Types of Rocks and Soil

Rocks are classified into three main types based on how they form: igneous, sedimentary, and metamorphic. Each type has distinct properties and comes from a different process in the rock cycle.

Igneous Rocks

• Form when magma (underground) or lava (above ground) cools and solidifies.
• Magma that cools slowly underground forms coarse-grained rocks like granite.
• Lava that cools quickly at the surface forms fine-grained rocks like basalt or glassy rocks like obsidian.

Sedimentary Rocks

• Form when layers of sediment (sand, silt, shells, or minerals) are compressed over millions of years.
• Often contain fossils — remains of plants and animals trapped in the layers.
• Examples: sandstone (compressed sand), limestone (compressed shells/marine organisms), shale (compressed mud).

Metamorphic Rocks

• Form when existing rocks are changed by intense heat and pressure deep underground.
• The original rock is not melted — it is transformed into a new rock type.
• Examples: marble (from limestone), slate (from shale), quartzite (from sandstone).

How Soil Forms

• Soil forms from weathered rock that is broken down into tiny particles over thousands of years.
• It is a mixture of rock particles, organic matter (humus), water, and air.
• Humus comes from the decomposition of dead plants and animals and makes soil fertile.