Plate Tectonics The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material.
Continental Drift This process continues as the current earthquake/volcano hotspots of the world reflect the edges of the moving plates atop which the continents sit.
historical perspective In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to build."
inside the earth The size of the Earth -- about 12,750 kilometers (km) in diameter-was known by the ancient Greeks, but it was not until the turn of the 20th century that scientists determined that our planet is made up of three main layers: crust, mantle, and core.
The continental crust is composed mostly of granite which allows them to ride on the denser oceanic plates of volcanic basalt. The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere .
tectonic plates The lithosphere floats upon the molten mantle.
Geologists came to the conclusion in the 1960's that the Earth's lithosphere was broken up into about 12 large pieces called plates that are moving relative to one another.
convection The means by which the Earth releases its internal heat. Hot mantle rises to the surface and spreads laterally, transporting oceans and continents as on a slow conveyor belt. The speed of this motion is a few centimeters per year.
convection The animation above shows the result of a computer simulation of simple convection. Hot areas of the fluid are in reds and seem to rise, colder in blues seem to sink. It takes place primarily because buoyancy forces are able to overcome viscous resistance.
The Earth is producing "new" crust where two plates are diverging.
Subduction zones appear as deep oceanic trenches. Most of the continental mountain belts occur where plates are pressing against one another.
creating trenches In this cross section of the Earth in the Southern Hemisphere, the map shows a subduction zone that has created the Peru-Chile Trench at the western edge of South America and the Andes Mountains along the west coast of South America.
Plate boundaries The red lines on this map of the world indicate c onvergent boundaries where two plates collide to form mountains or a subduction zone. The yellow lines indicate divergent boundaries where two plates are moving in opposite directions.
Mid-Atlantic Ridge The Atlantic Ocean is getting larger as the Western Hemisphere moves away from Europe and Asia while the Pacific Ocean is becoming smaller. This is occurring because the North American and South American plates are moving westward.
A satellite view of the Sinai Peninsula shows two arms of the Red Sea spreading ridge, exposed on land. This is a northern extension of Africa’s Great Rift Valley . spreading ridge
Great Rift Valley This geologic depression extends from Syria in southwestern Asia to Mozambique in southeastern Africa. It takes the form of a series of valleys and bodies of water that are bounded by parallel fault lines.
The Great Rift Valley The Great Rift Valley is widening, in the process causing many volcanic eruptions and earthquakes in the area.
plates dive beneath plates The Atlantic Ocean is getting larger as the Western Hemisphere moves away from Europe and Asia. The Pacific Ocean, on the other hand, is becoming smaller.
trench forms When the less dense, lighter continental plate overrides the oceanic plate a subduction zone forms. Because the oceanic plate is bent and driven down, a deep trench forms at this collision point.
Marianas Trench The island of Saipan rises more than 36,000 feet above the floor of the trench. On the floor of the trench the water pressure is more than 7 tons per square inch.
The pink lines on this map of the Pacific Ocean represent deep ocean trenches.
ocean floor Earthquake occurrence in different plate tectonic settings: This map of Earth's solid surface shows many of the features caused by plate tectonics . The oceanic ridges are the spreading centers, creating new oceanic crust.
meeting of the plates An aerial view shows probably the most familiar meeting of two plates in the United States, the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo, CA. Photograph by Robert E. Wallace, USGS.
The Farallon Plate, was consumed by subduction beneath the N. American and Caribbean plates, leaving the Juan de Fuca, Rivera, and Cocos Plates.
As the mantle rocks melt they form magma which pools. Because the magma is less dense than the surrounding mantle material it will rise.
forming volcanoes A volcano will form if the magma reaches the surface. Magma that has reached the surface.
Krakatau volcano lies in the Sunda strait between the islands of Java and Sumatra. In about 416 A.D., caldera collapse destroyed the volcano and formed a 4-mile wide caldera.
The eruption of the Krakatau volcano in 1883 produced one of the largest explosions on Earth in recorded time (VEI=6) and destroyed much of Krakatau island.
In 1960 scientists visited Anak Krakatau to record its renewed activity and to measure changes in the size and shape of the island.
ash rises over Krakatau They observed explosive eruptions of pyroclasts from ash to boulders in size
four years of explosions Explosive eruptions occurred at half to 10-minute intervals. The largest explosions produced turbulent clouds of ash that rose 4,000 feet above the vent. In this photo a column of ash is rising about 1,500 feet (450 m) above the crater.
Volcanic ash in the atmosphere contributes to the brilliance of Pacific Ocean sunsets
Most of Indonesia's volcanoes are part of the Sunda arc, a 3,000-km-long line of volcanoes extending from Sumatra to the Banda Sea, the result of subduction of the Australia Plate beneath the Eurasia Plate .
Volcanoes in the Banda Sea result from subduction of the Pacific Plate under the Eurasia Plate. Black "teeth" are on the overriding plate. Arrows show direction of movement along major transform faults.
Strike-slip fault In most subduction zones , motion of the subducted plate is nearly perpendicular to the trench axis. In Sumatra, where the motion is oblique to the axis, a strike-slip fault zone is seen, and is lying parallel to the volcanic chain. Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally.
A tsunami is formed when continental plates shift suddenly as it did last December off the coast of Indonesia. The huge waves killed thousands.
tsunami A natural phenomenon consisting of a series of waves generated when water in a lake or the sea is rapidly displaced on a massive scale. D ecember 26, 2004
"The Great Wave Off Kanagawa" - 19 th Century woodcut
tsunami wall Japan has been building tsunami walls of up to 13.5 ft high in front of populated coastal areas. Early warning systems enable low-lying populated areas to evacuate.
Destructive tsunamis originate primarily in subduction zones around the Pacific Rim. Model simulations combined with real-time tsunami measurements will help assess the hazard. D eep-ocean A ssessment And R eporting of T sunamis S ystem
collisional boundary When two land masses meet neither will slide under the other. Instead, the two crush together at what is known as a collisional boundary. When two land masses meet neither will slide under the other. Instead, the two crush together at what is known as a collisional boundary.
collisional ranges The Himalayas, the highest mountains in the world, were created this way. (In fact, they're still growing.) So were the European Alps. Even the Appalachian Mountains formed when two land masses came together. The Appalachian’s Blue Ridge Mts. The Swiss Alps
A whole lotta shakin’ goin’ on Transform boundaries neither create nor consume crust. Rather, two plates move against each other, building up tension, then releasing the tension in a sudden and often violent jerk. This sudden jerk creates an earthquake.
transform boundary The San Andreas Fault is undoubtedly the most famous transform boundary in the world. To the west of the fault is the Pacific plate, which is moving northwest. To the east is the North American Plate, which is moving southeast. Photograph by Robert E. Wallace, USGS.
slippin’ and a slidin’ Los Angeles, located on the Pacific plate, is now 340 miles south of San Francisco, located on the North American plate. In 16 million years, the plates will have moved so much that Los Angeles will be north of San Francisco!
Hawaii forms The vast majority of earthquakes and volcanic eruptions occur near plate boundaries, but there are some exceptions.
hot spots The Hawaiian Islands, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than 3,200 km from the nearest plate boundary.
Hawaiian Islands Artist's conception of the movement of the Pacific Plate over the fixed Hawaiian "Hot Spot," illustrating the formation of the Hawaiian Ridge-Emperor Seamount Chain.