How come almost all fossils are found in sedimentary rocks, and not in igneous or metamorphic rocks?

Question

Points to the best answer, thank you.

Answer 1

Look there are lots of good thoughts offered in previous answers, but they are off the point. Fossils are evidence of previous life, as impressions, bones, shells, casts, molds, etc. The term 'fossils' applies to evidence of very recent and very old life.

And these things can be found in all kinds of rocks. It's just that more are found in sedimentary rocks because the extreme heat and pressure that creates igneous rocks and metamorphic rocks tends to destroy the fossil evidence.

Take for instance the human footprints in cooled lava (that is an igneous rock) on the west side of the island of Hawai'i. Those are fossil footprints. Think about the casts of people found in Pompeii, those are fossils, too. And, it is not unusual to find calcified calyx of marine bottom dwellers in metamorphosed marble. Ergo, fossils can be found in all types of rocks, Q.E.D.

Answer 2

Bones, although hard, tend to melt at temperatures approching 500 degrees farenheit, organic plants at a temperature even less. The reason we find fossils are that the hard parts of an animal or plant are preserved in a matrix that causes decomposition to stop (by not having any oxygen or by not having any microorganisms around to feed on the organic matter.

Igneous rocks are formed under very high temperatures which would melt any organic tissues.

Fossils can be found in some metamorphic rocks. Metamoprosed rocks are sedimentary or igneous in origin (and sometimes even other metamorphic rocks) that have gone through a facies change due to the application of heat and pressure. A sedimentary rock containing fossils that goes through low grade metamorphism can still display evidence of the fossils. A prime example of this is slates and marbles where sea shells can still be seen when the stones are cut and polished. Depending on the temperatures that are expereinced by the rock bodies, the fossils will retain their shape if the parent material is not melted. In extreme pressures, fossils may no longer be recognizable to the untrained eye if stretched and folded.

Answer 3

Think about how the rocks are formed. Sedimentary are usually created in or around water. Great place to find living organisms. Also, the sedimentary rocks begin as sand, pebbles, grains, solute or whatever. Usually, these materials will cover and preserve the animal that will one day be a fossil.

Igneous rocks are formed from molten rock and then cool down. You don't normally find life around these when the rocks are being created. Many times, the rock was formed deep underground. If it is formed above ground, then lifeforms try to avoid molten lava. Even if the animal were to fall into the lava, it would be destroyed and not preserved.

Metamorphic is a different beast. It involves rocks changing form. So for a fossil to be found in metamorphic rock, it would have to be a fossil in either sedimentary or igneous rock and then with time, heat, and pressure becomes a different rock. Hopefully, the fossil was not destroyed in the process.

I have seen fossils in metamorphic rock. I have seen shells in marble. They actually look pretty cool. I have never seen fossils in ingneous rock, however, I have heard of them before.

Answer 4

because igneous rocks are formed with the solidification of magma meaning it is solid lava, if there were fossils it wouldn't be igneous. Metamorphic rocks are caused by intense heat and pressure so if there was a fossil at the start, there won't be after the metamorphism. Sedimentary rocks are rocks made out of other rocks, like fossils.

Answer 5

Igneous rocks form when molten rock solidifies, that would destroy any organic matter.

Metamorphic rocks are formed by the extreme heating and pressure of sedimentary rocks, enough to move crystals in the rocks and realign atomic structure. This process destroys fossils.

Fossils are therefore only found in sedimentary rocks where organic matter is trapped in the sediment as it is deposited and leaves an imprint.

Answer 6

From Wikipedia: "Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants."

Answer 7

It's the method of deposition. A dino may dy by a river and layers of mud (Sediment) build up over the bones. Over time this sediment morphs into rock which retains the strata of the sediment