Terrestrial World Surfaces

1. Terrestrial World Surfaces • Solid rocky surfaces shaped (to varying degrees) by: • Impact cratering • Volcanism • Tectonics (gross movement of surface by interior forces) • Erosion (by impacts or by weather)

2. The Earth's Surface • Unlike Mercury and the Moon, impact craters are few and far between on the Earth. • From a crater counting perspective the surface is “young” • The Earth has been impacted as frequently, if not more frequently than the Moon, yet those scars have been erased • Note that the Earth's atmosphere offers little protection against the large impactors that produce the craters seen above.

3. The Earth's Surface • The Earth's larger size means that it is still geologically active. • New surface is being created and old surfaces are being resurfaced regularly. • The Earth also retains an atmosphere which makes for efficient erosion of craters.

4. Geological Activity vs. Planetary Size • It's no coincidence that the smallest worlds above are the ones that are heavily cratered. • The larger a world is the more readily it retains its internal heat. • A pea cools off much more quickly than a potato • Earth and Venus are still hot in the interior and molten material can reach and re-surface the surface.

5. How Lightly Cratered?

6. How Lightly Cratered?

7. Two Distinct Terrains: Continents and Basins • The Earth has two distinct surface levels • Continents: Thick portions of the crust that occupy high elevations • Basins: Thiner crust occupying the “lowlands”

8. Two Distinct Terrains: Continents and Basins • Interestingly, there is just enough water on Earth to fill the basins.

9. Two Distinct Terrains: Continents and Basins • Interestingly, there is just enough water on Earth to fill the basins. • It didn't have to be this way, consider Jupiter's moon Europa • The world is submerged under a 60 mile deep ocean frozen at the top.

10. Two Distinct Terrains: Continents and Basins • In contrast, Venus is completely different

11. Crustal Plates: Unique to Earth? • Mountain chains and tectonic activity delineate regions of the Earth's surface.

12. Crustal Plates: Unique to Earth? • Mountain chains and tectonic activity delineate regions of the Earth's surface.

13. A Thin Crust Enables Activity

14. Mantle Convection • Motions in the Earth's interior (think Lava Lamp) fracture and move the crust about.

15. Mantle Convection • Motions in the Earth's interior (think Lava Lamp) fracture and move the crust about. • Twenty crustal plates move under this influence (about 1 inch a year)

16. Mantle Convection • Motions in the Earth's interior (think Lava Lamp) fracture and move the crust about. • Twenty crustal plates move under this influence (about 1 inch a year)

17. Mantle Convection • Motions in the Earth's interior (think Lava Lamp) fracture and move the crust about. • Twenty crustal plates move under this influence (about 1 inch a year)

18. Plate Motion Drives Geology

19. Making an Ocean • The Atlantic Ocean has swung open in the last 300 million years. • Given that amount of time one inch per year is plenty.

20. Earthquakes • Earthquakes are the result of “sticky” plate boundaries. • Stress builds up followed by a sudden slip.

21. Continental Drift Continues

22. Continental Drift Continues

23. Erosion and Continental Craters • Some of the continental crust is quite old. • However, erosion is a powerful force and fresh mountain range (e.g. the Rockies) can be worn down in just a hundred million years. Craters don't stand much of a chance.

24. How Lightly Cratered?