Plate Tectonics What is it and what makes it work? Oceanography

The Earth’s interior 1000 C 4000 C 7000 C

Pea-soup analogy

mantle convection Heat transfer from the core to the mantle Produces slow convection currents of the mantle material (in the order of centimetres per year)

Mantle convection currents produce lateral forces at the boundary between the mantle and the crust

Lateral forces at the crust-mantle boundary push the crust around – in different directions leading to the motion of crustal plates.

The plates are moving at rates of a few centimeters per year San Diego is moving at 5 cm/year vs Imperial valley is moving 1 cm/year

Plate motions

Divergent boundary

Convergent boundary Subduction

Transform boundary

Plates of the Earth’s crust

Plates around North America

The Theory of Plate Tectonics Unifying principle of geology. Evolved in the late 1960s, early 1970s. Realization that certain parts of the Earth were more affected by earthquakes and volcanoes. Also noted that large mountainous regions occur on margins of continents. Changes in sea level were related to these events. They were not a random occurrences.

Distribution of major volcanoes and earthquakes “Ring of Fire”

The Theory of Plate Tectonics Many hypotheses were presented to answer these questions. None answered all of the questions at all localities. This led to the hypothesis of PLATE TECTONICS. It became widely accepted in a few short decades.

Steps in Development The Theory of Plate Tectonics 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 million years ago. 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.

Marsupial Mammals Marsupials are the group of mammals commonly thought of as pouched mammals. They give live birth, but they do not have long gestation times like placental mammals. Instead, they give birth very early and the young animal, essentially a helpless embryo, climbs from the mother's birth canal to the nipples. There it grabs on with its mouth and continues to develop, often for weeks or months depending on the species. Like other mammals, the marsupials are covered with hair. Mothers nurse their young — a young kangaroo may nurse even when it has grown almost to the mother's size. The only naturally occurring marsupial in the United States is the opossum, Didelphis marsupialis. In the past, however, marsupials were quite common. During the Mesozoic marsupials were very common in North America; more common, in fact, than placental mammals. They persisted here until the mid- to late-Tertiary. In South America and Australia, however, marsupials continued to be an important group of land mammals.

Many South American forms are similar to the North American opossum Many South American forms are similar to the North American opossum. The marsupials of South America began to go extinct in the late Miocene and Early Pliocene when a land connection with North America formed, allowing placental mammals to cross into South America. In Australia, though, marsupials continue to be very diverse, and are the dominant native mammals. They include kangaroos, koalas, tasmanian devils, wombats, and other typical Australian mammals. Until recently, they also included the marsupial wolf, Thylacinus. Like the quagga, the marsupial wolf is now extinct. The last individual was seen in Tasmania in the 1950s.

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

Captain Harry Hammond Hess The Theory of Plate Tectonics Captain Harry Hammond Hess (later Professor of Geology at Princeton) Believed in Wegener’s hypothesis Conducted echo-soundings of the oceans during WWII Mapped the mid-ocean ridges (3000 m high and 2000 m wide) 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