Meteor Crater (Arizona): Meteor Impact Craters
1. Zoom from the North Magnetic Pole To Meteor Crater, Arizona.
2. Last lecture – talked about the aurorae – due to the solar wind bombarding
Earth’s magnetosphere (not all of Earth’s Geologic Wonders have terrestrial
origins!)
a. Continue this with the process of Meteor Impacts Meteor Crater,
Arizona
b. At only 1.2 km in diameter (less than a mile!) it is certainly Not the
Biggest, but it is the most recent
3. Meteor Crater is located approximately 43 miles east of Flagstaff, near
Winslow in the northern Arizona desert.
a. Scientists refer to the crater as Barringer Crater in honor of Daniel
Barringer who was first to suggest that it was produced by meteorite
impact. The crater is privately owned by the Barringer family through
their Barringer Crater Company, * "the first proven, best-preserved
meteorite crater on earth”
b. The crater was created about 50,000 years ago during the Pleistocene
epoch when the local climate on the Colorado Plateau was much
cooler and damper. At the time, the area was an open grassland dotted
with woodlands inhabited by woolly mammoths and giant ground
sloths.
c. The object that excavated the crater was a nickel-iron meteorite about
50 meters across
d. impacted the plain at a speed of several kilometers per second. The
speed of the impact has been a subject of some debate. Modeling
initially suggested that the meteorite struck at a speed of up to 20
km/s more recent research suggests the impact was substantially
slower, at 12.8 km/s (28,600 mph)
e. Had to be at least 11 km/sec!!
f. It is believed that about half of the impactor's 300,000 metric tons
(330,000 short tons) bulk was vaporized during its descent, before it
hit the ground.
g. The impactor itself was mostly vaporized; very little of the meteorite
remained within the pit that it had excavated
4. In 1891 Grove Karl Gilbert, chief geologist for USGS, investigated the crater
and concluded that it was the result of volcanic steam explosion (Volcanoes
can have craters or calderas!).
a. assumed that if it were an impact crater then the volume of the crater,
as well as meteoritic material, should be present on the rim.
b. also assumed a large portion of the meteorite should be buried in the
crater and that this would generate a large magnetic anomaly.
c. His calculations showed the volume of the crater and the debris on the
rim were roughly equivalent, mass of the hypothetical impactor was
missing.
d. Also, no magnetic anomalies.
5.
6.
7.
8.
9.
e. Gilbert argued that the meteorite fragments found on the rim were
coincidental.
f. publicized these conclusions in a series of lectures in 1895.
g. Ironic - In 1892 Gilbert was among first to propose that the moon's
craters were caused by impact rather than volcanism.
In 1903, mining engineer and businessman Daniel Barringer suggested that
the crater had been produced by the impact of a large iron-metallic
meteorite.
a. Barringer's company, the Standard Iron Company, received a patent
signed by Theodore Roosevelt for 640 acres (2.6 km2) around the
center of the crater in 1903.
b. In 1906 Roosevelt authorized the establishment of a newly named
Meteor, Arizona post office
c. Barringer to believe that the bulk of the impactor could still be found
under the crater floor.
d. Impact physics was poorly understood at the time and Barringer was
unaware that most of the meteorite vaporized on impact.
e. spent 27 years trying to locate a large deposit of meteoric iron, and
drilled to a depth of 419 m (1,376 ft), but no significant deposit was
ever found.
f. Barringer estimated from the size of the crater that the meteorite had
a mass of 100 million tons. The current estimate of 300,000 tons for
the impactor is only three-tenths of one percent of Barringer's
estimate.
1960, Gene Shoemaker – stishovite!!
a. (stability of quartz)
The impact created an inverted topography of rock layers around the rim –
oldest is on the top!
a. Coconino Sandstone (265 Ma) nearest the top of the rim
b. Toroweap Formation (255 Ma)
c. Kaibab Formation (dolomite 250 Ma)
d. Moenkopi Formation (mudstone 200 Ma) nearest the outer foot of the
rim
e. (Same layers as at Grand Canyon, to the north!)
f. In the interior of the crater the layers are in the expected order.
During the 1960s, NASA astronauts trained in the crater to prepare for the
Apollo missions to the Moon. remains visible to this day.
a. The crater was used as the setting for the climax of the 1984 film
Starman.
Why was it so important that Meteor Crater was young?
a. Compare it to Chesapeake Bay: A 35.5 Ma Impact Crater!
b. For the Moon, a Lack of erosion allows understanding of cratering
process
c. Moon: about 500,000 craters > 1 km
d. Talk about other planetary surfaces – can use the crater density as a
measure of how old a surface is when you can’t get there to do
radiometric dating
i. E.g. – Moon’s highlands are older than Maria – have higher
crater density
ii. Venus’ surface only has 1000 craters (but 10,000s of
volcanoes! Resurfacing by volcanism!)
10. Craters are always circular --- why?
a. Throw a baseball into some sand! This is NOT what happens!!
b. Talk about the physics of impacts – vaporization – shock wave –
seismic wave
c. Talk about the Structure of the Crater that results (central peak, ejecta
blanket, fractured bedrock, impact melt, breccia)
d. A nuclear bomb at the surface will do the same thing!
i. Like the 1962 Sedan surface test (Yucca Flats, Nevada): A 100
kiloton nuclear device produced this 372 m (1230 ft) diameter
crater
e. However, Meteor Crater is slightly squarish (could be due to jointing?)
11. Spectrum of impacts in terms of size (gradual to catastrophic):
a. Several tons of cosmic debris each day
b. Every few tens of millions of years, a real big one
12. Top 5:
a. Chesapeake Bay – largest in US (90 km in diameter)
b. Vredefort dome, South Africa - Largest in world (300km in diameter) 2 Ga!
c. Manicouagan (100 km - Quebec)/Saint Martin (40 km –
Manitoba)/Rochechouart(23 km – France) – all 214 Ma, and all line up
when you move the plates back into Pangaea! All part of one impact
that broke up during arrival???
d. Chixulub Crater, Yucatan, Mexico - Potentially most important – (170
km in diameter), 65 Ma – nuclear winter; extinction of the dinosaurs
and many other species! [show diagram of #genera again, from
Burgess Shale talk]
e. This Will happen again! We track the locations of >10,000 objects in
space!!! Just a matter of time! Though there are contests going on now
trying to figure out how to best divert a large asteroid that is on
course with Earth! Science fiction (or bad Hollywood!) is becoming a
reality here!
Questions:
1) You buy land in a remote area and the seller tells you that there was once a
meteor impact there. How do you tell if this might be true or not?
2) Jupiter acts as a good shepherd, gravitationally diverting out of the solar
system most of the large rogue objects that might otherwise one day strike
Earth. How do you think the course of evolution of life on Earth would have
gone if large impacts were much more frequent? How about much less
frequent?