There are seven continents on Earth now, but this was not always the case. The continents are floating around the globe (at an incredibly slow rate) on top of 10 plates.

Plate tectonics is a theory that explains how the Earth's continents float slowly on the surface, changing both their position and size over time. The theory was proposed by Alfred Wegener in 1912, but wasn't widely accepted until the 1960's.

The theory argues that the Earth's crust (the lithosphere) is broken up into about 10 plates. These plates float on the Earth's upper mantle, which is a partially-molten layer. Major geologic activity occurs at the edges of these plates where they push into one another, pull farther apart, or move sideways with tremendous power. At these rims there are earthquakes, mountain building, and/or volcanic activity.

Under the sea (where the Earth's crust is relatively thin but dense) and the plates are moving away from one another, new crust is formed at spreading ridges as lava flows from the upper mantle. This process is called sea-floor spreading, and increases the size of plates. When the rims of the thin, undersea tectonic plates come toward one another, one edge goes under the other (is subducted). This is a subduction zone, where plates get smaller.

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 an original, gigantic supercontinent 200 million years ago, which he named Pangaea, meaning "All-earth".

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).

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Fossils Evidence in Support of the Theory
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 this species 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, Edouard Suess's Gondawanland. This lent further support to A. Wegener's Continental Drift Theory

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

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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