The Fiery Origins: What Are Igneous Rocks and How Are They Formed?

Igneous rocks are one of the three main types of rocks along with sedimentary and metamorphic, formed through the cooling and solidification of magma or lava. They make up the majority of the Earth’s crust and can be identified by their texture, density, color, and mineral composition.

There are two main categories of igneous rocks: intrusive and extrusive. Intrusive igneous rocks form when magma cools slowly beneath the Earth’s surface, resulting in large, visible mineral grains. Common intrusive rocks include granite, gabbro, and diorite. Extrusive igneous rocks form when lava cools quickly on the Earth’s surface, creating fine-grained or glassy textures. Basalt, obsidian, and pumice are examples of extrusive igneous rocks.

Igneous rocks provide insights into the thermal and chemical evolution of the Earth. Their mineral composition and texture provide evidence of the environment in which they were formed. For example, the presence of olivine and pyroxene in basalt indicates that the lava cooled quickly on the Earth’s surface. The study of igneous rocks has helped scientists understand plate tectonic processes and the movement of magma in the Earth’s interior.

Types of Igneous Rocks

There are two main categories of igneous rocks:

Intrusive Igneous Rocks

Intrusive igneous rocks form when magma cools slowly beneath the Earth’s surface. As the magma cools over long periods of time, ranging from thousands to millions of years, it develops large mineral crystal structures. This results in a coarse-grained texture called phaneritic texture.

Common intrusive igneous rocks include:

• Granite – composed mainly of quartz and feldspar minerals. It has a speckled appearance due to diverse mineral crystal sizes. Granite often appears pink, white, or gray.
• Gabbro – a mafic intrusive rock. It is black or dark green from high concentrations of pyroxene, plagioclase, and olivine. Gabbro weathers into rounded boulders.
• Diorite – an intermediate intrusive rock with a salt-and-pepper appearance from black amphibole and white plagioclase feldspar.
• Dunite – an ultramafic rock composed almost entirely of olivine crystals, giving it a greenish color.

Other intrusive igneous rocks include dacite, pegmatite, and peridotite. Intrusive rocks cool more slowly than extrusive rocks, allowing for the growth of large, interlocking mineral grains visible to the naked eye. This gives intrusive rocks a coarse, granular texture.

Extrusive Igneous Rocks

Extrusive igneous rocks form when lava erupts onto the Earth’s surface and cools quickly. The fast cooling time, often just days or weeks, means lava doesn’t have time to produce large mineral crystals. This results in fine-grained or aphanitic textures.

Common extrusive igneous rocks include:

• Basalt – one of the most abundant volcanic rocks, it has an aphanitic texture with high amounts of plagioclase and pyroxene. Basalt often forms fluid lava flows and is black, brown, or gray in color.
• Andesite – an intermediate volcanic rock with a porphyritic texture of larger crystals in a fine-grained matrix. Andesite lava commonly forms thick, blocky flows.
• Rhyolite – a felsic volcanic rock that forms lava domes, flows, and pyroclastic debris. It has high silica content with quartz, feldspar, and ferromagnesian crystal fragments in an aphanitic matrix.
• Obsidian – a jet black volcanic glass that forms when rhyolite lava cools extremely quickly. Obsidian fractures into conchoidal shards.
• Pumice – a lightweight, vesicular volcanic rock produced when rhyolite lava is full of gas bubbles. Pumice can float on water.
• Tuff – consolidated volcanic ash made up of pyroclasts ejected from volcanic explosions. Tuff has varying textures based on the original composition and volcanic eruption style.

The rapid cooling of lava often creates smooth, glassy textures in extrusive igneous rocks. They also tend to be less dense and lighter in color than intrusive rocks.

Formation of Igneous Rocks

Igneous rocks originate from the cooling and crystallization of magma or lava. Magma is molten rock located beneath the Earth’s surface in the mantle or crust, while lava is magma that has erupted onto the surface through volcanoes or cracks in the crust.

