Igneous rocks are formed from the cooling and solidification of magma or lava. Their color can range widely based on their mineral composition. While some igneous rocks tend to be dark in color, others can be quite light. Let’s take a closer look at the diversity of igneous rock colors.
What are igneous rocks?
Igneous rocks are formed from magma (inside the Earth) or lava (on the Earth’s surface). Magma is molten rock material that originates deep within the Earth’s mantle. As magma rises towards the surface, it melts surrounding rock and incorporates new compounds. Eventually, it will erupt onto the surface as lava or cool slowly beneath the crust to form intrusive igneous rocks.
The minerals that crystallize as magma cools determine the eventual color and texture of the igneous rock. Igneous rocks composed mainly of dark-colored minerals like amphibole, pyroxene, and olivine tend to be darker in color. On the other hand, felsic igneous rocks made mostly of lighter minerals like quartz and feldspar are typically lighter.
Extrusive vs. intrusive igneous rocks
Igneous rocks are further classified as extrusive or intrusive based on where they form. Extrusive igneous rocks erupt onto the surface as lava or volcanic ash. They cool rapidly, so minerals have little time to form large crystals. Examples include basalt, andesite, rhyolite, and obsidian.
Intrusive igneous rocks cool slowly beneath the surface. Minerals have time to interlock and form large crystals, resulting in a coarse-grained texture. Granite and gabbro are common intrusive igneous rocks.
Common dark-colored igneous rocks
Many mafic and ultramafic igneous rocks tend to be dark in color. Let’s look at some of the most common dark igneous rocks:
Basalt
One of the most abundant volcanic rocks, basalt is typically black or dark gray. It is composed mainly of pyroxene and plagioclase feldspar. Iron-rich olivine and pyroxene give basalt its dark color. Extrusive basalts cool rapidly, resulting in a fine-grained texture.
Gabbro
The intrusive equivalent of basalt, gabbro is an extremely dark, coarse-grained rock. Like basalt, it contains a high percentage of iron- and magnesium-rich pyroxene and olivine.
Peridotite
Peridotite is a rare, dense ultramafic rock that originates from the Earth’s mantle. Its color ranges from dark green to black. It contains a high proportion of olivine, with lesser amounts of pyroxenes and amphiboles.
Obsidian
Obsidian is a naturally glassy volcanic rock that forms when felsic lava cools rapidly. It is typically jet black in color and has a smooth, glass-like texture. Small gas bubbles may be trapped in some obsidian, producing a dark gray color.
Common light-colored igneous rocks
While mafic and ultramafic rocks tend to be dark, felsic igneous rocks are often light in color. Here are some common light-colored igneous rocks:
Granite
Granite is a widely distributed intrusive felsic rock that has a coarse texture. It is primarily composed of light-colored minerals like quartz, feldspar, and muscovite or biotite mica. Granite can form in shades of gray, pink, white, or reddish brown.
Rhyolite
The extrusive equivalent to granite, rhyolite has a fine-grained texture and light color. It forms from high-silica lava rich in quartz and feldspar. Rhyolite can be white, pink, gray, or red-brown.
Pumice
Pumice is a porous volcanic rock that forms during explosive eruptions. It has a felsic composition high in silica and magnesium. Rapid cooling traps gas bubbles, giving pumice an extremely lightweight, frothy texture. Its color is typically light gray, tan, or pale yellow.
Felsite
Felsite is an extrusive igneous rock with a fine, glassy groundmass. It has a light color ranging from nearly white to light gray or buff. Felsite’s composition is close to that of granite, rich in quartz and alkali feldspar.
What affects igneous rock color?
The main factors that determine igneous rock color are chemical composition and cooling rate. Here is an overview:
- Chemical composition – Rocks higher in iron and magnesium-rich mafic minerals tend to be dark, while felsic rocks higher in silica are lighter.
- Cooling rate – Extrusive rocks that cool quickly are darker as minerals have less time to crystallize. Intrusive rocks that cool slowly are lighter.
- Crystal size – Fine-grained rocks appear darker than coarse-grained equivalents of the same composition.
- Weathering and alteration – Secondary mineralization and oxidation over time can darken or lighten a rock’s original color.
Igneous rock chemistry
The most important factor controlling igneous rock color is chemical composition. Rocks higher in iron and magnesium are generally darker, while rocks richer in silica are lighter.
Mafic and ultramafic igneous rocks contain over 45% mafic minerals like olivine, pyroxene, and calcium-rich plagioclase feldspar. The abundance of dark green to black iron- and magnesium-rich minerals gives mafic rocks like basalt and gabbro a dark color.
Felsic igneous rocks are higher in silica (over 65%) and contain abundant quartz and potassium feldspar, as well as muscovite, biotite, and amphibole. Their light color results from the predominance of clear to white minerals.
