Continental Drift Theory and Plate Tectonic 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 oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually. Tectonic plates are able to move because the Earth’s lithosphere has a higher strength and lower density than the underlying asthenosphere. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge and drag, downward suction, at the subduction zones.
Another explanation lies in the different forces generated by the rotation of the globe and the tidal forces of the Sun and the Moon. The relative importance of each of these factors is unclear, and is still subject to debate. The geoscientific community accepted the theory after the concepts of seafloor were developed in the late 1950s and early 1960s. Fun Facts: * The highest mountain range above sea level, the Himalayas, was formed 55 million years ago when the Eurasian and Indo-Australian continental plates converged.
Due to ongoing convergence, the Himalayas, including Mount Everest, continue to rise by approximately 2 centimeters (?1 inch) each year. * The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean. Along its crest, the ridge has a deep rift valley that, on average, is similar to the depth and width of the Grand Canyon: 1 to 3 kilometers (0. 6 -1. 8 miles) deep and 6. 5 to 29 kilometers (4-18 miles) wide. * Scientists are able to calculate average rates of tectonic plate movement for a given time period. These rates of movement range widely.
For example, the rate of spreading at the Mid-Atlantic Ridge near Iceland is relatively slow, about 2. 5 centimeters (1 inch) per year. This is similar to the rate at which fingernails grow. The fastest known rate of plate movement, 15 centimeters (6 inches) per year, occurs on the East Pacific Rise in the South Pacific. SEAFLOOR SPREADING Seafloor spreading is a process of plate tectonics. New oceanic crust is created as large slabs of the Earth’s crust split apart from each other and magma wells up to fill the gap. The large slabs of rock that make up the Earth’s crust are called tectonic plates.
As they slowly move away from each other beneath the ocean floor, hot magma from the Earth’s mantle bubbles to the surface. This magma is then cooled by seawater. The new rock forms a new part of the Earth’s crust. Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor. New bodies of water and even continents can be created through seafloor spreading. The Red Sea, for example, was created through seafloor spreading, as the African plate and the Arabian plate tear away from each other.
Today, the northern Sinai Peninsula connects the Middle East (Asia) with North Africa. Eventually, geologists predict, seafloor spreading will expand the Red Sea so that it will completely separate the two continents. . Seafloor spreading disproves an early part of the theory of continental drift. Continental drift was one of the first theories that the Earth’s crust was dynamic and always in motion. Supporters of continental drift originally theorized that the continents moved (drifted) through unmoving oceans.
Seafloor spreading proves that the ocean floor itself is the site of tectonic activity. Subduction is the opposite of seafloor spreading. Subduction happens where tectonic plates crash into each other instead of spreading apart. In subduction zones, the edge of the heavier plate subducts, or slides, beneath the lighter one. It then melts back into the Earth’s mantle. Seafloor spreading creates new crust. Subduction destroys old crust. The two forces roughly balance each other, so the shape and diameter of the Earth remains constant.