Formed in fire
Volcanoes are necessary to make igneous rocks
Columnar basalt in Mount Rainier National Park
Photo credit: brewbooks on Flickr
Igneous rocks are those formed by heat. Their name comes from ignis, Latin for fire. Almost all igneous rocks come from minerals that are crystallized from the cooling of magma, molten rock. The texture of igneous rocks is determined by whether the rocks solidify above the ground or below it. Volcanic, or extrusive, rocks cool quickly and have fine-grained textures. Plutonic, or intrusive rocks, cool more slowly, have larger crystals and are coarser.
Some Common Extrusive Igneous Rocks
Basalt, Obsidian, and Pumice
The most common type of volcanic glass is obsidian, named after Obsius, the man who described it. It cools so quickly the atoms don't have a chance to arrange themselves as crystals. Obsidian breaks into fragments with sharp edges, which made it a very common weapon as an arrow head or spear point or knife blade among ancient tribal peoples.
Basalt is the most abundant volcanic rock; it is under more of the Earth's surface than any other type of rock. It is also abundant on the Moon. The name dates back to ancient Egypt and was mentioned by Pliny the Elder. Many oceanic islands, e.g. Samoa, Hawaii, and Tahiti are composed primarliy of basalt. Basalt is often bubbly looking, dark gray to coal black in color, and fine-grained. It is widely used in construction: crushed as a road base, cut into tiles, or in buildings and monuments.
Pumice Photo credit: Genista on Flickr
Pumice from an ancient Greek word meaning worm-eaten, is a special kind of volcanic glass. It is extremely porous and very light weight. In fact, it is one of the few rocks whose density is less than water, therefore, it will float. Pumice is formed rapidly trapping an abundance of gas. It is a common household rock. Many people use pumice to clean stains from porcelain and for exfoliating hardened skin or calluses. Ground pumice is sometimes added to toothpastes and soap as an abrasive.
Some Common Intrusive Igneous Rocks
Granite, Diorite, and Gabbro
Granite is the best-known igneous rock. It is used for countertops and floor tiles, as paving stones, in many buildings both ancient and modern, and as tombstones and monuments. Granite is a relatively light-colored, coarse-grained plutonic rock mostly made of feldspar, quartz, and mica. It is hard, resists scratching and erosion, and strong enough to bear weight (like the island of Manhattan). For these reasons, it is easy to understand its widespread use throughout the centuries. Granite is ubiquitous in the Earth's crust and is crystallized from magma under the surface. The composition of the magma is what determines the final granite, whether it is green because of olivine or pink becuase of orthoclase feldspar.
A sculpture from 2100 BCE in southern Iraq, made of diorite
Photo credit: koopmanrob on Flickr
Diorite is another, less well-known, intrusive igneous rock. Its name comes from the Greek for divide. Diorite is considered an intermediate rock - half way between granite and gabbro. It is a drab grey rock, since it has little quartz or potassium feldspar, but is mostly composed of hornblende (a dark mineral) and feldspar. Diorite is extremely hard and durable. The most famous example of it is the Code of Hammurabi, which was carved in 1750 BCE. It has been used in many civilizations for road building and fortresses, including the Incas, the British (who used it for cobblestone streets), and medieval Islamic societies.
Gabbro, a name used for dark rocks used in Renaissance palaces and churches, is a plutonic rock. It is coarse-grained with large amounts of iron containing feldspars. Gabbro is similar in composition to basalt. What's the difference? Gabbro is intrusive and has larger crystals, while basalt is extrusive with fine grained crystals. Like granite, it is hard, polishes beautifully, and can withstand years of wear. Gabbro is sometimes marketed as black granite, even though geologically it is a different rock altogether.