Rocks are everywhere. You can find them along a beach. There are rocks in deserts, on mountains, and in fields. There is rock at the bottom of the ocean. You might find rocks in your own backyard. Even soil is made partly of ground-up rock. Most of Earth’s crust, or outer layer, is made of rock. If you dig down deep enough in the ground, you will find a layer of solid rock. This layer is called bedrock.
Rocks are chunks of solid minerals. Rocks can be red, brown, black, tan, gray, or other colors. Some rocks are plain looking. Some rocks are very beautiful. Some rocks have stripes or speckles.
Geologists (scientists who study rocks and the Earth) group rocks by how they formed. They put rocks into three main groups called igneous rocks, sedimentary rocks, and metamorphic rocks.
ROCKS FROM VOLCANOES
It would be exciting to watch igneous rocks form. Igneous rocks come from melted rock deep in the Earth where it is very hot. The melted rock is called magma. The hot rock moves up through cracks in Earth’s crust. Sometimes the magma cools and hardens before it gets to Earth’s surface. Sometimes the hot rock breaks out, or erupts, on the surface.
Magma that gets to the surface is called lava. When lava erupts again and again it piles up to form mountains called volcanoes. Erupting lava can run down a volcano’s sides like a river of fire.
Lava also oozes up through cracks in the ocean floor. The lava cools and makes new crust on the bottom of the sea.
Sometimes lava cools quickly to make a kind of glass rock called obsidian. Other important igneous rocks are granite and basalt. There is more basalt on Earth than any other kind of igneous rock.
Sedimentary rocks form in layers sort of like a layer cake. Sedimentary rocks called clastic rocks form from layers of sand and mud. Many layers of mud and sand build up on the bottoms of rivers and lakes over millions of years. The heavy top layers press down on the bottom layers. Minerals called quartz or calcite make a kind of cement that holds the mud and sand together. The layers get hard and turn into rock. Sandstone and shale are sedimentary rocks that form this way.
Some sedimentary rocks are called chemical rocks. They form from minerals in water. The water dries up leaving layers of these minerals that turn into rock. Gypsum and halite (used for table salt) form this way.
Other sedimentary rocks are called organic rock. Limestone is an organic rock made from smashed seashells or coral. Coal is an organic rock made from layers of plants that died millions of years ago.
Metamorphic rock comes from changes in other kinds of rock. Heat and high pressure inside Earth “cook” solid rock and change it. Slate is a metamorphic rock that comes from cooked clay or shale. Granite changes into a metamorphic rock called gneiss. Limestone changes into marble.
THE LIVES OF ROCKS
Rocks can change many times over millions of years. Wind and water can wear away igneous and metamorphic rocks. Tiny grains of these rocks can build up in layers to become sedimentary rocks. Sedimentary rock buried deep in Earth can “cook” and change into metamorphic rock. Metamorphic or sedimentary rock can melt, become lava, and turn into igneous rock. These changes are called the rock cycle.
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