How do fossils form?

How it works

Most plants and animals rot away or get eaten after they die. Only a lucky few are buried quickly by mud or sand at the bottom of a river, lake or sea, which keeps the air out. Hard parts such as bones, teeth and shells have the best chance of being preserved.

More layers of mud and sand pile on top, pressing the lower layers until they harden into rock. Groundwater carrying dissolved minerals seeps through. The minerals fill tiny holes in bone or wood, or slowly replace it, until the remains have become stone in the very same shape.

Fossils aren't only bones. Footprints, leaf prints, burrows and even dinosaur eggs can be fossilised too. Much later, the land may be slowly lifted up, and wind and rain wear away the rock above, so the fossils are exposed and someone finds them.

Where to see it in New Zealand

At Curio Bay in the Catlins, in the far south of the South Island, you can see a petrified forest at low tide. Back when dinosaurs lived, a forest was buried by volcanic mudflows, and its tree stumps slowly turned to stone. Visit with an adult, watch the tide, and leave everything where it is.

Try it together

Press a shell or a tough leaf firmly into playdough, then lift it out carefully. The print left behind is like a mould fossil. Fill the print with a different colour of playdough, press it in, then lift it out. Now you have a cast fossil too.