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Impact features

Meteor Crater

near Flagstaff, AZ

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Instructor has no financial or other interest in Meteor Crater.

It’s just a neat place to see.

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Characteristics• Bowl-shaped

• raised rim (~100 feet)

• diameter ~4000 feet

• depth ~600 feet

• fragments of an iron meteorite in vicinity (Canyon Diablo meteorite)

• 25,000 - 50,000 years old

• volcanoes nearby (5 million years old)

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Meteor Crater, Arizona

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History

• 1876 Mathias Armijo, cowboy– visited by prospectors, metal fragments– thought to be volcanic

• 1891 sample of metal sent for analysis– 77% iron– 2% lead– rest gold and silver

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• 1891 A E Foote, a mineralogist, visits– identifies metal fragments as a meteorite– diamonds found in meteorite– crater due to impact

• 1895 G K Gilbert, Director USGS– not an impact– volcanic

• 1902 D M Barringer, mining engineer– graduated Princeton at 19– law school

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– studied geology at Harvard– wanted to work in west– discovered Commonwealth silver mine, AZ– rich– visited crater site, was of meteorite origin– bought the place!

• 1906 published paper outlining his ideas

• B convinced that a large (several million tons) iron meteorite buried below the crater, worth ~$250,000,000

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• Barringer’s arguments in favor of impact origin– surrounded by millions of tons of debris

pulverized sandstone (Coconino)– oxidized chunks of iron– meteorite fragments mixed with rock debris– overturned rim– no volcanic rocks in crater– no magnetic effect - iron in small pieces

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Detailed history and info.

• www.barringercrater.com/adventure

• www.meteorcrater.com/– gdcinfo.agg.nrcan.gc.ca:80/crater/– index_e.html

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• Gilbert’s arguments in favor of volcanic origin– no evidence of a large meteorite– no magnetic effect from meteorite– not necessary to have lava in the crater– crater the result of high pressure steam

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Test:

A. volcanic - no magnetic effect; volumes equal

B. impact - should be a magnetic effect, unequal volumes

Gilbert measured and A was correct

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Barringer• Drilling in center - nothing

• decided that the meteorite buried beneath south rim

• 1919 Gilbert reiterates volcanic theory

• 1920 B raises more money; drill sticks and breaks - meteorite?

• 1925 mine shaft - floods, company sinks, B dies

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Geology

• Couldn’t be simpler (layer cake)

• horizontal layers of sedimentary rocks

• each of a different color

• only three rocks involved– lowest is the Coconino sandstone (white)– middle is the Kaibab limestone (buff)– uppermost is the Moenkopi sandstone (purple)

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Layer-cake arrangement of sedimentary rocks

youngest on top

excavation

rim

deformation of the rocks

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New player

• Eugene Shoemaker

• PhD from Princeton University

• doctoral dissertation, 1960 - Meteor Crater

• idea that a high velocity object hitting the Earth would be largely vaporized

• old idea - 1908

• now the ability to simulate with computer

3/6/02 16Mid 1960s

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Estimates

• Iron meteorite

• 100 feet in diameter

• 63,000 tons

• 15 km/sec

• energy ~120 megatons of TNT

• greater than the nuclear device dropped on Hiroshima

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High velocity; example

• Buffalo - New York is 450 miles

• =730 km

• at 15 km/sec, the trip would take 49 seconds

• meteorites can have velocities of 70 km/sec

• trip to NY would take 10 seconds

• And you would be driving a car that weighed 63,000 tons (don’t hit anything!)

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In spite of that - Meteor Crater is a small event.

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Underground nuclear test: Project Sedan

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Overturned rim; reversal of rocks

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Overturned rim: oldest rock on top

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Crater types:

bowl shaped, small

raised rim

Meteor Crater

central peak

larger

terraces

Copernicus

3/6/02 24Isidorus, Moon

3/6/02 25

Theophilus, Moon

3/6/02 26Gosses Bluff, Australia