Types of Rocks for Second Graders

There are three main types of rocks: sedimentary, igneous, and metamorphic. Each type of rock forms through changes that happen over time.

Sedimentary Rocks
Sedimentary rocks are made from tiny pieces of other rocks or materials that come from plants and animals. There are three kinds of sedimentary rocks: clastic, organic, and chemical.

• Clastic Rocks: Like sandstone, they form from small pieces of other rocks.
• Organic Rocks: Like coal, they come from plants and animals that are squished together into rock.
• Chemical Rocks: Like limestone, they form when water that has minerals in it evaporates and leaves the minerals behind.

These rocks start as bits of other rocks. Wind, water, and ice can move these bits, and when they settle down and get pushed together, they turn into solid rock!

Metamorphic Rocks
Metamorphic rocks are rocks that change from their original form due to a lot of heat or pressure. There are two kinds: foliated and nonfoliated.

• Foliated Rocks: These rocks, like gneiss, have layers because the minerals inside line up in one direction under pressure.
• Nonfoliated Rocks: These rocks, like marble, do not have layers because their minerals don’t line up.

Both types can form from sedimentary rocks when they are squeezed and heated.

Igneous Rocks
Igneous rocks are made when hot melted rock cools and becomes solid. They can form in two ways.

• Intrusive Igneous Rocks: These form inside the Earth, like granite, and cool slowly, creating large crystals.
• Extrusive Igneous Rocks: These form on the surface of the Earth, like basalt and obsidian, and cool quickly, resulting in small crystals.

Sometimes, extrusive rocks can have little holes in them, like pumice, because gas bubbles get trapped when they cool down.

Now you know about the three types of rocks and how they form!