CONTINENTAL DRIFT

SEAFLOOR SPREADING

THEORY OF PLATE TECTONICS

Plate Tectonics

Continental Drift

Evidence for Continental DriftPangea: Alfred Wegener (1912) suggested that

all the continents were joined together at some time in the past Continental Drift was suggested that this huge

land mass (called Pangea) began to break apart about 200 mya

Idea wasn’t accepted as he was unable to explain how they drifted apart

Long after his death (1930) was there enough evidence to accept Continental Drift

Pangea Videos

http://www.5min.com/Video/Learn-about-The-Continental-Drift---Plate-Tectonics-117505893

http://www.pond5.com/stock-footage/128542/pangea-earth-s-continents-separate.html

Clues to support Continental Drift

Fossil clues: fossils found (Mesosaurus) in S America and Africa that were freshwater land animals that couldn’t have swum sea water to both sides, therefore lived together before the split

Widespread Plants: Glossopteris fossil found on Africa, Australia, India, Antarctica, S America suggested that these plants thrived once when continents were attached

Clues to support Continental Drift

Rock clues: similar rock structures found on different continents

Climate Clues: glacier deposits found in tropical climates and tropical plants found on island in Arctic Ocean

How could continents drift?

Wegener’s radical idea of continental drift provided no evidence as to how the continents plowed through higher density ocean floor material

His proposal suggested it was caused by Earth’s rotational spin, but no evidence supported that

Only after discovery of seafloor spreading was his idea re-introduced as explanation for continental drift

Seafloor Spreading

Mapping the Ocean Floor: sound and radar waves were used to map the ocean floor in 1940’s and 50’s

Noticed that in middle of oceans were huge mountain ridges

This discovery caused scientists to ask why?

Seafloor Moves

Harry Hess suggested that the seafloor moves

Proposed that hot, less dense material below Earth’s crust rises toward the surface at mid ocean ridges , lows sideways carrying the seafloor away from te ridge in both directions

Evidence for Spreading

Research ship Glomar Challenger gathered data in 1968 by drilling ocean floor

Found younger rock are located at the mid-oceanic ridges and older rock found farther away

Mid-oceanic ridges have molten rock, heat, exotic life-forms

Evidence for Spreading

Magnetic clues: when molten rock cools, it cools with magnetic attractions to the north and south poles

Found that magnetic north has changed several times in earth’s history and the magnetic poles are preserved in the rock at mid-oceanic ridges

On each side of the mid-ocean ridges portrays these magnetic changes

Magnetometer detects the magnetic field reversals on both side of ridges indicating that the ridges are moving apart when new rock is formed

Seafloor spreading and Magnetic reversals

Theory of Plate Tectonics

Plate Movement: plate tectonics combine the knowledge of continental drift and seafloor spreading in the Theory of Plate Tectonics

Earths crust and upper mantle are broke into plates

These plates ride on top a plastic-like layer of the mantle much like a raft on water

Composition of Earth’s Plates

Lithosphere is the outer layer of earth’s crust

Made of less dense material than plastic-like layer below it (asthenosphere)

Plate Boundaries

When plates move they interact with each other in many ways

They can move toward, pull away or slide past each other.

When plates interact, the result of their movement is seen at the plate boundaries

Plate Boundaries

Plates Move Apart: Divergent boundaries are areas where plates move away from each other

Mid-Atlantic Ridge is divergent boundary between North American Plate and the Eurasian and African Plate

Great Rift Valley in eastern Africa is result of continental plate being pulled apart

Plate Boundaries

Plates Move together: Convergent boundaries are areas where plates move together

As crust is added in one area, crust disappears in other areas when seafloor cools, becomes denser and sinks (convergent boundaries)

When oceanic crusts converge with less dense continental crust, the denser oceanic plate sinks under the continental plate in subduction zones

Deep-sea trenches are associated with convergent boundaries

http://www.geocraft.com/WVFossils/subduction.html

Plate Boundaries

As ocean plates sink under continental plates, they warm and melt and magma forms. This less dense magma rises to surfaces and form volcanoes Cascade Mt ranges are result of convergent boundaries and

subduction zones

A subduction zone can form from two oceanic plates that collide where the older, denser plate sinks (Mariana Islands are chain of volcanic islands )

When 2 continental plates collide, usually they crumple up forming mountain ranges (earthquakes common, but no volcanoes) (Himalayan Mts)

Plate Boundaries

When plates slide past each other: transform boundaries are result where two plates slide past each other

They move in opposite direction and usually earthquakes are associated with transform boundaries

San Andreas fault is an example

Causes of Plate Tectonics

Question about what was force that would cause plates to move were finally answered by convection currents

Convection currents in earth’s crust are currents caused by warm less dense magma rising

Some magma reaches the surface at ridges, some is moved away from rising magma and drags the plates with them. They cool and settle back to earths center where it moves and is heated up and cycle repeats

Causes of Plate Tectonics

Causes of Plate Tectonics

Features Caused by Plate Tectonics

Interactions of plates build mountains, create ocean basins and cause volcanoes Earthquakes are associated with these interactions

Tension forces result in earth crust being pulled apart and form normal faults

Causes large blocks of crust to break, tilt, or slide down and large fault-block mountains formRift valleys and mid-oceanic ridges are associated with divergent boundaries where tension forces pull crust apart and normal faults occur

Tension forces and Normal faults

Rock layers above the fault move down when compared with rock layers below the fault

Compression Forces cause Reverse Faults

Where plates meet, compression force cause massive folding and faulting into mountain ranges (Himalaya) by reverse faulting

Rock layers above the fault surface is moved up relative to rock layers below the fault

Reverse and normal faulting

Reverse fault:Compression forces

Normal fault:Tension forces

Strike-slip Faults

Caused at transform boundaries where 2 plates move past each other (usually in opposite directions, but can be where both plates are moving in same direction but at different speeds)

These are called strike-slip faults

New technology and Plate tectonics

With GPS and modern technology we have been able to record crustal movement on plates as low as 1 cm per year

Pacific plate is moving at rate of 2.5 cm per year average