PLATE TECTONICS A DRIFTING CONTINENTS 1 Alfred Wegner

PLATE TECTONICS

A. DRIFTING CONTINENTS 1. Alfred Wegner proposed the Theory of Continental Drift in early 1900’s. • Wegner’s theorized that all the continents were once a single landmass about 245 million years ago that gradually drifted apart (are still drifting). þCalled this supercontinent Pangea which is Greek for “all Earth” þThe land masses Split again – Laurasia & Gondwana 180 million years ago

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2. Evidence for Pangaea a. Continents fit together like puzzle pieces b. Fossils of the same plants and animals are found on different continents **climate evidence supports this c. Mountains that appear on one coastline had similar formations and rocks on opposite coastlines BUT>>>>>Wegner couldn’t explain why the plates moved

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Matching Mountain Ranges

Glacier Evidence

3. How did plates move then? 1950’s-mapping of the seafloor found Midocean Ridges-huge underwater mountain chains found in every ocean around the Earth a. They formed along cracks in the crust where molten rock pushed up and out in a process called Sea Floor Spreading ü younger rock found closer to ridge than on continents. ü Earth DOESN’T get larger because oceanic crust is destroyed along deep-ocean where why the oceanic plates sink into If trenches, this is true, isn’t the Earth the asthenosphere getting bigger?

Sea Floor Spreading

4. How do plates move then? • Due to tremendous heat, rock in the asthenosphere is like hot taffy • This allows plates to ride on top of hot, flowing rock. • Plates move because heat is being released from deep inside the earth. • Convection currents causes hot material to rise and expand (plates diverge) and cooler material to sink and contract (plates converge). https: //www. youtube. com/watch? feature=player_embedded&v=ryr. XAGY 1 dm. E

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See Page 19 All of this new information helped to create the Theory of Plate Tectonics

Section 1. 3 C. Plates Move Apart

See Page 35 Boundaries are formed when tectonic plates move. The direction of the movement determines the type of boundary.

1. Divergent Boundary: plates move apart • See Page 23 a. Usually found in the ocean b. Mid-ocean ridges and rift valleys occur • Mid-ocean ridges form the longest mountain ranges on earth. • Most contain a rift valley along their center.

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Continued c. Earth’s magnetic poles have switched places several times at divergent boundaries. See Page 24

See Page 24 These magnetic reversals are caused by changes in Earth’s magnetic fields. Bands of rock record periods of magnetic reversals. As molten material cools, magnetic minerals line up with the magnetic field. When it hardens, the minerals act like tiny compass needles.

d. Hot spots can be used to track plate movements. • Hot Spot: an area of volcanic activity that develops above where magma rises in a plume from the mantle. – They don’t move but plates do – A fixed point to measure speed and direction of plates

The Hawaiian islands are located in the middle of the Pacific Plate. The largest island, Hawaii, is still over the hot spot.

See Page 28 When the plate moves on, it carries the first volcano away from the hot spot. Heat from the mantle plume will then melt the rock at a new site, forming a new volcano.

2. Convergent Boundary: plates push together. • They form three types of convergent boundaries. • One plate is subducted or pushed below the lighter plate

a. Continental-Continental 2 continental plates collide crumpling and folding the rock between them. Mountains could form.

Continued Oceanic-oceanic subductions: two oceanic plates collide and the older, denser plate sinks beneath the top plate, forming deep-ocean trenches and island arcs.

Continued Oceanic-continental subductions: an oceanic plate sinks beneath a continental plate, forming a deep-ocean trench and volcanic coastal mountains.

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3. Tectonic plates scrape past each other at transform boundaries. • Two plates move past each other in opposite directions. • No crust is formed or destroyed. • Occurs on the sea floor and on land.

The San Andreas Fault is a transform boundary and moves about 1 inch per year.

Comparing Boundaries Divergent *plates move apart *in ocean and on land *produce mid-ocean ridges, rift valleys, volcanoes, earthquakes Transform *plates move past each other in opposite directions *in ocean and on land Convergent *crumples and folds crust * produces mountains, earthquakes C-C Collision Convergent O-O Subduction Convergent O-C Subduction *older, denser plate sinks *produce deep-ocean trenches, island arcs *oceanic plate sinks under continental plate * forms deep-ocean trench, volcanic coastal mountains

The theory of plate tectonics helps geologists today. • The plate tectonics theory enables geologists to understand how Earth’s continents and ocean basins formed. • Helps scientists predict earthquakes and volcanic activity.