An igneous rock originated from the volcanoes is termed as volcanic rock. The texture of the volcanic rock is always fine and glassy. The Volcanic rock is often composed of Phenocrysts and Clasts of different rocks. The Phenocrysts are the visible crystals that are bigger than the matrix. The fractional crystallization of the molten magma that takes place before the extrusion leads to the formation of these Phenocrysts.
The names of the volcanic rocks are derived from their chemical compositions as well as their texture. The very commonly found rock is the Basalt rock, and it has a very low content of silica, whereas the Rhyolite rock, developed from the volcanoes has a high content of silica. The chemical composition of Basalt and Gabbro rocks are similar, and the Rhyolite rock has a similar composition to that of the Granite rock. The Dacite, Andesite and Latite rocks are included in the group of intermediate rocks. The vesicular texture of the volcanic rock is caused due to the void, which is left behind by the volatiles that escape from the liquid lava.
An example of explosive volcanic eruption is the Pumice rock and it possesses so many vesicles, that it can float in water. Similarly, the pyroclastic rock is formed as a result of explosive volcanism. Such rocks generally contain huge silica content. Some rocks that include in the pyroclastic rock group are Tuff and Ignimbrite.
Volcanic rock is formed by the cooling of the lava on the earth's crust when it comes in contact with the water or air. This causes the fine crystallization of the minerals present in the rock which in turn gives a fine texture to the rock. The texture of some volcanic rock is glassy due to the immediate cooling of the molten lava. Some rocks have Rhomb-like porphyry designed in a matrix, which consist of Phenocrysts that are visible to the naked eyes.
The volcanic rocks are mostly composed of similar minerals. The difference in the rocks depends on the quantity of the silica (SiO2). The well developed rocks are found to be rich in silica content and minerals like Tectosilicates such as Feldspar, Muscovite and Quartz Polymorphs. Sometimes, the molten magma pulls some crystals from the other magma. Such crystals are known as Xenocrysts. The Kimberlite rocks do not produce Diamonds, but it is occasionally found on the surface of the rock. These diamonds are the example for the Xenocrysts, as it is picked up and transported by the Kimberlite rocks to the earth's surface.
Types of volcanic rock
The Volcanic rocks differ in appearance, texture and sometimes chemical composition. Similarly, some are light, heavy, dark or even pure white. Even the rocks derived from the same deposits can be different in some aspects. In some places, like New Zealand, 3 types of volcanoes were formed from 3 different types of magmas. So, obviously, there will be differences in the rock structures and compositions, after they cool down to form the rock. Some of the types of rock formed out of volcanoes are,
The Earth's crust mainly consists of the commonly found Basalt rock. This rock has a grainy, dark and heavy structure. The Basalt magma usually forms a shield type of volcanoes. Great rock columns generally consist of this rock, for example, the Organ Pipes situated in the Dunedin or the Ireland's Causeway.
As compared to the Basalt rock, Andesite rocks are found in light colors because the iron content is less, whereas the silica content is more (about 50 to 60 % of silica). Volcanoes formed by the Andesite magma have a cone structure. Some kind of Scoria rock is included in the Andesite class due to its chemical classification.
Rhyolite rock is often found in light hues or white color which is an indication about the high silica content. It contains more than 70% of silica and less content of magnesium or iron. The Rhyolite magma is thick in consistency, due to which the release of the gases is obstructed. Depending on the presence of the gas, the Rhyolite magma forms the cone volcanoes; otherwise the volcanoes in the caldera form are usually seen. The rocks require low temperature of about 750 to 850 degrees to form a Rhyolite magma. Pumice rock, an example of Rhyolite rock is very light in weight, which is a proof of the gas bubbles trapped in the magma, before it solidified.
Composition of Volcanic Rocks
Igneous volcanic rocks are the subfamily of the volcanic rock. Rocks emerged out of different lava have different characteristics. When the Rhyolite lava cools quickly, a black and glassy substance is produced. This is referred to as Obsidian. When the same lava is filled with gas bubbles, it cools down to form a spongy and soft rock known as Pumice. If the same lava is allowed to cool down slowly, it will form a solid rock known as Rhyolite. The crystallization of the rock is the result of the quick cooling of the molten lava, when comes in contact with the air or water. This leads to the fine texture of the rocks. Researchers say that the freshly emitted molten lava consists of large crystals, and the small crystals that cling to the ground mass or matrix is developed in the post effusion stage. The glassy rocks have a special characteristic of the presence of Spherulited or the rounded bodies, which consist of divergent fibers that emerge from the center. This unique rock consists of unparalleled and imperfectly arranged crystals of Quartz, Tridymite or Feldspar in a mixed form. The variety in the glassy rock known as Perilite structure consists of concentric cracks in rounded shapes, due to the contraction caused during the cooling process. The Phenocrysts often show a different and unique composition, the layers of this rock are colored in different hues. The volcanic rocks developed in the initial stage are composed of basalt and the small difference in the chemical composition may cause varying types and structures of volcanic rocks in the following series. Examples of such a series are Tholeiitic, Calc-Alkaline, and Alkaline.
Basalt is an igneous rock of the extrusive group. It is mainly used in construction and building works and in aggregate. It is also used as a raw material in making sculptures and statues. Pumice is a volcanic igneous rock, which is used as a raw material in making emery board, hand soaps, etc. Obsidian rock is the main raw material in making sharp objects like knives, spears, arrowheads, scrapers and various sharp tools and weapons. Opaque Obsidian is mainly used in making jewelry.