Rock Cycle

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The Rock Cycle

What are rocks? Rocks are solid, natural elements made of minerals and other materials. Rocks can be as small as a pebble or as large as a canyon wall. Some rocks are smooth, while others are very rough. Thick layers of rock make up the earth. Scientists group rocks by how they form. The three main types of rock are igneous, sedimentary, and metamorphic. Igneous rock forms in the mantle below earth’s surface. Heat beneath the surface melts rock. The melted rock then mixes with hot gases to form magma. Magma changes into igneous rock when it cools. Igneous rocks that form below earth’s crust are called intrusive rocks. Igneous rocks that form outside on earth’s surface are called extrusive rocks. Intrusive rocks form when hot magma fills cracks in earth’s crust. Mineral crystals form as the magma cools. Diamonds and quartz are some minerals that form in intrusive rock. Rock that forms from quickly cooled lava, such as rhyolite, has small mineral grains. Rocks that form from slowly cooled magma, such as granite, have large mineral grains. Extrusive rocks form when magma flows to the earth’s surface through volcanoes. The magma that flows out of volcanoes is called lava. Lava cools
and hardens quickly. This process sometimes creates smooth and glassy rocks like black obsidian. Basalt is the most comma igneous rock. A layer of basalt lies underneath the ocean floor. The islands of Hawaii are made up of layers of basalt. Pieces of rocks, minerals, remains of living things, and dissolved minerals that come out of water (such as lime) are all kinds of sediment. Sediment is moved by wind and water. It piles up on land and on riverbeds, lake bottoms, and the ocean floor. New layers of sediment build up over time, pressing down on older layers underneath. Dissolved minerals such as calcite come out of the water and
cement the grains together. Rocks, mud, and decaying plants and animals stack up in layers to form sedimentary rocks. Each new layer presses down on the ones below it. The three types of sedimentary rock are clastic rocks, chemically formed rocks, and organically formed rocks. A chemical sedimentary rock, such as rock salt or halite, forms when minerals fall out of solution and settle on the ocean floor. An organic sedimentary rock such as chalk forms from the remains of once-living things. Layers of pebbles and sand make up clastic rocks. These rocks look like pieces of sediment with pebbles stuck in them. Sandstone is clastic rock. Tiny
grains of sand pressed together over time form sandstone. Chemically formed rocks often are found in caves. Water drips from cave ceilings and leaves minerals behind. Over time, long, spear-like mineral deposits hang from a cave ceiling. These are called stalactites. Layers of rotting plants and animals make up organically formed rocks. Over time, the rotting matter turns into rock. Coal is one type of organically formed rock. Scientists often find fossils in organically formed sedimentary rock. Metamorphic rock forms from igneous or sedimentary rocks. High heat and pressure change these rocks. But often they do not melt. Instead, the heat and pressure turn igneous and sedimentary rocks into metamorphic rocks. Heat and pressure can change any rock several times. Scientists call changed rocks metamorphic rocks. A metamorphic rock often is very different from the original rock. A rock’s color, hardness, or smoothness can change. Shale is a sedimentary rock. This soft rock can be black, gray, green, red, or yellow. With pressure, shale turns into a metamorphic rock called slate. Slate is a harder rock than shale. It breaks up into smooth, thin pieces. Slate can be black, green, purple, or even red. With even more pressure, slate turns into a metamorphic rock called schist. Schist is a very hard, glassy rock. It can be black, gray, green, red, or white. Marble is a metamorphic rock. Millions of years ago, all marble was limestone. Limestone is a sedimentary rock. Heat and pressure within the earth turned some of the limestone into marble. Marble is a soft rock. Wind and water erode marble more quickly than other types of rock. Marble does not break or split like other rocks. Its surface becomes smooth and shiny as the top layers wear away. Several colors of marble exist. Some types of marble are rare. Pure white and pink marble are hard to find. Black, gray, and mixed color marble are more common. Marble is found throughout the world. Most marble comes from quarries in Greece and Italy. Miners remove marble from the ground there. Rocks change into different types of rock over thousands or millions of years. Geologists, scientists who study the earth, call this pattern of change the rock cycle. The rock cycle can start anywhere, but let’s begin with igneous rock. Hot magma cools and hardens into igneous rock. Wind and rain erode igneous rock by sweeping away tiny bits of rock. Rivers and streams carry some of the rock to oceans. Layers of rock pile up on the ocean floor. The weight of the layers presses down to form sedimentary rock. Pressure and heat from below Earth’s surface change sedimentary rock. Pressure from earthquakes also changes the rock. The rock is pressed tightly together and changes into metamorphic rock.

Movements of earth’s crust can push rocks into the mantle. In the mantle,
rocks melt to form magma. Magma that is pushed toward the crust cools and forms igneous rock. Then the rock cycle continues. Layers of sedimentary rock are called strata. Many strata of rock can stack up over long periods of time. This process of making layers is called stratification.
Stratification occurs on land and in oceans. Wind and rain erode rocks on
land. The eroded pieces then settle in low areas of land. In oceans, water
currents wear away rocks. These eroded rocks then sink to the ocean floor. The top layer’s weight presses on the bottom layers to form the strata seen in many sedimentary rocks. People often can see several layers of strata in canyons. They also can see strata when roads are cut through mountains.
Scientists study strata to learn about earth’s past. They can learn about the
age of the earth by looking at the rocks that make up different strata. Scientists sometimes find fossils of plants and animals in strata. The fossils give clues about what earth was like thousands of years ago.

In the Conclusion section you will find your questions to answer.