Chapter 18 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 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 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 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.