Hello and thank you for your question.
There is an excellent resource on impact cratering at
IMPACT CRATERING ON EARTH
http://www.unb.ca/passc/ImpactDatabase/essay.html
There is also a very useful set of questions and answers at the Auburn
University Site, Wetumpka Impact Structure
http://www.auburn.edu/academic/science_math/geology/docs/wetumpka/gen.htm
The above site asserts that a true impact structure is formed by the
impact of a meteorite with the earth (or with another planetary body).
Structures similar to these that result from volcanic action are not
true impact structures:
"Although craters can be formed by meteor impacts, they can also
result form volcanic processes. Careful analysis of minerals in rocks
can help determine whether a depression is the result of a meteorite
impact or not. The presence of two shock indicators, the minerals
coesite and stishovite, in quartz-rich rocks have aided in the
positive identification of impact craters. Coesite and stishovite are
mineral pseudomorphs of quartz that form at extremely high pressures.
Volcanic explosions cannot reach the pressures needed to form coesite
and stishovite (greater than 15 and 30 GPa, respectively) (Melosh,
1989)."
Id.
You can investigate impact craters worldwide from the following
database:
Earth Impact Database
http://www.unb.ca/passc/ImpactDatabase/CILocSort.html
Although the earth has been ceaselessly struck by meteorites
throughout its history, evidence of most of these has been eroded
away, so there are far more volcanic structures to be seen than impact
structures:
Nonvolcanic craters
"Some well-exposed, nearly circular areas of intensely deformed
sedimentary rocks, in which a central vent-like feature is surrounded
by a ring-shaped depression, resemble volcanic structures in gross
form. As no clear evidence of volcanic origin could be found in or
near these structures, scientists initially described them as
"cryptovolcanic," a term now rarely used. Recent studies have shown
that not all craters are of volcanic origin. Impact craters, formed by
collisions with the Earth of large meteorites, asteroids, or comets,
share with volcanoes the imprints of violent origin, as evidenced by
severe disruption, and even local melting, of rock. Fragments of
meteorites or chemically detectable traces of extraterrestrial
materials and indications of strong forces acting from above, rather
than from below, distinguish impact from volcanic features.
"Other possible explanations for these nonvolcanic craters include
subsurface salt-dome intrusion (and subsequent dissolution and
collapse caused by subsurface limestone dissolution and/or
ground-water withdrawal; and collapse related to melting of glacial
ice. An impressive example of an impact structure is Meteor Crater,
Ariz., which is visited by thousands of tourists each year. This
impact crater, 4,000 feet in diameter and 600 feet deep, was formed in
the geologic past (probably 30,00050,000 years before present) by a
meteorite striking the Earth at a speed of many thousands of miles per
hour.
"In addition to Meteor Crater, very fresh, morphologically distinct,
impact craters are found at three sites near Odessa, Tex., as well as
10 or 12 other locations in the world. Of the more deeply eroded, less
obvious, postulated impact structures, there are about ten
well-established sites in the United States and perhaps 80 or 90
elsewhere in the world."
http://pubs.usgs.gov/gip/volc/structures.html
In a paper published in the Aug. 1 (2002) Nature, Simon Stewart of
British Petroleum (BP) and Philip Allen of Production Geoscience Ltd.
describe a 20-kilometer-diameter structure in the North Sea that bears
striking similarity to impact craters on Jupiter’s icy moons Callisto
and Europa. Yet, while it resembles craters found elsewhere in the
solar system, the newly discovered “Silverpit” crater is only
one-tenth to one-hundredth the diameter. Equally intriguing to
geologists, Silverpit’s concentric ring structure is unlike any other
impact crater found on Earth.
Geotimes
http://www.agiweb.org/geotimes/oct02/NN_crater.html
Here is the abstract of that paper:
"Most craters found on Earth are highly eroded, poorly preserved and
only exposed on land. Here we describe a multi-ringed impact structure
discovered in the North Sea from the analysis of three-dimensional
seismic reflection data. The structure is 20 km in diameter, and has
at least ten distinctive concentric rings located between 2 and 10 km
from the crater centre. The structure affects Cretaceous chalk and
Jurassic shales, and is well preserved below several hundred metres of
post-impact Tertiary strata, which constrains its age to be 60–65 Myr
old. The formation of concentric ringed impact structures at this
relatively small scale had not previously been thought possible,
especially on the terrestrial planets. We have mapped the ring
structures at a resolution of tens of metres both laterally and in
depth, and show that the rings are fault-bounded graben structures,
similar to fault arrays formed in low-strain-rate detachment tectonic
settings. Strata deeper than 500 m palaeodepth appear unfaulted, and
we infer that the concentric ring structures may have accommodated
post-impact extension towards the excavated crater, through detachment
on weak layers within the chalk."
A 20-km-diameter multi-ringed impact structure in the North Sea
Nature 418, 520 - 523 (01 Aug 2002)
http://www.nature.com/nature/
Sometimes it can be difficult to tell whether a structure was the
result of volcanic or meteoritic action.
THE POTENTIAL OF POTENTIAL FIELDS FOR DETECTING BURIED IMPACT
STRUCTURES: EARTH AND MARS
http://solid_earth.ou.edu/notes/solar_system/Ames3.htm
Saarijärvi
http://www.netppl.fi/~jarmom/geo/imp/saari_e.htm
Lappajärvi
http://www.netppl.fi/~jarmom/geo/imp/lappa_e.htm
IMPACT STRUCTURE IN JAPAN
http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_43973.htm
The controversy: The Spanish impact structures and the competing
models of an endogenetic origin
http://www.impact-structures.com/controversy.htm
Azuara impact structure
http://www.impact-structures.com/peculiarities.htm
The Gatun Structure
http://arc.cs.odu.edu:8080/dp9/getrecord/oai_dc/oai:USF:etd-10122001-142859
The discovery of the Chicxulub Impact Structure, and the determination
of both its very large size and geologic age clearly ended the
argument over whether there had been an impact at the end of the
Cretaceous. The legacy of the volcanists is, however, a very strong
one, with intense debate now centered on the contribution of the
massive volcanic eruptions to not just the K/T extinction, but to many
major extinction events, including the greatest biologic crisis in the
history of the earth - the Permo-Triassic extinction event.
Volcanism
http://faculty.plattsburgh.edu/thomas.wolosz/volcanism.htm
Search terms used:
volcanic "impact structure"
I hope this gives you the material you seek. If you need anything
more specific or if you find any of this unclear, please request
clarification. I would appreciate it if you would hold off on rating
my answer until I have a chance to reply.
Sincerely,
Google Answers Researcher
Richard-ga |
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