Mt. Fuji, Japan

The Theory of Plate Tectonics Steps in Development 1. Benjamin Franklin (late 1700s) Recognized that crust of Earth was a shell. Surface could be broken and parts moved about. 2. Alfred Wegener (1912) German meteorologist-geophysicist Proposed theory of Continental Drift. Proposed continents float on a denser underlying interior of the Earth. CONTINENTS periodically break up and DRIFT apart.

The Theory of Plate Tectonics Wegener believed all continents were joined together. Supercontinent of Pangaea existed about 200 myBP. Pangaea covered 40% of the Earth’s surface.

The Theory of Plate Tectonics Majority of Pangaea was in Southern Hemisphere. Pangaea was surrounded by a single ocean, the Panthalassic Sea. Pangaea broke up ~ 180 million years ago.

The Theory of Plate Tectonics Wegener first published his theory 1912.

The Theory of Plate Tectonics Evidence in support of Continental Drift 1. Continental Fit Sir Francis Bacon (1620) noted that the continents might fit together. Made observation after seeing some newly made maps.

The Theory of Plate Tectonics 2. Habitats of Modern Organisms Hippopotamus found in Africa and Madagascar. Marsupials in Australia. Indicate some migration and evolution took place before and after drift began.

The Theory of Plate Tectonics 3. Fossil Record Wegener used the fossil record. Found fossils of plants and animals that were found on several continents. Included animals, Cynognathus, Lystrosaurus, Mesosaurus, and plants Glossopteris.

The Theory of Plate Tectonics 4. Similar Rock Types Across Ocean Basins Mountains of Northern Hemisphere similar in Greenland, NA, and Europe. Also similar rocks between South America and Africa.

The Theory of Plate Tectonics 5. Ancient Climates Glacial striations found in India, Australia, South America and Africa. Radiate from a point in southern Africa. Also coal deposits found in presently cold regions, such as Norway.

The Theory of Plate Tectonics Wegener could not explain how the continents drift. He could provide no mechanism for the theory. Wegener seemed to have a great idea, and lots of supporing evidence. However, Plate Tectonics was not widely accepted until the early 1970’s. Why?

Captain Harry Hammond Hess Mapped the mid-ocean ridges (3000 m high and 2000 m wide) Conducted echo-soundings of the oceans during WWII (later Professor of Geology at Princeton) Located deep-sea trenches (10,000 m deep) associated with large continental mountain belts (the Andes) and island arcs (Aleutians, Japan) Hess presented sea-floor spreading as a mechanism in 1960’s Believed in Wegener’s hypothesis The Theory of Plate Tectonics

Led to the discovery of Oceanic Ridges and Trenches. Still no explanation.

Research vessels found alternating bands of rock on the sea floor on either side of the ridges. Rocks possessed iron that indicated magnetic field reversals.

The Theory of Plate Tectonics Basic Plate Tectonics Concepts 1. The outer portion of the Earth, the lithosphere (the upper mantle and crust), is composed of rigid units called plates. 2. Plates move slowly. 3. Most of the Earth’s large-scale geologic activity, such as earthquakes and volcanic eruptions, occur at or near plate boundaries. 4. Interiors of plates are geologically quiet, with fewer and usually milder earthquakes and fewer volcanoes than at plate margins.

The Theory of Plate Tectonics Earth’s Lithospheric Plates The Earth's surface is broken into ~15 crustal plates

The Theory of Plate Tectonics Earth’s Lithospheric Plates Continental plates are composite. Include both continental and oceanic crust. North American Plate - continental portions are thicker, lower density lithosphere. - ~ 120 km thick in continental regions. - oceanic portions are thinner, higher density lithosphere.

The Theory of Plate Tectonics When plates move, everything on them moves with them. North American Plate moving west, Eurasian Plate moving east. Rate is about 5-10 cm/yr. Over long periods of time, movement is large.

The Theory of Plate Tectonics So…. Has the size of the Earth increased? Why? Where plates move apart or diverge, at some other place on the Earth, they must come together or converge. This is where the Earth gets exciting.

The tectonic plates are continually moving Convection currents in the mantle drive plate movements The source of heat is radioactivity deep in the Earth Divergent (plates move away from each other) Convergent (plates collide with each other) Transform (plates move past each other) The Theory of Plate Tectonics

Types of Plate Boundaries convergent divergent transform convergentdivergent hot spot The Theory of Plate Tectonics

Divergent Plate Boundaries Occur along spreading centers where plates are moving apart The Mid-Atlantic Ridge: Spreading at ~2.5 cm/yr You are here

BRUCE visits the Mid-Atlantic Ridge

The Theory of Plate Tectonics Divergent Plate Boundaries East Pacific Rise

“Alvin!”

ALVIN visits the East Pacific Rise in January, 2002

East Pacific Rise Sea Floor at 3000 m

East Pacific Rise Black Smokers

Divergent Boundaries The East-African Rift Zone Oldoinyo Lengai erupting