Impact Crater
As a child,
thinking about the size, the speed – the sheer force of the meteorite that made
Meteor Crater – was just unbelievable. Period.
And I would
watch my friends doing a cannonball into the swimming pool, and try to compare,
to visualize a big rock crunching into the earth, sand and stone rising into
the air.... I even threw rocks into sand, and I guess that showed me best what
a high velocity impact might look like... but I knew it wasn't really close....
to the massive violence, the fire, the terror of it.
So, let's
look at this idea a bit more closely, because it teaches us important things.
In the
broadest sense, the term impact crater can be applied to any depression,
natural or man made, resulting from the high velocity impact of a projectile
with the Earth – or any larger body for that matter, like the moon, or other
planets, or asteroids....
Impact
craters typically have raised rims, and they range from small, simple,
bowl-shaped depressions to large, complex, multi-ringed impact basins.
Here's an
illustration that can help.
Meteor
Crater, in Arizona, is perhaps the best-known example of a small impact crater
on the Earth.
Next,
here's a picture of a simple crater on Mars:
Courtesy NASA: Mars Global Surveyor
You can see
that these craters fit with the first illustration.
You can
find Impact craters on many solid Solar System objects including the Moon,
Mercury, Callisto, Ganymede and most small moons and asteroids. However, on
other planets and moons that experience surface geological processes, such as
Earth, Venus, Mars, Europa, Io and Titan, visible impact craters are less
common because they become eroded, buried, or transformed by tectonics over
time.
Where such
processes have destroyed most of the original crater topography, the terms
impact structure or astrobleme are more commonly used.
 The Steps Of An
Impact
Here's a
great description from the Planetary Science Institute (PSI) about the steps
involved with an impact.
The diagram
to the right shows the stages of crater formation.
When an
impactor plows into a target, it brings a lot of energy with it.
That energy
drives the creation of the impact crater.
For
simplicity, let's split the formation of the crater into 3 stages:
1. Contact and
compression
2. Excavation
3. Modification
Stage
One, the energy
forces the target rocks down and compresses them.
Stage
Two, a transient
crater starts forming:
· Material is
melted,
· Even vaporized,
· Thrown from the
rapidly expanding crater.
For
relatively small impact events (craters < 2-4 kilometers across on Earth)
the transient crater is relatively stable, and we end up with a simple crater,
such as Meteor Crater (http://en.wikipedia.org/wiki/Meteor_Crater).
Stage
Three, for larger
impact events, the transient crater is unstable -- too deep and wide.
Rocks at
the bottom of these craters resist being compressed and deformed, and
eventually 'snap back' during this modification stage.
This is the
process that pushes up the central peak in complex craters.
Finally,
the ejecta falls to the ground, and the rim and center of the crater slump and
settle into their final shapes.
All of
this happens within a few minutes –
For larger craters the melted rocks can take a long time to cool and harden,
and the rim and peaks may fall and slump some more.
Then,
ultimately, Earth's processes – wind, water, hot and cold – changes the crater.
The Chesapeake Bay
Crater
Here's the
perfect example, with a side view illustration of the largest impact crater in
the United States. The Chesapeake Bay Impact Crater – And we didn't even know
it existed until 1987, and then only in 1992 did we understand the extent of
the event.
Two
illustrations tell it all.
First,
let's look down on the bay from a map view:
Quite a
large crater.
Let's look
at the side view:
There are
several special features to check out:
1. The Central peak
of the impact
2. The Peak ring
Both
of these are features of a complex impact crater.
3. The Outer Rim
edges
4. Breccia filling in
both the Inner basin and the rest of the crater.
5. The beds of
sedimentary rock over the crater, along with the Chesapeake Bay and Atlantic
Ocean on top.
It took 35
million years following the impact to bury the crater with those additional
sedimentary beds of rock.
Side Note The Chesapeake Bay crater is distinguished further by a cluster of at least 23
adjacent secondary craters.
And
researchers think that the North American tektite strewn field, a widespread
deposit of distal ejecta, is thought to be derived from the Chesapeake Bay
impact. See more below.
Here's another photo from Mars that shows a complex
crater:
Courtesy NASA: Mars Global Surveyor
Note the exceedingly tall central peak, the rim structures,
and the wild, almost flowery impact ejecta.
Click here to look at the crater making process.
A Few More Words
Let's go
back and look at the basic structure of a crater again. Here's the simple
crater.
In the
crater itself, you can see the symbol for Breccia, Impact Melts, and the
Fractured bedrock below. Around the sides you see the Impact Ejecta.
All of
these materials near, or beneath, an impact site are physically altered by the tremendous heat, pressure, and shock waves created by large meteorite impacts,
and as a result they are known as impactites.
