Why are seashells and fossils of marine animals found in sedimentary rocks?
Fossils are only found in sedimentary rocks because these are rocks which form at the earth’s surface, by deposition of sand and silt or calcium carbonate in a sea. The organisms live within the sediment (bottom feeders) or in the water column above, and when they die, they sink into the sediment. If they are buried quickly enough to prevent decomposition, they might form a fossil. You can’t get fossils in igneous rocks because they are formed from a melt and melting would destroy any fossils in the rock. This is also mostly true for metamorphic rocks because the recrystallisation of the rock usually destroys any fossils or fossil traces.
