Plate Tectonics

Living with a Dynamic Planet

Alfred Wegener:a man before his time

• Meteorologist

• Research on arctic climates – Greenland

• Evidence for past climates preserved in rocks

• A super-continent: Pangea

• Pangea broke apart 200 million years ago

• Continents drifted to their present locations

Wegener’s Continental Drift

Evidence for Pangea: glaciation

Evidence for Pangea: paleontology (fossils)

How Science Works:Continental Drift Disputed

• Lack of Mechanism – what makes continents move?

• Alternate explanations possible

• Anti-German sentiments in the U.S.

• What are the credentials of Wegener?

• Continental Drift largely dismissed until 1960’s

Oceanography: a new scienceand new evidence

• Following WW II – a convergence of questions, technology, manpower and money

• Defense needs – mapping the seafloor• Anti-submarine technology• GI Bill – a wave of scientists and engineers• Booming postwar economy and support for

science

A new view of the Seafloor

• Completely surprising!

• Mountains on the sea floor

• Trenches – near some continental edges, near islands

More puzzles: Magnetic stripes near the Mid-Ocean Ridges

A new hypothesis: Sea-Floor Spreading – Making New Oceanic Lithosphere

Continental Drift Revised:the New Global Tectonics

• Earth’s lithosphere is made of crust and uppermost mantle

• Lithosphere is broken into plates

• Plates float on moving asthenosphere (upper mantle)

• Plate motion driven by heat in mantle

• Plates interact – especially at edges

Lithosphere and Asthenosphere

Plates

• Made of Continental and Oceanic Crust – attached to uppermost part of mantle

• Plates move

• Plate interactions produce specific features (e.g., mountains, etc.) and processes (e.g., earthquakes)

• Motion is also related to heat dissipation and density differentiation

Balancing the System

• Sea Floor (oceanic lithosphere) is created at MOR (divergence)

• Either Earth is expanding (no evidence!) or

• Sea Floor must be destroyed somewhere – where plates converge

Subduction – destroying sea floor

The sinking of cold, dense, oceanic lithosphere into the asthenoshere, beneath an over-riding plate.

• Associated: Trenches, Volcanoes, Mountains, Earthquakes

Convergence Brings Continents into Collision – Forming Mountains

• Subduction of oceanic lithosphere brings landmasses together

• Collision of continents (or continent/island arc)

• Due to low densities, neither can subduct

• Crash!!

TRANSFORM BOUNDARIES – Two Plates Passing

• Transform boundaries link other types of plate boundaries:

• Transform boundaries allow a single plate edge to have multiple types of interactions w/other plates

• Transforms are major Faults – Earthquakes!

The Big Picture of Plate Interactions

Plate Tectonics Explains:• Formation of Oceans• Formation of Mid-ocean Ridges• Formation of Trenches• Formation of Mountains

• Formation of Metamorphic Rocks• Formation of Sedimentary Rocks as mountains

erode

• Earthquakes: cause and distribution• Volcanos: cause, types and distribution

• Formation of Igneous Rocks