The formation of igneous rocks involves the following steps:

1. Melting of existing crustal or mantle rocks due to high temperatures and/or low pressures. Factors like mantle plumes or subduction can cause melting. This produces magma of varying composition.
2. Magma accumulates in magma chambers beneath the surface and can differentiate and evolve through crystal fractionation and magma mixing.
3. Less dense, evolved magma may rise towards the surface through conduits, dikes, or sills due to decreases in temperature and pressure. More mafic magma tends to remain deeper.
4. When magma reaches the surface through a volcano or fissure, it is called lava. Lava may erupt non-explosively as fluid flows or form violently explosive eruptions.
5. As the lava or magma loses heat, it begins to crystallize into solid rock, forming igneous rocks. Cooling can happen slowly beneath the surface (intrusive) or rapidly on the surface (extrusive).
6. The final texture and composition of the igneous rock depends on the original magma chemistry and the cooling rate. Mafic magma produces mafic minerals while felsic magma creates felsic minerals.

The chemistry of the original magma or lava ultimately determines the mineral content, crystal size, and texture of the final consolidated igneous rock.

Characteristics of Igneous Rocks

Igneous rocks exhibit distinct characteristics that allow geologists to identify them:

Texture

Texture describes the size, shape, and arrangement of mineral grains or crystals in a rock.

• Intrusive igneous rocks like granite have visible individual crystals interlocked together. This is called phaneritic texture.
• Extrusive igneous rocks like basalt have microscopic crystals set in a fine-grained matrix, known as aphanitic texture.
• Rocks like obsidian solidify so rapidly that crystals don’t form, creating natural volcanic glass.
• Porphyritic texture contains larger crystals suspended in a fine-grained groundmass.

Mineral Composition

The minerals within an igneous rock provide clues about its origin and formation.

• Felsic igneous rocks are high in silica (over 65%) and contain minerals like quartz, potassium feldspar, sodium feldspar, muscovite mica, and amphiboles. Granite is a common felsic rock.
• Mafic igneous rocks have lower silica (45-55%) and high amounts of ferromagnesian minerals like olivine, pyroxenes, and calcic plagioclase feldspar. Basalt and gabbro are mafic rocks.
• Intermediate igneous rocks fall between felsic and mafic compositions. Andesite is a common intermediate volcanic rock.

Color

The color of an igneous rock indicates its composition:

• Light colored igneous rocks are typically felsic. Pink and white granite gets its color from quartz and potassium feldspar.
• Dark colored igneous rocks tend to be more mafic. Basalt is black from high levels of pyroxene and olivine.
• Green, brown, or red/black speckled rocks can be intermediate in composition, like diorite.

Density and Weight

The density or specific gravity of an igneous rock is related to its silica content and porosity.

• Mafic igneous rocks have higher densities due to heavier iron and magnesium-rich minerals.
• Felsic igneous rocks contain lighter minerals like quartz and feldspar, giving them lower density.
• Vesicular igneous rocks like pumice have many air pockets that lower the overall density.
• Obsidian has a lower than expected density because it cooled too quickly for mineral grains to form.

Vesicles

Some extrusive igneous rocks contain tiny holes known as vesicles. They form from gas bubbles that were trapped in the lava as it solidified after erupting onto the surface. High vesicle content indicates rapid cooling and degassing of the lava. Pumice and scoria are examples of vesicular igneous rocks.

Amorphous Glass

Rocks that cool extremely fast from lava may solidify into natural amorphous glass before crystals can grow. Obsidian, pitchstone, and some forms of rhyolite have glassy, smooth textures with no crystalline structure.

Hardness

Igneous rock hardness depends on composition and texture:

• Coarse-grained intrusive igneous rocks tend to be more resistant to abrasion due to tightly interlocking mineral grains.
• Fine-grained extrusive igneous rocks are generally softer as they lack large mineral crystals.
• Obsidian is an exception – its volcanic glass is very hard despite the lack of crystals.

Weathering

The mineralogy and texture of igneous rocks controls their weathering and erosion patterns:

• Granitic rocks weather chemically into rounded boulders and corestones.
• Columnar basalts weather into hexagonal pillars.
• Glassy rocks like obsidian weather into sharp fragments before chemical weathering round the edges.

