How impact craters are formed

This sounds a bit like a no-brainer really; the asteroid or comet crosses the path of Earth and hits the Earth’s surface causing an impact crater. That’s it isn’t it? No, nothing in life is quite as simple as that and the way impactors (asteroids, comets, space stations or astronauts) hit the Earth is quite complex and in some areas common sense tells us the wrong thing.

The first thing necessary to produce an impact crater is for the impactor to enter the atmosphere of the planet or satellite. The impact that will produce a crater will be greater than the speed of sound and occurs at hypervelocity.

There are two basic type of craters that can be formed depending on the impactors speed.

If the object impacting the Earth is less than 10 to 20 m then most of the velocity on passing through the atmosphere is lost. Most of the surface layers of these objects are lost because of the melting and vaporisation that occurs. On hitting the Earth a very small crater of only about 10 mm or below is formed.

If the object is roughly 10 m or more then most of the velocity that the poor old impactor had will be maintained. This makes a large difference in the type of impact crater and the damage that the impactor can cause. These pressures on hitting the Earth are much more than any of the pressures produced during geological processes such as plate tectonics.

Stages of Crater Formation

When the impactor does actually hit the surface there are some specific stages that occur-

1. The Contract and Compression Stage

On thumping into the target the impactor squeezes and compresses the surface. The energy in the impactor spreads out and tries to accelerate everything around it to the speed that it was travelling at. As the Earth is just that little bit larger the Earth puts up resistance and the impactor slows down. This means that a lot of energy is transferred to the surface of the Earth.

Shock waves start at the point where the impactor touches the surface. The pressure experienced is the same as the pressure a rock experiences if buried at greater than 2000 km. The target and the impactor are vaporised iron melted when the extreme pressure is released.

Once this stage of impact occurs the impactor has no more effect on the formation of the impact crater. This stage will only last about one second except for some of the extreme larger impacts.

2. The Excavations Stage

You would think that would have been enough for the surface of the Earth are no, it has to suffer some more. From the contact and compression stage a hemispherical shockwave surrounds the impactor and travels into the target. Waves of pressure are reflected from the original ground surface and continue downwards weakening, fracturing and shattering the rock.

Material is moved down and a hole is produced much larger than the diameter of the impactor. The material that has been dug out is thrown over the surface that surrounds the crater.

3. Modification

This all depends on gravity and the strength of the target materials. Small craters will maintain their shape because gravity doesn’t have such a large effect on them but larger craters will mostly collapse under gravity.

As time goes by the crater will probably flatten out and then it is only defined by the difference in material appearance. An interesting fact is that the impact causes the material beneath the impactor to rise up causing a higher bit in the middle of the crater, a bit like when they drop of water hits a smooth surface of water.

Crater with Rise in the Middle

After all that pressure release and shockwaves there is one seemingly odd thing about impact craters, except for the very small ones, at whatever angle they hit the body they are round. You would expect shape of the crater to be ellipse if the impactor hit at an angle other than 90° to the surface. This is because of the extreme speeds that the impactors hit the surface. (This does not apply to impactors that hit at an angle less than 10° to the surface but there are extremely few of these in the solar system anyway).

Circular Craters on the Moon

This is obviously the effects on a hard surface and not on the oceans. On the oceans Tsunami is formed and can do a lot more damage than an impact on land.

Impact craters can be very useful when trying to estimate the age of a planet or satellite. By counting the number of impacts the age of the body can be estimated. A large amount of craters an older body and the less the impact craters the younger the body. This isn’t totally true as volcanic activity can resurface a planet making it look as good as new.

Impactors used to hit the Earth all the time ages ago on the creation of the solar system but the comets, asteroids and meteorites are not so much of a problem anymore, so you can stop looking up at the sky waiting to dodge the next one.

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