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astroblemes(ass -trŏ-bleemz) Circular craterlike scars, usually ancient, left in the Earth's crust by meteorite impact. Criteria used for recognition are shatter cones, high-pressure silica polymorphs, shock microdeformations of quartz, impactite glasses, traces of nickel, and nickel-iron associations. All are of a geologically youthful age, none older than about 500 million years. The largest are Vredefort, South Africa (40 km diameter), Nordlinger Ries, Germany (25 km), Deep Bay, Saskatchewan, Canada (14 km), Lake Bosumtwi, Ghana (10 km), and Serpent Mound, Ohio (6.4 km).
The velocity of a large meteorite, i.e. one over thousands of tonnes, is dissipated very little by atmospheric braking effects and it hits the ground at its cosmic velocity of 11 to 74 km s–1 and explodes. Most of the kinetic energy goes to accelerate and excavate the material from the volume that eventually becomes the crater. Usually only an exceedingly small fraction (<0.01%) of the projectile survives. The resulting crater has a diameter between 20 and 60 times that of the impacting body, this factor depending on the incident velocity.