Acraman crater facts for kids
Acraman lake (circle), ringed by Lake Gairdner and others, outlining the Acraman crater depression
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| Impact crater/structure | |
|---|---|
| Confidence | Confirmed |
| Diameter | up to 90 km (56 mi) |
| Age | ~580 Ma Late Ediacaran |
| Exposed | Yes |
| Drilled | No |
| Bolide type | Chondrite |
| Location | |
| Location | Gawler Ranges |
| Coordinates | 32°1′S 135°27′E / 32.017°S 135.450°E |
| Country | Australia |
| State | South Australia |
 Location in South Australia
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The Acraman crater is a very old and deeply worn-down impact crater in the Gawler Ranges of South Australia. An impact crater is a bowl-shaped hole made when a large space rock, like a meteorite, crashes into a planet or moon. The spot where the Acraman crater is located today is marked by Lake Acraman. This lake is a circular, temporary playa lake, which means it's a shallow lake that often dries up. It is about 20 kilometres (12 mi) across.
Scientists first reported finding the crater and its scattered rock pieces (called ejecta) in 1986. They published their findings in the journal Science. The proof that this was an impact crater includes special broken rocks called shatter cones. They also found shocked quartz in the shattered bedrock on islands within Lake Acraman. Shocked quartz is a type of quartz that has been changed by the extreme pressure of an impact.
Contents
How Big Was the Acraman Crater?
The Acraman crater has been worn away a lot over millions of years. This means its original size is hard to figure out. Some scientists think it was once very large, possibly up to 85 to 90 kilometres (53 to 56 mi) wide. Other scientists believe it was smaller, maybe only 35 to 40 kilometres (22 to 25 mi) across. This smaller size is closer to the size of the depression where Lake Acraman sits today. If the crater was as large as 90 km, the impact would have released an enormous amount of energy.
When Did the Impact Happen?
The impact event that created the Acraman crater happened about 580 million years ago. This was during a time in Earth's history called the Ediacaran period. Scientists didn't figure out this age from the crater itself. Instead, they looked at where the scattered rock pieces (ejecta) were found in nearby rock layers. These layers are called sedimentary basins.
The Lake Acraman Impact Structure is considered an important historical site. It is officially listed on the South Australian Heritage Register.
What is the Ejecta Layer?
A wide layer of rock pieces, or ejecta, is believed to have come from the Acraman crater. This layer is found in Ediacaran rocks in the Flinders Ranges. The Flinders Ranges are at least 300 kilometres (190 mi) east of the crater. Ejecta has also been found in drill holes from the Officer Basin to the north.
When the impact happened, these areas were covered by a shallow sea. The ejecta settled down into the mud on the sea floor. These rock pieces contain shocked minerals and small shatter cones. They are made of rock that is similar in age and type to the rocks at the crater site. Scientists also found an iridium anomaly in the ejecta. This means there was more iridium than usual, which suggests the material came from space.
An increase in different types of tiny ocean living things (called acritarchs) happened just after this ejecta layer was formed. Some scientists think there might be a connection between the impact and this increase in life forms. The crater is also close to where the first complex life forms, known as the Ediacara Biota, were discovered. However, the impact was likely so big that its effects would have been felt all over the world.
Who Was John Acraman?
The Acraman crater, Lake Acraman, and the nearby Acraman Creek are all named after a person. They are named after John Acraman, who was a well-known businessman in South Australia during the colonial period.
Gallery
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Landsat image of Lake Acraman; screen capture from NASA World Wind
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Oblique Landsat image of Lake Acraman draped over digital elevation model (10x vertical exaggeration), looking east towards the Flinders Ranges where ejecta has been found; screen capture from NASA World Wind.
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Orange and brown soils mix with off-white saltpans, including Lake Gardiner and Lake Everard in this true-colour image.
See also
In Spanish: Cráter Acraman para niños
