Evidence for Impact Craters
There is clear evidence that asteroids have collided with many plannets and moons in the Solar System. The impact forms distinctive craters which you can see on the surface of the Moon. The craters all have a depression in a circular shape and a rim of broken rock and they cover most of the surface.

The Earth's surface does not show such extensive cratering though there are some, including the 50,000 year old Meteor Crater in Arizona. There is no reason to assume that the Earth is less prone to impacts than the Monn so there must be another reason. The key difference is the amount of tectonic activity on the Earth. The Earth's crust is subjected to weathering, erosion and long-term recycling by plate tectonics, all of which destroys craters.

The evidence for impact can be seen from:

quartz grains will be violently shocked (shocked quartz) and even melted (tektites)
rock strata will be tilted
material at depth is brecciated
the ejected material falls back to the surface but the sequence of rocks is inverted because the material closer to the surface is ejected first and falls back to the surface earlier
the presence of iridium from outer space
fragments of the meteorite

	Created by siobhan.quirk over 11 years ago

Undergraduate Geology - Part 1 (Earth Structure) Note on Meteorites and Impact Craters, created by siobhan.quirk on 11/05/2013.