why wouldn't igneous and metamorphic rocks contain fossils?

Answer 1

Simply put: igneous are formed deep in the earth or through volcanoes, so fossils would not exist because the rock was initially molten. Even if the igneous rock is formed by recycled surface rock and sediment, the fact it melts and than cools and hardens eliminates the possibility of fossils.

Metamorphic rocks are similarly formed by some degree of heat and pressure. They can form by the metamorphosis of igneous or sedimentary rocks. Typically they are so deformed by the process that fossils would have been destroyed if they were initially present (assuming sedimentary rocks are being subjected to metamorphic processes). However, if metamorphism of a fossiliferous sedimentary rock is not too severe some relict fossils may remain. It is likely they would be deformed somewhat, but it is possible and does occur regularly.

Thus, you can be relatively sure there won't be fossils in any igneous rock. You cannot make that assumption about metamorphic rocks, as they do occasionally contain fossils that were contained in the original sedimentary rocks.

Answer 2

Igneous rocks- formed by melting and re cooling- the process would roast and absorb and identifiable fossil material.

Think of stirring jolly ranchers into boiling syrup... no jolly rancher left.


Metamorphic- formed over time and under pressure through chemical transformation- wouldn't maintain the original structure of the fossil source.

Think of miking baking soda in vinegar- a new salt is formed through chemical reaction- no baking soda left.

Answer 3

They aren't formed with conditions which would retain or preserve fossils. Fossils can't survive high temperature molten rock or high temperature/ high pressures which dissolve and precipate different forms of rock.