Since volcanic rocks tend to form quickly and have little time to cool, the crystals within them tend to be very small, making volcanic rocks characteristically fine-grained. Igneous rocks can be classified in several ways including based on their chemical and mineralogical composition. Rocks that contain minerals with higher amounts of heavy elements such as iron and magnesium are considered mafic while rocks which have high silica content and minerals rich in lighter elements such as alkali feldspar are considered felsic.
There are also rocks which are intermediate between felsic and mafic. Basalt is a ubiquitous mafic volcanic rock. It forms in all active tectonic settings, but it often forms from lava originating from volcanoes that occur in continental rift basins, mid-oceanic ridges, and oceanic island arcs. Basalt also composes most of the surface rock of terrestrial planets and large asteroids. This makes it one of the most common, if not the most common, rock in the solar system.
An example of felsic volcanic rock is rhyolite which commonly forms at continental arcs. Rhyolite-forming lava tends to be more viscous and will occur alongside more explosive volcanic eruptions than those associated with basalt-forming mafic lavas.
Andesite is the intermediate between basalt and rhyolite in terms of its chemical composition. One place where andesites are found is at an oceanic island arc. Plutonic rocks are igneous rocks which form from subterranean chambers of molten rock or magma.
Plutonic rocks make up the base of continental crust as well as oceanic crust. Their slow rate of formation causes them to be coarse grained since there is plenty of time for large crystals to form before the magma solidifies into rock. They also tend to be very long-lasting.
Some of the oldest rocks on Earth are plutonic rocks. Plutonic rocks form beneath the surface of differentiated planets and large asteroids. They originate from pockets of magma within the crust that can intrude into other rocks. When a large body of molten rock beneath the surface solidifies, it is called a pluton. Plutonic rocks can be classified based on their composition. A common type of mafic plutonic rock is gabbro. Gabbro commonly forms at mid-oceanic ridges. It can also form wherever there is an especially mafic magma, such as the oceans of molten rock which once existed on the Moon after the large impacts that formed the lunar maria.
Beneath the abyssal plain and slopes of the mid-oceanic ridges, it composes the primary bedrock. Granite is a felsic plutonic rock composing the base of most of the continental crust. Mineralogically, granite contains quartz, various feldspars, and micas. Although there are many rocks that resemble granite, they are not all true granites.
They are also known as intrusive igneous rocks. These rocks form from magma that solidifies by slowly cooling below the surface of Earth. Furthermore, we can categorize plutonic rocks into two groups by observing if they are tabular or not. We can also classify them further depending on the shape of the rock. Tabular plutonic rocks have magma filled in a fracture or another plane that is weak.
Non-tabular plutonic rocks are larger than tabular rocks. This is because magma cools very slowly. It allows individual magma crystals to grow large via coalescing.
Therefore, we can observe that plutonic rocks are coarse-grained rocks. The main way to identify plutonic rocks is observing tightly packed mineral grains that have a medium to large grain size, typically ranging from 1 to 5 mm. These rocks have a phaneritic structure. Moreover, these grains are usually of the same size. We call it an equigranular mixture. Most magmas actually never reach the surface of the earth. The basic classification is the same as for plutonic rocks: with increasing silica content, they include: basalt, andesites, dacites, rhyolite, pumice and obsidian.
Main types of igneous rocks The most widely used and simplest classification of igneous rocks is according to the silica SiO 2 content in the bulk rock composition. Wikimedia Commons. Diorite sample image: Michael C. Rygel via Wikimedia Commons. Andesite Andesite from Methana, Greece. Show more. Try our free app! Android iOS version.
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