Craters: when big rocks hit
Small meteorites land with a thud, but large ones hit so hard that they blast out a crater. In this lesson you will find out how craters form and make some yourself.
- Explain how an impact crater forms.
- Describe a famous crater on Earth.
- Carry out a crater experiment and work out a mean from your results.
Links to: Year 5 science, forces: gravity; working scientifically: planning a fair test, repeating measurements and recording data; maths: finding the mean.
Why big rocks make craters
Air resistance slows a small rock down. A big one, many metres across, is too heavy to be slowed much, so it strikes the ground at many kilometres per second. It carries so much energy that the ground behaves almost like a liquid for a moment. Rock is thrown outwards, a rim is pushed up around the edge and a round hollow is left behind. This is an impact crater.
Craters are almost always round, even when the rock arrives at a slant, because the crater is made by the blast spreading out in every direction.
Meteor Crater
One of the best-preserved craters on Earth is Meteor Crater in Arizona, USA. It is about 1.2 kilometres wide and about 170 metres deep, and it formed about 50,000 years ago when an iron meteorite roughly 30 to 50 metres across struck the desert. Thousands of iron pieces, known as Canyon Diablo meteorites, have been found around it.
The Moon is covered in craters because it has no wind, rain or plants to wear them away. Earth has been hit just as often, but most of its old craters have been eroded or buried.
Finding the mean crater width
In a crater experiment, a marble was dropped three times from 1 metre. The craters measured 7 cm, 9 cm and 8 cm wide. What is the mean width?
- Add the three results: 7 + 9 + 8 = 24 cm.
- Divide by the number of results: 24 ÷ 3 = 8 cm.
- Repeating the drop and taking the mean makes the result more reliable than a single drop.
Answer: The mean crater width is 8 cm.
Make craters. Fill a deep tray with about 5 cm of flour, smooth it and sprinkle a thin layer of cocoa powder on top. Drop a marble from 50 cm, 1 m and 1.5 m, three times each, and measure each crater's width with a ruler. Record your results in a table and find the mean for each height. Does a higher drop make a bigger crater? Look for the rim and the rays of white flour thrown over the cocoa, just like real craters. Wear old clothes, and have an adult help with the dropping height.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1Why does a big space rock make a crater when a small one does not?
Air resistance slows small rocks to the speed of a car, but a large rock keeps most of its speed and energy.
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Question 2Why are craters nearly always round?
The crater is shaped by the explosion of energy spreading outwards, so even a slanting impact usually makes a round crater.
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Question 3About how wide is Meteor Crater in Arizona?
Meteor Crater is about 1.2 km across and about 170 m deep.
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Question 4Why does the Moon have many more craters we can see than Earth?
Earth's weather, rivers and moving rocks slowly erase craters. On the airless Moon they last for billions of years.
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Question 5Three craters measure 6 cm, 8 cm and 10 cm. What is the mean width?
6 + 8 + 10 = 24, and 24 ÷ 3 = 8 cm.