Plate Tectonics

Types of Plate

Boundaries

Divergent

Convergent

Transform

Divergent

Convergent

Transform Fault

Plates move apart, resulting in upwelling of material from the Mantle to create new sea floor.

Plates move together, causing one of the slabs of lithosphere to be consumed into the Mantle as it decends beneath the overriding plate.

Plates slide past each other, without creating or destroying lithosphere.

W. W. Norton. Modified from Cox and Hardt, 1986.

Each plate is bounded by a combination of these types of boundaries.

New lithosphere is continually being created at spreading centers: Since the total surface area of the Earth remains constant, lithosphere must also be destroyed somewhere else.

Zones of Plate Convergence

When two plates collide, the leading edge of one is bent downward, allowing it to descend beneath the other plate.

Upon entering the hot asthenosphere, the plunging plate (which is relatively cold) begins to warm and loses its rigidity.

W. W. Norton

The descending lithosphere reaches a depth of up to 700 km (depending on its angle of descent) before its leading edge becomes assimilated into the material of the upper mantle.

Three types of convergent plate boundaries:

The nature of convergent boundaries is influenced by the type of crustal material involved.

Oceanic - Continental

Oceanic - Oceanic

Continental - Continental

When the leading edge of a plate capped with continental crust converges with oceanic crust:

The less dense continental material (granitic) remains “floating”, while the more dense oceanic slab (basaltic) sinks into the asthenosphere.

A region where an oceanic plate descends into the asthenosphere is called a subduction zone.

As the oceanic plate slides beneath the overriding plate, the oceanic plate bends, producing a deep-ocean trench.

Oceanic-Continental Convergence

The oceanic crust is bent (at an angle of about 45º), permitting it to descend into the lithosphere.

Oceanic-Continental Convergence

Upon entering the hot asthenosphere, the downward moving plate and the water soaked sediments carried upon it begin to melt.

This newly formed magma is less dense than the surrounding mantle rocks, which causes the magma to rise to the surface.

Most of this magma will be emplaced in the continental crust and form intrusive igneous rocks.

Rock Cycle

The remaining magma will eventually migrate to the surface as volcanic eruptions and will form extrusive igneous rocks.

Deep earthquakes occur at subduction zones.

W. W. Norton

The deepest earthquakes occur at depths as great as 700 km below the surface.

The volcanic Andes Mountains were formed by oceanic-continental convergence, when the Nazca plate melted as it plunged beneath the continent of South America.

Plate Tectonics

The largest earthquake ever recorded occurred along the Nazca-South American plate boundary.

ChileMay 22, 1960

Magnitude 9.5

Berkeley, CA Seismogram

Fig. 6.18

W. W. Norton

Major Volcanoes of the World

The Cascade Mountain Range, in western US, is a volcanic mountain range that was formed as a result of oceanic-continental convergence.

Eruption of Mount St. Helens in 1980

The Cascade Mountain Range is a result of the “Juan de Fuca Plate” subducting under the western US.

Oceanic - Continental

Oceanic - Oceanic

Continental - Continental

Oceanic - Oceanic Convergence

When two oceanic plates converge, one descends beneath the other initiating volcanic activity (similar to the oceanic - continental case), but the volcanoes form on the ocean floor rather than on continents.

Volcanic Island Arc

Dry land emerges from the ocean depths, forming a chain of volcanic islands called a volcanic island arc.

Examples of an island arcs are the island chains of the western Pacific Ocean, such as Japan.

In the case of Japan, volcanic islands are being created by the collision of the Pacific plate with the Eurasian plate. The Pacific plate being subducted beneath the Eurasian plate.

The Pacific plate subducting beneath the Eurasian plate.

Japan Subduction Zone and Island Arc

The January 17, 1995 Kobe, Japan EarthquakeMagnitude 6.9

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W. W. Norton. Modified from Cox and Hardt, 1986.

Aleutian Islands

Example of an island arc off the coast of Alaska

The second largest earthquake ever recorded occurred along an oceanic - continental plate boundary in Alaska.

Great Alaska Earthquake

March 28, 1964Magnitude 9.2

Dallas, Texas Seismogram

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Elementary school in Anchorage, Alaska destroyed by the 1964 Alaska earthquake.

Oceanic - Continental

Oceanic - Oceanic

Continental - Continental