Chapter 1: Plate Tectonics

Section 1.2: Continents change position over time

I. The Big Idea and Key Concepts

A. Big Idea: The movement of tectonic plates causes geologic changes on Earth.

B. Key Concept 1.1: Earth has several layers.

C. Key Concept 1.2: Continents change position over time.

D. Key Concept 1.3: Plates move apart.

E. Key Concept 1.4: Plates converge or scrape past each other.

II. Section 1.2: Continents change position over time.

A. Vocabulary

1. continental drift- Wegener’s hypothesis that Earth’s continents were once joined in a single landmass and gradually moved, or drifted, apart

2. Pangaea- from the Greek word meaning “all lands”; Wegener’s name for the joined continents (supercontinent)

3. mid-ocean ridge- huge underwater mountain range

4. convection- heat transferred by the movement of material

5. convection current- heating, rising, cooling and sinking action that transfers heat in a material

6. theory of plate tectonics- theory that states that Earth’s lithosphere is made up of huge plates that move over the surface of the Earth

B. Continents join together and split apart.

1. Were the continents once joined together?

a. In the 1500’s mapmakers started including South America and Africa in their maps.

b. They noticed that the western Africa and eastern South America seemed to fit together like puzzle pieces.

2. Alfred Wegener (German scientist, late 1800’s)

a. He proposed the continental drift hypothesis.

b. Wegener gathered his evidence from fossils, studies of ancient climate and the geology of the continents.

c. People didn’t accept his theories until the mid-1900’s.

3. Wegener’s Evidence for Continental Drift

a. Fossils

i. Fossils of an ancient reptile were discovered in eastern South America and western Africa.

ii. These fossils were not found anywhere else in the world.

iii. The fossils were of a small reptile that lived 270 million years ago.

b. Climate

i. Fossils of tropical plants can be found on Greenland’s shores.

ii. South Africa’s rocks were deeply scratched by ice sheets that once covered the area.

c. Geology

i. Types of rock found in Brazil matched rock found in western Africa.

ii. Limestone rocks in the Appalachians were exactly like those in Scotland’s Highlands.

d. Which evidence do you think is most convincing? Explain.

4. Pangaea and Continental Drift

a. Wegener’s conclusion: The continents were once joined in a huge supercontinent.

b. He named this supercontinent Pangaea.

c. Pangaea began to split apart 200 million years ago.

C. The theory of plate tectonics explains how plates and their continents move.

1. Evidence from the Sea Floor

a. Scientists began mapping the seafloor in detail in the 1950’s.

b. They found huge mountain ranges called mid-ocean ridges.

c. These ridges appear in every ocean, lining the Earth like seams on a baseball.

d. Sea-floor Spreading

i. Ridges form cracks along the crust.

ii. Melted rock rises through these cracks, cools and forms new oceanic crust.

iii. These areas are called spreading centers.

e. Age of the Sea Floor

i. Evidence that the sea floor is spreading apart came from the age of rocks in the crust.

ii. The youngest rock is closest to the ridge, while the oldest rock is farthest away.

iii. Continental crust (4 billion years old) is much older than oceanic crust (160-180 million years old.

iv. If the seafloor is spreading, why isn’t the Earth growing?

f. Ocean Trenches

i. In these deep trenches, dense oceanic crust is sinking into the asthenosphere.

ii. Old crust is being destroyed at the same rate that new crust is being formed.

2. Causes of Plate Movement

a. Tectonic plates rest on the asthenosphere, a layer of soft, hot rock.

b. Rock in this layer moves by convection.

c. Rocks in the asthenosphere act in a similar way.

i. Hot, soft rock rises, cools and sinks, only to be heated and rise again.

ii. This sinking and rising motion, if it continues, is called a convection current. This current moves rocks in the mantle only a few centimeters a year. Over millions of years, this moves plates thousands of kilometers.

d. Slab pull and ridge push also move these huge plates.

i. slab pull- gravity pulls the edge of a cool, dense plate into the asthenosphere and the entire plate is dragged along.

ii. Ridge push- material from a mid-ocean ridge slides downhill from the ridge, pushing the rest of the plate.

3. Putting the Theory Together

a. Theory of plate tectonics- the Earth’s lithosphere is made up of huge plates that move over the surface of the Earth.

b. Plates can move apart, push together, or scrape past each other.

c. Most major earthquakes, volcanoes, and mountain ranges appear where tectonic plates meet.