| Rocks | Classification Chart | Igneous Rocks | Sedimentary Rocks | Metamorphic Rocks | Rock Property Tests |
| Types of Igneous Rocks |

The rocks in the forground are Navajo Sandstone created from great shifting sand dunes during the Jurassic Period, 150 million years ago.

Rocks are all around us. They make up the backbones of hills and mountains and the foundations of plains and valleys. Beneath the soil you walk on and the deep layers of soft mud that cover the ocean basins is a basement of hard rock.

What are rocks made of?

Rocks are made up mostly of crystals of different kinds of minerals, or broken pieces of crystals, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants.

We can learn something about the way a rock formed from by looking carefully at the evidence preserved inside. What a rock is made of, the shapes of the grains or crystals within the rock, and how the grains or crystals fit together all provide valuable clues to help us unlock the rock's history hidden within.

Where do rocks come from?

Rocks are divided into three basic types, igneous, sedimentary and metamorphic, depending upon how they were formed. Plate tectonics provides an explanation for how rocks are recycled from igneous to sedimentary to metamorphic and back to igneous again.

Rock Cycle

All rocks are connected in a cycle of creation, change, and destruction called the Rock Cycle. The rock cycle begins with molten rock (magma below ground, lava above ground), which cools and hardens to form igneous rock. Exposure to weathering and erosional forces, break the original rock into smaller pieces. The smaller material (now called sediment) is carried away by rivers, wind, glaciers, and other means and is eventually deposited elsewhere. These sediments can then be buried and lithified (hardened), forming sedimentary rock.

Sedimentary rock can be deeply buried, subjected to heat and pressure, which over time, cause it to change its structure into a new rock, a metamorphic rock. Metamorphism is a big word meaning change. Eventually, these metamorphic rocks may be heated to the point where they again melt into magma.

Note that the rock cycle doesn't always have to work in this order; sometimes igneous rocks can be buried and metamorphosed, skipping the sedimentary rock phase, and sometimes sedimentary and metamorphic rocks can be uplifted and eroded to form new sedimentary rocks. It is also possible for rocks to remain unchanged in stable regions for long periods of time.

Rock Cycle

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