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Asked by anon-190697 to Brendan on 8 Nov 2018.
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Brendan Marrinan answered on 8 Nov 2018:
Crikey that’s a big question. There are 3 types of rocks: Rocks that come from volcanoes, rock that get buried and crushed and turn into different rocks, and rocks that form over a very long time by sedimentation (a build up of layers). Rocks from volcanoes generally don’t have fossils. The rocks that get buried and crushed don’t usually have fossils because these get destroyed by the crushing underground. The rocks built up over a long period of time (Sedimentary Rocks) are usually full of fossils! A fossil is formed by a creature (a snail for example) that died in mud and got buried before something could eat it, once it’s buried more and more sand, silt and clay is put on top (which takes a very long time), over time the parts of the snail get replaced by minerals then after millions of years the sand, silt and clay form a rock with the fossil inside it. I hope that helps. This is a bit different with Chalk and Limestone which are completely made up of tiny little creatures that died in the sea millions of years ago.
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Comments
johnmidgley commented on :
Hello,
Fossils are either the actual preserved remains of a formerly living creature (these are real fossils), or occasionally, the imprint or evidence of a living creature (called trace fossils).
Real fossils form by the process explained by Brendan, trace fossils are formed when the actual fossil erodes, leaving behind the mold. This mold fills with sediment, or water holding minerals. The sediment goes hard leaving a copy of the creature, or the water evaporates and crystals form leaving a mineralised copy of the creature. Trace fossils can also be the imprints of footprints, or the burrows in which these creatures (like worms) lived.
Brendan commented on :
A more concise answer than mine. I haven’t studied fossils for years. Find a few nice ones now and again though.