17.1 Plate Tectonics

Objectives• Describe one piece of early evidence that led

people to suggest that Earth’s continents may have once been joined.

• Discuss evidence of continental drift.• Explain why continental drift was not accepted

when it was first proposed.

– continental drift– Pangaea

Vocabulary

• On the geologic time scale, Earth’s surface is changing at rates almost too great to imagine.

Drifting Continents

• Earth’s surface appears to remain relatively unchanged.

Three scientists made observations about the earths continents

– Abraham Ortelius

– Eduard Suess

– Alfred Wegener.

Abraham Ortelius - Dutch mapmaker

• 1500s• noticed the fit of continents on

either side of the Atlantic Ocean.

Eduard Suess - Austrian geologist• 1800s• hypothesized that present southern continents

had once been joined as a single landmass that he named Gondwanaland.

Alfred Wegener German scientist • 1912 proposed the

continents moved.

Wegner’s Hypothesis – continental drift

• continental drift, proposed all Earth’s continents had once been joined.(Pangaea)

• Pangaea, (Gr) Pan = all Gaea= Earth– (all the continents had once been joined) – Broke apart 200 million years ago.

Continental Drift

Evidence from Rock Formations

– Areas of similar rock types on opposite sides of the Atlantic Ocean.

Evidence from Fossils and Rock Formations

– Similar fossils found on widely separated continents.

– fossils older than 200 million years when Pangaea broke apart.

– Fossils of Glossopteris, a seed fern that resembled low shrubs, have been found on many continents, indicating that the areas had a single climate that was close to the equator.

Evidence from Fossils

Fossil Record

Ancient Climatic Evidence

Glacial deposits found in Africa, India, Australia, and South America suggested that these areas had once been covered by thick ice caps.

Coal deposits in Antarctica suggested that it must have been closer to the equator.

Wegener proposed that the continents were located near the south pole before Pangaea began to fracture.

A Rejected Hypothesis

• 1900s, most scientists rejected Wegener’s hypothesis of continental drift.

• Two unanswered questions:– What forces could move continents

– How continents could move without shattering

Section Assessment

1. How did Pangaea differ from the present layout of the continents?

During Pangaea, the continents were joined and essentially located in one hemisphere from the north to the south poles. Presently, the continents have separated and are distributed in both the eastern and western hemisphere, with the majority of landmass in the northern hemisphere.

Section Assessment

2. According to Wegener, what force caused the continents to move?

Wegener suggested that the rotation of Earth could be responsible for continental movement.

______ Similar rock types are located in South America and Africa.

______ Gondwanaland and Pangaea essentially refer to the same thing.

______ Coal located in India and Africa suggest that they were once located in warmer climates.

______ Geologic evidence suggests that North America, Greenland, and Europe were once connected.

Section Assessment3. Identify whether the following statements

are true or false.

true

true

false

true