Reading the rocks: minerals, rocks and fossils

Pick up any stone and you are holding a piece of history. It might have been melted deep inside the Earth, laid down on the sea floor, or squeezed under a mountain. Scientists who read that history are called geologists.

Minerals: the building blocks

A mineral is a natural solid whose tiny particles are arranged in a neat, repeating pattern. That pattern gives each mineral its own shape and properties. Quartz forms six-sided crystals and is hard enough to scratch glass. Mica splits into thin, shiny sheets. Rocks are mixtures of minerals. Granite, for example, is mostly quartz, feldspar and mica.

Three rock families

How a fossil forms

When a sea creature dies, its body can be buried quickly in mud or sand. The soft parts rot, but hard shells and bones can stay. Over thousands to millions of years, more layers pile on top, water carries minerals through the sediment, and the shell and the mud around it slowly turn to stone. That is why fossils are almost always found in sedimentary rock.

The Whanganui cliffs

The cliffs between Castlecliff and Kai Iwi are one of the few places in the world where you can see such a thick pile of young sea-floor layers on land. Many layers are packed with fossil shells. Because lots of these shellfish still live today, scientists know whether each layer formed in deep or shallow, warm or cool water. The layers record about 50 swings between ice ages, when the sea dropped, and warm times, when it rose again.