Abstract
The Earth is the most endogenically active of the terrestrial planets and thus, has retained the poorest sample of impacts that have occurred throughout geologic time. The current known sample consists of approximately 160 impact structures or crater fields and 20 impact events also registered as depositional events in the stratigraphie record, some of which are related to known structures. The sample is biased towards young ( 20 km diameter) impact structures on the geologically better know cratonic areas. The known terrestrial impact record is sufficient to estimate a terrestrial cratering rate for the last few hundred million years. The present resolution of the record, however, does not unequivocally support suggestions of periodic impacts. Approximately 30% of known impact structures are buried and were initially detected as geophysical anomalies and subsequently drilled to provide geologic samples. The recognition of terrestrial impact structures may, or may not, come from the discovery of an anomalous quasi-circular topographic, geologic or geophysical feature. In the geologically active terrestrial environment, anomalous quasi-circular features, however, do not automatically equate with an impact origin. Specific samples must be acquired and the occurrence of shock metamorphism or, in the case of small craters, meteoritic fragments, must be demonstrated before an impact origin can be confirmed. Terrestrial impact structures result in unusual local geologic conditions, which can lead to the concentration of natural resources, such as minerals and hydrocarbons; in some cases, the economic deposits are world-class. Impact represents a long-term hazard to human civilization and is responsible for at least, one mass extinction of the biosphere, at the Cretaceous-Tertiary boundary 65 Ma.
Published Version
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