Acraman
A crater found because its ejecta turned up 300 km away.
Key facts
- When
- about 580 million years ago
- Where
- Gawler Ranges, South Australia, Australia
- What hit
- an asteroid perhaps 4 to 5 km across
- Era
- Deep time
- Location
- 32.00° S, 135.45° E
Probably a few million megatons of TNT.
A few per cent of the Chicxulub impact, but still about a thousand times all the nuclear weapons in the world today.
What happened
Acraman is one of the few large impacts whose ejecta layer was discovered before anyone was sure of the crater. In the Flinders Ranges of South Australia, geologists found a thin layer of shattered volcanic rock within mudstones of the Bunyeroo Formation. The fragments were of a kind found only in the Gawler Ranges, about 300 km to the west, and some contained shocked quartz.
That pointed to an impact there, and the eroded remains of a large crater were identified around Lake Acraman, a salt lake that marks its central area. The original crater is thought to have been about 85 to 90 km wide.
The impact took place in the Ediacaran period, shortly before the first large, complex animals appear in the fossil record.
Effects
Debris was thrown across what is now southern Australia. Some researchers have noted changes in microscopic fossil plankton above the ejecta layer, but any link to the impact is still uncertain.
The evidence
The far-flung ejecta layer with shocked minerals, shatter cones and breccias near Lake Acraman, and the matching rock type of the debris.
Did you know?
The ejecta layer is only a few centimetres thick in places, yet it can be followed across hundreds of kilometres.
Acraman is one of the largest known impacts in Australia.
Related pages
How is impact energy measured? See energy explained, or try the impact calculator.