Published on March 18, 2014
The Continental Drift Hypothesis Chapter 14 Lesson 1 p494-500
New Vocabulary • Pangaea (495) – The supercontinent that all continents today were once a part of • Continental Drift (495) – A hypothesis that suggests that continents are in constant motion on the surface of Earth
Pangaea • Each year, North America moves a few centimeters farther away from Europe and closer to Asia. • Nearly 100 years ago Alfred Wegener began an important investigation to know whether Earth’s continents were fixed in their positions. – He proposed that all continents were once part of a supercontinent called Pangaea. – Over time Pangaea began breaking apart, and the continents slowly moved to their present positions. • Wegener proposed the hypothesis of continental drift, which suggested that continents are in constant motion on the surface of Earth.
Evidence That Continents Move • The most obvious evidence for continental drift is that the continents appear to fit together like pieces of a puzzle. • However, scientists were skeptical, and Wegener needed additional evidence – 1. Apparent fit of the continents – 2. Fossil Correlation – 3. Rock and Mountain Correlation – 4. Past Climate Data
Climate Clues • When Wegener pieced Pangaea together, he proposed that South America, Africa, India, and Australia were located closer to Antarctica 280 million years ago. • He suggested the climate of the Southern Hemisphere was much cooler at the time. – Glaciers covered most of these continents • Wegener studied the sediments deposited by glaciers in South America, Africa, India, and Australia. • He discovered glacial grooves, or deep scratches in rocks made as the glaciers moved across the land on neighboring continents today.
Fossil Clues • Animals and plants that lived on different continents can be unique to the continent alone. – Kangaroos are exclusive to Australia. Lions live in Africa but not S. America. – Because oceans separate continents, these animals cannot travel from one continent to another by natural means • However, fossils of similar organisms have been found on several continents separated by oceans.
Fossil Clues • Fossils of a plant called Glossopteris have been discovered in rocks from South America, Africa, India, Australia, and Antarctica. • Evidence suggests these plants grew in a swampy environment. Therefore, the climate of this region, including Antarctica, was different than it is today – Antarctica had a warm and wet climate and changed drastically from 55 milllion years earlier when glaciers existed.
Rock Clues • Wegener also observed that mountain ranges and rock formations on different continents had common origins. – Today geologists have determined that large- scale volcanic eruptions occurred on the western coast of Africa and the eastern coast of South America at about the same time hundreds of millions of years ago • These volcanic rocks are identical in both chemistry and age
Rock Clues • The Caledonian mountain range in northern Europe and the Appalachian Mountains in eastern N. America are similar in age, structure, and composition. • If you place the continents together, these mountains would meet and form on long, continuous mountain belt.
What is missing? • Wegener continued to support the continent drift hypothesis until his death in 1930 • His ideas were not widely accepted until nearly 4 decades later, because he could not explain how they moved • One reason scientists questioned continental drift was because it is a slow process • It was not possible for Wegener to measure how fast the continents moved – How could continents push their way through the solid rock of the sea floor? – However at that time, the world was only beginning to understand what the seafloor looked like. – It took many years after Wegener died before the evidence of plate tectonics hidden in the rifts on the seafloor to be discovered.
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