Making an Impact

Impact craters are formed when objects fall from height (from space or even from volcanic eruptions) and crash into the surface of a planet or moon. We call these falling objects impactors. This blog post focuses on impact craters caused by meteorites.

Impact craters
There are many objects travelling through our solar system that, at times, will make it to the surface of planets and moons as meteorites. When this occurs, the surface is changed and an impact crater is often created. There is evidence of this occurring on Earth, our Moon and many other planetary bodies in our solar system. The Earth has been bombarded by meteorites for billions of years and it still occurs today.
Wolfe Creek Crater, WA (Image: Wikipedia Commons)
Earth’s atmosphere partially protects us from meteorite impacts, as many will burn up due to the friction caused as they speed through the atmosphere. The features commonly known as ‘shooting stars’ are not in fact stars, but meteorites moving through the atmosphere. The Moon is covered in impact craters as its thin atmosphere offers little protection for the surface.
Unlike on Earth, where the surface is often changed over time, craters on the Moon are well preserved as there is minimal weathering and erosion occurring. Sometimes it is difficult to see ancient craters on Earth due to these surface changes or the fact that some craters may be under the ocean. Advances in remote sensing technology, such as gravity surveying, have allowed scientists to discover and study craters not usually visible on the surface.
You can find out more about meteorites on Earth in this AusEarthEd blog post: Meteorites and the history of Earth.

Anatomy of craters
Impact craters can vary greatly in diameter and depth according to the size of the meteor and the speed with which it hits. Scientists categorise craters as either simple or complex and describe the following common characteristics:
  • Floor – bottom of the crater and usually lower than surrounding surface
  • Wall – usually steep sides inside of the crater
  • Central uplifts – a peak sometimes found in large craters caused by the rebound effect of the surface from the large impact
  • Rim – edge of the crater consisting of rock that is pushed up by the impact
  • Ejecta – material immediately surrounding the crater that is thrown out of the crater area by the impact
  • Rays – streaks of ejecta material radiating out from the crater for some distance

You can learn how to create and measure your own impact craters from this PALMS demonstration video. The teacher notes to accompany this activity, Making Craters, can be found here and the student worksheet can be found here.

Famous craters
Impact craters can vary greatly in size according to the size and speed of the meteorite forming them. A large crater can be found near the town of Chicxulub in Mexico, with approximately half of the crater found on the sea floor. Scientists believe this crater was formed by a meteorite impact around 65 million years ago. It is thought this impact was part of events that led to a mass extinction of around 75% of all species on Earth at that time, including non-avian dinosaurs.
Until 2019, the largest and oldest impact crater on Earth was thought to be the Vredefort Dome in South Africa. The Vredefort Dome is currently still the largest crater found on Earth however scientists confirmed the discovery of an older crater last year, the Yarrabubba crater in Western Australia. Dating of zircon minerals in the structure yielded ages of 2.229 billion years old (± 5 million years). This makes the Yarrabubba crater 200 million years+ older than Vredefort.
Saturn’s moon Mimas has a large impact crater which often leads to comparisons with the Death Star from Star Wars. What do you think?
Would you like to try and work out how some of these surface features, including impact craters were formed? Where do you think they might be? Take a look at the photos from the PALMS How Did That Get There? Activity (downloads). The accompanying student worksheet can be found here and the Teachers Guide here.

More information
Some further information and resources on impact craters can be found at the following web pages: