The document discusses how the Earth has changed over geologic time according to the theory of plate tectonics. Approximately 237 million years ago, all the continents were joined together in a supercontinent called Pangea, which broke up about 152 million years ago. Continents have drifted over time as new crust spreads from oceanic ridges, pushing tectonic plates apart. An earlier supercontinent, Gondwana, existed over 500 million years ago and included most of the landmasses in the Southern Hemisphere before breaking up in the Silurian period.

1. Chapter 16
Evidence of Evolution
Section 16.5

2. © Cengage Learning 2015
16.5 How Has Earth Changed Over
Geologic Time?
• Theory of Plate Tectonics
– Continents were one big supercontinent called Pangea
• Formed about 237 mya (Triassic); broke up about 152 mya ago
(Jurassic)
– Continents drift over time
• Continental plates move no more than 10 cm/year
• New crust spreads outward from oceanic ridges, forcing tectonic plates
away from the ridge and into trenches

3. © Cengage Learning 2015
How Has Earth Changed Over Geologic
Time? (cont’d.)
fault hot spottrench trenchridge
123 4

4. © Cengage Learning 2015
How Has Earth Changed Over Geologic
Time? (cont’d.)
• Gondwana
– Supercontinent that existed before Pangea, more than 500 mya
• Identical layers of rock around the Southern Hemisphere hold matching
fossils of organisms that were extinct millions of years before Pangea
formed
– Included most land masses that are now in the Southern
Hemisphere, India and Arabia
– Broke up in the Silurian

5. © Cengage Learning 2015
How Has Earth Changed Over
Geologic Time? (cont’d.)
600 mya
430 mya
340 mya
240 mya
200 mya
150 mya
65 mya
present
Gondwana
Pangea