Uses of Igneous Rocks

Thanks to their diverse properties, igneous rocks have many uses:

• Construction Aggregates: Crushed basalt and granite are used to make concrete, asphalt, and road gravel. Granite sand adds strength to cement mixes.
• Architectural Stone: Granite, marble, and other igneous rocks provide beautiful and durable building materials. Granite facing, flooring, steps, and cladding provide style and longevity. Marble is prized for pillars, floors, fireplace surrounds, and other ornamental uses.
• Decorative Stone: People craft igneous rocks like granite, marble, and onyx into tiles, countertops, furniture, and art pieces that add beauty to homes and buildings.
• Jewelry: Translucent to transparent igneous rocks are faceted into valuable gemstones. Extrusive rocks provide many vibrant gems, like ruby (corundum), amethyst (quartz), fire opal, and agate (chalcedony). Obsidian also makes distinctive jewelry.
• Industrial Minerals: Certain igneous rocks are vital sources of key industrial minerals. Aluminum ore comes from anorthosite, while feldspar for glass and ceramic manufacturing is mined from granite and rhyolite. Abrasives like volcanic pumice powder are essential for polishing and scouring.
• Monumental Stone: Granite has been used since ancient times for monuments and statuary. Its hardness and ability to be finely polished makes it ideal for engraving and withstanding weathering in memorials and monuments. The pyramids were originally encased in white limestone and capped with granite.
• Geologic Research: Studying igneous rock features and chemistry provides critical insights into the evolution and inner workings of our planet. Scientists analyze igneous textures and mineral data to interpret past eruptive conditions, magma origins and evolution, and Earth’s dynamic processes occurring deep in the crust and mantle.

Examples of Igneous Rocks

There are endless varieties of igneous rocks found across the globe. Here are some of the most notable examples and their key characteristics:

Granite

This ubiquitous intrusive felsic rock has an interlocking phaneritic texture of quartz, feldspar, mica, and amphibole minerals. True granites contain 20-60% quartz with varying amounts of alkali feldspar and plagioclase. Granite forms as continental crustal rocks melt then slowly crystallize at depth. It weathers into rounded corestone boulders and produces sandy acidic soils.

Rhyolite

The extrusive equivalent of granite, rhyolite lava flows have high silica with quartz, sanidine, plagioclase, and ferromagnesian silicate mineral fragments in an aphanitic matrix. Rhyolite forms volcanic domes, short flows with steep sides, and dangerous pyroclastic ash explosions due to the high viscosity of its silica-rich lava.

Basalt

One of the most abundant volcanic rocks, basalt lava forms fluid flows across the seafloor and on continents. It has an aphanitic to glassy texture with plagioclase, pyroxene, and sometimes olivine. Basalt ranges from black to brown or gray. Columnar jointing during cooling creates the Giant’s Causeway pillars along coasts.

Andesite

This intermediate volcanic rock has a porphyritic texture of larger plagioclase, amphibole, and pyroxene crystals set in a fine-grained matrix. Andesite magma commonly generates thick, blocky lava flows and domes rather than runny flows. Composite volcanoes like Mount St. Helens produce andesite lava.

Obsidian

Obsidian is a jet-black volcanic glass formed when felsic lava cools so fast that mineral crystals can’t grow. Its smooth glass fractures into conchoidal shards. Obsidian was used by ancient cultures to craft sharp tools and weapons. Sources include Yellowstone and volcanoes along the Pacific Rim.

Pumice

This light, vesicular volcanic rock forms when rhyolite lava loaded with dissolved gas bubbles rapidly depressurizes. The lowering pressure enables bubbles to expand, creating a “foamy” rock texture filled with air pockets and vesicles. Pumice has enough air that it can float on water, allowing it to travel long distances from source eruptions.

Gabbro

This dark, coarse-grained intrusive mafic rock comprises mostly calcium-rich plagioclase feldspar and pyroxene, though olivine may also be present. Gabbro forms when basaltic magma slowly cools at depth into large interlocking crystals. It weathers into rounded boulders found along coasts. Gabbro is geologically equivalent to basalt.

