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The Continental Drift Theory
Transcript of The Continental Drift Theory
In 1912, German scientist Alfred Wegener hypothesized that the continents once formed part of a single large landmass that broke apart about 200 mya during the Mesozoic Era.
He called this supercontinent Pangaea. Despite the evidence that supported Wegener's hypothesis, his ideas were strongly opposed.
Wegener was unable to explain exactly how the continents drifted apart.
He suggested that the continents plowed through the rock of the ocean floor, but this idea was shown to be physically impossible. Missing Mechanisms MID-OCEAN RIDGES are underwater mountain ranges that run through the center of steep, narrow valleys.
They form when magma rises from the asthenosphere, creating new oceanic lithosphere as tectonic plate move apart.
Scientists noticed that thinner sediment was found closer to the ridge; they also determined that oceanic rocks are younger than rocks found on land.
Rocks closer to a mid-ocean ridge are younger than rocks farther from the ridge. Discovery of Mid-ocean Ridges SEA-FLOOR SPREADING is a process by which new oceanic lithosphere (sea floor) forms when magma rises to Earth's surface thorough a rift formed at mid-ocean ridges and solidifies, as older, existing sea floor moves away from the ridge.
Proposed in the late 1950s by geologist Harry Hess; later named by geologist Robert Dietz.
Hess suggested that if the ocean floor is moving, the continents may be moving too. He thought this process was the missing mechanism that Wegener had failed to find. A New Hypothesis: Sea-floor Spreading Scientists later discovered that the reversal patterns seen in rocks on the sea floor also appeared in rocks on land.
They reasoned that sea-floor spreading explains how the continents move over Earth's surface; magma from a rift cools, hardens and then moves away in opposite directions on both sides of the mid-ocean ridge.
Sea-floor spreading was the mechanism that proved Wegener's hypothesis of continental drift. Fossil Evidence Fossils of Mesosaurus, a freshwater reptile, were found in South America and Africa.
Fossils of Lystrosaurus, a land reptile, were found in Antarctica, India and Africa.
Fossils of Glossopteris, a tree with tongue-shaped leaves, were found in Australia, Antarctica, South America, Africa and Madagascar Wegener's Evidence Evidence from Rock Formations Similar types of rocks found in Africa and South America were identical.
Mountain chains that ended at the coastline of one continent seemed to continue on other continent across the ocean. Evidence from Glaciation Geologists discovered layers of debris deposited by ancient glaciers in Africa and South America.
This suggests that those continents were once joined together and positioned differently, resulting in changes in climatic patterns. Video: Seafloor Spreading with Bill Nye Evidence for Sea-floor Spreading: Magnetic Reversals
In the 1960s, scientists studying the ocean floor discovered magnetic patterns on the sea floor.
As magma solidifies to form rock at a rift in a mid-ocean ridge, iron-rich minerals in the magma align with Earth's magnetic field.
They concluded that throughout Earth's history, the north and south magnetic poles have switched places about 300 times. This process is called MAGNETIC REVERSAL.
Rocks with magnetic fields that point north are said to have a normal polarity; those whose magnetic fields point south have a reversed polarity.
The study of the alignment of magnetic minerals in rock as it relates to the reversals of Earth's magnetic poles is called PALEOMAGNETISM. Mapping the Ocean Floor During World War II, German scientists used sonar (sound waves) to locate submarines.
Sonar was later used to map the ocean floor; scientists found that the seafloor had an underwater system of ridges and trenches. The Earth's Magnetic Field Earth's magnetic field has a north pole and a south pole. Invisible lines of magnetic force leave Earth near the south pole and enters Earth near the north pole.
Currently, the lines of magnetic force travel south to north; this has not always been true.
At times during Earth's history, the lines of magnetic force have traveled in the opposite direction.