The Three Types of Rocks

All rocks are not the same, even though many of them look alike. There are three main types or classes of rock: sedimentary, metamorphic, and igneous. The differences between each of the rocks have to do with how they are formed. This is what determines the type of rock it is.

The first type of rock is sedimentary. A sedimentary rock is formed from particles of sand, shells, pebbles, and other pieces of material. All of these particles together make up sediment. Sediment is the result of material broken down by weathering or erosion.

Slowly, the sediment accumulates in layers and over a long period of time it will harden into rock. Sedimentary rock, usually, is fairly soft and may break apart very easily or crumble into pieces. If sedimentary rock is broken apart inside is sand, pebbles, or stones in the rock. Sedimentary rock is usually the only type of rock that also contains fossils. Fossils are the remains of once living animals or plants usually from long ago.

Examples of sedimentary rock types include limestone, shale, and sandstone.

The second type of rock is metamorphic. A metamorphic rock is formed under the surface of the earth from the change that takes place because of intense heat or pressure. In nature, it takes millions of years for a metamorphic rock to be formed. The rocks that result from this very long process often has ribbon-like layers and many shiny crystals. The crystals are formed by minerals which grow slowly over time on the surface of the rock.

Examples of metamorphic rock types include gneiss, marble, slate, and quartzite.

The third type of rock is igneous. Igneous rocks form when magma cools and hardens. Magma is the molten rock deep within the earth. The same material inside a volcano. Sometimes this magma, though, cools inside the earth. At other times it will erupt onto the surface from a volcano. When magma leaves the volcano it is called lava.

When this lava cools very quickly, no crystals form and the rock will look shiny and glasslike. Sometime there are gas bubbles trapped inside the rock during the cooling process of the magma. If this occurred there will be tiny holes and spaces left in the rock.

Examples of igneous rock types include basalt, obsidian, pumice, and granite.

In summary, even though many rocks may look alike, there are three types of rocks with different characteristics. The three types of rocks are sedimentary, metamorphic, and igneous. Sedimentary are form from sediments that may include particles of sand, pebbles, or other stones. Fossils may also be found in sedimentary rock. Metamorphic rock takes millions of years to form and is formed by intense heat and pressure. They often are ribbon-like and contain shiny crystals. The last type is igneous rock, which forms when magma cools and hardens deep inside the earth. Magma is the same material inside of a volcano. All three rock types can be found in many places. Some examples of each include limestone, marble, and granite. Rocks are definitely not all the same.