Continental Drift

Who is Alfred Wegener?

A German scientist who 1912 suggested that that the continents have and are moving – Theory of Continental Drift.

What was his hypothesis?

Wegener’s theory of continental drift proposed that the continents were once joined as a single landmass (Pangaea). Pangaea began to break apart 200 million years ago and has slowly been moving towards the current position of today’s continents.

What evidence did Wegener use to support his hypothesis?

1. Climate data • By studying sedimentary rock we figure out what the climate would

have been at the time the rock formed. For example, coal deposits in Antarctica

• Glacier evidence in now much warmer climates, so at some point those areas must have been closer to the poles.

2. Rock evidence• Similarities in the shapes of the continents, they look like puzzle

pieces• Specific rocks coincide on Africa & South America where they seem

to fit3. Fossil evidence – fossils of certain animals have been found on

several continents. Ex. Mesasaurus are only found in Brazil, South America & in South AfricaEx. Glossopteris found in Antarctica, Australia, South Africa, and South

America

Sea-Floor Spreading

• The theory that new ocean floor is created at mid-ocean ridges and destroyed at deep-sea trenches.

Features & Processes of Sea-Floor Spreading

• Mid-Ocean Ridge (A) – A long chain of mountains with a central rift valley that is located along a divergent plate boundary along the ocean floor.

• Sea-Floor Spreading (B) – Theory that new ocean floor is created at mid-ocean ridges and destroyed at deep-ocean trenches

• Deep-Sea Trench (C) – A long, narrow steep sided trough that runs parallel to continents or island chains.

• Subduction (D) – A convergent plate boundary were an oceanic plate is plunging under another plate.

Evidence to Support Sea-Floor Spreading

1. Magnetic Reversals have occurred where the present magnetic north pole has become the south magnetic pole.

• The symmetrical pattern in sea-floor rocks show that the rock formed at one place (the ridge) and then broke apart and moved away from the center.

2. Age2. Age• By using rock core

samples scientists determined the youngest rock is closest to the mid-ocean ridge and the oldest furthest away.

Convection in the Mantle

• Convection in the mantle occurs because heat from the inner & outer cores is transferred through the mantle.

• Convection is able to occur in the asthenosphere (upper portion of the mantle) because the asthenosphere is pliable (can move like a soft solid)

Rock material in the mantle is heated by the intense heat of the outer core. The heated rock begins to expand and rise up through the mantle towards the crust. Warmer mantle material pushes the cooler material out of the way – as the material moves its pushes and pull the lithospheric plates with it. Eventually the cooler, denser material begins to sink back down in the mantle towards the cores. The cycle then continues to repeat.

The Theory of Plate Tectonics

• The theory that the lithosphere is broken into several large plates that move and interact with each other at their boundaries.

• The continents are embedded in lithospheric plates and as the plates move they carry the continents with them.

• The theory of plate tectonics is supported by various evidence:– Fossil– Climate– Location of Earthquakes & Volcanoes– Sea-Floor Spreading

• The theory of plate tectonics explains why earthquakes and volcanoes occur in particular locations.

Transform Boundary

• A transform boundary is a point where two plates slip past each other, moving in opposite directions.

• Earthquakes occur frequently along these boundaries.

• The San Andreas fault in California is an example of a transform boundary.

Divergent Plate Boundary

• An area where two plates move apart.

• Divergent boundaries occur on land and on the ocean floor.

• If two oceanic plates move apart, new ocean crust is created along the boundary creating a mid-ocean ridge.

• If two plates move apart on land, a deep valley, called a rift valley, is formed. The Great Rift Valley in east Africa is a well-known example.

Convergent/Collision Boundary

• The area where two plates come together is called a convergent boundary. This movement of plates causes a collision.

• A collision between two continental plates squeezes the crust into mountain ranges.

• The Himalayan Mountains is an example of a convergent boundary between two continental plates. This mountain chain continues to form as the plate carrying India collides with the plate carrying Asia.

Subduction Boundary/Zone

• Denser ocean crust sinks below either an oceanic or continental plate. As the oceanic crust is forced back into the mantle a trench is formed. As the rock material in the ocean crust begins to melt in the mantle, some of the molten rock is forced upward producing volcanoes.

• Subduction zones are common in an area surrounding the Pacific Ocean known as “The Ring of Fire.” Due to this process, there is much earthquake and volcanic activity.