1. 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.

2. 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.

3. The eastern coast of South America mirrors the shape of the west coast of Africa.

4. 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.

5. 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.

6. The presence of an ice sheet covering Pangea could explain glacial features found on some continents today.

7. 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.

8. The orange area shows where fossils of Glossopteris have been found.
Fossils provide evidence for continental drift.

9. 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.

10. The Caledonia mountain range in northern Europe and the Appalachian Mountains in eastern North America are similar in age, structure, and rock type.

11. 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.

12. Lesson 1: The Continental Drift Hypothesis
The puzzle piece fit of continents, fossil evidence, climate, rocks, and mountain ranges supports the hypothesis of continental drift.
Scientists were skeptical of continental drift because Wegener could not explain the mechanism for movement.

13. Mapping the Ocean Floor
During the late 1940s scientists were able to determine the depth of the ocean using a device called an echo sounder.
Once ocean depths were determined, scientists used these data to create a topographic map of the sea floor that revealed vast mountain ranges, called mid-ocean ridges, that stretch for many miles deep below the ocean’s surface.

14. Seafloor Topography

15. Seafloor Spreading
By the 1960s, scientists discovered the process of seafloor spreading.
Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.
When the seafloor spreads, the mantle below melts and forms magma.

16. Seafloor Spreading (cont.)
Magma erupts on Earth’s surface as lava, which cools and crystallizes on the seafloor, forming rock.
Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lavas.
As the seafloor continues to spread apart, the older oceanic crust moves away from the mid- ocean ridge.

17. Seafloor Spreading (cont.)

18. Seafloor Spreading (cont.)
Scientists argued that if the seafloor spreads, the continents must also be moving.
The rugged mountains that make up the mid-ocean ridge system can form in two different ways.
Large amounts of lava can erupt from the center of the ridge, cool, and build up around the ridge.

19. Seafloor Spreading (cont.)
Or, as the lava cools and forms new crust, it cracks and the rocks move up or down along these cracks in the seafloor, forming jagged mountain ranges.
The abyssal plain, the smooth part of the seafloor, is made when the layer of sediment that accumulates far from the mid-ocean ridge becomes thick enough.

20. Continents move as the seafloor spreads along a mid- ocean ridge.

21. Development of a Theory
The first evidence used to support seafloor spreading was discovered in rocks on the seafloor.
Scientists studied the magnetic signature of minerals in these rocks.
Earth’s magnetic field today is described as having normal polarity—a state in which magnetized objects, such as compass needles, will orient themselves to point north.

22. Development of a Theory (cont.)
Sometimes a magnetic reversal occurs and the magnetic field reverses direction.
The opposite of normal polarity is reversed polarity: a state in which magnetized objects reverse direction and orient themselves to point south.

24. Development of a Theory (cont.)
Volcanic rock on the seafloor contains iron-rich minerals that are magnetic.
Magnetic minerals in cooling lava from the mid- ocean ridge record the direction of Earth’s magnetic field.
Scientists have discovered parallel patterns in the magnetic signature of rocks on either side of a mid- ocean ridge.

25. Minerals in fresh lava record Earth’s magnetic signature.

26. Development of a Theory (cont.)
Scientists studied magnetic minerals in rocks from the seafloor using a magnetometer to measure and record the magnetic signature.
They discovered parallel magnetic stripes on either side of the mid-ocean ridge.

27. Development of a Theory (cont.)
Each pair of stripes has a similar composition, age, and magnetic character.
The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges is carried away from the center of the ridges in opposite directions.

28. Seafloor Spreading Theory

29. Development of a Theory (cont.)
Other measurements made on the seafloor confirm seafloor spreading.
Measuring the amount of thermal energy leaving the Earth shows that more thermal energy leaves Earth near mid-ocean ridges than is released from beneath the abyssal plains.

30. Development of a Theory (cont.)
Sediment collected from the seafloor can be dated to show that the sediment closest to the mid-ocean ridge is younger than the sediment farther away from the ridge.