Cooling rate
The rate of cooling also impacts igneous rock color. Dark extrusive rocks cool much more rapidly than light intrusive equivalents of similar composition. For example, obsidian is jet black, while its intrusive equivalent granite can be pink or white.
As magma cools quickly on or near the surface, mineral grains have less time to grow. The resulting fine-grained texture appears darker than the same minerals would at a larger crystal size. Slower cooling underground allows light-colored minerals like feldspar and quartz to fully crystallize and grow.
Crystal size
For a given chemical composition, rocks with larger mineral grains tend to be lighter than fine-grained rocks. Coarse-grained intrusive igneous rocks allow more light to scatter from their component mineral crystals, giving them a lighter color.
Extrusive rocks cool quickly, so many minerals don’t have time to form. The dark basaltic glass of obsidian contains few mineral crystals to reflect light. Pumice has abundant light-colored minerals but is filled with tiny gas bubbles that also darken its color.
Weathering and alteration
After solidifying, igneous rocks undergo chemical and physical weathering at the Earth’s surface. Oxidation of minerals causes red, brown, and yellow discoloration in many igneous rocks over time.
Hydrothermal alteration and metamorphism driven by heat and pressure can also modify rock color long after formation. Greenschist and blueschist metamorphic rocks get their colors from secondary green amphibole and blue glaucophane minerals.
Conclusion
While some igneous rocks tend to be dark due to their high mafic mineral content, igneous rocks as a whole exhibit a diverse range of colors. Felsic rocks high in silica and quartz are frequently light gray, white, or pinkish. Factors like cooling rate, grain size, and post-formational weathering further influence igneous rock color.
Rapidly cooled extrusive igneous rocks will be darker than their intrusive equivalents of the same composition. Fine-grained rocks also appear darker than coarse-grained rocks with the same minerals. Overall, the predominant minerals in an igneous rock, as determined by original magma chemistry, are the most fundamental determinants of its eventual color.
References
The following references were used in compiling this article:
- Best, Myron G. Igneous and Metamorphic Petrology. 2nd ed., Wiley-Blackwell, 2013.
- Blatt, Harvey, and Robert J. Tracy. Petrology: Igneous, Sedimentary and Metamorphic. 3rd ed., W.H. Freeman, 1996.
- Harris, Nicholas, et al. “Classification of Igneous Rocks.” Geoscience Australia, 2009, www.ga.gov.au/scientific-topics/minerals/mineral-resources-and-advice/australian-resource-reviews/igneous-rocks.
- Helmke, Phillip A., and Stephen Marshak. Earth Systems of the American Northwest: An Integrated Approach. Oxford University Press, 2022.
- Philpotts, Anthony R. Principles of Igneous and Metamorphic Petrology. 2nd ed., Cambridge University Press, 2018.
| Igneous Rock Type | Color | Texture | Mineral Composition |
|---|---|---|---|
| Basalt | Black to dark gray | Fine-grained | Pyroxene, plagioclase, olivine |
| Gabbro | Black to dark green | Coarse-grained | Pyroxene, plagioclase, olivine |
| Granite | Light gray, pink, white | Coarse-grained | Quartz, alkali feldspar, plagioclase |
| Rhyolite | White, pink, light gray | Fine-grained | Quartz, alkali feldspar, plagioclase |
| Obsidian | Black | Glassy | Silica-rich volcanic glass |
| Pumice | Light gray, tan, yellow | Fine, porous | Quartz, feldspar, glass |
Additional Questions
What is the darkest igneous rock?
The darkest igneous rocks tend to be mafic rocks like basalt and gabbro that contain abundant dark green to black pyroxene, olivine, and amphibole minerals. Obsidian, a natural volcanic glass, is also extremely dark due to its glassy texture and lack of light-reflecting mineral crystals.
What is the lightest igneous rock?
Felsic igneous rocks like granite and rhyolite tend to be the lightest colored due to their high silica content and abundance of light minerals like quartz and feldspar. Pumice, a porous volcanic glass, is also very light in color.
What minerals make igneous rocks dark?
Iron- and magnesium-rich mafic minerals like olivine, pyroxene, amphibole, and biotite mica give igneous rocks a dark color. In particular, olivine and pyroxene are common dark minerals in basalts and gabbros.
Why is obsidian black?
Obsidian is black because it is volcanic glass that cooled so rapidly from lava that mineral crystals didn’t have time to grow. With few light-reflecting mineral grains, light passes through the glassy groundmass resulting in a dark color. Tiny gas bubbles may also contribute to its dark appearance.
Can felsic rocks be dark?
Most felsic igneous rocks are light in color due to abundant quartz and feldspar. However, some intermediate felsic lavas may appear dark gray due to rapid cooling that inhibits crystal growth. Accessory minerals like biotite mica and amphibole can also lend a darker color to some felsic igneous rocks.
