Introduction:
Continental Drift: From Idea to Theory

Scientists use theories to explain everything from the origins of the universe to why objects fall to the ground. However, when first proposed, such theories often challenged commonly held beliefs. What evidence would you need to convince you to accept a new explanation of how the world works?

The purpose of this activity is to look at one scientist's controversial idea that Earth's continents were once joined, but drifted apart over millions of years, to help you understand how scientific theories are developed.

Next: Prove It!

Theories are built on evidence, a collection of physical objects and observations. Scientists collect evidence as part of their research.

You might know that ice melts at temperatures above 0ºC (32ºF) because you've watched an ice cube from your freezer turn into a puddle on your kitchen table.

In the box below, write down one other example of something you believe to be true and the evidence you have for believing this. Click "save notes" before moving on to the next page.

Next: Wegener's Theory: Watch This!

The history of science is full of examples where one explanation of how the world works has been replaced by another. Often, the more radical the new idea, the more opposition there has been to accepting it.

Watch this video segment about Alfred Wegener, whose scientific findings challenged the long-held belief that Earth's continents were fixed in place.

Note: You will hear the following words in the video segment. Click on each word to see its definition.

• continental drift
• continental shelf
• meteorologist

In the box below, write down any additional questions that you have about Wegener or continental drift, then click "save notes" before moving on to the next page.

Next: New Discoveries That Challenged Old Ideas

You've just learned how Alfred Wegener developed his groundbreaking theory of continental drift. Here are examples of some other long-held beliefs that were changed by new discoveries.

• For thousands of years, people believed that Earth's surface was a flat disk floating in the ocean. The Greek philosophers of the 6^th century BCE (before the common era) were the first to suggest that Earth is a sphere.
• Until the 1600s, the world's most learned people were convinced that Earth was the center of the universe. Thanks to Copernicus, Galileo, and Newton, it's now widely known that Earth and the other planets in our solar system orbit the Sun.
• Also until the 1600s, fossils were thought to be natural features that "grew" in rocks. Then Danish geographer Nicolas Steno determined that fossils came from living organisms, and that the deeper an object was found underground, the older it was.

Next: Read About It!

Think about what you have watched and learned about Alfred Wegener's theory of continental drift. In this activity, you will read an edited transcript of the video. As you read, consider how Wegener supported his theory and why the scientific community of the time had trouble accepting it.

The following words will appear in the reading. Click on each word to see its definition.

• coal deposit
• glaciers
• incandescent
• intuition
• sea level

Click "View" on the left to begin the activity. Be sure to print your notes before you exit the activity. Your notes will not be saved.

If you want, you can also watch the video again.

Plate Tectonics: The Scientist Behind the Theory

QuickTime Video
Length: 3m 56s

Next: Write About It!

You have now explored Alfred Wegener's theory of continental drift. Next you will have the chance to share your ideas with others.

• Print out your notes. (After you print, your notes will be erased. Make sure you are done before printing.)
  
  
• Print out Plate Tectonics: The Scientist Behind the Theory (PDF). You can read the transcript of the video again and/or read an optional text about what scientists ultimately learned that led to a wider acceptance of Wegener's theory. Check with your teacher if you are not sure which text to read.
• Then, select a writing assignment from page 2 of the printout.

Note: The following words will appear in the readings. Click on each word to see its definition.

Video Transcript

• coal deposit
• glaciers
• incandescent
• intuition
• sea level

Optional Reading

• convection currents
• crust
• deep-ocean trenches
• mid-ocean ridges