Download Chapter 6 – Plate Tectonics and Earthquakes

Chapter 10 – Plate Tectonics and
Earthquakes
If the continents drift, where are
we going?
I. Once Upon a One-Land Planet
A. Abraham Ortelius- in 1587 this
famous mapmaker noticed that South
America and Africa looked like mirror
images. In his book Thesaurus
Geographicus he argues that the
continents were once joined.
B. Alfred Wegener- 1912 – the
thirty-two year old
meteorologist unveiled his
theory of continental drift
1930 Greenland. The last photo
of Alfred Wegener alive. He is
on the left. He was 50 years
old.
1. Wegener theorized that 200
million years ago Earth was one large
continent called Pangaea (meaning “all
Earth”) and one large ocean called
Panthalassa.
2. Gradually the continent split
into two large landmasses
a. Laurasia in the northern
hemisphere
b. Gondwanaland in the southern
hemisphere
C. Response from scientists at
the time was very negative, but
Wegener persisted
D. About 30 years after Wegener’s
death, evidence convinced almost
all scientists that continental
drift was an acceptable, useful
theory
II. Evidence Supporting Continental
Drift
A. Fossils
1. Glossopteris- an extinct plant
fossil located in rocks about 250
million years old are found in
South Africa, Australia, India,
and Antarctica
a. Seeds of this plant are too
large to be carried by wind,
too fragile to be carried by
waves
b. This suggests that places the
plant fossils are found were once
closer together.
c. Glossopteris fossils found in
Antarctica suggest a climate change,
which indicates a position change in
the land.
2. Many other reptile fossils support continental
drift such as; Cynognathus, Lystrosaurus, and
Mesosaurus
B. Evidence from Rocks
1. Rock formations in Africa match
those in South America
2. A folded mountain chain in
South Africa matches a folded
mountain chain in Argentina.
3. Coal fields with distinctive layers in
Brazil are similar to those in Africa
4. Glacial deposits are found in
South America, Africa, India,
Australia, and Antarctica
Glacial Deposits
5. Rock deposits of coal, salts,
and limestone derived from coral
reefs suggest changes in climate
caused by continental drift
C. Evidence from the Ocean Floor
1. New mapping techniques of
the 1950’s and 1960’s allowed
scientists a closer look at
the ocean floor.
2. Harry Hess, a geologist and
Navy submarine commander,
proposed the theory of seafloor spreading in 1960.
3. Scientists discovered the
midocean ridge system, an undersea
mountain range that snakes around
the globe.
a. At the midocean ridge, a
great deal of volcanic activity
causes lava to erupt from the
rift valley.
b. The lava hardens and forms new
ocean floor. This is called ocean-floor
spreading, and helps to explain how the
continents drift.
c. Old ocean floor that is pushed away
from the ridge eventually moves down
deep in the Earth along trenches
through a process called subduction.
Subduction Zone
III. Earth’s Moving Plates
A. Lithosphere- contains the crust
and the uppermost part of the
mantle and is composed of plates
1. There are 7 major plates named
after their associated continents,
or surface features (Ex. North
American plate, South American
plate)
2. The Pacific is the largest
plate, covering one-fifth of
Earth’s surface.
B. Plate Boundaries- where two
plates meet
1. Divergent boundary- plates
move apart (diverge) ex.
Midocean ridges
2. Convergent boundary- plates
come together (converge)
a. creates pressure and
friction
b. can cause earthquakes and
volcanoes
3. Strike-slip boundary- two
plates grind together and slip
past one another horizontally (Ex.
