Rocks, Minerals & Soil

Earth is built from rocks. Some rocks are old enough to remember the time of the dinosaurs, while others are formed fresh in volcanoes today. Rocks themselves are made from minerals — solid, natural building blocks like quartz, feldspar, and mica. When rocks break down at the surface they help make soil, the thin living layer that feeds plants and crops.

The Three Rock Groups

• Igneous rocks form when hot magma or lava cools and hardens. Examples: granite, basalt, obsidian, pumice.
• Sedimentary rocks form when layers of sand, mud, or shells are pressed and cemented together over time. Examples: sandstone, limestone, shale, conglomerate.
• Metamorphic rocks form when an existing rock is heated and squeezed deep underground until its minerals change. Examples: marble, slate, gneiss, quartzite.

The Rock Cycle

Rocks do not stay the same forever. Lava cools into igneous rock. Wind and water break that rock into tiny grains called sediment, which press together into sedimentary rock. If sedimentary rock is buried deep and heated, it becomes metamorphic rock. Push it deeper still and it can melt back into magma — and the cycle starts again.

• Cooling — magma or lava hardens into igneous rock.
• Weathering and erosion — wind, water, and ice break rock into sediment.
• Compaction and cementing — sediment layers turn into sedimentary rock.
• Heat and pressure — any rock can change into metamorphic rock.
• Melting — deep heat turns rock back into magma.

Minerals — The Building Blocks

A mineral is a solid that forms naturally and has a fixed chemical recipe. Granite, for example, is built from three minerals you can spot with your eye: pink or white feldspar, glassy gray quartz, and shiny black mica. Geologists test minerals by their color, hardness, streak, and the way they break.

• Quartz — hard, glassy, often clear or milky white.
• Feldspar — pink, white, or gray; the most common mineral in the crust.
• Mica — splits into thin shiny sheets.
• Calcite — soft; the main mineral in limestone and marble.
• Hematite — heavy and reddish-brown; an important iron ore.

Soil and Its Layers

Soil forms slowly as rock weathers and decaying plants and animals add humus. A soil profile shows clear horizontal bands called horizons. The topsoil is dark and rich because it holds the most humus and is where roots grow. Below it sits subsoil with clay and minerals, then a layer of partly broken parent material, and finally solid bedrock — the unweathered rock underneath.

1. Topsoil — dark, rich in humus; most plant roots live here.
2. Subsoil — lighter, full of clay and minerals washed down from above.
3. Parent material — partly weathered rock fragments.
4. Bedrock — the solid unbroken rock at the bottom.