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Dinosaur and Paleontology Dictionary
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CONTINENTAL DRIFT Go to an interactive plate tectonics puzzle

Go to a printable quiz on continental drift

In 1915, the German geologist and meteorologist Alfred Wegener first proposed the theory of continental drift, which states that parts of the Earth's crust slowly drift atop a liquid core. The fossil record supports and gives credence to the theories of continental drift and plate tectonics.

Wegener hypothesized that there was a gigantic supercontinent 200 million years ago, which he named Pangaea, meaning "All-earth".

LaurasiaPangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents.

Wegener published this theory in his 1915 book, On the Origin of Continents and Oceans. In it he also proposed the existence of the supercontinent , and named it (Pangaea means "all the land" in Greek).



Fossil Evidence in Support of the Theory


Glossopteris, a tree-like plant from the Permian Period through the Triassic Period. It had tongue-shaped leaves and was about 12 ft (3.7 m) tall. It was the dominant plant of Gondwana.
Eduard Suess was an Austrian geologist who first realized that there had once been a land bridge connecting South America, Africa, India, Australia, and Antarctica. He named this large land mass Gondwanaland (named after a district in India where the fossil plant Glossopteris was found). This was the southern supercontinent formed after Pangaea broke up during the Jurassic period. Suess based his deductions on the fossil plant Glossopteris, which is found throughout India, South America, southern Africa, Australia, and Antarctica.

Fossils of Mesosaurus (one of the first marine reptiles, even older than the dinosaurs) were found in both South America and South Africa. These finds, plus the study of sedimentation and the fossil plant Glossopteris in these southern continents led Alexander duToit, a South African scientist, to bolster the idea of the past existence of a supercontinent in the southern hemisphere, Eduard Suess's Gondwanaland. This lent further support to A. Wegener's Continental Drift Theory



Earth's Plates:
Earth's Crust

The Earth's crust is divided into huge, thick plates that drift atop the soft mantle. The plates are made of rock and are from 50 to 250 miles (80 to 400 km) thick. They move both horizontally and vertically. Over long periods of time, the plates also change in size as their margins are added to, crushed together, or pushed back into the Earth's mantle.

PLATE TECTONICS
The theory of plate tectonics (meaning "plate structure") was developed in the 1960's. This theory explains the movement of the Earth's plates (which has since been documented scientifically) and also explains the cause of earthquakes, volcanoes, oceanic trenches, mountain range formation, and other geologic phenomenon.

Type of Crust Average Thickness Average Age Major Component
Continental Crust 20-80 kilometers 3 billion years Granite
Oceanic Crust 10 kilometers Hundreds of millions of years Basalt
The plates are moving at a speed that has been estimated at 1 to 10 cm per year. Most of the Earth's seismic activity (volcanos and earthquakes) occurs at the plate boundaries as they interact.

The top layers of the plates are called the crust. Oceanic crust (the crust under the oceans) is thinner and denser than continental crust. Crust is constantly being created and destroyed; oceanic crust is more active than continental crust.

TYPES OF PLATE MOVEMENT: Divergence, Convergence, and Lateral Slipping
At the boundaries of the plates, various deformations occur as the plates interact; they separate from one another (seafloor spreading), collide (forming mountain ranges), slip past one another (subduction zones, in which plates undergo destruction and remelting), and slip laterally.

Divergent Plate Movement: Seafloor Spreading
Seafloor spreading is the movement of two oceanic plates away from each other, which results in the formation of new oceanic crust (from magma that comes from within the Earth's mantle) along a a mid-ocean ridge. Where the oceanic plates are moving away from each other is called a zone of divergence. Ocean floor spreading was first suggested by Harry Hess and Robert Dietz in the 1960's.
Convergent Plate Movement:
When two plates collide, some crust is destroyed in the impact and the plates become smaller. The results differ, depending upon what types of plates are involved.
   Oceanic Plate and Continental Plate - When a thin, dense oceanic plate collides with a relatively light, thick continental plate, the oceanic plate is forced under the continental plate; this phenomenon is called subduction.
   Two Oceanic Plates - When two oceanic plates collide, one may be pushed under the other and magma from the mantle rises, forming volcanoes in the vicinity.
   Two Continental Plates - When two continental plates collide, mountain ranges are created as the colliding crust is compressed and pushed upwards.
Lateral Slipping Plate Movement:
When two plates move sideways against each other, there is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure build up to incredible levels. When the pressure is released suddenly, and the plates suddenly jerk apart, this is an earthquake.


Earth's Major Plates:


The current continental and oceanic plates include: the Eurasian plate, Australian-Indian plate, Philippine plate, Pacific plate, Juan de Fuca plate, Nazca plate, Cocos plate, North American plates, Caribbean plate, South American plate, African plate, Arabian plate, and the Antarctic plate. These plates consist of smaller sub-plates.

Since the Earth's crust solidified billions of years ago, plates of its crust have been drifting all over the globe . The map of the Earth is always changing; not only are the underlying plates moving, but the plates change in size. Also, the sea level changes over time (as the temperature on Earth varies and the poles melt or freeze to varied extents), covering or exposing different amounts of crust.

Continental Drift
Forward Backward
Key

LINKS: The Great Continental Drift Mystery from the Yale-New Haven Teachers Institute, by Lois Van Wagner.

The Mesozoic Era
248 - 65 million years ago - The Age of Reptiles
The Triassic Period
248 - 206 million years ago
The Jurassic Period
206-144 million years ago
The Cretaceous Period
144-65 million years ago


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