Chapter 2 Plate Tectonics

Basic premise of plate tectonics

The lithosphere is divided into sections that move relative to one another and relative to the asthenosphere at slow cm/yr rates; plate boundaries become deformed.

Relative velocity

A plate's velocity with respect to another plate.

Absolute velocity

A plate's velocity with respect to a fixed point.

Plate

A distinct piece of lithosphere with plate boundaries on all sides.

Continental margin

The edge of a continent where it meets an ocean or sea.

Active margin

A continental margin that is also a plate boundary.

Passive margin

A continental margin that is not a plate boundary.

Major tectonic plates

Pacific, North American, South American, Eurasian, African, Australian, Indian, and Antarctic plates.

Minor tectonic plates

Juan de Fuca, Cocos, Caribbean, Scotia, Arabian, Philippine, Nazca, Iran, and Bismarck plates.

Microplate

A very small tectonic plate occurring as a non-rigid segment along plate boundaries; Tonga Plate is an example.

Plate boundaries and earthquakes

Earthquakes commonly occur along plate boundaries.

Mid-ocean ridge

A 2-km-high submarine mountain belt formed along a divergent oceanic plate boundary.

Seamount

A submarine mountain.

Trench

A deep elongate trough bordering a volcanic arc; it marks a convergent plate boundary.

Ridge axis

The crest of a mid-ocean ridge; it marks the location of a divergent plate boundary.

Fracture zone

A band of vertical fractures in ocean floor roughly perpendicular to a mid-ocean ridge; its actively slipping part is a transform fault.

Volcanic arc

A curving chain of active volcanoes formed adjacent to a convergent plate boundary.

Three plate-boundary types

Divergent, convergent, and transform.

Divergent plate boundary

Two plates move away from one another; true divergent boundaries occur at mid-ocean ridges.

Convergent plate boundary

Two plates move toward one another; trenches and volcanic activity commonly occur where oceanic crust is involved.

Transform plate boundary

Two plates move past one another; examples include the San Andreas Fault and transforms offsetting mid-ocean ridges.

Seafloor spreading

New oceanic crust forms at a mid-ocean ridge and older crust is carried progressively farther away from the ridge.

Magnetic stripes on seafloor

Basalt records Earth's magnetic polarity when it erupts, producing striped magnetic patterns that can help indirectly date ocean floor.

Ocean-floor age pattern

The farther from a mid-ocean ridge, the older the ocean floor becomes.

Rifting

A continent stretches and splits along a belt; successful rifting can separate a continent and develop into oceanic spreading.

Continental-rift examples

The Arabian Peninsula separated from Africa by rifting; Africa is currently rifting along the East African Rift.

Subduction zone

Region along a convergent boundary where one plate bends and sinks into the mantle beneath another.

Oceanic-continental convergence

An oceanic plate subducts beneath a continental plate.

Nazca-South America example

The Nazca Plate subducts beneath the South American Plate, producing the Andes Mountains.

Oceanic-oceanic convergence

One oceanic plate subducts beneath another oceanic plate; volcanic island arcs can form.

Volcanic island arc

A chain of volcanic islands formed at oceanic-oceanic convergence.

Back-arc basin

A marginal sea formed behind some oceanic volcanic arcs.

Tonga convergence example

The Pacific Plate subducts beneath the Tonga microplate.

Downgoing plate

The plate being subducted beneath the overriding plate.

Overriding plate

The less-dense plate that is not sinking or being subducted.

Accretionary prism

Wedge-shaped mass of sediment and rock scraped off the downgoing plate and pasted onto the overriding plate.

Forearc basin

Depression in front of a volcanic arc where sediment is commonly deposited.

Subduction-zone volatiles

Downgoing oceanic crust carries H2O and other volatiles deep into the mantle; these lower the mantle's melting temperature.

Partial melting

Melting of minerals with the lowest melting temperatures while other minerals in the rock remain solid.

Cascade volcanoes

Mt. Rainier, Mt. St. Helens, and other Cascades form from mantle partial melting caused by Juan de Fuca subduction beneath North America.

Wadati-Benioff zone

Zone of earthquakes that marks the top of a downgoing subducting plate.

Continental collision

When two continental plates collide; their low density prevents normal subduction.

Suture

A linear belt of highly deformed rocks formed where continental plates collide and the former trench closes.

Collisional mountain belt

Mountain belt formed where two continental plates collide and the downgoing plate breaks off.

San Andreas Fault

Transform boundary between the North American Plate and Pacific Plate.

Transform boundary vs fracture zone

Active sliding between plates is a transform boundary; a section with no active movement is a fracture zone.

Hot spot

Special location within a plate where a deep plume of magma rises from the mantle and breaks through the lithosphere.

Hot-spot volcano chain

As a plate moves over a mostly stationary hot spot, a chain of volcanoes forms; old volcanoes can sink below sea level and become seamounts.

Hot spots and absolute plate motion

Because hot spots are thought to move very little, their volcanic chains can record absolute plate movement.

Ridge push

Plate-driving force caused by gravity acting on elevated ridge areas.

Slab pull

Gravity-driven force in which a subducting plate helps pull the rest of the plate; the lecture identifies it as the largest plate-driving force.

Mantle drag

The asthenosphere pushing at the bottom of a tectonic plate.

Supercontinent

A continental mass including all or most present-day continents; forms as convergence brings continents together.

Pangea

The most recent supercontinent, formed about 250 million years ago.

Evidence for Pangea: matching geology

Matching rock types and mountain belts of the same age on different continents support that the continents were once joined.

Evidence for Pangea: fossils

Mesozoic fossils on different continents help connect the continents back together.

Glossopteris evidence

Glossopteris plant fossils indicating a cool climate occur on five different continents and support reconstruction of Pangea.

Pangea climate belts

When Pangea is reconstructed, rock types reveal several climate belts that existed during the Mesozoic.