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The plates are made of rock and drift all over the globe; they move both horizontally (sideways) and vertically (up and down). 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. These plates are from 50 to 250 miles (80 to 400 km) thick.
|Type of Crust||Average Thickness||Average Age||Major Component|
|Continental Crust||20-80 kilometers||3 billion years||Granite|
|Oceanic Crust||10 kilometers||Generally 70 to 100 million years old||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 (volcanoes and earthquakes) occurs at the plate boundaries as they interact.
The top layer of the Earth's surface is called the crust (it lies on top of the plates). Oceanic crust (the thin 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.
Under the crust is the rocky mantle, which is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. The upper mantle is rigid and is part of the lithosphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The asthenosphere is a part of the upper mantle that exhibits plastic properties. It is located below the lithosphere (the crust and upper mantle), between about 100 and 250 kilometers deep.
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 (at a divergent plate boundary), 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 (at a convergent plate boundary), 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 (at a transform plate boundary), 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.
Wegener hypothesized that there was an original, gigantic supercontinent 200 million years ago, which he named Pangaea, meaning "All-earth". Pangaea was a supercontinent consisting of all of Earth's land masses. It existed from the Permian through Jurassic periods. It began breaking up during the Jurassic period, forming continents Gondwanaland and Laurasia, separated by the Tethys Sea.
Pangaea 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 Pangaea, and named it (Pangaea means "all the land" in Greek).
Eduard Suess was an Austrian geologist who first realized that there had once been a land bridge between 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. He based his deductions on the 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
Glossopteris, a tree-like plant from the Permian 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.
Label Subduction (Plate Convergence) Label the destruction of crust as two plates converge.
The Ring of Fire
WEB LINKS ON THE EARTH'S CONTINENTAL PLATES
The Great Continental Drift Mystery from the Yale-New Haven Teachers Institute, by Lois Van Wagner.
Questions and answers about continental drift from Monash University Earth Sciences.
Plate tectonics from the University of Tennessee (Knoxville).
Speed of the continental plates from Zhen Shao Huang.
Plate tectonics from the US Geological Service
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