What are Rocks Made of and Types of Rocks - For Kids

The earth is one big rock factory. According to the scientists, the earth has been making rocks for billions of years.
The earth makes three different kinds of rocks.
One kind is made from hot, syrupy liquids, deep inside the earth. Sometimes, some of this liquid rock pushes its way between two layers of solid rock—making a sort of rock sandwich. Then the liquid cools off and becomes solid, too. Sometimes, when volcanoes erupt, some of the liquid rock is pushed up out of the earth. When it reaches the earth’s surface, it cools and becomes solid.
Rock that was once a hot liquid is called igneous rock.
Igneous means “of fire.”
Granite, the hard, light-colored sparkly rock used on the outside of many buildings, is an igneous rock. And so is the black glassy rock called obsidian that some prehistoric people made into knives and arrowheads.

Granite – an igneous rock

 Another kind of rock is made out of “rock powder.” Wind and rain wear off tiny, powdery bits of rock from mountains. Rivers carry the powdered rock to the sea, where it sinks to the bottom. Over thousands of years, the bottom layers of powder are squeezed together by the weight of new layers. Slowly, the powdery bits on the bottom are turned into a layer of solid rock. Over millions of years, earthquakes and other forces may lift up the layers of new rock and they become dry land.
Rocks that are made this way are called sedimentary rocks.
Sedimentary comes from the word sediment, which means “to settle.”
Limestone and sandstone are sedimentary rocks.

Sandstone – sedimentary rock

There is also rock that is changed deep in the earth.
The heat and weight of the earth slowly change it into a different kind of rock. 
Rocks changed this way are called metamorphic rocks.
Metamorphic means “changed.”
Slate, a gray-black rock from which blackboards used to be made, is a metamorphic rock that was changed from clay.
Marble is a metamorphic rock that was changed from limestone. Most metamorphic rocks are very old. They stay buried unless erosion, an earthquake, or the birth of a mountain lifts them to the earth's surface.

Marble—a metamorphic rock

In fact, all the rocks we see were made long, long ago.
The oldest rocks ever found on earth are more than three billion years old.
But the earth hasn't stopped making rocks—it's making them right now.
It takes a long, long time to make a rock.