A rock can be as small as a pebble or as big as a mountain. Every rock is made of minerals. There are three main kinds of rock: igneous, sedimentary and metamorphic. Which kind a rock is depends on how it was made. Each kind has its own story, and the three stories join together in one long loop. Let's follow each one, starting with fire.
Igneous rocks begin as melted rock. Melted rock that erupts onto the surface is called lava. When hot melted rock cools, it hardens into solid rock. The speed of cooling matters a great deal. Rock that cools slowly deep underground grows big crystals; lava that cools fast grows tiny ones. So the size of the crystals is a clue to where the rock cooled.
Granite is a rock with crystals big enough to see. Obsidian is a rock that is a kind of volcanic glass. Pumice is a bubbly, holey volcanic rock. These three are all igneous, yet they look very different from each other. Each one has its own look because it cooled in its own way. You can hold one in your hand and see the story of how it cooled.
Wind and rain slowly break big rocks into smaller pieces. Sedimentary rocks are made from bits of older rocks and from living things. They often form in layers. Sandstone is a rock made of sand grains stuck together. Think of a sandstone cliff as a giant pile of tiny grains that became one rock. Most limestone forms in the sea, and some is made from the shells and skeletons of sea animals. Coal is a rock that comes from plants that lived long ago.
Metamorphic rocks are rocks changed by great heat or pressure. They start out as another kind of rock. Marble is a rock made when limestone is changed by heat and pressure. Gneiss is a metamorphic rock with stripes or bands. The rock is not made from nothing; it is an older rock that was changed. A metamorphic rock always has an earlier life as some other rock.
Minerals are the building blocks of rock. A mineral is a natural solid with a set chemical makeup and an orderly crystal structure. Minerals are made by nature and are never alive. The Mohs scale ranks how hard minerals are by what can scratch what. Quartz has a Mohs hardness of 7. Diamond is the hardest natural substance known. Which mineral is hardest? Diamond wins.
Over very long times, one kind of rock can turn into another kind. The rock cycle is the never-ending change of rocks from one type into another over geologic time. Earth's outer layer is broken into huge slabs of rock called tectonic plates that slowly move. Mountain ranges can form where tectonic plates push together. Rock that cooled deep underground can end up on the surface after uplift and erosion. Even the hardest mountain is only one stop on a very long trip.
Words to know
- igneous: from melted rock
- sedimentary: pressed bits
- metamorphic: changed rock
- mineral: natural solid
- lava: melted surface rock
- crystals: orderly mineral shapes
