2 of 7

Continental Drift

Who: Alfred Wegener, German Scientist�What: Proposed the Theory of Continental Drift�When: January 6, 1912�Why: Believed to have evidence that the continents were once connected in one super-continent, called PANGAEA, but have since DRIFTED apart.� -His evidence included: Fossil Patterns & Coinciding Shapes of Continents ��However, his theory was eventually rejected due to the discovery of sea-floor spreading which showed that the PLATES are moving, and not the continents.

Watch This Video (2.5 minutes)

3 of 7

Evidence of Continental Drift�

1) The Continents Fit Together Like a Puzzle�Wegener looked for other connections between the matching coastlines. Where coastlines appeared to ﬁt together, Wegener noticed that certain geologic formations also appeared to ﬁt together. For example, a mountain range on one coastline appears to connect with a mountain chain on the opposite coastline. Several deposits of sediments left behind by glaciers also line up along the matching coastlines of continents.

2) Fossil Evidence�In addition to geologic evidence, Wegener found fossil evidence that the continents may have once been connected. Fossils of a freshwater reptile called mesosaurus were found on both the eastern coast of South America and the western coast of South Africa. It is highly unlikely these freshwater animals swam across the entire Atlantic Ocean while they were alive. Fossils of a land reptile called lystrosaurus were also found along matching coasts of eastern Africa, central India, and Antarctica. These land animals also could not have crossed the oceans separating these continents. Equally unlikely is the possibility that the same species evolved independently on multiple continents. Instead, Wegener �reasoned the continents must have once been joined. As the continents moved apart, they took �mesosaurs and lystrosaurs with them.

4 of 7

Sea-Floor Spreading�Who: Harry Hess, Naval Scientist�What: Discovery of Sea-Floor Spreading�-Sea-Floor spreading is caused by convection currents beneath the Earth’s crust, which causes large slabs of Earth's lithosphere—split apart from each other causing cracks in the Earth’s surface. As this occurs, magma from beneath the lithosphere seeps into the cracks, cool, harden and create new crust. The Mid-Ocean Ridge is the �largest example of Sea-Floor Spreading on Earth. Sea floor spreading �creates new crust, which can be found at the center of the ridge. → → �When: 1946�Why: His discovery eventually disproved the Theory of Continental Drift, and caused a revision in the way “Science” thought about the changing Earth.

Watch this Video (4 minutes)��

5 of 7

Theory of Plate Tectonics �Who: Dan McKenzie

What: Theory or Plate Tectonics�When: 1968�Why: After Harry Hess’ discovery of Sea-Floor Spreading, Dan McKenzie used this discovery, along with Wegener’s original theory of Continental Drift to write the current “Theory of Plate Tectonics.” This theory states that the Earth’s lithosphere (crust) is broken into PLATES, which are constantly in motion due to convection currents in the asthenosphere. Convection occurs when hot, molten magma is churning beneath the crust, and pushes up on the lithosphere, causing the plates to move.

This motion can be seen at plate boundaries (Convergent, Divergent and Transform).

6 of 7

Plate Boundaries

Watch this Video (1.5 minutes)

7 of 7