Types of Rocks and What Soil Is Made Of: Two Wren and Sitka Lessons, Grades 3-6

Igneous rock was once melted. It cooled and hardened, either underground slowly or on the surface fast after a volcano. Slow cooling makes big visible crystals like in granite. Fast cooling makes smooth glassy rock like obsidian.

Sedimentary rock is built from pieces. Sand, mud, shells, and gravel settle in layers, get buried and pressed, and cement together. Sandstone and limestone are like this. Fossils are almost always found in sedimentary rock, because the other two kinds involve heat or pressure that destroys them.

Metamorphic rock was changed. Heat and pressure deep underground squeezed and cooked an existing rock into something new without melting it. Limestone becomes marble. Shale becomes slate.

The three kinds turn into each other. Heat and pressure cook one kind into another, weather and water break rock into pieces that settle and press into new rock, and melting starts the whole thing over. That loop is called the rock cycle, and it never finishes.

Soil is not one thing. It is broken rock, dead plants and animals, water, air, and an enormous number of living things, all mixed together.

The broken rock part comes in three sizes. Sand is the biggest and feels gritty. Silt is medium and feels smooth like flour. Clay is the smallest and feels sticky when wet.

The dark crumbly part made of rotted plants and animals is called humus, and it holds water and feeds plants. It is the part that makes soil good.

A single handful of healthy soil holds more living things than there are people on Earth. Bacteria, fungi, worms, and mites are all breaking things down and putting nutrients back.

Dig a hole and you see layers. Dark topsoil on top, paler subsoil under it, then broken rock, then solid bedrock. Making one inch of topsoil can take hundreds of years.