Meteor Crater, Arizona kilometers (0.7 miles) 40,000 years old

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

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

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

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

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

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

A convective thunderstorm

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 spreading hypotheses in late 1960s

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

The ‘Ring of Fire’

San Andreas Fault Juan de Fuca plate

Seafloor Spreading  In 1962, Harry Hess proposed seafloor spreading Seafloor moves away from the mid- oceanic ridge due to mantle convection Seafloor moves away from the mid- oceanic ridge due to mantle convection Convection is circulation driven by rising hot material and/or sinking cooler material 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 Ridge elevation, high heat flow, and abundant basaltic volcanism are evidence of this

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 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

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 Can occur in the middle of the ocean or within a continent Divergent motion eventually creates a new ocean basin Divergent motion eventually creates a new ocean basin  Marked by rifting, basaltic volcanism, and eventual ridge uplift During rifting, crust is stretched and thinned During rifting, crust is stretched and thinned Graben valleys mark rift zones Graben valleys mark rift zones Volcanism common as magma rises through thinner crust along normal faults Volcanism common as magma rises through thinner crust along normal faults Ridge uplift by thermal expansion of hot rock Ridge uplift by thermal expansion of hot rock

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

Nile Delta and Sinai Peninsula (MODIS)

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

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

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

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

Mantle Plumes and Hot Spots  Mantle plumes may form “hot spots” of active volcanism at Earth’s surface Approximately 45 known hotspots 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 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 Age of volcanic rocks can be used to determine rate of plate movement Hawaiian islands are a good example Hawaiian islands are a good example