Rocks........ sedimentry to metamorphic? (Homework problem)?

Question

If a sedimentry rock has a fossil in it and is exposed to high heat and pressure, haveing it become a metamorphic rock, would the fossil still be there? Why would it be there or why would it not be there?

Answer 1

Sedimentary rocks that have been strongly metamorphosed in general do not usually contain fossils. The main reasons why fossils would be destroyed are as folows:

1) Deformation. High grade metamorphic rocks (particulary metamorphosed sediments) commonly get strongly deformed during metamorphism, obliterating all primary structures (and any fossils).
2) Recrystallisation. Metamorphism involves recrystallisation of the original minerals in the rock, and in the case of high-grade metamorphism, the mineral grains oftem become substantially larger. Again this tends to destroy small-scale primary structures/fossils
3) Melting. During high-grade meamorphism, many sedimentary rocks will partially melt.

Fossils are however common in low grade metamorphic rocks - the highest grade rocks I've personally seen fossils in are of greenschist facies (quite a low metamorphic grade). This is because at low grade the degree of deformation and recrystallisation is often rather limited, so the fossils often survive.

At higher metamorhpic grades you would need special circumstances for any evidence of fossils to be preserved...

1) Little or no deformation of the host rock.
2) The fossls would have to be quite large (much larger than the size of the crystals forming the metamorphic rock) for any trace to survive recrystallisation.
3) a lack of partial melting of the host rock.

I have seen sedimentary structures preserved in rocks of amphibolite facies (a medium grade of metamorphosm), where the rocks have escaped strong deformation. Under similar circumstances, I would imaging trace fossis such as burrows etc could also survive, as long as they were quite large.

Answer 2

The first answer is correct, despite a few spelling errors. Fossils do not usually survive metamorphism, those that do are found in very mildly metamorphosed rocks and they are very distorted. The reason that most of them do not survive is due to the high temperatures and pressures at which metamorphism takes place. I have seen many metamorphic rocks and have yet to see any fossils.

Answer 3

it is unlikely that the fossil will remain in the rock unless the rock was minimaly metamorphosed. The pressure would crush and deform the fossil ad the heat would probably cause the minerals that make up the fossil to dissolve and reform into other minerals.

Answer 4

I think all the earlier answers are correct, but maybe would add that microscopic fossils may survive the metamorphic process relatively intact.

Answer 5

nope,the heat and pressure destroys the fossil-that's why there are no fossils in metamophic rock.

Answer 6

Sedimentary - Sandstone, limestone. those are extremely certainly eroded. Igneous - Pumice (a weaker rock) and granite (a better rock through it is smaller pores and extra compact debris)