Chapter 18Chapter 18

Plate tectonics Plate tectonics

History of plate tectonics

• The earth’s surface is divided into several major and minor plates and the interaction between these plates is known as plate tectonics.

There are 8 major and about 30 minor plates.

–N. American–S. American–African–Eurasian–Indian–Pacific–Australian–Antarctic

• Intense geologic activity occurs at the plate boundaries where plates;

–Collide with one another

–Move away from one another

–Slide past one another

• The concept of plate tectonics was developed in 1960s by combining two theories;

–Continental Drift

–Sea floor spreading

1. Continental Drift: proposed by Alfred Wagener in 1912.

• He found similarities in S. America, Africa, India, Antarctica and Australia.

• On this basis he proposed;

–All the continental landmasses were once joined together in one supercontinent—Pangea. All the oceans formed one super ocean—Tetheys.

–The Pangea then split into a northern Laurasia and a southern Gondwanaland.

• Revival of continental drift: Wegener’s mechanism of continental drift was not very convincing and his theory remained discarded till 1960s.

• It was revived with the advent of paleomagnetism, which confirmed that the continents have moved relative to one another.

• Additional evidence for cont. drift:

–The continents fit like a jigsaw puzzle

–Rock similarity

–Fossils and age similarity

2. Sea floor spreading: proposed by Harry Hess in 1962.

• He proposed that the sea floor moves away from the crest of a mid ocean ridge and finally disappears beneath a continent or an island arc (subduction).

• Mantle convection is responsible for ocean spreading.

Plates and plate boundaries

• Rigid lithospheric plates move over plastic asthenosphere.

• Plate boundaries: 3 types;

1. Divergent plate boundary: plates move away from one another. Also known as Constructive P.B. or spreading center.

2. Convergent P.B: plates move towards one another. Also known as Destructive P.B.

3. Transform P.B: plates move horizontally past one another. Also known as Conservative P.B.

Do plates really move?

• Plate motion is very slow but predictable. 1-10 cm/year.

• Movement is measured by satellites, lasers and GPS.

Do plates really move?

• The magnetic anomalies at the sea floor and movement along a transform fault indicate plate motion.

Divergent Plate Boundaries

• Two plates move away from one another.

• Can occur within a continent or an ocean. If it occurs in a continent—rifting.

Divergent Plate Boundaries

• Generally basaltic magma erupts and spreads on either side of the fractures causing the plates to push away from one another.

• Examples:

–Red Sea.

–East African Rift

• A passive continental margin forms when one of the moving portions of the plate are covered by sediments.

• A mid ocean ridge is formed when divergent P.B. occurs in the middle of an ocean—mid Atlantic ridge.

Convergent P.B.

• Two plates move towards each other and collide forming mountains.

Convergent P.B.

• Three types of convergent p.b;

–Ocean-ocean convergence

–Ocean-continent convergence

–Continent-continent convergence

• Ocean-ocean convergence: one plate subducts under the other, a trench and a volcanic island arc are formed.

• Ocean-continent convergence: oceanic plate subducts beneath the continent and an active continental margin is formed.

• Continent-continent convergence: neither plate subducts and a collisional mountain chain is formed.

3. Transform P.B: plates slide past one another an no significant material is created or destroyed.

• Transform faults generally connect two divergent p.b. or two trenches.

Why plates move?

• Mantle convection.

• Ridge Push

• Trench Pull

Mantle plumes and Hot Spots

• Mantle convection moves towards the earth surface in the form of a plume and appear on the surface as a Hot Spot.

Plate tectonics and ore deposits

• Valuable metallic ores are associated with divergent boundaries and volcanism on the sea floor.