by Alexandra H., Brooke T., help from Tom L., Tim D., and Mr. Brown Edited by Mr. Ledoux

Theory of Plate Tectonics Earth’s crust is composed of large plates These plates float on the mantle below These plates have always moved (slowly) This movement shapes our Earth’s surface

What it Explains The theory explains the how and why of… Mountains Volcanoes Earthquakes Similar prehistoric animals lived at the same time on now widely separated continents

History of Plate Tectonics Continental drift was originally proposed by Alfred Wegener, a German meteorologist, in 1912 His idea was supported by: The fit of the continents The distribution of fossils Similar rock sequences Ancient climates The apparent wandering of the Earth's polar regions

Pangaea Wegener used his observations to hypothesize that all of the present-day continents were once part of a single supercontinent called Pangaea.

Species Fossils of the same species were found on several different continents. Wegener proposed that the species dispersed when the continents were connected and later carried to their present positions as the continents drifted apart.

Rock Sequences Rock sequences in South America, Africa, India, Antarctica, and Australia are very similar. Same three layers, same order, in areas now separated by great distances Why?

Correct! He proposed that the rock layers were made when all the continents were part of Pangaea.

Drifting Magnetic Poles Wegener did not believe it possible that the magnetic poles could move far from the Earth’s poles. Wegener did not believe it possible that the magnetic poles could move far from the Earth’s poles. So the apparent drift over time could be explained by the continents moving. So the apparent drift over time could be explained by the continents moving.

Problems with Wegener’s Theory Wegener’s Theory was not accepted by most geologists. They said that ocean currents or winds could scatter the ancient plants and animals. They said that ocean currents or winds could scatter the ancient plants and animals. They also thought that maybe it was possible for the poles to wander. They also thought that maybe it was possible for the poles to wander.

The Big Problem Wegener could not explain how the continents could move. He couldn’t explain what forces could actually cause the continents to move.

The Big Solution Arthur Holmes, (Scottish geologist) made this proposal in He believed heat trapped in the Earth caused convection currents, areas where fluids beneath the Earth's crust rise, flow laterally, and then fall. The currents would rise beneath continents, spread laterally, then plunge beneath the oceans. Unfortunately, Wegener died in 1930

Theory of Plate Tectonics Totally revolutionized our understanding of the Earth, past and present. Theory ranks with the theories on evolution, relativity, the Big Bang, and Newton’s Laws.

The Ocean Floor Landforms on the ocean floor are as diverse as those on land These landforms are also formed by moving plates

Ocean Floor Cont. The ocean floor, just like land, has mountains, volcanoes, fault lines, ridges, valleys, and many other landforms.

The Break Up of Pangaea Pangaea was the large landmass that separated to form the continents millions of years ago

Scientific Basic Facts CONTINENTAL CRUST Less Dense (lighter weight): average density = 2.7 gm per cubic centimeter Thicker: 0 to 40 km, average 35 km under high mountains, the crust can be thickened, up to 65 km. Under the craton, the. Thicker: 0 to 40 km, average 35 km under high mountains, the crust can be thickened, up to 65 km. Under the craton, the average thickness is 30 km. Mineral Composition: granitic (felsic) rocks such as granodiorite and granite enriched in Potassium (K), Sodium (Na), Aluminum (Al), and Silica (Si) "Floats" on top of the Mantle

Oceanic Crust OCEANIC CRUST OCEANIC CRUST More dense (heavier): average density = 3.0 gm per cubic centimeter More dense (heavier): average density = 3.0 gm per cubic centimeter Thinner: 0 to 10 km, Thinner: 0 to 10 km, average 5 km, thinnest at Mid-Ocean Ridges Mineral Composition: mafic rocks such as basalt and gabbro enriched in Magnesium (Mg) and Iron (Fe) Mineral Composition: mafic rocks such as basalt and gabbro enriched in Magnesium (Mg) and Iron (Fe) Underlies Ocean Basins Underlies Ocean Basins Also “floats” on the mantle Also “floats” on the mantle

Convection Currents Inside the Earth The tectonic plates float on the magma that lies in the core of the earth This magma is just one big convection current

Earth’s Plates

About Earth’s Plates The different plates were formed when Pangaea broke up. In all there are 14 (depending on your source)

Continental Drift

Continental Drift Continued Continental drift means that the plates move across the magma. It happens very slowly…2 inches a year is considered very fast!

4 types of plate boundaries: new crust is created as the plates pull away from each other 1. Divergent boundary -- new crust is created as the plates pull away from each other crust is destroyed as one plate dives under another 2. Convergent boundary -- crust is destroyed as one plate dives under another crust is neither produced nor destroyed as the plates slide horizontally past each other 3. Transform boundary -- crust is neither produced nor destroyed as the plates slide horizontally past each other broad belts in which boundaries are not well defined and the effects of plate interaction are unclear 4. Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear

Transform Boundary Two plates slip past each other, moving in opposite directions Edges of crust is not damaged. Earthquakes often occur Example: San Andreas fault in California

Divergent Boundaries Plates are moving away from each other Magma from below is pushing the plates apart. This creates new crust. Creates mid-ocean ridges and rift valleys The rift valley in east Africa is about 3,000 km long.

Rifting

Convergent Boundaries Two plates crash together More dense plate slides under the less dense plate. Old crust is recycled back into the mantle. Creates: A trench (at least 1 oceanic plate) Mountain ranges (2 continental plates)

Subduction

Oceanic Subduction

Continental Subduction

Mountain Building Convergent boundaries can also cause mountains to be built up by the crust folding. Himalayan Mountains are formed by India pushing up into Asia

Borders This is where all the action is: This is where all the action is: Volcanoes Volcanoes Earthquakes Earthquakes

Plate Boundaries

Hot Spots Plates move over an area where magma is close to the surface. Volcanoes occur at the same place, but because the plate is moving, a chain of volcanoes can occur. Hawaiian Islands were formed this way.

Earth’s Crust

Future Earth What do you think the world will look like in a million years??? What do you think the world will look like in a million years??? It is predicted that the continents will eventually crash together on the other side of the world, creating a new Pangaea. It is predicted that the continents will eventually crash together on the other side of the world, creating a new Pangaea.

Future World

Bibliography s/astronomy/planets/earth/Continents.sht ml s/astronomy/planets/earth/Continents.sht ml onics/GRAPHICS/plate_opening.jpg