CRATER REFERENCE

CRATER REFERENCE

by: Charles O’Dale

  • VELOCITY OF IMPACT (earth) – On earth the lowest impact velocity with an object from space is the gravitational escape velocity of 11 km / sec. The highest velocity impact is more than 70 km/s:
    earth escape velocity [11 km/s] + sun escape at the earth’s orbit [30 km/s] + earth’s orbit velocity [30 km/s] = ~70 + km/sec.
    The average earth impact velocity is 17 km/s.
    At these speeds, impacts produce shock waves in solid materials. Both meteorite and target rocks are compressed to high density, then rapidly depressurized and exploding violently. Since craters are caused by explosion they are always circular with only very low angle impacts causing elliptical craters.
  • IMPACT CRATER/STRUCTURE IDENTIFICATION – Geologic confirmation for an impact event:

– GEOMORPHOLOGY – One of the first indicators of a possible impact site is “circular geology”.
– SHATTER CONES/SLICKENSIDES – Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events.
– BRECCIA – Breccias are extremely common in the central uplift, in crater-fill deposits, and in the ejecta blanket of meteorite impact craters.
– FRACTURED ROCK – While travelling toward impact sites I documented fractured rocks increasing in magnitude as we neared the crater site.
– GRAVITY ANOMALIES – Gravity contours illustrate anomalies caused by fractured country rock under an impact site.
– MAGNETIC ANOMALIES – Magnetic studies  document the magnetic disturbances within impact structures.
– SHOCK METAMORPHOSIM – The extreme pressures and temperatures at hypervelocity impacts have caused shock metamorphic effects on target rocks.
– CRATER EJECTA – When the crater formation process ends, the resulting circular structure and the surrounding area is covered by an ejecta blanket.
TEKTITE – Tektites are small, glassy pebble-like objects that form during meteorite impact. They represent droplets of molten target rock that are ejected up into the Earth’s atmosphere, which then fall back to the surface up to several hundred kilometers from where their source impact crater. They often acquire aerodynamic shapes as they fly through the atmosphere
– CRATER RIM – Documentation of surviving crater rims on this planet.
CRATER STRUCTURE – Why are impact craters almost always round? (or polygonal)?
  • CRATER IMPACT AGE vs  EXTINCTION EPOCH
  • CRATER FORMATION – Impact involves the transfer of massive amounts of energy to a relatively small area of the Earth’s surface, in an extremely short period of time (Kinetic energy). Three stages of crater formation:
    – contact & compression;
    – excavation; and
    – modification.
  • CRATER CLASSIFICATION – On this planet, impact craters are divided into three basic morphologic subdivisions:
    – simple craters;
    – complex craters; and
    – peak ring craters.
  • METEORITE
    – introduction
    – meteorite categories;
    – dating meteorites;
    – meteorite glossary; and
    – reference.
  • CANADIAN CRATERS – Documented in 1967 & 1975.
  • 2019 UPDATES
  • UPDATE ARCHIVES

Reference:

Osinski G. The Geological Record of Meteorite Impacts Canadian Space Agency 2018

ANN M. THERRIAULT – RICHARD A. F. GRIEVE – MARK PILKINGTON The recognition of terrestrial impact structures Natural Resources Canada