Igneous Rocks Explained: Intrusive vs Extrusive

What Are Igneous Rocks?

Igneous rocks are formed from molten rock material¹. They are one of the three main classes of rocks, alongside sedimentary and metamorphic rocks.

Molten rock beneath the Earth’s surface is called magma. When magma reaches the surface, it is known as lava. Igneous rocks that solidify below the surface are called plutonic (or intrusive) rocks, while those that solidify at or near the surface are called volcanic rocks.

Explosive volcanic eruptions produce fragmental volcanic rocks known as pyroclastic rocks. In contrast, non-explosive or less explosive eruptions result in lava flows that solidify into lava rocks.

Classification of Igneous Rocks

Igneous rocks are primarily classified based on their composition—either mineralogical or chemical. However, there are exceptions where classification is based on texture rather than composition.

Plutonic Rocks

Plutonic rocks are coarse-grained igneous rocks that crystallized slowly within the Earth’s crust. These rocks are typically classified according to their mineral composition. The most important minerals used for classification include quartz, feldspars, pyroxene, olivine, hornblende, and feldspathoids.

The classification of most plutonic rocks relies on the QAPF diagram, which considers the proportions of quartz (Q), alkali feldspar (A), plagioclase feldspar (P), and feldspathoids (F). Mafic and ultramafic plutonic rocks are classified using separate diagrams because they typically lack quartz and feldspars—the key components of the QAPF system.

A notable exception is pegmatite. Although most pegmatites have a granitic composition, their defining feature is texture—pegmatites are extremely coarse-grained plutonic rocks, often with crystal sizes exceeding several centimeters.

The QAPF diagram for plutonic rocks is a double ternary plot. Q, A, P, and F represent quartz, alkali feldspar, plagioclase, and feldspathoids, respectively. Note that quartz and feldspathoids are mutually exclusive — a single rock sample cannot contain both. This is not an arbitrary rule; these minerals truly do not coexist in the same rock due to chemical incompatibility. Feldspathoid-bearing rocks are relatively rare, while rocks rich in quartz and feldspar are much more common. The best-known example is granite.

Monzonite from La Palma — Monzonite is a plutonic rock that resembles granite but contains less quartz and more plagioclase (monzonite plots in the center of the QAPF diagram). Plutonic rocks are coarse-grained or at least visibly crystalline because they solidified deep underground where cooling was slow. This sample is from La Palma, Canary Islands. Although La Palma is a volcanic island, some plutonic rocks like monzonite and gabbro crop out in small erosional windows. Width of sample: 6 cm.

Volcanic Lava Rocks

Volcanic rocks are typically fine-grained or glassy due to rapid cooling. As a result, they are classified based on chemical composition rather than mineralogy, which is difficult to determine under the microscope.

Volcanic rocks such as basalt, andesite, and phonolite are classified using the TAS diagram (Total Alkali-Silica diagram), which plots the relative proportions of alkalis and silica. These boundaries are strictly defined and provide a standardized system for naming volcanic rocks.

Porphyry — Rhyolite porphyry from Scotland with K-feldspar and quartz phenocrysts.
Rhyolite is a volcanic rock with granitic composition. Volcanic rocks are fine-grained (individual crystals are not visible to the naked eye), but they often contain larger crystals (phenocrysts) embedded in the fine-grained groundmass. These phenocrysts formed before the magma was expelled from the volcanic vent. Width of sample: 8 cm.

Pyroclastic Rocks

Pyroclastic rocks form during explosive volcanic eruptions when magma is violently fragmented and ejected into the atmosphere. They are composed of volcanic ash, lapilli, bombs, and blocks.

These rocks are classified based on grain size and the relative proportions of different components. For example, a tuff is dominated by ash-sized particles, while an agglomerate consists mainly of larger clasts such as bombs or blocks. Pyroclastic rocks are technically distinct from both lava rocks and sedimentary deposits, though they may resemble either under certain conditions.

Pyroclastic rocks classification scheme — Classification of pyroclastic rocks follows principles similar to those used for clastic sedimentary rocks. Volcanic ash is analogous to sand, lapilli to gravel, and volcanic bombs and blocks to boulders. The most important pyroclastic rock type is tuff, which is lithified volcanic ash — a volcanic analogue of sandstone. These three diagrams are based on classification principles defined by the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks.².

