Igneous Rocks: How Are They Formed?

When it comes to the composition of the Earth, three main types of rock come into play. These are known as metamorphic rock, sedimentary rock, and igneous rock, respectively. Also known as “fire rock” (derived from the Latin “ignus”), these type of rock are the most common type of rock in the Earth’s surface. In fact, combined with metaphoric rock, igneous rock makes up 90 to 95% of all rock to a depth of 16 km from the surface.

Igneous rocks are also very important because their mineral and chemical makeup can be used to learn about the composition, temperature and pressure that exists within the Earth’s mantle. They can also tell us much about the tectonic environment, given that they are closely linked to the convection of tectonic plates. But just how are these rocks formed?

In essence, igneous rocks are formed through the cooling and solidification of magma (or lava). As hot, molten rock rises to the surface, it undergoes changes in temperature and pressure that cause it to cool, solidify, and crystallize. All told, there are over 700 known types of igneous rock, the majority of which are formed beneath the surface of the Earth’s crust. However, some are also formed on the surface as a result of volcanic activity.

Those that fit into the former category are known as intrusive (or plutonic) rocks, while those that fit into the latter are known as extrusive (or volcanic) rock. In addition to these, there is also hypabyssal (or subvolcanic rock), a less common form of igneous rock that is formed within the Earth between plutonic and volcanic rocks.

Intrusive (Plutonic) Igneous Rock:

Intrusive igneous rock is formed when magma cools and solidifies within small pockets contained within the planet’s crust. As this rock is surrounded by pre-existing rock, the magma cools slowly, which results in it being coarse grained – i.e. mineral grains are big enough to be identifiable with the naked eye. The most common types of plutonic igneous rock are granite, gabbro, or diorite.

The central cores of major mountain ranges consist of large bodies of intrusive igneous rocks – also known as batholiths –  since they are the result of magma cooling within preexisting solid rock on the surface. In addition to batholiths, other types of igneous rock structures include stocks, laccoliths, lopoliths, phacolith, chonliths, sills, dikes, and volcanic pipes (or necks). All of these are to be found in subterranean layers, but can sometimes breach the surface due to tectonic activity.

Extrusive (Volcanic) Igneous Rock:

Extrusive rocks are so named because they are the result of magma pouring onto the surface of the planet and cooling. When it reaches the surface, either on a continental shelf as a volcano or on the ocean floor as a submarine volcano, it becomes lava, by definition. The viscosity of lava depends upon the temperature composition and crystal content of the molten rock itself.

Therefore, the lava can flow slowly, forming short steep flows; or it can flow rapidly, forming long, thin flows. It can also explode violently, dispersing magma into the air that falls back to the surface as ash and tuffs. Compared to intrusive rock, this type of igneous rock cools and crystallizes at a much faster rate due to it being exposed to air or water, which results in it being fine-grained.

Sometimes, the cooling is so rapid as to prevent the formation of even small crystals after extrusion, resulting in rock that may be mostly glass (such as obsidian). If the cooling of the lava happened more slowly, the rocks would be fine-grained or porphyritic – where the crystals differ in size, with at least one group of crystals obviously larger than another group.

Basalt is a common form of extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Extrusive igneous rocks include andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff. Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks.

Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination with a microscope, so only an approximate classification can usually be made in the field.

Hypabyssal (Subvolcanic) Igneous Rock:

Hypabyssal rock is a form of intrusive igneous rock that solidifies at medium to shallow depths within the crust, usually in fissures as dikes and intrusive sills. These rocks typically have an intermediate grain size and texture between that of intrusive and extrusive rock. As might be expected, they show structures that intermediate between those of extrusive and plutonic rocks. Common examples of subvolcanic rocks are diabase, quartz-dolerite, micro-granite and diorite.

Classification of Igneous Rocks:

Igneous rocks are classified according to their mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. Two important variables that are used for the classification of igneous rocks are particle size and the mineral composition of the rock. Feldspar, quartz, olivines, micas, etc., are all important minerals in the formation of igneous rocks, and are important to their classification.

Types of igneous rocks with other essential minerals are very rare. In simplified classification, igneous rocks are separated by the type of feldspar present, the presence or absence of quartz, and – in cases where feldspar or quartz are not present – by the type of iron or magnesium minerals present. Rocks containing quartz are silica-oversaturated, while rocks with feldspathoids are silica-undersaturated.

Igneous rocks which have crystals large enough to be seen with the unaided eye are classified as phaneritic, while those with crystals too small to be seen are aphanitic. Typically, rocks belonging to the phaneritic class are intrusive in origin, while aphanitic rocks are extrusive.

An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is classified as porphyry. Porphyritic textures develop when lava cools unevenly, causing of some of the crystals to grow before the main mass of the molten rock.

So the next time you find yourself somewhere, just standing about, remember that the ground you walk on was formed under from a pretty hellish process. It began deep in the Earth, where silicate rock, tormented by extreme heat and intense pressure, became a hot, oozing mess. Once it was churned up to the surface. it either exploded into the atmosphere, or melted a path across the landscape before cooling in place.

In short, our world was born of conditions that make Dante’s Inferno look boring and cheerful by comparison!

We have written many articles about igneous rocks for Universe Today. Here’s an article on How Rocks are Formed, What is the Earths’ Mantle Made From?, and What is the Difference Between Magma and Lava?

And for a more detailed look at the Earth, here’s What is the Lithosphere?, and What are the Earth’s Layers?

If you’d like more info on igneous rocks, check out U.S. Geological Survey Website. And here’s a link to Geology.com.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.