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Chapter 9 The History of Life on Earth
Earth’s Changing Continents
Key Concept Movements of Earth’s tectonic plates
have affected climate, geographic connections, and the
distribution of organisms.
The surface of Earth on which we live is constantly moving. Sometimes, we feel this movement as earthquakes. But did you know that Earth’s surface has changed so much during Earth’s long history that the continents have changed locations?
Plate Tectonics The thin, cool “skin” of Earth is called the lithosphere. This layer is broken into several smaller blocks called tectonic plates. These plates rest on a thick layer of solid rock called the mantle. Earth’s mantle is solid, but it moves very slowly. As the mantle moves, it drags on the bottom of the cold tectonic plates lying on top of it. As a result, the tectonic plates move. Earth’s sur- face currently has about 12 large plates and many small ones. Some of the large plates are labeled in Figure 1. Most plates move as fast as your fingernails grow—between 2 cm and 5 cm per year. Over geologic time scales, this movement can cause large cumulative effects—plate movements may total thousands of miles. The theory that explains how Earth’s tectonic plates move and change shape is called plate tectonics.plate tectonics. What is plate tectonics? 7.4.a, 7.4.f
What You Will Learn
• Earth’s continents have moved
around Earth’s surface through- out Earth’s history and have only recently arrived at their current locations.
• Rocks and fossils provide evidence
of continental drift. They also provide evidence of the changes in life and climate that have occurred during Earth’s history.
Why It Matters
Understanding the history of climate change and life on Earth will help you better understand today’s Earth.
Vocabulary
- plate tectonics
- continental drift
Graphic Organizer In your Science Journal, make a Spider Map that organizes the types and details of plate boundaries.
Figure 1 Earth’s Tectonic Plates
Section 2 Earth’s Changing Continents 271
Where Tectonic Plates Meet
Tectonic plates may contain oceanic lithosphere, conti- nental lithosphere, or both types of lithosphere. As tectonic plates move, they collide, separate, and grind past each other. Places where two or more tectonic plates meet are called plate boundaries. There are three main types of plate boundaries. The type of plate boundary that forms is a result of how the plates move relative to each other.
Convergent Boundaries
Plates move toward each other at a convergent boundary , as shown in Figure 2. If both plate edges are continental litho- sphere, the rocks are forced together until they crumple to make great mountain belts. But if one plate is thin, dense oceanic lithosphere, it may sink downward into the mantle. As the plate sinks, surrounding rock may melt. Some of this molten rock rises to the surface and makes a line of volcanoes.
Divergent Boundaries
Plates move apart at a divergent boundary , as shown in Figure 2. This process forms a rift—a giant crack in the litho- sphere. Volcanic eruptions fill the crack with lava that cools to form new oceanic lithosphere. If a rift tears apart a continent and then widens for millions of years, a new sea forms. The sea may gradually grow into a new ocean.
Transform Boundaries
Two plates slide horizontally past each other along a trans- form boundary , as shown in Figure 2. The movement of the plates can cause earthquakes in the area of a transform boun- dary. One of the world’s most well known transform boundaries is the San Andreas fault, which cuts right across California.
plate tectonicsplate tectonics (PLAYT tek TAHN iks) the theory that explains how large pieces of Earth’s outermost layer, called tectonic plates, move and change shape
7.4.a Students know Earth processes today are similar to those that occurred in the past and slow geologic processes have large cumulative effects over long periods of time. 7.4.e Students know fossils provide evidence of how life and environmental conditions have changed. 7.4.f Students know how movements of Earth’s continental and oceanic plates through time, with associated changes in climate and geographic connections, have affected the past and present distribution of organisms.
Figure 2 Tectonic Plate Boundaries
Lithosphere
Convergent boundary
Divergent boundary
Transform boundary
Section 2 Earth’s Changing Continents 273
History of Continental Drift Figure 4^ The Breakup of Pangaea
By putting together all of the evidence, scien- tists can draw maps that show how Earth’s geog- raphy has changed over time. For example, all of Earth’s continents made up a supercontinent called Pangaea (pan JEE uh) about 245 million years ago. At the same time, Earth also had a single super- ocean. Pangaea split into several new plates begin- ning about 200 million years ago. As the plates drifted apart, those new continents separated, and new oceans formed between them. The breakup of Pangaea is shown in Figure 4. These huge changes moved rocks and fossils all over Earth. The rocks and fossils give scientists evi- dence of the plate movements. In addition, plate movements changed Earth’s climate and affected evolution , or how populations of species have changed over time.
Changes in Climate
As continents moved, they changed the way land and sea were placed on Earth’s surface. If con- tinents moved toward the equator, they received more energy from the sun and developed warmer climates. Continental drift caused ocean currents and winds to flow differently. These changes affected heat flow. As a result, temperature and precipitation patterns around the planet changed. For example, Antarctica was not frozen 40 mil- lion years ago. But as the other continents moved, Antarctica was left surrounded by the cold water near the South Pole. As cold water currents moved around Antarctica, the polar icecap formed. Antarc- tica slowly became the icy land we see today.
Changes in Life
When Pangaea split apart, the organisms living on each continent were separated. As their environ- ments changed, the organisms that lived in those environments also changed. And as new oceans formed, changes also occurred in sea life. This explains why different organisms live on different continents. It also explains why fossils of the same organisms are found on different continents.
How have tectonic plate motions affected the distribution of organisms? 7.4.f
About 245 million years ago The continents were one giant landmass called Pangaea.
About 135 million years ago As Pangaea broke apart, the North Atlan- tic and Indian Oceans began to form.
About 65 million years ago The continents continued to drift apart toward their modern locations.
Today The continents continue to move at a rate of about 2 cm to 5 cm per year.
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274 Chapter 9 The History of Life on Earth
Figure 5 After North and South America were joined by the Panama Land Bridge, animals migrated across the bridge and ocean currents changed dramatically.
Case Study: The Panama Land Bridge North and South America drifted close together about 3 mil- lion years ago. At that time, a narrow strip of land joined North and South America for the first time, as shown in Figure 5. This strip of land was called the Panama Land Bridge.
Changes in Life
Animals could now walk across the Panama Land Bridge. As they migrated, they competed with one another. Many animals became extinct, but successful ones flourished. Opossums and armadillos invaded North America, whereas camels and cats invaded South America. At the same time, creatures in the sea were separated by the new land bridge. Some populations of clams, corals, whales, and sea urchins evolved into separate species on the Pacific and Caribbean coasts of Panama. How did the formation of the Panama Land Bridge affect the distribution of organisms in the Americas? 7.4.f
Changes in Climate
The land bridge forced warm, tropical water that had once flowed between the continents to flow around the Gulf of Mexico and north past Florida. The new flow of water formed the Gulf Stream—a strong ocean current. The Gulf Stream changed the climate of western Europe by transporting warm water across the Atlantic Ocean. This water heats the air and makes climates milder.