· Sand can melt into
impact glasses, such as Libyan Desert Glass
· Rocks may be
shattered and compressed into breccias,
Melt glasses (black) and
fragments of sediments (bright) are embedded in the fine grained, gray matrix,
the cement of the breccia.
· Shatter cones occur in bedrock beneath the point of
impact and have been deformed by shock waves.
· Tektites, such as the Moldavite below, are
glassy objects found in strewn fields and are associated with large, ancient
impacts.
Let's look
at these a little more closely.
Libyan
Desert Glass
Sometimes
called the great sand sea glass, Libyan Desert Glass is a natural silica glass and is found strewn over large areas of the Libyan Desert.
The origin
of the glass is a controversial issue for the scientific community, with many
feeling that the glass was the result of a meteorite impact; however, no crater
has ever been found.
Some
geologists associate the glass not with impact melt ejecta, but with the explosion of an asteroid above the earth hot enough to
melt surface material without leaving an impact crater. This would be similar
to an air burst event like the 1908 Tunguska explosion over Siberia.
Regardless
of its origins, Libyan Desert Glass is indeed beautiful.
Here's
an article about the glass being used in the middle of one of Tutankhamen's necklaces... and about it's origins:
http://news.bbc.co.uk/2/hi/science/nature/5196362.stm
Impact
Breccia
A Breccia (pronounced Bre she a) is a
rock made up of angular fragments of minerals or other pieces of rocks that are
mixed and held together by a cement type rock called a matrix.
Highly shocked
breccia from the Azuara (Spain) crater.
Impact
breccias are used as a diagnostic tool to determine an impact event, such as an
asteroid or comet striking the Earth. And as a result, they are usually found
at impact craters.
Breccia of
this type may be present on or beneath the floor of the crater, in the rim, or
in the ejecta expelled beyond the crater. Impact breccia may be identified by
its occurrence in or around a known impact crater
However,
Impact breccias are not only formed during the process of impact cratering –
when large meteorites or comets impact with the Earth – they are also formed
during impact cratering with other rocky planets or asteroids... which in turn
can eventually reach the Earth.
And as a
result, you can find impact breccia meteorites.
Shatter
Cone
This is a fascinating rock, and one used to determine Impact Craters.
A Shatter
Cone has a conical shaped pattern of regular thin grooves (striae) that radiate
from the top (apex) of the cone.
Shatter
cones range in size from less than one centimeter to more than one meter across
and are formed as a result of the high pressure, high velocity shock wave
produced by an impacting meteorite.
Years of
hunting for and mapping of natural shatter cones and their orientations have
revealed that the apex of the cones, from all sides of an impact site, point to
the exact center of impact.
Consequently,
in the early 1960s shatter cones and impact minerals were both considered
adequate criteria for suggesting an impact site.
Tektites
Tektites
are some of the most beautiful and fascinating impactites. They are pieces of
natural glass that form during a meteorite impact. Most tektites are high in
silica (68-82%) and very low in water content (average 0.005%).
Their name
comes from the Greek word "tektos," meaning molten because they are
droplets of molten rock that are ejected up into the Earth's atmosphere and
then fall back to the surface several hundred kilometers away from the impact.
As a result, they frequently acquire aerodynamic shapes flying through the
atmosphere.
What's
really neat about tektites is that they are found in "strewn
fields." Look at the illustration below.
The four
main strewn fields in the world are the
· 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) and
· Australasian
(source crater still unknown, although a large crater in Western Cambodia, Lake
Tonle Sap, has been proposed).
North
American Strewn field
The
southeastern portion of the United States has two regions where tektites are
found. The tektites from each location are quite distinct from each other.
Texas is the area where the Bediasites are found.
These
are dark in color and often round with deep grooving. Georgia as the name
states is the location of the Georgia Tektites. These are very translucent and green
in color.
Owned by The
Meteorite Exchange
One
Georgia Tektite was found on Martha's Vineyard; however, the lack of others may
indicate that it was taken there by someone in the past.
The
Moldavite Strewn field
The Moldavite strewn field is divided into two
parts and the tektites from each of these parts are distinctive in color from
each other.
Owned by The
Meteorite Exchange
These
areas are quite small by comparison to some of the other strewn fields. But,
none the less great amounts of Moldavites have been found. The most prized are
the deeply grooved and clear green pieces. The green Moldavites have been and
continue to be used for stones in jewelry.
Ivory
Coast Strewn field
The
western coast of Africa is the location of the Ivory Coast tektites, and its source is the Bosumtwi
Crater.
Photograph from
the NASA Space Shuttle.
Owned by The
Meteorite Exchange
The
strewn field extends out into the Atlantic ocean for some distance based on
microtektites recovered from cores of the sediments. These tektites are
extremely rare because of the difficulty in recovering them from the forested
areas. They are black in color and often have an egg shape or nearly spherical
shape.
