Lava rock, also known as igneous rock, is formed when volcanic lava or magma cools and solidifies. It is one of the three main rock types found on Earth, along with metamorphic and sedimentary. Typically, eruption occurs when there is an increase in temperature, a decrease in pressure or a change in composition. There are over 700 types of igneous rocks, all of which have diverse properties; however, they can all be classified into three categories.
Extrusive, also known as volcanic, rocks are a type of igneous rock that form at the crust's surface as a result of volcanic activity. This type of rock occurs when lava flows on or above the Earth's surface and cools down rapidly. The lava comes from the upper mantle layer, 30 to 90 miles beneath the surface, and cools within a few weeks. Because the magma cools and solidifies quickly, the crystals that form do not have time to grow very large, and therefore most extrusive rocks are finely grained. The most common type of extrusive rock is basalt.
Intrusive, or plutonic, igneous rocks form underneath the Earth's surface when magma flows into underground chambers or tunnels. The rock is not exposed to the atmosphere above surface, so the magma cools slowly which allows large mineral crystals to form within the rock. It takes thousands of years for Intrusive rocks to form. A mass of this rock type is called an “intrusion.” Granite is the most common type of intrusive igneous rock.
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Hypabassal, or subvolcanic, rock derives from magma that has solidified at a shallow depth of the volcano, mainly in dykes and sills. This type of rock is formed in between extrusive and intrusive rock, and similarly has a texture in between that of intrusive and extrusive rock. This type of rock is rarer than extrusive and intrusive varieties, and often occurs at continental boundaries and oceanic crusts. Andesite is the most common type of hypabassal rock.
Over 700 different types of igneous rocks have been discovered to date. These vary in terms of appearance, grain size and amount of time that it takes for the lava to cool. A common igneous rock rule is that if lava cools at a faster rate, the rock formed will have finer grains and have a glassy appearance; if rock cools at a slower rate, the grains will be larger and more coarse. Porphyritic rock is a type that has a combination of large and small grains; this occurs when a rock has a mixed cooling history.