Chapter 9Plate Tectonics

Section 9.2

Plate Tectonics

Plate tectonics

• By the 1960’s, accumulated evidence supporting the hypothesis of continental drift and seafloor spreading led to the formation of a more far-reaching theory.

• This theory is called plate tectonics.• The theory of plate tectonics not only describes

continental movement but also proposes a possible explanation of why and how continents move.

• The term tectonics comes from the Greek word tekonikos, meaning “construction.”

• Tectonics is the study of the formation of features in the earth’s crust.

Plate tectonics

• According to the plate tectonics theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer.

• This layer is known as the lithosphere.• This outer shell lies over a weaker region in the

mantle known as the asthenosphere.– The lithosphere floats upon the asthenosphere and

permits plate motion.• The lithosphere is divided into segments called

plates, which move and continually change shape and size.– There are 7 major plates and many smaller plates.

• Seven Major Plates: Eurasian, African, Australian-Indian, Antarctic, Pacific, North American, and South American.

Plate tectonics

Plate tectonics

• The largest plate is the Pacific plate, which covers most of the Pacific Ocean.

• Several of the plates include an entire continents plus a large area of the seafloor.– Continental and Oceanic lithosphere.

• This is a major departure from Wegener’s hypothesis of continental drift, which proposed that the plates moved through the ocean floor, not with it.

Plate tectonics

• The lithospheric plates move relative to each other at a very slow but continuous rate that averages about 5 centimeters per year.

• This movement is driven by the unequal distribution of heat within Earth (Earth’s convection cycle).– Hot mantle rises, cooler mantle sinks, setting

Earth’s outer shell into motion.

• This grinding movement generates earthquakes, creates volcanoes, and deforms large masses of rock into mountains.

According to the theory of plate tectonics,

A. The asthenosphere is divided into plates.

B. The lithosphere is divided into plates.

C. The asthenosphere moves over the lithosphere.

D. The asthenosphere is strong and rigid.

Which of the following statements correctly

describes the asthenosphere?A. It is a thin, cold, and rigid layer.

B. It is the source of Earth’s heat.

C. It permits plate motion.

D. It occurs only near subduction zones.

In the plate tectonics theory, the lithosphere is divided into

A. 100 major plates.

B. 7 major plates and many smaller plates.

C. Many small plates, but no large plates.

D. 50 major plates and many smaller plates.

The lithospheric plates move an average of

A. 5 inches per year.

B. 50 inches per year.

C. 5 centimeters per year.

D. 50 centimeters per year.

A tectonic plate consists of

A. The crust and uppermost mantle.

B. The oceanic and continental crust only.

C. The crust and entire mantle.

D. The asthenosphere only.

In the plate tectonic theory, a plate can be made up of

A. Continental lithosphere only.

B. Oceanic lithosphere only.

C. Both continental and oceanic lithosphere.

D. Both continental and oceanic asthenosphere.

Plate tectonics

• All major interaction among individual plates occur along their boundaries.

• There are three main types of boundaries.– Convergent.– Divergent.– Transform.

Plate tectonics1. Convergent:• Form where two plates move

together.• This results in oceanic

lithosphere plunging beneath an overriding plate, and descending into the mantle.

• Scientists refer to the region along a plate boundary where one plate moves under another as a subduction zone.

• A deep ocean trench generally forms along a subduction zone.

– Ex: The Andes in South America.

Plate tectonics

2. Divergent:• Occur when two plates

move apart.– Also called spreading

centers.

• Results in upwelling of material from the mantle to create new seafloor.

• This formation is called a rift valley.

– Ex: The Mid-Ocean Ridge and the East African Rift Valley.

Plate tectonics

3. Transform Fault:• Margins where two

plates grind past each other without the production or destruction of lithosphere.– Ex: San Andreas Fault

Plate tectonics

• Each plate contains a combination of these three types of boundaries.

• The total surface area of the Earth does not change, but the plates may shrink or grow.

• This shrinking/growing depends on the locations of convergent and divergent boundaries.– Ex: The Antarctic plate is getting larger while the Phillipine

plate is becoming smaller.

• New plate boundaries can be created because of changes in the forces acting on these rigid slabs.

What kind of plate boundary occurs where two plates grind

past each other without destroying or producing

lithosphere?A. Divergent boundary.

B. Convergent boundary.

C. Transitional boundary.

D. Transform fault boundary.

A divergent boundary at two oceanic plates can result in a

A. Rift valley.

B. Volcanic island arc.

C. Continental volcanic arc.

D. Subduction zone.

What type of boundary occurs where two plates move together, causing one plate to descend into

the mantle beneath the other plate?

A. Transform fault boundary.

B. Divergent boundary.

C. Convergent boundary.

D. Transitional boundary.

Which of the following is a geographic example of a

transform fault boundary?

A. The East African Rift valley.

B. The San Andreas Fault.

C. The Mid-Atlantic Ridge.

D. The Andes Mountains.

New ocean crust is formed at

A. Divergent boundaries.

B. Convergent boundaries.

C. Continental volcanic arcs.

D. Transform fault boundaries.