Lapillistone, tuff, ignimbrite — Lapillistone, tuff and ignimbrite are all pyroclastic igneous rocks. The image is from Tenerife. The thickness of the tuff layer is 15 cm.

Rocks that do not fit neatly into the system

Obsidian is composed almost entirely of volcanic glass and therefore lacks a mineralogical composition. While its chemical composition can be measured, its defining feature is its glassy appearance. All rocks with this appearance are called obsidian or volcanic glass regardless of their chemical makeup. Obsidian is usually extrusive, but some pyroclasts also have a glassy texture. The picture above shows glassy sand grains from Hawaii, which are small basaltic fragments of obsidian. Most obsidian, however, has a felsic composition. Width of view: 20 mm.

Diabase with notable plagioclase phenocrysts. — Diabase is an example of a rock that challenges conventional classification schemes.
Compositionally, it is identical to basalt and gabbro, but texturally it falls somewhere between the two. These rocks usually form in the crust at relatively shallow depths, near the surface. Diabase and similar rocks are sometimes referred to as hypabyssal, though that term is now somewhat outdated. The white crystals are plagioclase phenocrysts. Sample from La Palma, Canary Islands. Width of sample: 5 cm.

Scoria rock sample — Scoria is a highly vesicular volcanic rock. It generally has a basaltic composition, although this is not a defining characteristic. The main feature is its high porosity. Sample from Tenerife, Canary Islands. Width of sample: 7 cm.

Carbonatite is a true oddity among igneous rocks. It can be either volcanic or plutonic, but its most distinctive feature is its composition. Unlike most igneous rocks, which are made primarily of silicate minerals, carbonatite is compositionally similar to limestone or marble—hence the name, which refers to its carbonate content. These exotic igneous rocks have their own classification schemes and are best treated separately; otherwise, we risk shattering the illusion that we understand igneous rocks. Carbonatites remain mysterious. They are rare but not so uncommon that they can be ignored. Carbonatite is not the only exotic igneous rock type. Other rare types such as lamproites, kimberlites, and lamprophyres are also treated separately for various reasons. However, all of them are volumetrically insignificant and can be safely ignored when aiming for a general overview of igneous rocks. Sample from Germany.

Composition of igneous rocks

The range of chemical compositions found in igneous rocks reflects the average bulk composition of the Earth’s crust. The most important chemical elements are oxygen and silicon. Common igneous rocks contain between 40% and 77% silica (SiO₂). Other important oxides include alumina (Al₂O₃), magnesia (MgO), lime (CaO), soda (Na₂O), and potash (K₂O).

Average chemical composition of granitic and basaltic rocks based on 2,485 and 3,594 analyzed rock samples, respectively³:

Rock type	SiO₂	TiO₂	Al₂O₃	Fe₂O₃	FeO	MnO	MgO	CaO	Na₂O	K₂O	P₂O₅
Granite	71.84	0.31	14.43	1.22	1.65	0.05	0.72	1.85	3.71	4.10	0.12
Basalt	49.97	1.87	15.99	3.85	7.24	0.20	6.84	9.62	2.96	1.12	0.35

The numbers given in the table above represent weight percentages. Granite contains significantly more silicon and potassium than basalt. Basalt, on the other hand, is richer in iron, titanium, and magnesium.

Real igneous rocks do not generally contain these chemical compounds (potash, lime, etc.) as discrete substances. This is simply a traditional way of expressing the chemical composition of igneous rocks—one that has persisted despite lacking much logical justification today. In reality, igneous rocks are composed primarily of minerals. Silica, for example, does occur as the mineral quartz, but there is no such mineral as “magnesia” or “potash.” Common magnesium-bearing minerals in mafic igneous rocks include olivine and pyroxenes. The most important potassium-bearing minerals are K-feldspar and micas.

The chemical composition of a rock determines its mineralogical composition because chemical elements are the building blocks of minerals. Granite is rich in quartz and K-feldspar because it contains high amounts of silicon and potassium. Basalt, by contrast, contains large amounts of iron and magnesium, which are incorporated into pyroxenes and olivine. Basalt also has much more calcium than granite, which is why the dominant feldspar in basalt is Ca-bearing plagioclase rather than K-feldspar.

