World Builders™                                                                               Session Two  --  Geology



Igneous Rocks    

"Igneous" comes from the Latin word for fire.  Igneous rocks begin as fiery liquid magma.

    Igneous rocks are formed by the cooling of molten material called magma. Magma lies deep below the surface of the earth.  It is a very hot, very, very thick liquid made mostly of iron, magnesium, aluminum, silicon, and oxygen.

Rocks that Cool Deep in the Earth

     The mantle of the earth is very thick.  The magma that is near the surface cools slowly, becoming part of the crust. As it cools, the minerals crystalize, giving the rocks a slightly rough texture and/or speckled appearance. Later erosion removes the older rocks above the newly cooled rock, and the now-solid minerals appear on the surface.

Examples of igneous rocks that cool slowly include

granite, a light colored speckled rock, sometimes used for buildings
basalt, a darker colored rock
quartz, which may be white, pink (rose quartz), yellowish, and other colors.

Igneous rocks are hard, and often used for building because they are durable. The Sierra mountain range is made of granite. It will take a long time for these magnificent mountains to be eroded away!

Rocks that Cool Quickly at the Surface

volcanic dome

     Not all igneous rocks cool deep in the earth.

     Sometimes hot rock breaks through to the surface. It spurts out and forms cones, like the one in this picture. The cones are made of cinders and rock, and can be recognized by their distinctive shape.

     Volcanism, the activity that leads to the eruption of hot rock, is often accompanied by earthquakes. It often occurs where tectonic plates meet. The area around the Pacific Ocean, called the Ring of Fire, is dotted with volcanoes, and is earthquake prone. Click here for information on Volcanic Eruptions.

    Although we tend to think of the surface of the planet as being unchanging, it is actually quite active, and constantly undergoing reconstruction.

     This picture shows a place where lava once flowed like a river. You can see a sort of little island in the middle, and smooth streams flowing around it. On the left, near the front of the picture, you can see a series of ledges where the hot, flowing rock paused and cooled a little before flowing on.  Volcanic landscapes contain interesting features, showing that the molten rock flows much as water does.

     After an eruption, most life forms have been killed by the intense heat. However, once the rock cools, living organisms begin to reclaim the devastated land.

    Another kind of igneous rock that comes out of volcanoes is called obsidian. It is a kind of volcanic glass, and usually black and shiny. Sometimes small mountains are made of this rock. It has very sharp edges -- it should be handled with care. Our ancestors used obsidian to make knife blades and arrow heads.

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Header photograph from DHD Multimedia Gallery 
Photos © E Viau   taken at Craters of the Moon National Monument, Idaho, USA
Copyright ® 1999, 2003.   Elizabeth Anne Viau and her licensors.  All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .