Everything You Ought to Know about Andesite Rock (2024)

Andesite is a fine-grained, intermediate volcanic or extrusive igneous rock. Its composition lies between basalt and dacite, and it has mainly sodium-rich plagioclase and one or more mafic minerals, especially pyroxenes or hornblende.

As McBirney (1989) narrates, Leopold Von Buch coined the term andesite in 1827 to describe porphyritic rocks with albite and hornblende. These rocks were common in the Andes, the longest mountain range in the world. However, the name didn’t become popular.

Later, in 1861, J. Roth defined it as a porphyritic Cenozoic rock with mainly plagioclase and hornblende or augite, making it popular.

Besides these two descriptions, andesite rocks were initially associated with the andesite line.

What is the andesite line? It was a hypothetical like that separated the Central Pacific Basin’s basaltic (mafic) from partially submerged andesitic volcanic rocks on the continental margins. It parallels deep oceanic trenches and subduction zones in the Pacific basin.

However, over time, the definition has become broader to exclude age and mean any sub-alkaline rock with more sodic plagioclase than labradorite. Also, this rock is no longer associated with the andesite line.

What does andesite mean today? What is its chemical or mineral composition? How does it form, and where does it occur? What are some of its properties and uses? We have all the answers to these questions and many others.

Quick facts and properties

• Rock name: Andesite
• Type: Igneous
• Origin: Extrusive or volcanic
• Texture: Fine-grained or aphanitic but can be porphyritic.
• Cooling history: Fast. The small grains tell you that cooling happened fast on or near the Earth’s surface.
• Chemical composition: Intermediate with 57-63% silica.
• Colors: Mostly light to dark gray, but may vary.
• Mineral composition: Mainly sodium-rich plagioclase (andesine or oligoclase), pyroxenes, and a lesser amount of hornblende
• Mohs hardness scale: 6-7
• Density: 2.40 – 2.80g/cm³
• Extrusive equivalent: It has the same mineral composition as diorite, a plutonic rock.
• Andesitic lava viscosity: 3.5 × 10⁶cP at 1,200°C (2,190°F)
• Tectonic environment: Convergent plate boundaries – mainly continental margin and island arc subduction zones

What is andesite?

Andesite is a fine-grained, intermediate extrusive igneous rock. It has mainly sodic plagioclase, hornblende, and pyroxene with small amounts of other minerals.

Intermediate rocks lie between felsic or acidic rocks like rhyolite or its intrusive equivalent granite and mafic or basic rocks like basalt or its equivalent gabbro. These rocks have 52-63 wt.% silica. However, andesite has a slightly higher silica range, i.e., 57-63 wt.%.

This rock belongs to the basalt-andesite-rhyolite suite and is the most common extrusive rock after basalt. Magmas that form this suite differ in viscosity, silica, and mafic mineral content.

How do you identify this rock? Appearance (texture and color) is vital in identifying this rock while in the field. Usually, rhyolite is a massive, fine-grained, light-colored rock.

The typical andesite rock color is light to dark gray but can be greenish-black, orangish, bluish, greenish-gray, or black. Also, this rock may be greenish, reddish, reddish-brown, or brownish, especially upon weathering.

You didn’t expect this rock to be very dark, being intermediate. However, this isn’t always true for fine-grained or glassy rocks. A good example is obsidian, which is felsic but mostly jet black.

On the other hand, the fine-grained texture of rhyolite will have less than 1/16 mm grain size. Such grains are too small to see with the unaided eye. However, this rock can have a porphyritic texture, with vesicular, trachytic, amygdaloidal, ophitic, or sub-ophitic textures less common.

Let us talk more about porphyritic, vesicular, and amygdaloidal textures.

1. Porphyritic andesite

Porphyritic texture describes larger phenocrysts embedded in finer-grained matrix or groundmass. It shows two-stage cooling, i.e., an earlier slow cooling before extrusion that forms phenocrysts and a later faster during extrusion that forms groundmass.

Porphyritic andesite will have mostly plagioclase phenocrysts and sometimes hornblende (oxyhornblende) or pyroxenes (especially augite). Some of these phenocrysts are clustered, i.e., show glomerophyric texture.

These phenocrysts will occur in a mainly fine-grained (holocrystalline) groundmass of plagioclase (andesine or oligoclase). In this groundmass, plagioclase will be more sodic than in phenocrysts.

Also, the groundmass may have smaller amounts of pyroxene, magnetite, iron-titanium oxides, and sometimes glass with a low refractive index. Any quartz and tridymite is usually interstitial.

