plate tectonics overview i. the theory of plate tectonics the earth’s surface is divided into...
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Plate Tectonics
Overview
I. The Theory of Plate Tectonics The Earth’s surface is divided into plates that move and
interact with one another.
The plates include Continental Crust and Oceanic Crust.
The plates are made up of brittle slabs called the Lithosphere.
This includes the crust and the uppermost region of the upper mantle
The lithospheric plates slide around on top of a plastic-like layer of the upper mantle, called the Asthenosphere.
Convection
Material moves from regions of high heat (low density) to regions of low heat (high density).
This creates convection currents in the mantle
This movement causes the overlying lithospheric plates to move!
II. The Mechanism that Drives Plate Tectonics
Divergent
Lithospheric plates are moving apart
New crust and lithosphere is “created”
Associated with ocean ridges, volcanism, earthquakes and high heat flow
Most commonly found on the seafloor
III. Plate Boundary Interactions
Examples found on Earth: Mid-Atlantic Ridge, African Rift Valley, Iceland
Convergent
Plates are moving towards each other
Associated with trenches, island arcs and mountains
Old crust and lithosphere is “recycled”
Three types of convergent boundaries exist:
1. Continental - Continental (colliding)
2. Continental - Oceanic (subducting)
3. Oceanic - Oceanic (subducting)
Examples found on Earth: Himalayas, Ring of Fire, Andes Mountains, Marianas Trench
Sliding
Lithospheric plates slide horizontally past each other
Crust is deformed or fractured
Characterized by long faults and earthquakes
Examples found on Earth: San Andreas Fault
Plate Interactions Overview
IV. Evidence For Plate Tectonics
A. Continental Drift
In 1912, Alfred Wegener proposed that the Earth’s continents had once been joined as a large landmass called Pangaea.
• Continents fit together like a puzzle.
• Fossil remains of Mesosaurus (lived 270 million years ago) are found in Brazil and in South Africa, but nowhere else in the world.This hypothesis was rejected
because he could not propose an explanation for how this happened!
B. Earthquakes
Do not occur randomly throughout the world
Occur in the same regions as volcanoes
C. Volcanoes and Hot Spots
Do not occur randomly throughout the world
80% occur at convergent plate boundaries
15% occur at divergent plate boundaries
The remaining 5% do not occur at a plate boundary and form as the result of hot spots.
D. Magnetism
The Earth’s magnetic poles have often been reversed.
Rocks in the Earth’s crust contain magnetic minerals that provide a record of the direction of Earth’s magnetic poles at the time the rock was formed.
Magnetic reversals reveal themselves in symmetrical banding patterns in the rocks on the ocean floor.
What do you notice about the age of the rocks?
E. Age of the Ocean Floor
Further analysis of rocks on the sea floor revealed that the ages of the rocks vary.
The rocks are youngest near ocean ridges and get older as you move away in opposite directions
F. Heat Flow
Heat flow is a measure of the amount of heat leaving the rocks of the lithosphere.
The values are unusually high near ocean ridges and decrease as you move away from the centers in either direction.
The values are unusaully high in the subduction zone where plates converge.
What conclusions can be drawn from this evidence?
Harry Hess compiled all the evidence together and proposed the theory of Seafloor Spreading in 1962.
The theory states that new ocean crust is formed at ocean ridges and is destroyed at deep-sea trenches.
The Theory of Seafloor Spreading
A compilation of all this evidence and past theories has lead scientists to develop the current theory of Plate Tectonics.
V. Examples of Land Formation due to Plate Tectonics
A. Formation of the Andes Mountains
Nazca plate subducts beneath the South American
plate
B. Formation of Japan
Pacific Plate subducts beneath the Eurasian
Plate
C. Formation of Iceland
North American plate and Eurasian plate diverge
D. Formation of the Galapagos Islands
Nazca plate and Cocos Plate diverge
Is the Geology of the Galapagos more similar to
Iceland or Hawaii?