Comets

Comets are found beyond Neptune in the Kuiper Belt.

The nucleus is made up of ice and dust.

The tail is made of fragments of ice which have broken off the nucleus as it neared the sun in its orbit - as they have a highly elliptical orbit.

• CRATER = a bowl shaped depression with a RAISED RIM that is mostly made up of ejecta - the explosion causes the bedrock layers to tilt upwards.
• INVERTED STRATA = the rocks that were previously at the surface are ejected first and therefore settle first as the impact ejecta in the raised rim. The deeper rocks that were previously hidden below the surface are blasted out afterwards and therefore settle last on top of the previous ejecta layer and are now seen at the surface.
• IMPACT MELT = TEKTITES - these are glassy igneous rocks that formed when the rocks melted during the impact and cooled forming spheres.
• BRECCIA = the rocks are fractured into angular clasts due to the impact.
• SHOCKED QUARTZ = crystals of quartz contain lots of micro fractures which can only be caused by an impact.
• IRIDIUM = is a metal element which is rare at the Earth's surface but common in meteorites and therefore we find higher levels of iridium at an impact site and can confirm that this was the previous site of an meteorite impact.

We cannot date the Earth using Earth rocks as they were originally molten and therefore cannot give an as accurate date.

Meteorites contain radioactive elements which begin as UNSTABLE PARENT ISOTOPES.
These overtime decay to form there STABLE DAUGHTER ISOTOPES.
This happens at a known steady rate called the HALF LIFE.
1 half life means that half of the remaining unstable parent isotopes have decayed to form stable daughter isotopes.
By measuring the ratio of parent isotopes to daughter isotopes we can calculate how many half life's have occurred and therefore give the rocks an age.

E.g. 235 Uranium decays to 207 Lead (Pb) at a half life of 710 million years (ma).