A huge circular crater at least 1 km, and often much more, in diametre. They are found all over the earth's surface; the largest so far, 350 km in diametre, was recently discovered in Siberia.
The term astrobleme was coined in the 1960s by R. S. Dietz, and means, quite literally, star wound. Dietz argued that many of these huge craters are of extraterrestrial origin. Dietz's detractors, particularly Walter Bucher, argued instead that these craters are of endogenetic origin; Bucher called them "cryptovolcanic" or "cryptoexplosive" structures: he couldn't explain them, but no way was he going to admit they weren't terrestrial. In the decades since then this heated debate has subsided, and most accept Dietz's argument. Astroblemes are thought to result from the impact of a meteorite, or possibly an asteroid or even a comet, hitting the earth.
Astroblemes are distinguished from volcanically-inspired structures by several unique features. The most obvious are conical fracture patterns called shatter cones; their apices were originally oriented towards the meteorite, and the amount of tilt they subsequently experience indicate how the earth's plates have since shifted after the impact. Another feature of an astrobleme is the presence of materials such as coesite, a superdense form of quartz that is only produced under very high pressure and temperature. Meteorite fragments are commonly found inside the area covered by astroblemes. Finally, many astroblemes show rebound patterns where a central core of rock has been brought up to the surface by the force of the blow. The same central uplift patterns can be observed on lunar astroblemes as well.
Obviously such an impact would be catastrophic; in fact, many scientists now believe that a meteor crashing into the Yucatan peninsula was responsible for the global climate change which wiped out the dinosaurs. But while such events make for cheesy science fiction movies, I am happy to report that the astroblemes on the surface of the earth are very, very old: hundreds of millions of years old, in fact.
A.Word.A.Day brought this odd vocabularly gem to my attention, and www.grisda.org/origins/03085.htm provided me with details so technical my eyes glazed over.