Meteorites and Impact Craters

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Most meteorites come from the Asteroid Belt, a few are thought to come

from the Moon and perhaps Mars. Meteorites from the Moon are identified

because they have a composition similar to the rock samples brought back

from the Moon.

Iron (Metallic): composed of an alloy and iron and nickel. 6% of all know meteorites are this type. Though to represent the core of small planet-like objects which formed early in the history of the Solar System

Stony (Siliceous): composed of silicate minerals including olivine, pyroxene and plagioclase feldspar. 93% of known meteorites are this type. Thought to represent the mantle of a small planet-like object which formed early in the history of the Solar System.

Carbonaceous chondrites: a type of stony meteorite which contains water and organic compounds. The composition of these meteorites is thought to represent the material from which the Solar System was formed.

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

Meteorites

Impact Craters