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Igneous rocks make up 95% of the rocks of the crust of the earth. They are also some of the oldest rocks that are found at the surface of the earth. Igneous rocks form from molten rock which is either ejected at the surface of the earth or cools off underground. When volcanoes erupt, they can eject molten rock as shown in the following picture. Igneous rocks are classed on the basis of where they form. Igneous rocks are sub divided into two types of classifications:"intrusive" and "extrusive.”
Extrusive igneous rocks form when molten rock reaches the earth's surface and cools. Air and moisture cool the lava rapidly. The quick cooling doesn't allow the formation of large crystals, so most extrusive rocks have small crystals or none at all. In some extrusive rocks, like pumice and scoria, air and other gases are trapped in the lava as it cools. We can see holes left in the rock where the bubbles of gas were located. The most common extrusive rock is basalt. The black beaches of Hawaii are formed from eroded basalt.
Another kind of extrusive rock is known as pumice. Pumice cools off very quickly and also is associated with gaseous eruptions. Therefore pumice is very lightweight, and has no crystals. Some pumice will float in water.
Obsidian is also an extrusive igneous rock. Obsidian cools so rapidly that it has no crystals. Apache Tears (from Arizona) or Pele's tears (from Hawaii) are examples of small pieces of obsidian.

Some other common extrusive igneous rocks are Scoria, which is like basalt but has a lot more air bubbles trapped in the rock, and Rhyolite a very light colored extrusive igneous rock.

With intrusive igneous rocks, the molten rock cools before it reaches the surface. Molten rock that is still underground is called magma. Magma originates from the melting of the earth's crust and upper mantle. This melting occurs about a depth of 60 to 200 km. Molten rock that cools before it reaches the surface hardens to become intrusive igneous rock. Because it forms deep beneath the earth's surface, it has more time to cool and develops large crystals

Sedimentary rocks occur as a result of weathering and erosion which continually break down and rearrange the rocks. Small rocks, fragments, and organic remains that have been moved by water, wind, or other agents of erosion are called sediment. Over a period of time, sediment is cemented together to form sedimentary rocks. Sedimentary rocks can be formed from metamorphic, igneous, or other sedimentary rocks that have been broken down by weathering. Sedimentary rocks account for 75% of the rocks exposed at the earth's surface. In nature, sedimentary rocks are usually found in layers, with the oldest layer on the bottom.

There are three types of sedimentary rocks. The first are referred to as clastic. Sedimentary rocks that have been formed from the fragments of other rocks are clastic rocks. The word clastic comes from the Greek word klastos which means "broken." The following are examples of clastic rocks.

Conglomerate rocks are made of large rock pieces that have been cemented together. When looking at this rock type, you can easily see the parts that it has been made of. Pushing hard on single rock pieces in the conglomerate may result in these pieces being broken off of the rock.
Sandstone rocks are made of fine sand grains. Like conglomerate, the grains of sand have been cemented together. Rubbing the surface of sandstone rocks may result in small grains of sand being rubbed off of the rock.
Shale is made up of very fine grains of clay like particles. The texture of shale is very smooth to the touch. If you apply pressure to the shale, it is possible to break it into pieces that also have smooth textures.

The second type of sedmintary rocks are the result of chemical deposits. Halite is an example of a rock that formed when water evaporated and left behind minerals. Sedimetary rocks that result from chemical deposits are called chemical rocks.

The formations you see in caves are made when water drips from the top of the cave and leaves behind minerals. These formations are another example of chemical rocks.


The third form of sedimentary rocks are known as organic rocks. Organic rocks are formed from the life processes of living organisms or the remains of the organisms. Limestone that is formed from the shells of clams and other organisms is organic. When you write with chalk on the chalkboard, you are actually using the shells of organisms that lived long ago!

Coal is made from plants that died in swamps millions of years ago. Coal, therefore, is another example of organic rocks.


Watch the QuickTime video at the right. Has this ever ever happened to you? You take a peanut butter and jelly sandwich to school. You check your backpack at lunch and find that your books have been sitting on the sandwich all day! The heat of the day and pressure of the books have mushed the sandwich into a completely different shape. Your lunch has experienced what rocks experience deep in the interior of the Earth, the process of metamorphism.

Rocks that have changed shape or composition due to intense heat and pressure are called metamorphic rocks. Metamorphic rocks are created from sedimentary, igneous, or other metamorphic rocks. They are formed deep inside the earth. Heat and pressure from the rock above flatten and bend the rock. The texture of metamorphic rocks are classified into two categories: foliated and non-foliated.

In foliated metamorphic rocks, the composition of the original rock is not apparent. This is because the heat and pressure have combined to form new minerals in the rock. The individual mineral grains may recrystallize to form larger crystals that make parallel bands or lines in the rock. Three common foliated rocks are slate, gneiss and schist. Notice the foliation, or lines, in the rocks below.

Slate is formed by the heating and pressure on shale. Because slate separates so easily along the bands, it is used to make roof and floor tiles.

Drag your mouse over the image to view the result of adding heat and pressure to the rock.

Heat and pressure on basalt can make schist.

Drag your mouse over the image to view the result of adding heat and pressure to the rock.

Granite can also undergo heat and pressure to make a rock called Gneiss.

Drag your mouse over the image to view the result of adding heat and pressure to the rock.

The crystals in non-foliated rocks combine and rearrange so they don’t form any bands.


Marble is formed from limestone, which is formed from calcite. Even though limestone and marble are made of the same thing, artists would never use limestone for making statues. Marble is used because it is compact and has bigger crystals from all of the heat and pressure

Drag your mouse over the image to view the result of adding heat and pressure to the rock.

Quartzite forms from sandstone, which is mostly the mineral quartz that has been broken into very small pieces of sand. The heat and pressure makes the sand melt together.

Drag your mouse over the image to view the result of adding heat and pressure to the rock.v

So you can see Metamorphic rocks are changed rocks.

Show your understanding by making a concept map. A concept map is used to break a larger idea into smaller parts for better understanding. For example, if I gave you the words food, apple, fruit, meat, dairy, orange, and bread your map might look like this:

Now, take these words and make your own concept map: clastic, igneous, metamorphic, intrusive, organic, rock, extrusive, foliated, sedimentary, chemical, non-foliated.

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Updated October 24, 2008 by: Glen Westbroek

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