unit 7 lesson 1 the theory of plate tectonics copyright © houghton mifflin harcourt publishing...

Unit 7 Lesson 1 The Theory of Plate Tectonics

Copyright © Houghton Mifflin Harcourt Publishing Company

Drifting Apart


What is continental drift?

• Continental drift is the hypothesis, proposed by Alfred Wegener, that all of today’s continents were once part of a single landmass.

• Wegener called this single landmass Pangea and proposed that broke into pieces that “drifted” to the continents’ present locations.

• Alexander du Toit suggested that Pangea first separated into Laurasia and Gondwana.


What evidence supports continental drift?• Wegener developed his hypothesis by looking at

the shapes of the continents, climatic evidence, rocks, fossils, and land features.

• As mapmaking advanced, maps began to show more accurate shapes of the continents.

• The matching coastlines support continental drift because they suggest that the continents were once connected.



What evidence supports continental drift?• Fossils of the same plant and animal species are

found on continents separated by vast oceans.

• Scientists inferred the continents were joined when these plants and animals were alive.

• Different continents also have matching rock layers and land features.



What evidence supports continental drift?• Locations of coal deposits and past glacial activity

provide climatic evidence for continental drift.

• Coal deposits found in cooler climates suggest that these continents were once closer to the equator.

• The past movement of glaciers across South America, India, Africa, and Australia only made sense if the continents were connected when the glaciers existed.



What is plate tectonics?

• At first, continental drift was not widely accepted by the scientific community, but the hypothesis evolved into the theory of plate tectonics.

• Plate tectonics states that Earth’s surface is made up of giant, moving slabs called tectonic plates.

• A theory requires a great deal of evidence and is a system of ideas that explains many related observations about the natural world.



What evidence supports plate tectonics?• The theory of plate tectonics explains how moving

plates shape land features and how the motion relates to Earth processes.

• During sea-floor spreading, new sea floor forms and pushes existing rock in opposite directions.

• The far edges of the spreading sea floor are pushed into deep-ocean trenches through subduction, the process by which one tectonic plate is pulled beneath another plate.



What evidence supports plate tectonics?• The north and south poles have repeatedly

switched throughout Earth’s history, resulting in a a magnetic reversal.

• As new sea floor forms, magnetic minerals align with the current arrangement of Earth’s poles and become stuck in that position as the rock hardens.

• This alignment switches each time Earth’s poles shift, resulting in a magnetic reversal pattern. This provides evidence for sea-floor spreading and plate tectonics.



What evidence supports plate tectonics?• The locations of earthquakes and volcanoes also

provide evidence for plate tectonics.

• Most active volcanoes are found near plate boundaries, and most earthquakes take place near or on these boundaries as well.



Plates Make It Possible!


How does research related to plate tectonics affect society?• Knowledge about plate tectonics allows scientists

to identify areas at risk for earthquakes and volcanic eruptions.

• Scientists are also able to locate valuable resources, such as minerals and fossil fuels.


How does research related to plate tectonics affect science?• The theory of plate tectonics has had a great

impact on scientific thought because it has unified many theories of Earth science.

• Plate tectonics help scientists understand fossil distribution, earthquake patterns, volcanic eruptions, and climatic changes

• Scientists also use plate tectonics to observe how Earth’s surface has changed and continues to change.



unit 7 lesson 1 the theory of plate tectonics copyright © houghton mifflin harcourt publishing...

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