Plate Tectonics

Tectonics refer to the deformation of Earth’s crust and results in the formation of structural features such as mountains

Alfred Wegener• Proposed hypothesis in 1915 • Published The Origin of

Continents and Oceans

Continental drift hypothesis • Supercontinent Pangaea (meaning “all

lands”) began breaking apart into smaller continents about 200 million years ago

Continental drift: An idea before its time

Pangaea approximately 200 million years ago

Ocean opens between rigid plates

As the plates moves, the distance between two cities on the same Plate, New York and Denver, for example, remains relatively constant while the distance between New York and London iscontinually changing

Continental Drift EvidenceGeometry of continental margins

Similarity of rocks and structures

Correlation of glacial deposits

Distribution of fossils

Paleomagnetism and “polar wander”

Topography and other features of the seafloor

South America and Africa fit almost perfectly.

Not at the shoreline, but at the true edge of the continental margin (at the bottom of the continental slope).

South America and Africa were joined in the past and split apart when the Atlantic Ocean opened

Paleozoic glacial deposits and features are common among the now separate southern continents…

But they are now near the equator! Huh?

Also, the direction of ice movement is from what is now the sea towards the interiors! Huh?

These observations only make sense if the southern continents were once joined and were at the south pole!

The great debate

Objections to drift hypothesis • Inability to provide a mechanism capable

of moving continents across globe • Wegner suggested that continents broke

through the ocean crust, much like ice breakers cut through ice

The great debate

Continental drift and the scientific method

• Wegner’s hypothesis was correct in principle, but contained incorrect details

• For any scientific viewpoint to gain wide acceptance, supporting evidence required

Plate tectonics: The new paradigm

More encompassing theory than continental drift

Mix of ideas that explained motion of Earth’s lithosphere by subduction and seafloor spreading

Plate tectonics: The new paradigm

Earth’s major plates • Associated with Earth's strong, rigid

outer layer– Known as the lithosphere

– Consists of uppermost mantle and overlying crust

– Overlies a weaker region in the mantle called the asthenosphere

Basal tractions drive plate motions