Plate Tectonics

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PLATE TECTONICS

Alfred Wegener

- Wrote a book in 1912: The Origin of Continents and Oceans

- First proposed hypothesis in 1915

* Continental drift hypothesis

- Supercontinent called Pangaea began breaking apart about 200 million years ago (He gave the name)

- Continents “drifted” to present positions

- Continents “broke” through the ocean crust

Continental Drift: An idea before it’s time

- Wegner Continental drift hypothesis

- Evidence used by Wegener

* Fit of South America and Africa

* Fossils match across the seas

* Rock types and structure match

* Ancient climates

The Great Debate

*Rejection of the drift hypothesis

- Main objection to Wegener’s proposal was its inability to provide a mechanism

Theory of Plate Tectonics

* More encompassing than Continental drift

*Associated with Earth’s rigid outer shell

- Called Lithosphere

- Consists of several plates

* Plates are moving slowly

* Largest Plate is the Pacific plate

* Plates are mostly beneath the ocean

(Guatemala is affected by three plates: North American plate, Caribbean plate and Cocos plate)

Harry Hess

Harry Hess proposes sea-floor spreading

* Earth is divided in 12 plates.

Harry Hess was a geologist and navy submarine commander during World War II.

Part of his mission had been to study the deepest parts of the ocean floor.

He proposed in 1960 that the movement of the continent was a result of sea-floor spreading.

It was possible, he said, that molten magma from beneath the earth’s crust could ooze up between the plates in the Great Global Rift. As this hot magma cooled in the ocean water, it would expand and push the plates.

Hess proved Wegener’s basic idea right and clarified the mechanism that broke the once-joined continents into the seven with which we are familiar.

The plates themselves shift and change shape, carrying the continents along.

Evidence:

1. There are active fractures in the lithosphere along the ocean floor, in a pattern that mimics the shapes of the continental coastlines.

2. The age of the seafloor rock increases as you get further away from the mid-ocean ridge.

3. The thickness of the layer of sediments deposited on the ocean floor increases as you get further away from the mid-ocean ridge.

4. Patterns of seafloor magnetism on either side of mid-ocean ridges match up with one another.

*Hot spots are the emergence of magma and the formation of volcanoes at areas away from plate boundaries. Example: Hawaii

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