astroblemes


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Related to astroblemes: Impact craters

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.

References in periodicals archive ?
The Gravberg-1 superdeep borehole drilled in the Siljan astrobleme in southern Sweden reached a depth of 6.
Key Words: geomagnetism, astroblemes, tectites, polarity inversion.
Les astroblemes se caracterisent par une quantite de materiel lithologique transforme brutalement par les hautes temperatures degagees lors de l'impact.
Two subsurface features in central and southwestern Southern Michigan have been interpreted by some researchers to be astroblemes dating to the Ordovician Period.
Previously unknown among world primary diamond deposits are the astroblemes discovered in Russia in the 1970s (Masaitis et al, 1980).