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
Image from NASA Impact Crater activity
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?
Mimas (Image: NASA/JPL-Caltech/Space Science Institute)
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:
- This WA Department of Mines, Industry Regulation and Safety web page gives further information on craters in Western Australia and you can even do a virtual tour of impact craters in WA using Google Earth.
- This article from The Conversation discusses some of the many other craters found in Australia.
- Learn more about how Indigenous people describe the formation of impact craters in Australia.
- The Earth Impact Database from the Planetary and Space Science Centre at the University of New Brunswick, Canada is a wealth of information on impact craters all over the world.
- This NASA STEM activity shows you how to make moon craters and includes a video and background information.
- This Scientific American article discusses why impact craters are round.
- Further explanation of the parts of an impact crater can be found here.
- The ABC Catalyst episode – Asteroid Hunters discusses current research on impact craters and shows you how to find your own micro-meteorites.
- Watch this video to see what happens when a meteorite strikes Earth.
- You can look at impact craters on Earth as well as the Moon and Mars using Google Earth.