How Fossils Form Diagram – Label the Steps, Mold and Cast

All 9 steps on the diagram

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From death to burial 4

The animal dies A fish dies and sinks to the sea floor.
Soft parts decay Skin and muscle rot away. Bones, teeth and shells last longer.
Sediment buries the remains Mud, sand or silt covers the remains. Burial slows decay.
More layers build up More sediment settles on top, layer after layer.

From rock to the surface 3

Sediment turns to rock The weight above squeezes the layers into sedimentary rock.
Minerals replace the bone Water in the rock leaves minerals in the bones. They turn to stone.
Erosion uncovers the fossil Rock is worn away until the fossil shows at the surface.

Mold and cast 2

Mold A hollow in rock, left when a shell or bone dissolved.
Cast A copy of the shell, made when a mold filled up. Minerals or sediment can fill it.

How this list was compiled: Every step and name checked on 2026-10-03 in two independent sources. (1) Natural History Museum, London, How are dinosaur fossils formed?, https://www.nhm.ac.uk/discover/how-are-fossils-formed.html : the five key steps (the animal dies; soft parts rot away; burial by mud, sand or silt; more layers build up and turn into sedimentary rock; water leaves minerals in the bones), mold and cast, erosion bringing fossils to the surface, nearly all fossils being of animals that lived in water. (2) Australian Museum, How do fossils form?, https://australian.museum/learn/australia-over-time/fossils/how-do-fossils-form/ : death, soft tissues decomposing, burial, more sediment, the layers hardening into rock, bones replaced by minerals from groundwater, erosion revealing the fossil; a fossil as remains or traces of past life. (3) British Geological Survey, Fossils, https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/fossils/ : rapid burial, minerals filling and replacing a buried shell, bone or tooth, molds and casts. (4) US Geological Survey, Fossils, Rocks, and Time: Rocks and Layers, https://pubs.usgs.gov/gip/fossils/rocks-layers.html : particles burying animals on the lake or sea bottom, the bottom of the pile becoming rock, skeletons becoming fossils. (5) National Park Service, Glossary of Geologic Terms, https://www.nps.gov/subjects/geology/gri-glossary-of-geologic-terms.htm : fossil, mold, lithification, erosion. Licence: own work

The drawing: Drawn for this site on 2026-10-03 from plain geometry. Own original drawing; nothing is traced or copied. Licence: own work

How it works

This how fossils form diagram follows one fish in seven panels, from the sea floor to a fossil that shows in a hillside. An eighth panel shows the mold and the cast a shell can leave.

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Buried fast

Most dead animals rot away and leave nothing. A fossil can form when sediment such as mud or sand buries the remains soon after death.

Turned to stone, then found

More layers settle on top and their weight presses the sediment into rock. Water moving through the rock leaves minerals in the bones. The fossil stays inside the rock until erosion wears the rock away and it shows at the surface.

The small arrows give the order: left to right, then down.
Bones, teeth and shells are the parts that most often become fossils.

Questions and answers

How do I label a diagram showing how fossils form?

Read the seven panels in order, then press Quiz me and tap the panel each step names. For paper, print the diagram blank and write the steps in, or print it with the names as a key.

What are the steps of how a fossil forms?

An animal dies. Its soft parts decay. Sediment buries what is left. More layers build up and turn to rock. Water leaves minerals in the bones and they turn to stone. Much later, erosion wears the rock away and uncovers the fossil.

What is the difference between a mold and a cast?

A mold is the hollow left in rock when a buried shell or bone dissolves. A cast forms when minerals or sediment fill that hollow and harden into a copy of the shell or bone.

Why do most living things never become fossils?

Most remains rot away or are eaten before anything buries them. Fossils form mainly where sediment covers remains quickly, which is why nearly all fossils found are of animals that lived in water.