Less common phenocrysts in porphyritic andesites are magnetite, biotite, ilmenite, titanomagnetite, and apatite. Quartz phenocrysts may occur but are rare.

Also, olivine phenocrysts may occur but are often replaced by orthopyroxene. Such orthopyroxene phenocrysts may have olivine inclusions in their cores. Also, pigeonite may rim these orthopyroxene phenocrysts.

In porphyritic andesite, plagioclase phenocrysts are larger and the most abundant, appearing light-colored or whitish on a hand specimen. In contrast, the darker, i.e., dark, brownish, and black, are mafic minerals.

Usually plagioclase often shows normal zoning from calcium-rich cores to sodium-rich rims. However, some plagioclase phenocrysts may be oscillatory, with reversal increasing sodium and calcium. In such phenocrysts, cross-polarized light on thin sections may show abrupt concentric bands in oscillatory zoning. Others are discontinuous or even convoluted.

The complex zoning patterns and phenocrysts assemblage may indicate magma mixing as andesite formed. However, although zoning is popular in this rock, it can occur in diorites and tonalites.

Also, plagioclase phenocrysts may have glassy melt inclusions trapped in zones as crystals grow rapidly. These phenocrysts show evidence of melting along cleavage planes, creating a sieve texture.

Lastly, naming follows the most abundant phenocrysts. For instance, biotite, pyroxene, or hornblende andesite imply they form most phenocrysts, respectively. If it has more than one phenocryst, start with the less abundant and place a hyphen between the phenocryst mineral names.

2. Vesicular and amygdaloidal andesite

Some andesite rocks may develop a vesicular texture due to trapped gas bubbles. Other minerals may fill these vesicles, forming amygdaloidal andesite.

For instance, chlorite may occur from hydrothermal alteration. Also, mesolite, heulandite, and other zeolite minerals can fill these vesicles.

Mineral composition

The mineral composition of andesite is mainly sodium-rich plagioclase, pyroxene, and lesser amounts of hornblende (amphibole). Also, it may have smaller amounts of alkali feldspar, feldspathoids, and interstitial quartz or tridymite. Biotite, muscovite (micas), and glass are less common, and orthopyroxene often replaces any olivine present.

Accessory minerals include zircon, apatite, ilmenite, magnetite (sometimes titanian magnetite), and garnet.

In andesites, plagioclase is the most abundant mineral. It ranges from anorthite to oligoclase but is often andesine. Usually, anorthite accounts for 40% of the molar proportion of plagioclase.

On the other hand, the common mafic mineral in andesite is pyroxenes. These pyroxenes are usually augite, pigeonite (low calcium clinopyroxene), and orthopyroxene (enstatite).

Some minerals may occur in this rock as phenocrysts. Others as a fine-grained matrix, or both, as already seen in the section under porphyritic andesite.

On the QAPF diagram for extrusive igneous rocks, andesite is a volcanic rock with less than 20% quartz and 10% feldspathoids by volume of QAPF contents, with plagioclase accounting for at least 65% of the feldspar. This definition plots diorite and basalt in the same area on the QAPF diagram.

Considering mineral assemblage, diorite is the plutonic or intrusive equivalent of andesite. However, based on the QAPF diagram, this rock plots more accurately as dioritoids.

Chemical composition

Andesite is an intermediate volcanic rock. It has a 57-63 wt. % silica and is relatively low in alkali metal oxides, i.e., K₂O + Na₂O is less than 7 wt. %. However, intermediate rocks can have up to 52% silica. Those with 52-57% silica are basaltic andesites.

Typical andesite weight percentage chemical composition from Ragland (1989) data is 60.1% SiO₂ (silica), 16.1% Al₂O₃ (aluminum oxide), 6.9% Fe₂O₃ (ferric oxide), 5.9% CaO (calcium oxide), 3.8% Na₂O (sodium oxide), 2.8% MgO (magnesium oxide), 2.5% K₂O (potassium oxide) and 0.7% TiO₂ (titanium oxide).

From the above chemical composition of andesite, we can confirm it is a mesocratic rock with intermediate amounts of mafic elements (iron and magnesium) and felsic (silicon, aluminum, oxygen, potassium, and sodium). However, it is relatively high in aluminum.

Since it is a fine-grained rock, it may be impossible to classify this rock using only QAPF, which IUGS recommends. An alternative approach is using the TAS (Total Alkali-Silica) diagram.

