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'Solid as a rock.' Have you heard someone say that before? Rocks have a reputation for being solid, hard, and indestructible. Rocks line river beds and jut above the landscape as mountain peaks; they are fun to collect and sometimes are very beautiful. Each rock is different - some are smooth and round, some are sharp and dangerous. They come in all colors: pink, green, orange, white, red. They are everywhere, and we take their presence for granted and assume that they are unchangeable.
But rocks are not unchangeable! Just like the water cycle, rocks undergo changes of form in a rock cycle. A metamorphic rock can become an igneous rock, or a sedimentary rock can become a metamorphic one. Unlike the water cycle, you can't see the process happening on a day-to-day basis. Rocks change very slowly under normal conditions, but sometimes catastrophic events like a volcanic eruption or a flood can speed up the process. So what are the three types of rocks, and how do they change into each other? Keep reading to find out!
Three types of rock:
Igneous rocks are formed when hot magma (melted rock) is rapidly cooled, either by hitting underground air pockets or by flowing from the mouth of a volcano as lava. Granite, obsidian, and pumice are all common examples of igneous rocks. Pumice is a very porous rock, because when the lava cooled, pockets of air were trapped inside. Because of all those air pockets, pumice can actually float!
Sedimentary rocks are formed by layers of sediment (dirt, rock particles, etc.) being mixed and compressed together for extended periods of time. Common examples of these rocks are limestone, sandstone, and shale. Sedimentary rocks often have lots of fossils in them because plants and animals get buried in the layers of sediment and turned into stone.
Metamorphic rocks are a combination of rock types, compressed together by high pressure and high heat. They usually have a more hard, grainy texture than the other two types. Schist, slate, and gneiss (pronounced like 'nice') are metamorphic rocks.
These rocks change over hundreds of years in the six steps of the rock cycle:
Weathering & Erosion. Igneous, sedimentary, and metamorphic rocks on the surface of the earth are constantly being broken down by wind and water. Wind carrying sand wears particles off rock like sandpaper. Rushing river water and crashing surf rub off all the rough edges of rocks, leaving smooth river rocks or pebbles behind. Water seeps into the cracks in mountain rocks, then freezes, causing the rocks to break open. The result of all this: large rocks are worn down to small particles. When the particles are broken off a rock and stay in the same area, it is called weathering. When the particles are carried somewhere else, it is called erosion.
Transportation. Eroded rock particles are carried away by wind or by rain, streams, rivers, and oceans.
Deposition. As rivers get deeper or flow into the ocean, their current slows down, and the rock particles (mixed with soil) sink and become a layer of sediment. Often the sediment builds up faster than it can be washed away, creating little islands and forcing the river to break up into many channels in a delta. The Mississippi delta in Louisiana deposits lots of sediment in the Gulf of Mexico!
Compaction & Cementation. As the layers of sediment stack up (above water or below), the weight and pressure compacts the bottom layers. (Try making a stack of catalogs and watch how the bottom one gets squished as you add more on top - this is the same idea as the compaction of layers of sediment.) Dissolved minerals fill in the small gaps between particles and then solidify, acting as cement. After years of compaction and cementation, the sediment turns into sedimentary rock.
Metamorphism. Over very long periods of time, sedimentary or igneous rocks end up buried deep underground, usually because of the movement of tectonic plates. While underground, these rocks are exposed to high heat and pressure, which changes them into metamorphic rock. This tends to happen where tectonic plates come together: the pressure of the plates squish the rock that is heated from hot magma below. (Tectonic plates are large sections of the earth's crust that move separately from each other. Their movement often results in earthquakes.)
Rock Melting. Can you imagine 'rock hard' rocks melting? That's what they do in the depths of the earth! Metamorphic rocks underground melt to become magma. When a volcano erupts, magma flows out of it. (When magma is on the earth's surface, it is called lava.) As the lava cools it hardens and becomes igneous rock. As soon as new igneous rock is formed, the processes of weathering and erosion begin, starting the whole cycle over again!
See if you can find sedimentary, igneous and metamorphic rocks where you live. As you study them, think about how they have undergone many slow changes to become what they are. Draw a picture of the rocks you find and then draw a diagram of the whole rock cycle. Next, do some rock cycle experiments.