8.6: Igneous Rock Formation: Plutonic vs Volcanic
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The different crystal sizes and the presence or absence of glass in an igneous rock are primarily controlled by the rate of magma cooling. Magmas that cool below the surface of the earth tend to cool slowly, as the surrounding rock acts as an insulator, not unlike a coffee thermos. Most of us are aware that even the most expensive coffee thermos does not prevent your coffee from cooling; it just slows the rate of cooling. Magma that stays below the surface of the earth can take tens of thousands of years to completely crystallize depending on the size of the magma body. Once the magma has completely crystallized, the entire igneous body is called a pluton. Sometimes portions of these plutons are exposed at the earth’s surface where direct observation of the rock is possible, and upon inspection of this “plutonic” rock, you would see that it is composed of minerals that are large enough to see without the aid of a microscope. Therefore, any igneous rock sample that is considered to have a phaneritic texture (or porphyritic-phaneritic), is also referred to as a plutonic rock. A plutonic rock is also called an intrusive rock as it is derived from magma that intruded the rock layers but never reached the earth’s surface.
If magma does manage to reach the earth’s surface, it is no longer insulated by the rocks around it and will, therefore, cool rapidly. Magma that reaches the earth’s surface through a fissure or central vent (volcano) will lose some of its dissolved gas and becomes lava, and any rock that forms from lava will have an aphanitic texture due to fast cooling (or have a glassy texture due to very fast cooling). Flowing lava may continue to release gas while cooling; this is typical of mafic lava flows. If the lava hardens while these gases are bubbling out of the lava, a small hole or vesicle may form on the rock, and even though the rock is aphanitic (due to fast cooling), the name of the rock can be given the term “vesicular” to indicate the presence of these vesicles. For example, a basalt with vesicles is called vesicular basalt (Figure 8.8). Aphanitic rocks and rocks with glassy texture are also known as volcanic igneous rocks, or extrusive igneous rocks, as the magma was “extruded” onto the surface of the earth. Porphyritic-aphanitic rocks are also considered to be volcanic or extrusive rocks; these rocks may have begun crystallizing under the earth’s surface as a slowly cooling magma (as evident by the small amount of visible phenocrysts), but this magma was extruded onto the surface as lava before finally crystallizing completely on the surface of the earth to form an extrusive igneous rock with a porphyritic texture, or more accurately, a porphyritic-aphanitic texture.
A summary of the terms used to classify the igneous rocks are provided in Figure 8.9 in order to help with the identification of the igneous rock samples provided in your rock kit. Refer to the preceding figures for further help.