Welcome to the Geotour Igneous Rocks Module. This module is designed to allow you to learn about igneous rocks at your own speed using images of hand samples and rock outcrops in their natural settings.
InstructionsUse the up and down-pointing arrows directly to the right of this text to scroll the text up or down. You can click on each picture to enlarge the image for better viewing of details. To continue to the next card or to jump to another, scroll down to the bottom of this card and click on the subject of your choice. There is a self-quiz at the end of Igneous Rocks Tour which will allow you to test your knowledge of igneous rocks.
Igneous rocks form when molten rock cools and solidifies (crystallizes). Molten rock trapped underground is called magma. Molten rock erupted at Earth's surface is referred to as lava. The pictures below illustrate the significant textures and compositions that enable geologists and geology students to classify igneous rocks. For convenience of presentation the igneous rocks are divided into intrusive rocks (or plutonic rocks) which form underground, and extrusive rocks (or volcanic rocks) which form at Earth's surface. Intrusive rocks are featured in cards 2 through 6. Extrusive rocks are featured in cards 7 through 10.
Texture of Igneous Rocks The texture of an igneous rock is determined by its rate of cooling. Intrusive rocks, formed due to slow cooling of magma underground, are composed of large crystals because crystals had a long time to grow before the crystallization process was finished. This forms a coarse-grained or phaneritic texture. Observe picture 1 to see how this texture appears. (Click on picture 1 for a better view.) Also note the interlocking nature of the mineral crystals comprising this rock.
Extrusive rocks, formed due to rapid cooling of lava at Earth's surface, are composed of tiny crystals (fine-grained) because crystallization was completed within a few seconds, hours, or perhaps days. Click on pictures 2 and 4 to see how a fine-grained texture appears. Finally, click on picture 3 to view the special igneous rock texture referred to as porphyritic. The large white crystals formed as magma began to slowly crystallize underground. The rest of the rock, the gray matrix, is composed of tiny crystals which formed as the still-molten magma was erupted at Earth's surface where it cooled very rapidly.
Composition of Igneous Rocks Igneous rocks are divided into three compositional groups:
1) felsic - composed mostly of the minerals quartz and potassium feldspar which are generally white to pink in color. The rocks in pictures 1 and 2 are felsic in composition.
2) intermediate - composed mostly of the minerals amphibole and sodium-rich plagioclase feldspar, with some quartz and pyroxene possible. Intermediate igneous rocks are usually light to dark gray in color. The rock in picture 3 is intermediate in composition.
3) mafic - composed mostly of the minerals pyroxene, calcium-rich plagioclase feldspar and olivine. Mafic rocks are usually black to dark green in color. The rock in picture 4 is mafic in composition.
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| coarse-grained texture | fine-grained texture | intermediate composition | mafic composition |