The rock factory
The earth is a rock factory. Scientists believe it 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.
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.
There is also rock that is changed deep in the earth.
granite—an igneous rock
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.
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.
sandstone—a sedimentary rock
marble—a metamorphic rock
