Meteor Crater, Arizona

1. http://www.solarviews.com/eng/tercrate.htm 1.2 kilometers (0.7 miles) 40,000 years old Meteor Crater, Arizona

2. 0.9 kilometers (0.5 miles) 300,000 years old Wolfe Creek, Australia

3. 17 kilometers (10.5 miles) 200 million years old Aorounga, Chad, Africa

4. 100 kilometers (62 miles) 212 million years old Manicouagan, Quebec, Canada

5. Chicxulub The one that killed off the dinosaurs Diameter = 170 kilometers (105 miles) 65 million years old Chicxulub, Yucatan Peninsula, Mexico

6. So where did the rest of the Earth’s impact craters go? Answer: They have been destroyed by tectonic activity (creation and destruction of crust) and by erosion

7. Heat driven convection 1. Bottom water is warmed 2. It expands an is therefore less dense 3. It rises to the surface and then spreads out 4. Cooler water at the sides descends to fill the void

8. A convective thunderstorm

13. Plate Tectonics • Basic idea of plate tectonics- Earth’s surface is composed of a few large, thick plates that move slowly and change in size • Intense geologic activity is concentrated at plate boundaries, where plates move away, toward, or past each other • Combination of continental drift and seafloor spreadinghypotheses in late 1960s

14. Where do we see deep earthquakes? What is happening there?

16. The ‘Ring of Fire’

19. Juan de Fuca plate San Andreas Fault

20. Seafloor Spreading • In 1962, Harry Hess proposedseafloor spreading • Seafloor moves away from the mid-oceanic ridge due to mantleconvection • Convection is circulation driven by rising hot material and/or sinking cooler material • Hot mantle rock rises under mid-oceanic ridge • Ridge elevation, high heat flow, and abundant basaltic volcanism are evidence of this

22. Seafloor Spreading • Seafloor rocks, and mantle rocks beneath them, cool and become more dense with distance from mid-oceanic ridge • When sufficiently cool and dense, these rocks may sink back into the mantle at subduction zones • Downward plunge of cold rocks gives rise to oceanic trenches • Overall young age for sea floor rocks (everywhere <200 million years) is explained by this model

23. Divergent Plate Boundaries • At divergent plate boundaries, plates move away from each other • Can occur in the middle of the ocean or within a continent • Divergent motion eventually creates a new ocean basin • Marked by rifting, basaltic volcanism, and eventual ridge uplift • During rifting, crust is stretched and thinned • Graben valleys mark rift zones • Volcanism common as magma rises through thinner crust along normal faults • Ridge uplift by thermal expansion of hot rock

24. N Africa, Europe, the Mediterranean, the Middle East: (MODIS)

25. Nile Delta and Sinai Peninsula (MODIS)

26. Normally the orientation of the Earth’s magnetic field is like this. North South

27. But every once in a while (~100,000 years) the magnetic field flips South North

28. Hot magma ‘erupts’ from the center of a divergent zone and spreads out laterally as it cools and subsides

29. Mantle Plumes and Hot Spots • Mantle plumes - narrow columns of hot mantle rock rise through the mantle • Stationary with respect to moving plates • Large mantle plumes may spread out and tear apart the overlying plate • Flood basalt eruptions • Rifting apart of continental land masses • New divergent boundaries may form

30. Mantle Plumes and Hot Spots • Mantle plumes may form“hot spots”of active volcanism at Earth’s surface • Approximately 45 known hotspots • Hot spots in the interior of a plate produce volcanic chains • Orientation of the volcanic chain shows direction of plate motion over time • Age of volcanic rocks can be used to determine rate of plate movement • Hawaiian islands are a good example