Plate Tectonics

Jonathan Moss

Continental Drift• Wegner’s theory- Wegner

proposed that the continents were once joined together and through time separated by movements on the ocean’s basin.

• Evidence of his theory included the shape of the Atlantic coastlines of Africa and South America. They fit almost like a puzzle.

• A problem with his theory was that he could not say why the continents moved, just the observations.

Supercontinent Theory• Supercontinent Cycle- the explanation that the continents

form one massive land mass, separate into many smaller ones, then form a massive continent again.

• Pangaea was the massive supercontinent of all the continents together and Panthalassa was the massive ocean that surrounded.

• Accretion is the adding of materials to a plate, and one plates hit one another one may subduct under the other.

• Rifting is the resulting of divergence (plates moving apart)

Theory of Plate Tectonics• Lithosphere- thin outer shell of earth that forms the crust

and upper part of the mantle.• Asthenosphere- layer of “plastic” rock right under the

lithosphere.• The plates move by being carried along the

asthenosphere.• The three types of plate boundaries are…

– Divergent boundaries- when two plates move away from one another

– Convergent boundaries- when two plates move toward each other– Transform boundaries- when two plates slide across one another.

• Isostatsy is when the forces of gravity and pushing down and buoyant force pushing up on the lithosphere are equal.

• The movements to reach isostasty are all isostatic adjustments.

• The amount of force exerted when the lithosphere moves is called stress.

Divergent Boundary• Sea-floor spreading-when new ocean

lithosphere forms as magma rises and solidifies at the ocean ridge. – Mid ocean ridges are the land mass formed

around the rift in which sea floor spreading occurs.

• Paleomagnetism is the residual magnetism of rock.– Magnetic reversal is when the magnetic polarity

shifts from its current direction to another.– The new layers of sea floor in the ocean are

closer to the center and the oldest is furthest away is magnetic symmetry.

• Normal Faults- fault which the hanging wall moves downward relative to the footwall.– The Great Rift Valley of East Africa is formed

from a normal fault.

Convergent Boundary• Subduction Zones- region along the plate

boundary where one plate moves under another.– The oceanic crust subducts under the the

continental crust forming an ocean trench. – Subduction zones form ocean trenches.– The oceanic crust goes underneath another ocean

crust and deep ocean trenches form and form island arcs (a chain of volcanic islands)

– These collisions form deep ocean trenches and island arcs.

• Collision Zones- region where two plates collide and uplift forming large mountains.– The continental plate collides with another

continental plate forming large mountains.– Compression is the force of the two plates colliding

and uplift is the upward movement due to the collision.

– Large mountains are produced such as the Himalayan Mountains.

Volcanoes• Magma-liquid rock that forms under Earth’s

surface.• Three conditions in which magma will form:

– The temperature of rock rises above the melting point of minerals the rock is composed of.

– Enough pressure is removed from the rock the melting point will be reduced.

– Addition of fluids, such as water may decrease the melting point of some minerals in the rock.

• Lava-magma on earth’s surface.• Volcano- the vent in earth’s surface in

which magma and gases are expelled.– Commonly formed along convergent and

divergent boundaries.– The Pacific Ring of Fire is a major zone of active

volcanoes encircling the Pacific Ocean.

More Volcano Info….• Pyroclastic Material- fragments of rock that

form during a volcanic eruption.– Viscosity- resistance to the flow of magma, and

affects the force with which a particular volcano will erupt.• Mafic magma (describes magma or rock rich in

magnesium and iron) cause quiet eruptions. • Felsic magma (describes magma or rock that is rich in

light-colored silicate materials) cause explosive eruptions. This is because it is high in viscosity.

Types of Volcanoes• Shield- volcanic cones that are broad at

the base and have gently sloping sides. Usually form quiet eruptions.– The shield volcanoes that form the Hawaiian

islands.

• Cinder Cones- volcano that has very steep slopes. Formed from explosive eruptions.– One of the more famous cinder cones is the

Paricutin volcano.

• Composite- made of alternating layers of hardened lava flows and pyroclastic material. Usually form large volcanic mountains.– Mount Fuji

• Caldera- when the magma chamber empties, the cone may collapse forming a large basin-shaped depression.– Yellowstone caldera

Transform Boundary• Reverse Faults- when compression causes

the hanging wall to move upward compared to the footwall.

• Strike-slip faults- when the rock on either side of the fault plane slides horizontally in response to sheer stress.

• Tension- stress the stretches and pulls a body apart.

• Sheer stress- distorts a body by pushing parts of the body in opposite directions.

• The Rockies are an example of reverse faults, and the San Andreas Fault is a strike slip fault.

Earthquakes

• Elastic Rebound- the sudden return of elastically deformed rock to its undeformed shape.

• Seismology is the study of earthquakes and seismic waves.– A seismograph is a device that records vibrations

in the earth.

The form of earthquakes…• The location within the Earth where the first motion

of an earthquake occurs is the FOCUS.

• The point on Earth’s surface directly above the focus is called the EPICENTER.

Seismic Waves

• Seismic waves are the energy vibrations released when faults move.• Body Waves- waves that travel though the body of a medium.

– P Waves (primary waves)- fastest seismic waves and are always the first to be detected. Can travel through solid, liquid, or gas. Cause rocks to move in back-and-fort direction.

– S Waves (secondary waves)- second fastest waves, arrive at detection sites after P waves. Rock particles move side to side. Can only travel through solid material.

• Surface Waves- waves that travel along a surface of a body, instead of traveling through the middle.

• Seismic waves lead to determination of earth’s interior because certain waves travel faster through certain material, and by studying those speeds scientists can learn about the makeup of earth’s interior.

More Earthquake info…• Earthquakes that don’t result from

movement along plate boundaries include the New Madrid earthquake that was so strong vibrations were felt in South Carolina.

• Magnitude- the measure of the strength of the earthquake.

• Intensity- measure of the effects of an earthquake.

• Tsunami- earthquake whose epicenter is on the ocean floor that causes a giant wave which could crash onto land. The Indian Ocean tsunami in 2004 resulted in 200,000 people being dead or missing.