Igneous rocks also contain a number of less common or volumetrically minor minerals that utilize trace chemical components. Common accessory minerals in igneous rocks include apatite, titanite, magnetite, ilmenite, zircon, tourmaline, beryl, epidote, rutile, allanite, garnet, chromite, spinel, monazite, and others⁴.

Common igneous minerals

Large crystals of quartz, muscovite, plagioclase, microcline from a pegmatitic rock — Quartz (W), muscovite mica (N), plagioclase (O), and microcline (S) are all very common minerals in igneous rocks. These crystals are exceptionally large for a typical igneous rock—each is roughly 10 cm across. They come from a pegmatitic granite from Evje, Norway. Many examples below are also from pegmatites, chosen because their large minerals are easily visible.

Gabbro — This rock sample demonstrates that pegmatite need not have a granitic composition.
Here is an example of pegmatitic gabbro from Cyprus, composed of light-colored plagioclase and dark-colored pyroxene (augite). Both plagioclase and pyroxenes are widespread minerals in the Earth’s crust and especially common in basaltic rocks.

Picrite — Olivine is an important green mineral found in mafic and ultramafic igneous rocks. This is a sample of picrite from La Palma, Canary Islands. Width of sample: 5 cm.

Micas are significant minerals in many igneous rocks, especially those with a felsic composition. They are easy to recognize by their characteristic appearance. The two most common mica minerals are brown biotite and greenish-gray muscovite.

Nepheline syenite — Feldspathoid-bearing igneous rocks occupy the lower triangle of the QAPF diagram.
The most important feldspathoid mineral is nepheline. The rock sample shown above is nepheline syenite (foyaite). Gray areas represent alkali feldspar, while the darker gray parts are nepheline. Width of sample: 16 cm.

Igneous rocks in the field

Migmatite anatexis — Igneous rocks are formed from molten material. So, the process begins with something melting. Here’s an example of this process, known as anatexis—the partial melting of pre-existing rocks. Rocks melt only partially because different minerals have different melting points. Therefore, the source rocks and the molten material extracted from them differ in composition. In this case, you can see a light-colored melt alongside a dark-colored metamorphic residue. The photo was taken in Western Norway. The rock formed through this process is called migmatite. Width of view: over one meter.

Granite and biogenic sand in Dog's Bay, Ireland. — Intrusive granite on the western coast of Ireland. This outcrop represents a coarse-grained plutonic igneous rock that solidified slowly beneath the Earth’s surface.

Rapakivi — Boulder on a beach on the northeastern coast of the Gulf of Finland in Karelia.
This boulder originates from the Vyborg batholith, which is largely composed of rapakivi granite—a distinctive variety of granite characterized by large, rounded alkali feldspar crystals.

Columnar basalt at Giant's Causeway — Igneous rocks can form beautiful columns, such as these at the Giant’s Causeway. These iconic basalt columns are found on the northern coast of Northern Ireland. Columnar jointing forms when magma or lava cools and contracts.

Sheeted dikes — Volcanic vents are supplied with molten rock through tabular intrusions called dikes. This outcrop in Cyprus is composed entirely of sheeted dikes—one intruding into another. Such sheeted dike complexes are typical features of oceanic crust. Cyprus is among the best places in the world to observe former oceanic crust exposed at the surface. These dikes are composed of basalt and diabase.

pillow lava, Caldera de Taburiente, La Palma — Pillow lava forms underwater.
When lava erupts beneath water, its surface rapidly quenches, forming a hard crust while the interior remains molten and expands into a pillow-like shape. Modern seafloors are covered in such structures. This pillow basalt outcrop in La Palma formed when the island was still a submerged seamount. It is now visible due to uplift and erosion within the Caldera de Taburiente.

Aa lava on La Palma — Igneous rocks that solidify at the surface often form lava flows.
This photo shows the edge of a lava flow in La Palma that resembles a massive pile of rocks.

Pahoehoe — Some lava flows have smooth, continuous surfaces. Such lava is called pahoehoe, a Hawaiian term that describes its ropy, undulating appearance.

