Meteor Crater, Arizona

http://www.solarviews.com/eng/tercrate.htm

1.2 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

Earthquakes60-95_Nasa.mpg

Plate TectonicsPlate Tectonics

Basic idea of Basic idea of plate tectonicsplate tectonics - - Earth’s surface is composed Earth’s surface is composed of a few large, thick plates of a few large, thick plates that move slowly and change that move slowly and change in sizein size

Intense geologic activity is Intense geologic activity is concentrated at concentrated at plate boundariesplate boundaries, where plates move , where plates move away, toward, or past each otheraway, toward, or past each other

Combination of Combination of continental driftcontinental drift and and seafloor spreadingseafloor spreading hypotheses in late 1960s 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 SpreadingSeafloor Spreading In 1962, Harry Hess proposedIn 1962, Harry Hess proposed

seafloor spreadingseafloor spreading Seafloor moves away from the mid-Seafloor moves away from the mid-

oceanic ridge due to oceanic ridge due to mantlemantle convectionconvection ConvectionConvection is circulation driven by rising is circulation driven by rising

hot material and/or sinking cooler hot material and/or sinking cooler materialmaterial

Hot mantle rock rises under Hot mantle rock rises under mid-oceanic ridgemid-oceanic ridge

Ridge elevation, high heat flow, and Ridge elevation, high heat flow, and abundant basaltic volcanism are abundant basaltic volcanism are evidence of thisevidence of this

Seafloor SpreadingSeafloor Spreading Seafloor rocks, and mantle rocks beneath them, cool and Seafloor rocks, and mantle rocks beneath them, cool and

become become more densemore dense with distance from with distance from mid-oceanic ridgemid-oceanic ridge When sufficiently cool and dense, these rocks may sink When sufficiently cool and dense, these rocks may sink

back into the mantle at back into the mantle at subduction zonessubduction 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 Overall young age for sea floor rocks (everywhere <200 million years) is explained by this model million years) is explained by this model

Divergent Plate BoundariesDivergent Plate Boundaries

At At divergent plate boundariesdivergent plate boundaries, plates move , plates move away from each otheraway from each other

Can occur in the middle of the ocean or within Can occur in the middle of the ocean or within a continenta continent

Divergent motion eventually creates a new Divergent motion eventually creates a new ocean basinocean basin

Marked by Marked by riftingrifting, basaltic volcanism, and , basaltic volcanism, and eventual ridge uplifteventual ridge uplift

During rifting, crust is stretched and thinnedDuring rifting, crust is stretched and thinned Graben valleys mark rift zonesGraben valleys mark rift zones Volcanism common as magma rises through thinner Volcanism common as magma rises through thinner

crust along normal faultscrust along normal faults Ridge uplift by thermal expansion of hot rockRidge 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 SpotsMantle Plumes and Hot Spots

Mantle plumesMantle plumes - narrow columns of - narrow columns of hot mantle rock rise through the hot mantle rock rise through the mantlemantle Stationary with respect to moving Stationary with respect to moving

platesplates Large mantle plumes may spread out Large mantle plumes may spread out

and tear apart the overlying plateand tear apart the overlying plate• Flood basalt eruptionsFlood basalt eruptions• RiftingRifting apart of continental land masses apart of continental land masses

New New divergent boundariesdivergent boundaries may form may form

Mantle Plumes and Hot SpotsMantle Plumes and Hot Spots

Mantle plumes may formMantle plumes may form “hot spots”“hot spots” of active volcanism at Earth’s surfaceof active volcanism at Earth’s surface Approximately 45 known hotspotsApproximately 45 known hotspots

Hot spots in the interior of a plate Hot spots in the interior of a plate produce produce volcanic chainsvolcanic chains OrientationOrientation of the volcanic chain shows of the volcanic chain shows

directiondirection of plate motion over time of plate motion over time AgeAge of volcanic rocks can be used to of volcanic rocks can be used to

determine determine raterate of plate movement of plate movement Hawaiian islands are a good exampleHawaiian islands are a good example