Transcript 7-1 Summary
Chapter Introduction Lesson 1 The Continental Drift Hypothesis Lesson 2 Development of a Theory Lesson 3 The Theory of Plate Tectonics Chapter Wrap-Up Arctic_Images/Getty Images What is the theory of plate tectonics? The Continental Drift Hypothesis • What evidence supports continental drift? • Why did scientists question the continental drift hypothesis? The Continental Drift Hypothesis • Pangaea • continental drift Pangaea • Nearly 100 years ago, Alfred Wegener proposed that all the continents were once part of a supercontinent called Pangaea. • Over time, Pangaea began breaking apart and the continents slowly moved to their present position. Pangaea (cont.) • Wegener proposed the hypothesis of continental drift, which suggested that continents are in constant motion on the surface of Earth. • Wegener observed the similarities of continental coastlines now separated by oceans and how they could fit together like pieces of a puzzle. The eastern coast of South America mirrors the shape of the west coast of Africa. Evidence That Continents Move • Evidence to support Wegener’s hypothesis is found in • climate clues; • fossil clues; • rock clues. • When Wegener pieced Pangaea together, he proposed that the continents were located closer to the South Pole 250 million years ago. Evidence That Continents Move (cont.) • Wegener suggested that a large sheet of ice covered the continents. • Wegener studied the sediments left behind and the glacial grooves that formed when the ice sheets melted and Pangaea spread apart. • This provided climate evidence for continental drift. The presence of an ice sheet covering Pangea could explain glacial features found on some continents today. Evidence That Continents Move (cont.) • Animals and plants that live on separate continents can be unique to that continent alone. • Fossils of similar organisms have been found on several continents separated by oceans. • Fossils of a plant called Glossopteris have been found on continents that are now separated by oceans. • The orange area shows where fossils of Glossopteris have been found. • Fossils provide evidence for continental drift. Evidence That Continents Move (cont.) • Wegener observed that mountain ranges and rock formations on different continents had common origins, providing rock evidence for continental drift. • Volcanic rock that is identical in chemistry and age has been found on both the western coast of Africa and the eastern coast of South America. The Caledonia mountain range in northern Europe and the Appalachian Mountains in eastern North America are similar in age, structure, and rock type. Evidence That Continents Move (cont.) How were similar rock types used to support the continental drift hypothesis? What was missing? • Wegener’s ideas were not widely accepted until nearly four decades later. • Scientists questioned continental drift because it was a slow process and Wegener could not measure how fast continents moved or how they moved. • Scientists could not understand how continents could push their way through the solid rock of the mantle and the seafloor. What was missing? (cont.) mantle Science Use the middle layer of Earth, situated between the crust above and the core below Common Use a loose, sleeveless garment worn over other clothes What was missing? (cont.) Why did scientists argue against Wegener’s continental drift hypothesis? • All continents were once part of a supercontinent called Pangaea. • Alfred Wegener proposed that continents move around on Earth’s surface. What term did Wegener use to describe the constant motion of continents on the surface of Earth? A. Pangaea B. continental drag C. continental movement D. continental drift In which of these did Wegener observe similarities that suggested continents might fit together like the pieces of a puzzle? A. fossils B. ice sheets C. plates D. continental coastlines Wegener proposed that 250 million years ago, South America, Africa, India, and Australia were located closer to what? A. equator B. South Pole C. North Pole D. Pacific ocean Do you agree or disagree? 1. India has always been north of the equator. 2. All the continents once formed one supercontinent.