Plate Tectonics

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.

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.

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.

Continental - Continental Convergence When two plates carrying continental crust collide, a continental collision occurs.

Continental - Continental Convergence Continental collisions occur because of the relatively low density, and thus the buoyant nature, of continental crust (granitic).

Fig.4.28 W. W. Norton. Modified from Cox and Hardt, India Colliding with Asia

A continental collision occurred when the once- separated continent of India collided with Asia, forming the Himalayan mountains.

Prior to a continental collision, the landmasses are separated by oceanic crust, formed during an earlier episode of sea- floor spreading. As the continental blocks converge, the intervening sea floor is subducted beneath one of the plates.

The decending oceanic slab generates either a volcanic arc (on one of the continents) or an island arc (at some distance into the ocean). Erosion of newly formed volcanic rocks adds sediment to the continental margins.

As the continents collide, the sediments are squeezed, folded, and deformed. This process results in a new mountain range composed of deformed and metamorphosed sedimentary rocks and fragments of volcanic rocks.

Earthquakes Plate Tectonics

Location, year and number of fatalities (in parenthesis) for earthquakes in India in the past 200 years. (From Bilham and Gaur, 2000).

Magnitude 7.7 Gujarat, India January 26, 2001

Fig.4.28 W. W. Norton. Modified from Cox and Hardt, 1986.

From “Written in Stone” by Chet Raymo and Maureen Raymo The Appalachian Mountains (along the eastern seaboard of North America) were formed by a continental collision in which Africa and Europe collided with North America about million years ago.

From “Written in Stone” by Chet Raymo and Maureen Raymo Many of the geological structures found in and around the Boston area were formed during the continental collision in which Africa and Europe collided with North America about million years ago.

From “Written in Stone” by Chet Raymo and Maureen Raymo

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.

1906 San Francisco Earthquake

This picture, taken near Bolinas in Marin County by G.K. Gilbert, shows a fence that was offset about 8.5 feet along the trace of the fault (from Steinbrugge Collection of the UC Berkeley Earthquake Engineering Research Center ) San Francisco Earthquake

“Elastic Rebound” Theory of Earthquakes

Earthquake Probabilities ???

Collapse of the Cypress Street elevated section of I-880 (near downtown Oakland). October 17, 1989 Loma Prieta ( “World Series”) California Earthquake.

Izmit, Turkey August 17, 1999 Magnitude 7.4

Plate Tectonics

Northridge, CA 1994

Earthquakes Plate Tectonics

Fig.4.28 W. W. Norton. Modified from Cox and Hardt, 1986.

Magnitude  3