Impact structure
An impact structure is the remains of an impact crater, often eroded or buried so that the original bowl is no longer visible.
An impact structure is the geological mark left by an asteroid or comet impact. Fresh, young impacts leave obvious craters, but over millions of years erosion wears them down, sediments fill them, and earth movements tilt or bury them. What remains may be a ring of hills, a circular lake, a buried basin found by drilling, or simply an area of shattered and melted rock.
About 200 confirmed impact structures are known on Earth. To be confirmed, a site must show evidence such as shatter cones, shocked quartz with planar deformation features, high-pressure minerals, impact melt rocks or meteorite fragments. A circular shape on its own is not enough, because volcanoes and other processes can also make round features.
The oldest well-dated large structures, such as Vredefort in South Africa, about 2 billion years old, are deeply eroded, and their original crater rim has long gone. Scientists reconstruct their original sizes from the pattern of shocked rocks.
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