Ignimbritic lithic breccia (tuff breccia) in Tenerife. Width of view 0.8 meters. — Pyroclastic density currents are the most dangerous volcanic phenomena.
This rock deposit in Tenerife was laid down by a fast-moving, fiery cloud of ash, gas, and rock fragments that surged down a volcano’s slopes, incinerating everything in its path. Width of view: 0.8 meters.

Sequence of pyroclastic rocks in Tenerife. Pumice layer is a product of violent plinian eruption farther away. Scoriaceous mafic dark-colored lapilli were ejected from nearby vents (Strombolian eruptions). Width of view is 12 meters. — A pyroclastic rock sequence in Tenerife. The white pumice layer was produced by a distant Plinian eruption, while the dark, scoriaceous mafic lapilli came from nearby Strombolian vents. Width of view: 12 meters.

Volcanic bomb. Cumbre Vieja, La Palma. Width of sample 15 cm. — Volcanic bomb from La Palma. This rounded mass of lava was ejected during an eruption and solidified in flight. Width of sample: 15 cm.

Mineral resources associated with igneous rocks

Igneous rocks are important sources of many metals and diamonds, and they also make excellent ornamental and building stones.

Felsic, coarse-grained pegmatites often contain minerals that are rich in relatively rare chemical elements. Elements such as tin, fluorine, tungsten, zinc, thorium, lithium, and beryllium may be extracted from felsic pegmatites. Ultramafic igneous rocks, on the other hand, can be rich in chromium, titanium, iron, vanadium, and nickel⁵.

Magmatic hydrothermal fluids can transport significant amounts of metals such as copper and gold away from their source rocks and deposit them in other geological environments. For example, gold in quartz veins originates from magma. It is typically associated with chlorine, which enables gold to dissolve in water and be carried away. When magmatic fluids react with granitic rocks, they may produce greisens, which often contain appreciable amounts of elements like tin and fluorine. Similarly, reactions between magmatic fluids and carbonate rocks result in the formation of diverse calc-silicate minerals. These rocks are known as skarns, and they frequently contain valuable metal-bearing minerals as well.

Diabase is a widely used material for tombstones. This fine- to medium-grained mafic igneous rock is durable and visually appealing. Width of sample: 25 cm.

Chromite crystals extracted from a chromite concentrate.
The concentrate is from the Rustenburg mine in the Western Bushveld, South Africa. Width of view: 5 mm.

Kimberlite is a rare but economically important igneous rock.
It contains diamonds and fills pipe-like structures (diatremes) in the crust that formed during explosive, gas-rich eruptions. Although small tuff rings sometimes form on the surface, most kimberlitic magma never erupts. Diamonds are typically mined from deep below the surface. This sample is from Kimberley (“blue ground”), South Africa. Width of sample: 11 cm.

Other Uses of Igneous Rocks

In addition to their economic importance as sources of metals and minerals, igneous rocks are widely used for various practical and aesthetic purposes. Their strength, durability, and resistance to weathering make many of them excellent materials for construction. Granite, for example, is a popular choice for countertops, flooring, building facades, and monuments. Its coarse-grained texture and attractive appearance also make it a favored decorative stone.

Basalt is another commonly used igneous rock. Its high density and compressive strength make it ideal for use as crushed stone in road construction and railway ballast. It is also used in the production of rock wool, a material used for thermal and acoustic insulation.

Obsidian, due to its glassy nature and sharp edges when fractured, was historically used to make tools and weapons. Even today, it is occasionally used in surgical scalpels due to its extremely fine cutting edge. In addition, polished igneous rocks are often used in jewelry, ornamental carvings, and interior design, demonstrating that their value goes far beyond geology and industry.

References

1. Raymond, Loren A. (2007). Igneous rocks. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 9. 14-20.
2. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.
3. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
4. Nesse, William D. (2011). Introduction to Mineralogy, 2nd Edition. Oxford University Press.
5. Robb, L. (2005). Introduction to Ore-Forming Processes. Blackwell Science Ltd.

2 thoughts on “Igneous Rocks: Types, Formation and Examples”

Bahman
September 10, 2015 at 17:47
I would be very pleased to receive fotos or slides of igneous , sedimentary and metamorphism rocks……thanks much and hope success for you
norah
November 9, 2016 at 04:00
This helped me a lot with my rock project for school and it has a lot of information in it thank u 🙂

Igneous Rocks: Types, Formation and Examples