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Igneous rocks form when magma, or melted rock from inside the Earth, cools and hardens. They can form above ground when magma erupts from a volcano as lava. They can also form deep inside the Earth's crust and then be exposed at the surface by the overlying rocks wearing away. The crystals found in igneous rocks have angular shapes. They have not been worn like sedimentary rocks or squished like metamorphic rocks.

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Next time you're at a beach, grab a handful of sand. This material is sediment. Sediment carried by water and wind is deposited in layers. When sediment becomes cemented together it forms sedimentary rocks. These rocks are the best places to search for fossils.

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock.

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Deep inside the Earth, intense heat and pressure can change rocks into entirely new metamorphic rocks. In fact, the conditions are so extreme, they can change the rock's texture, color, and the shape of its crystals. Even the rocks minerals are broken down and new ones are formed. This can happen to any rock igneous, sedimentary, even other metamorphic rocks. It's all part of the Earth's rock cycle.

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One important use is as stone for buildings and statues. Diorite, a type of igneous rock was used extensively by ancient civilizations for vases and other decorative artwork and is still used for art today . Granite, a type of igneous rock is used both in building construction and for statues.

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It is used in many different ways: as a building stone, in the production of lime (an important material to improve soil for farming), glass making, industrial carbon dioxide and cement.

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Marble, a type of metamorphic rock is used for statues and decorative items like vases . Quartzite, a type of metamorphic rock is very hard and is often crushed and used in building railroad tracks. Schist and slate are sometimes used as building and landscape materials.

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1. 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 the rock like sandpaper. Rushing river water, crashing surf, and precipitation all rub off the rough edges of rocks, leaving smooth 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.

2. Transportation. Eroded rock particles are carried away by wind or by rain, streams, rivers, and oceans.

3. 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.

4. 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.

5. Metamorphism. Over very long periods of geologic time, sedimentary or igneous rocks end up buried deep underground, usually because of the movement of plate tectonics. 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.)

6. 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 the new igneous rock is formed, the processes of weathering and erosion begin, starting the whole cycle over again!

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3 Types of Rocks | #aumsum #kids #science #education #children