Australasian
Strewn field
By
far the largest; the Australasian tektite area encompasses most of Southeast
Asia, including Vietnam, Thailand, Southern China, Laos and Cambodia. It
stretches across the ocean to include the islands of the Philippines,
Indonesia, Malaya and Java. It reaches far out into the Indian Ocean and south
to the western side of Australia. Approximately one tenth of the Earth's
surface is accounted part of the strewn field.
Australites
are generally very dark in color, for the most part essentially black. Thin
edges or broken parts will have a yellow or brown color when examined with back
lighting. They have a wide range of forms. Teardrops, dumbbells, spheres, rods,
discs and all types of irregular shapes. In Australia are found aerodynamic
button shaped tektites and their cores that remain when they fly apart in the
passage through the atmosphere.
A
Bit of Geologic History
In the
early Solar System, rates of impact cratering were much higher than today.
The large
multi-ringed impact basins, with diameters of hundreds of kilometers or more,
retained for example on Mercury and the Moon, record a period of intense early bombardment in the inner Solar System that ended
about 3.8 billion years ago.
Since that
time, the rate of crater production on Earth has been considerably lowered, but
it is appreciable nonetheless; Earth experiences from one to three impacts
large enough to produce a 20 km diameter crater about once every million
years on average. This indicates that there should be far more relatively young
craters on the planet than have been discovered so far.
Although
the Earth’s active surface processes quickly destroy the impact record, about 170
terrestrial impact craters have been identified. These range in diameter from a
few tens of meters up to about 300 km, and they range in age from recent
times (e.g. the Sikhote-Alin craters in Russia whose creation was
witnessed in 1947) to more than two billion years, though most are less than
200 million years old because geological processes tend to obliterate older
craters. They are also selectively found in the stable interior regions of
continents. Few under sea craters have been discovered because of the
difficulty of surveying the sea floor, the rapid rate of change of the ocean
bottom, and the subduction of the ocean floor into the Earth's interior by
processes of plate tectonics.
Impacting at speeds in excess of 10
mi/sec (16 km/sec), a meteorite creates pressures on the order of millions of
atmospheres, producing shock waves that blast out a circular hole and often destroy
the meteorite.
The
Effect of Friction
The air might
not seem all that thick to you and me, and it's even less so at high altitudes,
but coming from the nothing of outer space, it's a big change.
Let's imagine
this:
First, you
are driving a car through space.
There is
nothing outside. There is no atmosphere. If you could stick your hand out the
window, you would feel nothing, even though you were traveling at thousands of
miles an hour. The Reason – no molecules of anything rushing by except for the
occasional meteorite.
Now imagine
driving a car down the freeway at 65 miles an hour.
You stick
your hand out the window, and it is immediately yanked backward. The Reason –
it is encountering our atmosphere, which is made up of molecules of all kinds
of things.
The impact
of all these molecules is the same impact that a meteorite or spacecraft
encounters as it penetrates the Earth's atmosphere, and it is what is referred
to as friction.
The
difference is the speed at which this impact occurs.
When it
occurs at 65 miles an hour, the heat generated by this friction is barely
noticeable. When it happens at thousands of miles an hour, the heat is not only
noticeable, it is enough to melt and burn up whatever it is that is entering
the atmosphere. This is why meteorites burn up in the atmosphere, and it is
also why NASA lost the last space shuttle.
And it is
not just objects that enter the Earth's atmosphere either. Just flying through
it at high speed will cause the skin of an aircraft to heat up, and in some
cases burn. The SR71 gets so hot that its outer skin has to be able to expand
and contract, and aircraft such as the X-15 and others routinely came back from
missions with severe heat damage and holes actually melted in their structures.
What is basically happening is that when an object enters the atmosphere, all
that speed (kinetic energy) is being dissipated into the atmosphere as heat.
Educational
Links
Here's a
few links to check out. They will get you started in using the net for more of
your own research and learning.
Basic
Science Studies II: Impact Cratering
http://rst.gsfc.nasa.gov/Sect18/Sect18_1.html
Explorer's Guide to Impact Craters
http://www.psi.edu/explorecraters/front.htm
Impact
Rocks
http://www.psi.edu/explorecraters/impactrocks.htm
Georgia
Tektites
http://www.meteorite-times.com/Back_Links/2002/May/Tektite_of_Month.htm
Meteorite
impact
http://geopanorama.rncan.gc.ca/nsask/meteorite_e.php
Terrestial
Impact Craters, Second Edition
There's a
great slide show here. When you get to the index of photos, click the first
photo to begin the slide show, and then click Next located below the pictures.
http://www.lpi.usra.edu/publications/slidesets/craters/
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