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

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.