Continental Drift Theory and Plate Tectonic Theory
CONTINENTAL DRIFT THEORY
In the early 20th century, German scientist Alfred Wegener published a book explaining his theory that the continental landmasses, far from being immovable, were drifting across the Earth. He called this movement continental drift.
Wegener noticed that the coasts of western Africa and eastern South America looked like the edges of interlocking pieces of a jigsaw puzzle. He was not the first to notice this, but he was the first to formally present evidence suggesting that the two continents had once been connected.
Wegener was convinced that the two continents were once part of an enormous, single landmass that had split apart. He knew that the two areas had many geological and biological similarities. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of mesosaurus suggests a single habitat with many lakes and rivers.
Wegener believed that all the continents—not just Africa and South America—had once been joined in a single supercontinent. This huge ancient landmass is known as Pangaea, which means “all lands” in Greek. Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their present positions as the continents.
PLATE TECTONIC THEORY
Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen "plates" that move around across the earth's surface relative to each other, like slabs of ice on a lake. There are seven or eight major plates and many minor plates.
Where plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, activity, mountain-building, and
In the early 20th century, German scientist Alfred Wegener published a book explaining his theory that the continental landmasses, far from being immovable, were drifting across the Earth. He called this movement continental drift.
Wegener noticed that the coasts of western Africa and eastern South America looked like the edges of interlocking pieces of a jigsaw puzzle. He was not the first to notice this, but he was the first to formally present evidence suggesting that the two continents had once been connected.
Wegener was convinced that the two continents were once part of an enormous, single landmass that had split apart. He knew that the two areas had many geological and biological similarities. For example, fossils of the ancient reptile mesosaurus are only found in southern Africa and South America. Mesosaurus, a freshwater reptile only one meter (3.3 feet) long, could not have swum the Atlantic Ocean. The presence of mesosaurus suggests a single habitat with many lakes and rivers.
Wegener believed that all the continents—not just Africa and South America—had once been joined in a single supercontinent. This huge ancient landmass is known as Pangaea, which means “all lands” in Greek. Pangaea existed about 240 million years ago. By about 200 million years ago, this supercontinent began breaking up. Over millions of years, Pangaea separated into pieces that moved away from one another. These pieces slowly assumed their present positions as the continents.
PLATE TECTONIC THEORY
Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into a couple of dozen "plates" that move around across the earth's surface relative to each other, like slabs of ice on a lake. There are seven or eight major plates and many minor plates.
Where plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, activity, mountain-building, and