The Rock Factory
The earth is a huge rock factory. Scientists believe it has been making rocks for billions of years.
The earth makes three kinds of rocks. One kind is made from hot, syrupy liquid rock, deep inside the earth. The melted rock is called molten rock. 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 and becomes solid, too. Other times, 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.
This type of rock that was once a hot liquid is called igneous rock. Igneous means “formed by fire”. Granite, the grey 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 arrowheads.
Another kind of rock is made out of “rock powder”. Wind and rain wear away bits of larger rocks. Rivers carry the powdery bits to the ocean. There, with other bits of things, they sink to the ocean floor and form a layer called sediment. Sediment comes from a word that means “to settle”. 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 until they become dry land.
Rocks that are made this way are called sedimentary rocks. Limestone and sandstone are sedimentary rocks.
The third kind of rock the earth makes is made deep under the surface. The heat and the weight of other rocks slowly change these rocks into a different kind of rock. Their new form is called metamorphic rock. Metamorphic means “changed”.
Marble is a metamorphic rock that changed from limestone. Slate is a metamorphic rock that was changed from mud. Most metamorphic rocks are very old. They stay underground unless erosion, an earthquake, or a new mountain brings them to the earth’s surface.
All the rocks you see were made long, long ago. The oldest rocks ever found on the earth are more than 3 billion years old. But the earth hasn’t stopped making rocks. It’s making them right now. It’s always wearing away old rocks and building up new ones.
Picture Credit : Google
