Theory of Plate Tectonics

Plates Meet at Boundaries Hot, mantle moves the plates. Tectonic Plates

Continental Drift  Wagener’s first theory  Continents are on crustal plates  Continents are moving  Did not explain “how”  Did not include ocean crust

Evidence for Continental Drift Evidence for Continental Drift 1. The shapes fit together ! Alfred Wegener: 1915

2. Rocks Match

3. Glaciers left scratches

4. Fossils match: land plants & animals

5. Climates have changed: New Mexico Climate through Time (Shows the continent Was closer to the equator.)

NEW THEORY: Plate Tectonics Ocean Floor Evidence

Evidence Continues 6. Mid-Ocean Ridges 7. Paleomagnetism (Magnetic field changed!) 8. Sea-Floor Spreading

Three Boundary Types: 1.Divergent 2. Convergent 3. Transform

Divergent Boundary: 1.Plates move apart. 2.New Crust from Mantle RED SEA: Young divergent boundary

Divergent Boundaries Continued 3. Most are underwater 4. Look for Mid-ocean ridge

1. Sea Floor Spreading 2. Volcanic Ridge Under the Ocean Evidence

Lava Flows Harden New Rock Records the Magnetic Field Each side is a mirror image of the other 3. Magnetic Pole Reversal

Divergent Boundary & Sea Floor Spreading Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart. As the plates move apart, the rocks break and form a crack between the plates. Earthquakes occur along the plate boundary. Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano.

As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor. This chain is called a Mid-ocean ridge. Mid Ocean Ridge

The new rock at the edge has no sediments like the sand or mud, since it is formed only recently. Farther away from the ridge, sand and mud gradually settle on it, in an ever-thickening blanket. The oldest rocks may have 14,000 feet of sand and other sediments resting on top of it. Black Smokers

Africa’s Rift Valley

Formation of Divergent Boundary 1.Continents spread 2.Plates sink 3.Volcanic Activity 4.New sea forms

Divergent Boundary Lithosphere: Crust & Moho (Stone-sphere) Asthenosphere: Upper Mantle with plasticity (weak sphere) Mid-Ocean Ridge: A mountain range underwater Magma Volcanoes Mid-ocean Ridge

Causes: 1. Heat Energy travels in Convection Currents 2. Less Dense Material goes up Convergent Boundary:Plates collide

Plates Crash Together Ocean Plate Continental Plate Trench Ocean Plate with Continental Plate

2. Ocean crust is denser 3. Ocean crust sinks under the continent.

Convergent: 2 Ocean Plates Island Chain Forms Convergent: 2 Ocean Plates Island Chain Forms

3. Volcanic Mountains form as magma melts its way up.

Subduction Zone  Ocean plate collides with continental plate to form a Subduction Zone. Signs of the zone are: 1.Deep Trenches in the Ocean 2.Mountain Range on the Continent 3. Volcanoes and Earthquakes

 Places where plates crash or crunch together are called convergent boundaries. Plates only move a few centimeters each year, so collisions are very slow and last millions of years. Even though plate collisions take a long time, lots of interesting things happen. Two continental plates collided to form the Himalaya Mountains.

 The edge of the continental plate can fold into a huge mountain range while the edge of the oceanic plate bends downward and digs deep into the Earth. A trench forms at the bend. All that folding and bending makes rock in both plates break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth's hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions when it finally reaches the surface. Mt. St. Helen Devil’s Punchbowl

Transform Fault Boundary  Plates slide past.  No spectacular features  Long valleys  Displacement of rivers & mountains  Plates slide past.  No spectacular features  Long valleys  Displacement of rivers & mountains

San Andreas Fault: Most studied

Displacement Pictures