Periodite is a dark green, coarse-grained igneous rock that many scientists believe is the main rock of the mantle. Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows.
Some basalts are intrusive having cooled inside the Earth's interior. This is a vertical columnar basalt formation. When basaltic lava cools it often forms hexagonal six sided columns. Pumice is a very light colored, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes. Pumice is so light that it actually floats on water.
Huge pumice blocks have been seen floating on the ocean after large eruptions. Some lava blocks are large enough to carry small animals.
Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes. Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende.
Gabbros are dark-colored, coarse-grained intrusive igneous rocks. They are very similar to basalts in their mineral composition. They are composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine. Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture. Notice in the photo to the left how it fractures.
Obsidian is produced when lava cools very quickly. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite.
Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained aphanitic matrix. The result is an aphanitic rock with some larger crystals phenocrysts imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.
Figure 5. Different cooling rate and gas content resulted in these different textures. Let us start with textures associated with rocks formed by lava flows. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye.
This is sometimes referred to as a fine-grained igneous texture. Some lava flows, however, are not purely fine-grained.
If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix. This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy.
Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice.
Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in.
An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff. A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture. However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye.
These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic.
Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture. Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water. The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it.
A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter and sometimes is much larger. The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite.
The most common igneous compositions can be summarized in three words: mafic basaltic , intermediate andesitic , and felsic granitic. Felsic composition is higher in silica SiO 2 and low in iron Fe and magnesium Mg. Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions. Composition influences the color of igneous rocks. Felsic rocks tend to be light in color white, pink, tan, light brown, light gray.
Mafic rocks tend to be dark in color black, very dark brown, very dark gray, dark green mixed with black. The color distinction comes from the differences in iron and magnesium content.
Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors green, gray, brown. The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition.
If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic. This color rule works most of the time but there are two problems that you need to keep in mind. First, the rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition.
That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly. The second problem is that when igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color. If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition. Igneous rocks with quartz in them are usually felsic. If a media asset is downloadable, a download button appears in the corner of the media viewer.
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