Scientists collect evidence, including objects they have found and things they have observed, as part of their research. Then, based on this evidence, they come up with a theory that explains how something occurs—or has occurred—in the natural world.

Scientists use theories to explain everything from how the universe began to why objects fall to the ground. In 1915, German scientist Alfred Wegener published a book to present a theory he called "continental drift." Wegener believed that the large pieces of land we call continents move over long periods of time. Even though Wegener found strong evidence to support his theory, not everyone accepted it from the start.

Goals
Here are the big ideas you will learn about in this lesson:

• The continents are in slow but constant motion, riding on top of pieces of Earth's crust called tectonic plates
• The scientific community often refuses to accept new scientific theories that challenge existing ones
• Scientists often use new evidence to build on other scientists’ work and strengthen theories

Strategies
You will be using many reading and writing strategies to complete this lesson. The key strategies that you will focus on are:

• Identifying and using text features
• Determining important information
• Categorizing basic facts and ideas

• geologist
• theory

According to Wegner's theory, there was only one large continent 300 million years ago. He named this continent Pangaea (from the Greek for "all the Earth," and pronounced "pan-GEE-uh"). Pangaea then split, and its pieces drifted, or moved, apart over millions of years to form today's continents. In fact, the continents are still drifting today. This video shows Wegener's evidence and explains the difficulty he had getting his ideas across to geologists—scientists who study the history and structure of Earth. Click "View" on the left to watch the video.

Then show what you've learned by answering these four questions. Be sure to save all your answers before you go to the next screen.

Scientists collect evidence, look for patterns in the evidence, and then test different ideas to explain these patterns. Only then do they put forth a theory. However, not every theory is accepted by other scientists.

As you read this passage, consider what Wegener was unable to prove to other scientists that caused them to reject his ideas at first. Click "View" on the left to read the passage.

After reading the passage, write one to two sentences to answer each question in the box below.

• What was the major weakness of Wegener's theory of continental drift?
• What did scientists eventually learn that led them to accept Wegener's theory?

You will now check your understanding of the vocabulary words for this lesson by placing them in sentences. Click "View" on the left to begin the activity.

Before moving to the next screen, choose two words from the vocabulary list and write a new sentence for each word.

Vocabulary words: continent, evidence, fossils, geologist, theory

As you saw and read on the previous screens, evidence plays an important role in turning an idea into a theory. In this activity, you will sort information on a concept map to show how different pieces of evidence support an idea or theory. In some cases, a piece of evidence will support more than one theory.

Click "View" on the left to begin the activity. Your teacher will review the answers with you.

Then, before moving on to the final assignment, you may want to watch this lesson's videos again or reread the passage.

Plate Tectonics: Scientist Behind the Theory

Video: 3m 53s

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Plate Tectonics—Proof That Wegener Was Right

Document

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In this lesson, you have learned how an idea that started with a man looking at a map of the continents became a scientific theory. You have also learned that theories can sometimes be made stronger as new pieces of evidence are discovered. You will now write an essay that shares what you have learned and what you think about it.

Begin by reviewing your notes. Click "review my work" above and view or print out the screen that pops up.

Final Assignment Instructions

• Click Continental Drift—Final Assignment (PDF).
• Choose one of the topics in the PDF and write an essay.
• Organize your response so that it includes at least three supporting details that you have learned from the reading passage, video, and other materials in this lesson.
• Use at least three vocabulary words (or form of the words) as part of your supporting details. For example, for the word jump, you could use jumps, jumped, or jumping.