Why do fossils form more easily in sediment than in igneous rock?

When plants or animals are quickly buried by mud, sand, or volcanic ash, oxygen and scavengers are kept away and the remains are more likely to survive long enough to mineralize. Sediments can harden into sedimentary rock while preserving the shape or imprint of the organism. By contrast, igneous rocks crystallize from magma or lava, and those temperatures are usually too high for fossil preservation. Metamorphic rocks also often distort or erase fossils because heat and pressure alter the original material. That is why fossils are most common in sedimentary layers such as shale, limestone, and sandstone.