Just like the ore families in Unit 2, all rocks are sorted by how they formed. There are three big families — learn these and you've got the backbone of geology.

The three families

• Igneous rock — born from heat. It forms when molten rock cools and hardens. (Think "ignite," as in fire.) Granite and basalt are igneous.
• Sedimentary rock — built from bits and layers. It forms when small pieces of older rock and shell settle in layers, get buried, and are pressed and glued together over long stretches of time. Sandstone, limestone, and shale are sedimentary.
• Metamorphic rock — changed by heat and pressure. It forms when an existing rock is cooked and squeezed deep underground until it transforms into something new — without fully melting. ("Metamorphosis" means transformation.) Limestone becomes marble; mud-rock (shale) becomes slate.

The rock cycle

None of this is a one-way street. Over millions of years, a single rock can travel through all three families. Lava cools into igneous rock; weather breaks it into bits that settle and harden into sedimentary rock; heat and pressure cook that into metamorphic rock; and if it melts, it becomes magma and the loop starts over. That endless loop is called the rock cycle.