There are three main types of rocks: sedimentary, igneous, and metamorphic. Each type of rock has its own characteristics and properties, are part of the rock cycle, and can be found all over the world. Igneous rocks are those formed from molten magma, which is underground melted rock. The melted rock above ground is called lava when it erupts from a volcano. Lava cools and forms several types of igneous rocks. In addition, the underground magma cools to form rocks under the earth's surface.
About 10 miles of the Earth's upper crust is composed of about 95% igneous rocks, with the remaining 5% only a thin, widespread covering of sedimentary and metamorphic rocks. The formation of igneous rocks occurs in two different ways. Those that form underground are called intrusive igneous rocks, and those that form above the earth's surface are called extrusive igneous rocks.
Extrusive igneous rock forms very quickly and is a bit glassy in texture. The rocks formed on the surface may take just a few hours to cool. Sometimes the extrusive rocks are called volcanic rocks. The atoms and molecules in the magma rearrange themselves into mineral grains, and as the magma cools down, the grains grow together and harden. Some examples of extrusive igneous rocks are pumice, basalt, obsidian, rhyolite, and many more.
Intrusive igneous rocks take a much longer time to form under the surface of the Earth. Those that form deep within the Earth's crust may take thousands of years to cool down. The crystals tend to be much larger. The rocks have a grainy feel and two examples include granite and gabbro. Huge bodies of intrusive rocks sometimes form and are found inside mountains called batholiths. They can be as large as 60 miles across. Other intrusive igneous rocks include diorite, pegmatite, periodate, and others.
There are three main types of volcanoes: shield, stratovolcanoes, and cinder cones. Each volcano forms different kinds of igneous rocks. A shield volcano spews lava that was formed at a very high temperature and has a lower amount of silicon, the main ingredient of the mineral quartz. The lava will flow smoothly, cool, and form the rock called basalt.
A stratovolcano's lava is thicker and cooler than from a shield volcano and has a lot more silica (mineral quartz) in them. An example of this type of rock is called rhyolite and is almost the same as granite except with much larger crystals.
Cinder cone volcanoes are basically big mountains of ash and rocks formed by splattering lava. Pumice and rhyolite are common rocks made by this type of volcano.
There are over 700 different kinds of igneous rocks and they can be used as crushed stone for construction, polished for floor tiles, monuments, building veneer, concrete blocks, landscaping, horticulture, and much more. Obsidian rocks were first used as cutting tools like knives, arrowheads, spear points, and other weapons and tools; and used for surgical scalpels. It is also a popular gemstone.
In summary, as with all three major types of rocks, the various types go through many changes during the rock cycle and some sedimentary rocks may become igneous or metamorphic rocks. Igneous rocks are the most common of the rocks that make up the crust of the Earth.