Below It All -- Samples of Impact Rocks
This shows you samples of impact rocks from several meteorite impact structures around the world.
Shatter Cones
 Shatter cones are distinctive cone or fan-shaped features in
rocks, with radiating fracture lines that resemble a horsetail. They are found
in two places:
· In Nuclear test sites
· Meteorite impact structures.
They are formed as a result of the
high pressure, high velocity shock wave produced by a large impacting object or
a large explosion. They range in size from less than 1 centimeter to more than
5 meters across and indicate that the original rock was shattered -- like a
car's windscreen being hit by a stone.
 The exact mechanism of formation of shatter cones is still
not well understood. Besides large asteroid or comet impacts, only nuclear
tests generate enough heat and pressure to even roughly mimic the process.
The first man-made shatter cone was produced in 1959 during
an underground nuclear explosion. As such, as long as you know that you're not
in a nuclear test site, if you see a shatter cone in a rock, it is definitive
evidence for a meteorite impact event. Shatter cones are actually the only
shock indicators that can be seen with the naked eye. Others, such as so-called
planar deformation features (PDFs) can only be seen with the aid of a
microscope.
Shatter cones can be found within the central uplifts of
large impact structures and occasionally within the crater-fill deposits.
Impact Melt Breccia
A breccia (from a Latin word meaning "broken") is a rock
that is composed of angular fragments of other rocks surrounded by a
fine-grained "matrix" that may be of a similar or a different
material. Think of concrete when you think of matrix.
Breccias can be formed by a number of geologic processes
(tectonic, volcanic, sedimentary) and from a variety of materials.
This is a specimen of impact rock
from Ries Germany. This area near Nordlingen Germany was subjected to a
massive impact by a meteorite roughly 15 Million years ago. It created a
crater almost 15 miles across. The massive force the local rock was
subjected to (a force 50 times greater than tectonic forces) melted the Earth
rock and subjected it to metamorphism, called shock-metamorphosis.
Some of the airborne molten material
landed in the Moldau River valley Czechoslovakia and is known as Moldavite.
Suevite, a name first used in reference to rock at the Ries
Germany site, is shocked Breccia that contains glass, sometimes consisting of
50% or more of the whole.
This is a specimen of Suevite from Polsingen (though Polsingen
material is more accurately called Lithic Melt Breccia due to low, or
absent, glass content). Polsingen Suevite (or Lithic Breccia) has a
reddish color to it from Hematite.
In its most general definition, "impact melt
breccia", means the matrix cementing the fragments is crystallized impact
melt. It is the primary evidence for a cataclysmic impact event, where the heat
generated from the impact shatters and melts the target rock.
 Suevite
Suevite is
an impact breccia composed of angular fragments of different rock types as well
as glass inclusions, set in a fine-grained matrix. This type of rock was first
recognized at the Ries impact structure.
 In the suevite sample and in the photo above, you can see lots
of glass fragments (black glassy areas, generally smoother than typical rock
fragments) and crystalline fragments (white, speckled).
These glasses are derived from rocks that were heated to
such high temperatures during the impact event that they melted. They then
cooled very rapidly (quenched) to form glass -- if a melt cools slowly, it can
form an impact melt rock (see above).
Suevites at the Ries structure form the crater-fill deposits
and are also found in the ejecta blanket.
What science has studied is the type of transport or
movement of the Suevites at the Ries Crater. What's particularly frightening is
that researchers have shown a suevite like pyroclastic flow deposit.
For those of you who haven't seen a pyroclasitc flow, go
here. It's a short video of the Soufriere Hills
volcano in the Carribean.
http://www.youtube.com/watch?v=8sPyyHY57H4&NR=1
Imagine not only rocks being ejected hundreds of miles away
but pyroclasitc like flows coming from the impact.
Impact Breccia
As described above, a breccia is a rock that is composed of
angular fragments of other rocks surrounded by a fine-grained
"matrix" that may be of a similar or a different material.
Breccias are extremely common in meteorite impact craters
and attest to the destructive power of the impact event. 
Impact melt breccias and suevites both contain melt derived
from the melting of target rocks; however, not all breccias contain melt.
Impact breccias can be found in many different settings
within impact structures, such as within the central uplift, in crater-fill
deposits, and in the ejecta blanket.
Fallback Breccia
This breccia formed when a large meteorite crashed into the Earth's surface at
Gosse Bluff, Australia.
The impact melted, shattered and altered rocks for many miles, throwing tons of
rock into the air -- likely forming an atomic bomb like mushroom cloud -- and of
course creating a crater.
This mushroom cloud material settled and solidified into
breccia, such as the sample above.
Tektites
Tektites are small, glassy pebble-like objects that form
during meteorite impact. And we've see these previously. They represent
droplets of molten target rock that are ejected up into the Earth's atmosphere,
which then fall back to the surface up to several hundred kilometers from where
their source impact crater.
For example, The Ries crater impact event is believed to be
the source of moldavite tektites found in Bohemia and Moravia (Czech Republic). The tektite melt originated from a sand-rich
surface layer and was ejected to distances up to 450 km downrange
of the crater. They often acquire aerodynamic shapes as they fly through the
atmosphere.
Their name comes from the Greek word 'tektos', meaning
'molten'. The first written reference to tektites was about one thousand and
fifty years ago, by Liu Sun in China, who gave them a name meaning 'Inkstone of
the Thundergod'.
Tektites often occur in so-called strewn fields (see map above),
areas over which tektites with similar chemical and physical properties are
found.
The four main strewn fields known are
- Central European (linked to the Ries crater in Germany),
- Ivory Coast (linked to the Bosumtwi crater in Ghana, West
Africa),
- North American (linked to the Chesapeake crater, North
America),
- Australasian (source crater still unknown, although a large
crater in Western Cambodia, Lake Tonle Sap, has been proposed).
Tektites do not contain any water. They can be mistaken for
obsidian or pitchstone (black volcanic glasses), but these will emit some water
on strong heating. Their density is similar to, or a little lighter than,
quartz beach sand.
Other Impact Glasses
What's frequently overlooked in the general discussion of
impacts is the production of different types of glasses other than tektites.
Here's a few unique samples of impact glass:
 The impact
velocity of the meteorite that produced the Noerdlinger Ries Crater is thought
to have been about 20 km/s (45,000 mph), with the resulting explosion the
power of 1.8 million Hiroshima
bombs.
Here's another example of the power of
that impact. Stone buildings in Nördlingen -- quaried from the impact structure
-- contain millions of tiny diamonds, all less than 0.2 millimeter across. The impact created an
estimated 72,000 tons of them when it impacted a local graphite deposit.
From the land down under comes the following sample:
The Henbury Meteorite Impact Glass is generally found as frothy rock melt,
created by the explosive impact of the nickel/ iron Meteorite at Henbury
Meteorite Craters.
These impact
glasses give us an insight into the enormous forces occurring at a glimpse of a
second at impact. It is in fact the compressed air in front of the meteorite
which creates this unimaginable heat.
|
