Download Theory of Plate Tectonics Cornell Notes

Topic: Chapter 9: Plate Tectonics.
Sec 5: Theory of Plate Tectonics. Sec
6: Changing of Earth’s Surface
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What are Plates?
The Earth’s lithosphere is not solid, but actually cracked into approx. 12
pieces that cover the globe carrying the continents or parts of the ocean floor,
or both. These pieces are called plates.
Geological theory that states that the pieces of the Earth’s lithosphere are in
constant, slow motion driven by convection currents in the mantle. The
theory explains the formation, movement, and subduction of Earth’s plates.
Plates fit closely together so when the convection currents in the magma
move them, they either collide, pull apart, or grind past each other. “Ridge
push” and “slab pull” may work with convection currents to move the plates.
The edges of the plates meet each other at “plate boundaries” FAULTS –
breaks in the Earth’s crust where rocks have slipped past each other – form
along plate boundaries.
a. Divergent Boundaries
b. Convergent Boundaries
c. Transform Boundaries
Each type has a different type of plate movement.
Two plates slip past each other, moving in opposite directions. Crust is
neither created nor destroyed.
What is the Theory of Plate
Tectonics?
Explain plate movement:
What are plate boundaries?
What are the (3) kinds of plate
boundaries?
Transform Boundaries?
Causes earthquakes and creates faults
Example: San Andreas Fault - California
Divergent Boundaries?
Place where two plates move apart or diverge. Found in both oceans and on
land, but occur mostly at the mid-ocean ridge.
When a divergent boundary occurs in the ocean, magma rises at the middle of
the ocean and forms mid-ocean ridge.
Ex. Iceland-islands or volcanic mountains
When a divergent boundary occurs on land, two plates slide apart and a deep
valley called a “rift valley” forms. Ex. Great Rift Valley in east Africa.
Convergent Boundaries?
Place where two plates come together or converge. The result is a collision.
When two plates collide, the density of the crust determines which one comes
out on top. Oceanic crust (basalt) is more dense than continental crust
(granite).
3 kinds of convergent boundaries:
a. continental crust/continental crust (c/c)
Ex: folded mountains - Himalayas
b. oceanic crust/continental crust (o/c)
Ex: Subduction: Andes Mtns, South America
c. oceanic crust/oceanic crust (o/o)
Ex: Subduction: Aleution Islands, Alaska (Island Arc)
What causes changes in Earth’s
Surface?
Plate movement can alter Earth systems and produce changes in Earth’s
surface. Ex: deformation of Earth’s crust, faults, mountain building, land
subsidence, and volcanoes.
Plate movements produce powerful forces that push, pull, bend, and twist the
lithosphere which produces stress in rock. Stress leads to deformation.
Deformation takes place so slowly you cannot obvserve it directly. Over a
long time, deformation changes Earth’s Surface.
1. Shearing
2. Tension
3. Compression
What is the result of stress in rock?
Three types of deformation?
What is shearing?
What is tension?
Shearing pushes a mass of rock in two opposite directions. If the rock breaks,
it causes a fault. During plate movement, stress builds up along the fault,
storing potential energy in the rock. If the rock breaks, potential energy is
converted to kinetic energy and an earthquake results.
Tension pulls on rock, making it thinner in the middle.
What is compression?
Compression squeezes rock, making it thicker in the middle.
What are faults?
Faults are breaks in the crust where slabs of crust slip past each other. The
rocks on each side of the fault can move up, down, or sideways. Faults
usually occur along plate boundaries where the forces of plate motion
compress, pull, or shear the crust so much the crust breaks.
1. Strike-Slip Faults: Caused by shearing. Rocks on either side of the
fault slide past each other sideways with little up or down motion.
This type of motion results in a transform boundary between plates.
EX: San Andreas Fault, California.
2. Normal Faults: Tension forces cause normal faults. The fault is at an
angle with one block of the rock lying above the fault and the other
block of rock lying below the fault. Tension faults occur where
plates diverge or pull apart. Ex: Rio Grande Rift Valley, New
Mexico.
3. Reverse Faults: Compression forces produce reverse faults. They
are the reverse of a normal fault. The footwall goes down while the
hanging wall moves up. Ex: Canadian Rockies
Folding and Faulting result in mountain building. Folding occurs when two
continental plates collide. The collison squeezes the two plates together and
the solid rock on top of the plates fold like a rug when its ends are pushed
toward each other. Faulting can cause mountains to form when one side is
pushed up considerably higher than the other. Fault-block mountains are
formed. Ex: Teton Mountains in Wyoming
When the land surface sinks, or subsides, as a result of geologic processes or
human activities. Plate movements along diverging plate boundaries cause
land subsidence. Forms rift valleys and ocean basins. Sometimes an uplift in
one area results in subsidence in another. Ex: Texas to North Canada about
65-70 million yrs ago – the sea covered this entire area!
A volcano is a weak spot in the crust where molten magma comes to the
surface. Plate movements determine where volcanoes develop on Earth. Ex
Ring of Fire on rim of Pacific Ocean. Most, but not all, occur along diverging
plates such as mid-ocean ridge or subduction zones Ex: o/c convergence:
Andes Mts/South America. Some form at converging boundaries (o/o
convergence):.Ex: Island arcs like Japan, New Zealand, Indonesia, Caribbean
Islands, Philippines, and Aleutians (Alaska).
Some volcanoes result in hot spots in Earth’s mantle. A hot spot is an area
where magma from deep in the mantle melts through the crust like a
blowtorch through steel. Hot spots lie in th middle of continental or oceanic
plates away from any plate boundaries. Ex: Hawaiian Islands. Formed over
millions of years as the Pacific Plate drifted over a hot spot.
Ex: Yellowstone National Park, Wyoming – hot spot under a continental
plate. Last volcanic eruption – 75,000 yrs ago.
3 Types of faults?
What results in Mountain Building?
What is Land Subsidence?
What creates volcanic mountains?
What are Hot Spots?