9.1

Continental Drift

*Plate Tectonics

I. Evidence for Continental Drift

A. Pangaeai. Alfred Wegener – proposed continental drift as a theory

ii. Continental Drift – continents have moved slowly to their current locations

iii.Pangaea – one large landmass that broke apart 200 million years ago.

iv.Pangaea means “all land”

v. Continental drift was accepted long after Wegener’s death in 1930.

B. Fossil Cluesi. Mesosaurus found on both Africa and South America.

ii. Glossopteris found on Africa, Australia, India, South America, and Antarctica.

C. Climate Cluesi. Warm weather plants were found on Spitzbergen in the

Artic Ocean.

ii. Wegner hypothesized that Spitzbegen drifted from the tropic regions.

iii.Glacial cluesa. South America, Africa, India, and Australia were onece covered

in glaciers

D. Rock Cluesi. Rock make up of the continents should be the same.

ii. Similar rock sturctures are found on different continents.

iii.Appalachian Moutnains in US are similar to Greenland and western Europe

II. How could continents drift?A. Although there was evidence, Wegener couldn’t

explain how or why continental drift happened.

B. Continental drift was rejected.

C. After Wegner’s death, more clues were found.

D. Seafloor spreading helped explain how continental drift could occur.

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9.2

Seafloor Spreading

*Plate Tectonics

I. Clues on the Ocean FloorA. Little was known before 1950s

B. Has mountains and valleys

C. mid-ocean ridges form an underwater mountain range that stretches along the center

D. The Seafloor Movesi. Seafloor spreading

a. Hot les-dense material below Earth’s curst is forced upward toward the surface

b. Turns and flows sideways carrying the seafloor away

c. As seafloor spreads apart, magma moves upward and flows from the cracks

d. Becomes solid as it cools and forms a new seafloor

e. The seafloor cools, contracts, and becomes more dense.

f. Colder seafloor sinks down

E. Age Evidencei. Glomar Challenger – drilling rig that allowed scientists

to drill into the seafloor to obtain rock samples

ii. Seafloor rocks are no older than 180 million years

iii.Youngest rocks are at the mid-ocean ridges

F. Magnetic Cluesi. Earth’s magnetic field has a north and a south pole

ii. Magnetic lines leave Earth near the north pole and enter near the south pole

iii.Magnetic reversal – magnetic forces run the opposite way

iv.Earth’s magnetic field has reversed many times

v. Iron-bearing minerals show the reversal of magnetic field

vi.Magnetic reversals form parallel strips to the mid-ocean ridges

9.3

Plate Tectonics

*Plate Tectonics

I. Plate TectonicsA. Plate Movements

i. 1969s – combined continental drift and seafloor spreading.

ii. Plate tectonics – Earth’s crust and upper mantle are in sections that move.

iii.Plates – the sections of the Earth’s crust and upper mantle

B. Composition of Earth’s Platesi. Lithosphere – crust and part of the upper mantle

ii. Asthenosphere – plastic like layer below the lithosphere

iii.Plates of the lithosphere “float” on the asthenosphere

II. Plate BoundariesA. Plates Moving Apart

i. Divergent boundarya. Seafloor spreading

ii. North American pate is moving away from the Eurasian and African plates – Mid-Atlantic Ridge

iii.Great Rift Valley in eastern Africa

B. Plates Moving Togetheri. Convergent boundary

ii. 3 typesa. Subduction zone – oceanic plate forced under a

continental plates and volcanoes form

b. Two seafloor plates collide, one sinks, new magma forms island arc of volcanoes

c. Two continental plates collide, forming mountain ranges. Earthquakes are common

C. Plates Sliding Past Each Otheri. Transform boundaries

a. Two plates slide past one another, either in the same directions at different rates or opposite directions

b. Pacific plate and North American plate form San Andreas Fault