Astrobleme
An old, heavily eroded scar left by an ancient impact, often no longer looking like a crater at all.
The word astrobleme means 'star wound'. It was coined in 1960 by the American geologist Robert Dietz for the deeply worn-down remains of ancient impact structures. Over tens or hundreds of millions of years, rain, rivers, ice and moving continents can strip away a crater's rim and fill its bowl, so the original shape disappears. What survives is often only the crater's roots: broken and melted rock deep below the old floor.
Astroblemes matter because Earth hides its impact history so well. The Moon keeps its craters for billions of years, but on Earth most have been erased. Finding astroblemes lets researchers estimate how often big impacts happened and link some of them to changes in life and climate.
Rochechouart in France is a classic example: around 200 million years old, it has no visible crater, yet its quarries and buildings are full of impact breccias. Vredefort in South Africa is another. Geologists recognise an astrobleme by shock evidence that only impacts make, such as shatter cones, shocked quartz with planar deformation features, and high-pressure minerals like coesite.
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