Continental Drift Theory

Proposed by Alfred Wegener in million years ago, all of the continents were combined into one super-continent called “Pangaea” The continents gradually drifted apart to where they are today

“ Puzzle Pieces” Continents look like they could be part of a giant jigsaw puzzle

Distribution of Fossils Plant and animal fossils found on the coastlines of different continents

Ancient Climates Tropical plant remains (coal deposits) found in Antarctica Glaciation in Africa, South America, India, and Australia during the same time

Sequence of Rocks Same rock patterns found in South America, India, Africa, Antarctica and Australia

Problems With The Theory Theory was not accepted by scientists…why? –Because Wegner could not identify the cause of continental drift

Causes of Plate Tectonics

Convection Currents Hot magma in the Earth moves toward the surface, cools, then sinks again. Creates convection currents beneath the plates that cause the plates to move.

Heat Transfer The Driving Force of Plate Tectonics

Theory of Plate Tectonics Plate Tectonics Plate Boundaries Causes of Plate Tectonics

Plate Tectonics

What is Plate Tectonics The Earth’s crust and upper mantle are broken into sections called plates Plates move around on top of the mantle like rafts

What is the Lithosphere? The crust and part of the upper mantle = lithosphere –100 km thick –Less dense than the material below it so it “floats”

What is the Asthenoshere? The plastisity layer below the lithosphere = asthenosphere The plates of the lithosphere float on the asthenosphere

2 Types of Plates Ocean plates - plates below the oceans Continental plates - plates below the continents

Plate Boundaries

Divergent Boundaries Boundary between two plates that are moving apart or rifting   RIFTING causes SEAFLOOR SPREADING

Divergent Boundaries create… Mid-ocean ridges rift valleys fissure volcanoes

Convergent Boundaries Boundaries between two plates that are colliding   There are 3 types…

Type 1 Ocean plate colliding with a less dense continental plate Subduction Zone: where the more dense plate slides under the less dense plate VOLCANOES occur at subduction zones

Andes Mountains, South America

Type 2 Ocean plate colliding with another ocean plate The more dense plate slides under the less dense plate creating a subduction zone called a TRENCH

Aleutian Islands, Alaska

Type 3 A continental plate colliding with another continental plate Have Collision Zones: –a place where folded and thrust faulted mountains form.

Transform Fault Boundaries Boundary between two plates that are sliding past each other EARTHQUAKES along faults Another example of a transform boundary is the San Andres Fault

San Andreas Fault, CA

Sea-Floor Spreading

Evidence Magnetic Stripes. How does the pattern of matching stripes show evidence of sea- floor spreading?

Mapping the mid-ocean ridge The East Pacific Rise has a mid-ocean ridge. Curves around like a baseball. Lies hidden under hundreds of meters of water.

Sonar A device that bounces sound waves off underwater objects and then records the echoes of these sound waves. Sonar mapped mid- ocean ridges.

Harry Hess An American geologist who studied mid-ocean ridges. He suggested that the ocean floors move like conveyor belts, carrying the continents along with them.

At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. As the molten material cools, it forms a strip of solid rock in the center of the ridge. Then more molten material flows into the crack.

Sea-floor spreading The process that continually adds new material to the ocean floor. Evidence molten material, magnetic stripes, and drilling samples.

Evidence #1 - Molten Material The submersible, Alvin, found strange rocks shaped like pillows or like toothpaste squeezed from a tube. Such rocks can form only when molten material hardens quickly after erupting under water.

Evidence #2 - Magnetic Stripes Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”. They hold a record of reversals in Earth’s magnetic field.

Evidence #3 - Drilling Samples The Glomar Challenger did a drilling sample and found rocks that the farther away from the ridge the older the rocks were. The younger ones were in the center of the ridge.

Subduction at Deep-Ocean Trenches Wider & wider? Deep- ocean trenches Ocean floor plunges into deep underwater canyons are deep-ocean trenches. Subduction is the process by which the ocean floor sinks beneath a deep- ocean trench and back into the mantle.

Subduction in the Pacific & Atlantic Deep ocean trenches are swallowing more oceanic crust than the mid-ocean ridge can produce. Thus, the width of the Pacific will shrink. The Atlantic is expanding. It has short trenches. In some places, the oceanic crust is attached to the continental crust which moves the continents.

Subduction At deep-ocean trenches, subduction allows part of the ocean floor to sink back into mantle, over tens of millions of years. Subduction and Earth’s Oceans Earth’s ocean floor is renewed about every 200 million years.