Igneous rocks are formed from lava (or magma), though they are not easily definable, as they exist in over seven hundred varieties.

The life of a rock has to begin somewhere. The question, it seems, is simply where and how.

This is, amongst other things, what separates one type of rock from the other. Surely everyone must already realize that there are countless different types of rock out there, from limestone to shale to dolomite and even to diamond, there is a plethora of different types.

But when it all gets boils down to the most basic, every single rock on the face of this planet can be divided into three main categories – igneous, metamorphic and sedimentary – defined as such by means of formation.

Of these three, it may be igneous rocks which have the most tumultuous beginnings.

Volcanic Formation

After all, what could be more exciting than forming from pure molten lava? This is the fundamentally unifying feature of the igneous classification.

When lava (or magma, if this takes place beneath the surface of the Earth) cools, it hardens into rock – but not just a single type of rock.

Plutonic

More than 700 different igneous rocks have been classified and named, most of them falling into a subcategory called plutonic (or intrusive) igneousrocks, meaning that they are formed (hardened) first beneath the surface of the Earth and then pushed upward by the tectonic motions of the plates in Earth’s crust, where they can then be studied by geologists and volcanologists alike.

The most common examples of this type of rock are the vast multitudes of granite formations that litter the Earth. Granite is a very crucial form of plutonic rock, occurring very commonly across the entire planet.

Volcanic

Those igneous rocks which are not formed below the Earth’s surface are known as volcanic (or extrusive) igneous rocks, so named for reasons which should be fairly obvious (that is, they are formed by volcanic eruptions).

Volcanic rocks form by far the most interesting forms of igneous rocks on the planet, such as obsidian, which forms when felsic lava cools so quickly that the resulting rock does not have time to crystallize, forming a very glassy surface. Obsidian can be so pure and lacking crystalline structure that it can be sharpened to one of the sharpest edges of any material known to man (its edge can be filed down to what is called molecular thinness), and thus has been historically used as tips for arrows and spears, and is even today utilized in medicine in the form of scalpel blades (hardened steel or titanium blades are usually used, but there are certain situations where metallic blades cannot be used in surgeries due to the electronic equipment associated with the procedure, in which case “natural” alternatives are utilized).

Pumice

More explosive volcanic eruptions often result in an igneous formation known as pumice, which is a very porous, usually light colored stone of very low density, formed when a highly viscuous lava cools down quickly enough that the magma nucleate bubbles don’t have time to escape prior to hardening into stone. Some of the more porous pumice even possesses an ability wholly unique amongst rocks – it floats on water.

After the eruption of the Indonesian volcano Krakatoa in 1883, entire rafts of pumice floated in the Pacific ocean for more than 20 years, carrying along entire tree trunks and other vegetation. Such rafts even become support vessels for certain marine species at times. Indeed, pumice holds the distinction of being the only floating rock – a title it surely bears proudly.

Pumice is also used in many household applications – as a mild abrasive in certain solvents and household cleaners, as well as an additive to concrete or as a skin exfolient (one of those few positive ways to look at an erupting volcano.

How Igneous Rocks are Classified

And these three varieties so far mentioned only scratch the surface of the 700 varieties such as basalt, diorite, rhyolite, gabbro and komatiite, and hundreds more.

All of these different formations have been analyzed by geologists, and are classified according to a great many very detailed measurements, such as the acidity of the rock, the predominance of quartz, the predominance of feldspars (a mineral which makes up nearly 60% of the Earth’s crust, and which can be found in all types of rock known to man), and silica content. With these and other chemical features in mind, geologists have been able to further subdivide volcanic (extrusive) rocks into three further categories, based on the type of lava that they originated from:

Felsic rock – Lightly colored, low density, quartz-heavy rocks

Mafic rock – Darkly colored, high density, silicate heavy rocks

Ultramafic rock – Rock possessing more than 90% of the mafic (silicate) chemicals

Surely, volcanic rocks are more diverse than most people realize at first. And this only one of the three types of rocks known on Earth. In this light, it is not surprising why geologists find things as seemingly mundane as rocks so utterly fascinating.

See Also:

Sedimentary Rocks

Metamorphic Rocks

An Examination of Lava

References:

“Igneus Rocks.” United States Geological Survey.

Alden, Andrew. “About Igneus Rocks.”