Craters: when big rocks hit

Most meteorites land quite gently, but every so often a much larger space rock hits the ground hard and blasts out a huge hole called an impact crater.

How a crater forms

Small meteoroids are slowed down by the air. But a very large space rock, many metres or more across, is so big and heavy that the air cannot stop it. It slams into the ground at enormous speed.

In a fraction of a second, its huge energy of movement turns into heat and a powerful shock wave. The shock wave pushes rock outwards and upwards, digging a bowl-shaped hole much wider than the space rock itself. Most of the space rock is smashed, melted or even turned into vapour. Rock thrown out of the hole lands around it in a raised rim.

That is why the biggest craters usually have very little meteorite left in them. Smaller impacts, though, can leave many pieces scattered nearby.

Famous craters on Earth

One of the best-known impact craters is Meteor Crater, also called Barringer Crater, in Arizona, USA. It is about 1.2 kilometres wide and roughly 170 metres deep. It was made about 50,000 years ago by an iron meteoroid perhaps tens of metres across. Many pieces of iron, known as Canyon Diablo meteorites, have been found around it.

Much older craters are much bigger. The Chicxulub crater, buried under Mexico's Yucatán Peninsula, is about 180 kilometres across. It formed about 66 million years ago, at the time the dinosaurs, apart from birds, died out. Many scientists think the impact played a major part in that great extinction.

Scientists have confirmed around 200 impact craters on Earth. Some are easy to visit, while others are buried, worn down or covered by water and can only be studied with drills and special instruments.

Why the Moon has more

Look at the Moon through binoculars and you will see it is covered in craters. Earth has been hit just as often, in fact more often, because it is bigger. So why do we see so few craters here?

The answer is that Earth is always changing. Wind and rain wear rocks down. Rivers carry away soil and fill holes with mud. Plants and forests cover the ground. Oceans hide much of the surface. Slowly, over millions of years, the rocky plates that make up Earth's surface move and recycle old rock deep underground.

The Moon has no air, no rain and no moving plates, so its craters stay for billions of years. That makes it a record book of impacts in our part of the Solar System.

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