Rocks, Minerals and Fossils

Pick up a stone. It feels like the most permanent thing imaginable, but it is a traveller. It has probably been molten, crushed, dissolved, drifted across continents and buried kilometres deep more than once, and given a few hundred million years it will be again. Geology is the art of reading that journey, and the alphabet is minerals, rocks and the fossils they preserve.

Minerals: the ingredients

A mineral is a naturally occurring solid with a fixed chemical recipe and a regular arrangement of atoms, a crystal. Quartz is silicon and oxygen. Halite, table salt, is sodium and chlorine. Diamond is pure carbon with its atoms locked in a rigid three-dimensional lattice; graphite, the soft grey stuff in pencils, is also pure carbon, with its atoms in slippery sheets. Same ingredient, different arrangement, utterly different mineral.

About 6,000 minerals have been named, and new ones are found every year, but the Earth's crust is dominated by about a dozen. Most of them are silicates, combinations of silicon and oxygen with metals such as aluminium, iron, magnesium, calcium, sodium and potassium. Feldspar and quartz alone make up more than half the crust. The glittering gem minerals are rare precisely because they need unusual chemistry or extreme conditions to form.

Rocks: three ways to make them

A rock is a mixture of minerals, and every rock on Earth was made in one of three ways.

Igneous rocks began as molten magma. If the magma erupted and cooled quickly at the surface, the crystals had no time to grow and the rock is fine-grained, like basalt, the dark rock of the ocean floor and of Hawaii. If it cooled slowly underground over thousands of years, the crystals grew large, as in granite, with its speckles of pink feldspar, grey quartz and black mica.

Sedimentary rocks are made of pieces of older rocks, or the remains of living things, that settled in layers and were pressed and cemented together. Sandstone is ancient beach or desert sand. Shale is compacted mud. Limestone is largely the shells and skeletons of sea creatures; the white cliffs of Dover are chalk, made of trillions of microscopic plankton shells that drifted to a sea floor 90 million years ago. Sedimentary rocks form in layers, oldest at the bottom, which makes them the pages of Earth's history book.

Metamorphic rocks are rocks that were changed by heat and pressure without melting. Bury limestone deep enough and it recrystallises into marble. Squeeze shale and it becomes slate, then schist, then gneiss as the pressure and temperature mount. The streaky, folded rocks of ancient mountain roots are metamorphic.

These are not separate families but stages. Mountains erode into sediment; sediment is buried and becomes rock; rock is dragged into the mantle and melts; magma erupts and cools. This endless loop is the rock cycle, and it has been turning for four billion years.

Fossils: life turned to stone

Occasionally something living is buried before it rots. The soft parts decay, but bones, shells and teeth can be mineralised, their pores filled with dissolved stone until the original is replaced by rock in the same shape. Sometimes only an impression survives, a footprint or the outline of a leaf. Very rarely, an animal is preserved whole in amber, ice or tar.

Fossilisation is extraordinarily unlikely. It needs rapid burial, usually in water, and then millions of years of luck. The vast majority of species that ever lived left no trace at all. Yet enough survived to tell the story of life: the first shells 540 million years ago, the first fish, the first creatures to crawl onto land, the first forests, the reptiles, the dinosaurs, the mammals and, in the last two million years, ourselves. Because sedimentary layers stack oldest to youngest, and because certain fossils appear in only certain layers, fossils allowed geologists to put Earth's history in order long before anyone could measure its age in years.

Telling time in rocks

That measurement came in the twentieth century from radioactivity. Some elements decay at perfectly steady rates, and by comparing the amount of a radioactive parent with the amount of its decay product in a crystal, geologists can read how long ago the crystal formed. Zircon crystals from Western Australia give 4.4 billion years, the oldest known pieces of the Earth. Meteorites, leftovers of the solar system's birth, give 4.57 billion years, which is how we know the age of the planet itself.

Why it matters

Rocks are the only record of the deep past. They tell us when oxygen first filled the air, when ice ages came and went, where continents stood and how life responded. They hold the ores, fuels and building materials of every civilisation. And a pebble on a beach, once you can read it, is a far more interesting thing than a pebble.