity or damage caused.
Mercalli Scale
The modified Mercalli scale expresses intensity in Roman numerals from I to XII (12 being most) and provides a
description of the intensity effects of an earthquake on humans at the surface.
Evolving Technology
However, comparing earthquakes is complicated because some are gentle (roll), others are violent (jolt).
Intensity also varies the farther from the epicenter of the earthquake.
Richter Scale
The Richter magnitude scale (Local Magnitude) was developed in 1935 by Charles F. Richter of Caltech to compare the
size and magnitude of earthquakes.
The magnitude of an earthquake is based on the amount of seismic energy released and recorded by a seismograph.
Richter Scale Simplified Everything
It is calculated by measuring the amplitude of the s- wave (a.k.a.- the distance the seismograph needle moved).
A graphical device (a nomogram) can be used to simplify the process and to estimate the magnitude based on the
distance and amplitude measurements.
Richter had to create an adjustment factor to allow for near versus distant quakes.
His logarithmic scale assigns values from 1-10 to the magnitude of any earthquake – “10” being the most powerful.
Richter Scale Failures
The main drawback to the Richter scale is its accuracy.
It is not accurate at estimating earthquake magnitudes where the epicenter is too far away (~350 miles)
or where the earthquake magnitude was greater than 7.0.
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While the Richter scale was widely used for most of the 20th century, scientists now prefer to use the moment
magnitude scale because it is more precise.
Tsunamis
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water typically caused by
earthquake.
Tsunami waves resemble a rapidly rising tide.
Tsunamis generally consist of a series of waves with periods ranging from minutes to hours.
Although limited to coastal areas, their destructive power can be enormous –
2004 Indian Ocean tsunami killed over 230,000 people.
2011 Tohoku tsunami of Japan killed over 19,000 people, damaged estimated at $235 billion.
Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water.
This will occur at a convergent plate boundary where there is a thrust or mega-thrust fault.
80% of all Tsunamis occur along the Ring of Fire and until recently tsunami’s where rare (5 every century).
California is not under any threat of a tsunami because we are not on the Ring of Fire.
However we are surrounded by it and the nearest threat is the Juan de Fuca plate.
Earthquakes and Society
Most earthquake injuries result from the collapse of buildings and or from falling objects.
Other dangers include a tsunami, landslides, explosions caused by broken electric and gas lines, and floodwaters
released from collapsing dams.
Destruction to Buildings and Property
We will tend to see more destruction to buildings that are designed poorly.
Nowadays most cities have earthquake codes including Los Angeles.
Predicting Earthquakes
While some claim to predict earthquakes scientist have not found anything or anyone that is reliable yet.
Pat Regan – believes that earthquakes are related to the preponderance of UFO sightings prior to seismic activity.
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