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Rock Guide

Rock Guide

The earth's crust is made of rock. These rocks are not all the same. Rocks are divided into three groups. They are sedimentary, igneous, and metamorphic rock. How the rock is formed determines what type of rock it will be.

Igneous Rock

Examples of igneous rock

Basalt     Granite     Obsidian     Pumice

Animation - Igneous Rock

Igneous comes from a word that means 'fire'. Deep inside the earth it is very hot. It is so hot that the rock is in liquid form. This hot, liquid rock is called magma.

Igneous rock forms when hot magma cools and hardens. Hot magma may slowly cool and harden deep underground. This forms intrusive igneous rock. Intrusive igneous rock cools and hardens very slowly. This allows time for crystals to develop. Granite is a type of intrusive igneous rock.

Sometimes hot, liquid magma flows to the surface of the earth through cracks called volcanoes. Magma that reaches the earth's surface is called lava. When the lava cools and hardens it forms extrusive igneous rock. Extrusive igneous rock cools very quickly. Sometimes it cools so fast that no minerals form. Obsidian, a dark glassy rock, is extrusive igneous rock.

Compare these two rocks. Do you see the many crystals in the granite? Notice the lack of crystals in the obsidian rock. These are excellent examples of rocks that cool slowly below and quickly above the ground.


Metamorphic Rock

Examples of metamorphic rock

Gneiss     Marble    Schist     Slate

Animation - Metamorphic Rock

Metamorphic comes from a word that means 'to change form'. That is exactly what happens to make metamorphic rock. Rocks, deep inside the earth are changed by heat and pressure. When this occurs, metamorphic rock is formed.

Where does the heat and pressure come from?

Rocks may be heated when magma seeps into the rock around it. The heat and pressure from the magma change the rock.

Movement in the Earth's crust place a great amount of pressure on the nearby rock. This movement and pressure cause the rock to change.


Sedimentary Rock

Examples of sedimentary rock

Conglomerate     Limestone     Sandstone     Shale

Video - How Sedimentary Rocks are Formed

Sedimentary comes from a word that means 'to settle'.

Rocks are constantly being "eroded" or broken into smaller pieces by wind and rain. These pieces (dust and sand) are blown by wind or moved by water into lakes and oceans.

The dust and sand settle to the bottom in layers called sediments. Small shelled sea animals also get mixed with the dirt and sand. Water trickles through the layers.

Every day more sediment piles on top of what is already there. Over time layers and layers of rock pieces and sea animals are deposited. The weight of the top layers press down on the bottom layers. These bottom layers compact (press together) and form sedimentary rock.