Comparison of Igneous Rocks

Igneous Rock	Type	Texture	Mineral Composition	Color	Fun Fact
Granite	Intrusive felsic	Phaneritic, coarse-grained	Quartz, feldspar, muscovite mica	Pink, white, gray	Tends to weather into rounded boulders and corestones. The large crystals make it durable for monuments.
Rhyolite	Extrusive felsic	Aphanitic, fine-grained; may be glassy	Quartz, sanidine and plagioclase feldspars	Light gray, pink, red	Produces obsidian when it cools extremely fast. Associated with violent volcanic eruptions and ash flows.
Basalt	Extrusive mafic	Aphanitic	Plagioclase, pyroxene, olivine	Dark gray to black	Forms over 90% of oceanic crust. Also creates fluid lava flows on land and columnar joint patterns like the Giant’s Causeway.
Andesite	Extrusive intermediate	Porphyritic	Plagioclase, amphibole, biotite mica	Gray to dark gray	Forms thick, blocky lava flows and domes rather than runny flows. Erupts from stratovolcanoes like Mount St. Helens.
Obsidian	Extrusive felsic	Glassy	Non-crystalline volcanic glass	Jet black	Formed by the extremely rapid cooling of rhyolite lava. Fractures into conchoidal shards and was used by ancient peoples to make tools.
Pumice	Extrusive felsic	Vesicular	Quartz, feldspar, ferromagnesian minerals	White, yellow, gray	Its vesicular texture makes it lightweight enough to float on water. Formed by rapid depressurization of gas-rich silicic lava.
Gabbro	Intrusive mafic	Phaneritic, coarse-grained	Pyroxene, plagioclase, olivine	Black or dark green	The intrusive equivalent of basalt. Weathers into rounded boulders like granite.

Frequently Asked Questions and Answers about Igneous Rocks

What is the difference between intrusive and extrusive igneous rocks?

Intrusive igneous rocks cool slowly underground, allowing large mineral crystals to form. Extrusive rocks cool rapidly at the surface, forming small crystals or glass.

How do igneous rocks form?

Igneous rocks are formed by the cooling and solidification of molten magma or lava. Magma cools slowly underground to form intrusive rocks. Lava cools quickly on the surface to form extrusive rocks.

What minerals are found in igneous rocks?

Common minerals in igneous rocks include quartz, feldspars, micas, amphiboles, pyroxenes, and olivine. Felsic rocks have more quartz and feldspars. Mafic rocks are higher in pyroxene, olivine and amphiboles.

Why do different igneous rocks have different colors?

The color depends on the minerals present. Light colored felsic rocks have abundant quartz and feldspars. Dark mafic rocks are higher in ferromagnesian minerals like pyroxene and olivine.

What are some uses for igneous rocks?

Igneous rocks are used as building stone, monuments, jewelry, industrial minerals, and aggregates in concrete. Granite and basalt have many construction uses. Obsidian was used historically for tools.

How does crystal size relate to cooling rate?

Rocks that cool quickly have small crystals that form rapidly, like basalt. Slow cooling underground produces large, interlocking crystal grains seen in granite.

What causes vesicles in some igneous rocks?

Gases dissolved in the original magma come out of solution as lava erupts and depressurizes at the surface. The gas bubbles get trapped as the lava solidifies quickly.

Why is obsidian glassy rather than crystalline?

Obsidian cools from lava far too quickly for mineral crystals to grow. The magma solidifies as natural volcanic glass before crystals can develop.

How do igneous rocks provide information about Earth’s interior?

Geologists study their mineral contents, chemistry and texture to interpret conditions within the magma chambers they formed from deep underground.

Conclusion

In summary, igneous rocks originate from cooled and crystallized magma or lava, comprising over 90% of the Earth’s crust. This diverse rock group exhibits unique textures, mineral compositions, colors, densities, hardnesses, and more based on their formation environment and original magma chemistry. Studying igneous rocks provides scientists with invaluable clues about geologic processes happening deep within our dynamic planet. These critical rocks also supply mankind with essential natural resources, durable building materials, and aesthetic appeal.