Different Directions

Different Directions

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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