| Since 26 Nov 1996 |
Homepages of Jarmo Moilanen |
| Since 26 Nov 1996 |
Homepages of Jarmo Moilanen |
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14 Dec 2002 | |
ImpactitesDifferent Types of ImpactitesHere are examples of several basic types of impactites. An impactite is a rock formed by meteorite, asteroid, or comet impact on Earth. Impactites are usually composed mostly of material from the impact site, often called target rock. This material often suffers shock metamorphosis, which happens when a high-pressure shock wave goes through target rock. The pressure may be as high as over 400 GPa at the impact point. This is enormous pressure. By comparison, volcanic and tectonic events on Earth generally produced pressures below 1 GPa. An impactite may also contain some anomalies, extraterestrial materials, such as iridium (Ir).
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| Projectile | |
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Projectiles are not impactites. A projectile is
a celestial body that impacts Earth and excavates a crater. There are two major
groups of projectiles: asteroids and comets. Asteroids are composed of rock, and
comets are more or less ice. Meteorite variations provide evidence of different
kinds of
asteroids. Sample: a 90 gr Canyon Diablo iron. Canyon Diablo irons are portions of the iron body which impacted in Arizona, USA some 49,000 years ago and excavating the famous 1.186 km wide Barringer crater (also known as Arizona crater or the Meteor Crater). |
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| Impact glass | |
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Impact glass is formed when impact melt rapidly cools down. These and
other similar impact products are often called impactites. Sample: impact melt glass from Aouelloul, Mauritania. |
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| Impact melt | |
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Impact melt is a clast-free melt that cooled down slowly. The slow cooling produce
a crystalline structure. Impact melts can resemble certain igneous rocks. Sample: dark impact melt from Sääksjärvi structure, Finland is almost clast free. |
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| Melt matrix breccia | |
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Most impact melts are melt matrix breccias. That means the rock matrix is impact-produced melt
with a lots of clasts. Sample: melt matrix breccia from Charlevoix, Canada |
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| Suevite | |
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Suevite is formed in material ejected into air by an impact. It usually has some
melt droplets or veins within lithic matrix. There are primarily two kind of
suevites: fall-back suevite and fall-out suevite. Fall-back suevite fell back
into the crater, and fall-out fell outside the crater rim. Sample: fall-back suevite from Ries, Germany. The dark veins are glass. |
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| Lithic breccia | |
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Lithic breccia has practically no melt. Sample: monomict Gardnos breccia from Gardnos, Norway |
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| Dyke breccia | |
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Dyke breccia is usually lithic breccia found as a dykes in the bedrock below the
crater. Sample: lithic polymict dyke breccia from Söderfjärden, Finland |
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| Pseudotachylite | |
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Pseudotachylite is term used frequently and often inapproproately. It is friction melt usually found as
dykes in
bedrock below the crater. Sample: pseudotachylite from Rochechouart, France. |
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| Impactite (or instant rock) | |
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When I usually talk about impactites, I mean all
rocks formed from impacts. However, some
material from small and fresh craters is often called impactite. This material
is combination of
melted target rocks and projectiles, or is made of pure glass (see impact glass,
above). Some of this material contains quite a bit of the projectile. Because
the melt has cooled down rapidly, it usually has a glassy matrix. Sample: impactite from Monteraqui, Chile. |
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| Spherules | |
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Spherules are odd, small teardrop-shaped or spherical material associated with
fresh craters. They are usually formed from melt droplets or by condensation of
the vaporized projectile or target rocks. Sample: spherules from Barringer crater (the Meteor Crater), USA |
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| Melt bombs | |
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Melt bombs are melt material blow out from the crater through the air. They usually
cool
down rapidly and are glassy. Melt bombs are usually trapped in
suevites. These are very similar to impact glasses and the terms are sometimes
used interchangeably. Samples: black melt bombs from Ries, Germany. |
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| Ejecta | |
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Ejecta is material ejected outside the crater that contains both shocked and
unshocked material. Sample: piece of ejecta from Cow Creek formation associated with the Manson impact structure in the USA. |
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| Ejected rocks | |
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When impact occurs, many rocks are ejected from the impact
site. Some of these materials may be thrown into ballistic trajectories, and some may even
briefly escape the upper atmosphere. Sample: a carbonate rock from Chicxulub impact. This rock found in Belize probably went in ballistic trajectory (the rounded shape may be due to in atmospheric ablation). |
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| Tektites | |
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Tektites are almost pure silicate glass. They form during impact and they
can be strewn over great distances. Sample: a tektite from Thailand. No one has yet discovered the impact crater that produced these tektites. It may no longer exist, because of erosion. |
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| Distal ejecta | |
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Distal ejecta contain shocked material and microtektites. It is usually
found as a microtektite layer in geological strata. While several layers are known,
not all of the
impact structures have has not yet been identified. Some may no longer exist,
because of erosion. Sample: K/T boundary clay from Gubbio, Italy. This material formed when a huge asteroid hit the Earth 65 million years ago, resulting in mass extinction of dinosaurs and many other life forms. The impact occurred in Chicxulub, Mexico. The Chicxulub crater is about 180-km in diameter. Distal ejecta from it has been found all over the World. |
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| Resurge breccia | |
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When an impact occurs in the ocean, the impact forces water away
from the impact site. It may rush back to the site as a huge resurge waves. Sample: Lockne resurge breccia from Lockne, Sweden. |
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| Shatter cones | |
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Shatter cones are formed when a intense shock wave travels through target rocks. Shatter cones
points toward the center of the impact. Shatter cones are some
of the best macroscopic evidence for an impact. Sample: nice shatter cones in sandstone from Kara, Russia. |
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| Text edited by Richard Dreiser. | |
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