San Andreas Fault in California)
C. Plate Motion Hypothesisconvection currents caused by heat
deep within the Earth may be
responsible for the movement of
continents
1. The layer below the
lithosphere is called the
asthenosphere and can flow like a
liquid due to the intense heat
and pressure in the mantle
2. Material closer to the core is
warm and rises, mantle material
closer to the lithosphere is
cooler and sinks
3. The rising and sinking cycle
of the mantle material creates a
circular motion that carries the
plates of the Earth
Convection Currents
IV. Theory of Plate Tectonics
A. Links together the ideas of continental
drift and ocean-floor spreading
B. Explains how the Earth has evolved
(changed) over time
C. Helps explain formation, movements,
collisions, and destruction of the Earth’s
crust
D. Helps scientists understand the past
and helps predict the future
V. Earthquakes
A. Shaking and trembling resulting from
movements of Earth’s crust or plate
tectonics
1. Most common cause of earthquakes is
faulting- a break in the Earth’s crust
a. Most faults lie between the surface and a
depth of 70 kilometers
b. Focus- point below Earth’s surface where
rocks break and move
c. Epicenter- point above Earth’s surface
directly above the focus
San Andreas Fault
2. Movement at faults occur due to
convection currents
3. Energy is released as the crust
moves in the form of seismic waves
B. Types of Faults ( a break or
crack where rocks move)
1. Normal faults- cracks where tension
causes one block of rock (hanging wall) is
sliding downward and away from
another rock (foot wall)
Normal Fault
a. Hanging wall- block of rock
above the fault
b. Foot wall- block of rock below
the fault
2. Reverse
causes one
to move up
rock (foot
faults- compression
rock (hanging wall)
relative to the other
wall)
Reverse Fault
3. Strike-slip faultsshearing forces cause cracks
between two plates to slide
past one another
C. Types of Seismic Waves
(Earthquake waves)
1. Primary Waves or P waves- travel fastest,
travel through solids, liquids and gases
a. push-pull waves that push rock particles ahead of
them
b. speed up when passing through denser material
2. Secondary Waves or S waves- slower than P
waves, travel through solids, but not liquids
and gases
a. cause rocks particles to move from side to side
b. speed up when passing through denser material
3. Surface Waves or L wavesslowest-moving seismic waves
a. originate at the epicenter
b. cause most of the damage
during an earthquake
c. earth’s surface moves up
and down as each L wave passes
D. Measuring Seismic Waves
1. Seismographs- instrument that detects
and measures seismic waves
2. Today's high-technology, digital
seismographs record ground shaking
over a large band of frequencies and
seismic amplitudes.
3. Seismologists determine the
strength of an earthquake by
studying the height of waves
recorded by seismographs .
a. Richter Scale- an open-ended
theoretical scale used to
measure the magnitude of
earthquakes (8.9 is largest
magnitude assigned to an
earthquake).
i. The scale is logarithmic.
Therefore an increase of 1
magnitude unit represents a factor
of ten times in amplitude.
ii. For example, the seismic
waves of a magnitude 6 earthquake
are 10 times greater in amplitude
than those of a magnitude 5
earthquake.
b. Modified Mercalli Scale
Measures earthquake intensity.
Describes damage to structures.
Ranges from I (felt by only a
few) to XII (total destruction).
VI. History of Seismic Activity in
North Carolina
A. The first earthquake recorded in NC
history was on March 8, 1735 near Bath,
NC.
B. From February 10 to April 17, 1784
there were 75 earthquakes felt in
McDowell County, NC.
C. April 31, 1886 an
earthquake with an epicenter
in Charleston, SC caused the
most property damage in North
Carolina’s history.
Earthquake damage in Charleston
Charleston Earthquake
D. February 21, 1916 Asheville, NC
- the most intense earthquake in
NC history, measuring a 6 on the
Mercalli scale.
E. June 5, 1998 an earthquake
in Moorseville, NC measured 3.2
on the Richter scale, but there
was no reported damage.
VII. Use of Seismic Waves in
Developing a Model of Earth’s
Structure
A. Solid Inner Core
1. When analyzing the activity of
seismic waves through the Earth,
scientists found that P waves
began to speed up at 5150
kilometers. This indicates the
waves were passing through solid
material, proving the solid
composition of the inner core
2. Scientists theorize the inner core is
composed of iron and nickel which under
great pressure become solid.
B. Liquid Outer Core
1. Scientists found that S waves
terminated at a depth of 2900 kilometers.
This indicates the outer core is liquid.
2. P waves also slow down as they pass
through liquids to support this conclusion.
3. Scientists also theorize the outer core is
composed of iron and nickel, but in liquid
form.
C. Mantle- directly above the outer core
1. Plasticity is the property of a solid
with the ability to flow like a liquid at
very, very slow rates.
a. High temperature and pressure allow
the rock to flow like a liquid
b. This also allows the plates of Earth
(lithosphere) to move on the mantle.
2. The mantle is composed of silicon,
oxygen, iron and magnesium.
3. Moho- boundary between the crust and
the mantle.
a. Change in the speed of seismic waves
moving through the Earth led to its
discovery.
b. Discovered in 1909 by a Yugoslav
scientist, Andrija Mohorovicic.
4. The Mantle can be divided into 3 layers.
a. Lithosphere- the upper mantle and part
of the crust.
b. Asthenosphere- the middle to upper part
of the mantle.
c. Lower Mantle- the lower part of the
mantle.
1. Oceanic crust – (less than 10
kilometers thick), made mostly of
silicon, oxygen, iron and magnesium
2. Continental crust- (average
thickness of 32 kilometers) made
mostly of silicon, oxygen, aluminum,
calcium, sodium and potassium