Text lesson

Unit 4 — Dating Planet Earth

Science — Third Preparatory

How a fossil forms, step by step

An organism dies in a place where sediment is being deposited, such as a river delta, a lake bottom or the sea floor.
It is buried quickly by sand, clay or mud, which keeps out oxygen and scavengers, so decay is slow or stops.
Over a very long time, more layers pile on top. Pressure and the minerals in the water turn the sediment into sedimentary rock.
The hard parts may be dissolved (leaving a mold), filled with minerals (cast) or slowly replaced by minerals (petrification).
Later, movements of the crust and erosion bring the rock to the surface, where a scientist finds the fossil.

This sequence explains why most fossils are found in sedimentary rocks (rather than igneous rocks, where the heat of lava would destroy a body) and why fossils of hard parts are far more common than fossils of soft bodies.
It also shows how rare fossilisation is only a tiny proportion of the organisms that ever lived became fossils, so the fossil record has gaps.

Unit 4 — Dating Planet Earth · 19 / 28

How a fossil forms, step by step