On the TAS classification, andesite falls in the area shown below, just after basaltic andesites. It has a 57–63 wt.% silica content and (Na₂O + K₂O) less than 7%. Also, it is in the zone just below trachyandesite with a composition between trachyte and andesite.

More classifications

Based on silica content, andesite falls into the intermediate category, i.e., between acidic/felsic and basic/mafic. We have already talked about that.

Also, it may be categorized into the subalkaline category of either calc-alkaline or tholeiitic, depending on the redoxstate as crystallization occurs. The former is iron-poor, the latter iron-rich.

Lastly, you will find low potassium and high potassium andesites. We don’t intend to go deeper into these types in this post.

How does andesite differ from basalt?

Winter (2014) notes that the IUGS recommends distinguishing andesite from basalt based not on plagioclase but on silica content and color index.

Following these criteria, andesite is a lighter rock with a color index below 35 and a silica content greater than 52 wt. %. In contrast, basalt is a darker rock with a color index above 35 and less than 52% silica. However, you cannot determine silica content in the field.

Another distinction is the kind of phenocrysts present. Usually, olivine and augite are the dominant phenocrysts in basalts. However, as you move to andesite, dacite, and rhyolites, orthopyroxene, biotite, and hornblende become more widespread.

Lastly, andesite has more sodium-rich plagioclase than calcium-rich plagioclase and hydrous minerals like biotite, amphibole, and rarely olivine. In contrast, calcium-rich plagioclase dominates in basalts and has olivine, pyroxene, and rarely amphibole.

Boninite

Boninite is a primitive andesitic rock high in silica and magnesium. It forms at early stages of subduction on fore-arcs, likely from a metasomatized mantle.

This rock has more than 52% silica, is magnesium-rich, i.e., about 8-25% magnesium oxide, and has less than 0.5 wt. % TiO₂. Its phenocrysts or microphenocrysts are clinopyroxenes, orthopyroxenes, chrome-bearing spinels, and olivine in a glassy matrix rich in crystallites.

Lastly, bonitite is named after the Izu-Boninarc, where it occurs.

How is andesite rock formed?

Andesite rocks form from fairly evolved or intermediate subalkaline magma that erupts and cools on the Earth’s surface. The quick cooling gives little time for large crystals to grow. Thus, it has a fine-grained texture.

Andesitic magma originates from fractional crystallization of mantle magmas or partial melting of Earth’s crustal rocks.

Also, magma mixing, such as granite and basalt, or contamination with crustal material can give rise to such magma composition. Evidence for magma mixing is the presence of phenocrysts not in chemical equilibrium with melt in these rocks.

Andesitic volcanism

Andesitic magma usually erupts explosively, effusively, or both at 900 and 1100 °C temperatures.

This eruption and those of other intermediate and felsic or silica-rich magma, like dacitic and rhyolitic, often form stratovolcanoes, not shield volcanoes.

Examples of stratovolcanoes associated with andesitic lava include Mt. Pelée, Tehama, Ngauruhoe Shasta, Hood, Adam, Volcán de Colima, Rainier, and Pinatubo. Others are El Chichon, Redoubt, Novarupta, Mt. St. Helens, etc.

Furthermore, stratovolcanoes often have a summit crater or caldera. It results from eruptions characteristic of periodic episodes of effusive and explosive eruptions. This is evidence that both effusive and explosive eruptions can occur.

Let us look at effusive and explosive andesitic magma eruptions.

1. Explosive eruption

Volatile-rich andesitic magmas will erupt explosively, ejecting pyroclasts, includingash, lapilli, scoria, and other debris. Some pyroclasts may consolidate, compact, and harden, forming andesite tuff or agglomerates.

These eruptions are mostly strombolian or Vulcanian types. Plinian occurs in more viscous, silica-rich dacite-rhyolite magmas but can occur in andesitic or basaltic magmas, too, notes Best (2013).

Usually, explosive eruptions are driven by escaping and expanding gases. Remember, as these magmas rise, depressurization occurs, resulting in exsolution, rapid expansion, and escape of gases.

Some violently explosive kinds often result in hazardous pyroclastic flows, surges, or huge eruption columns.

2. Effusive eruption

Effusive andesitic eruptions occur in magmas low in volatiles. These eruptions will form viscous, slow-moving, andesitic blocky lava flows with larger, less angular blocks than aa lava flows.

Usually, andesitic blocky lava flows may be as high as 100 meters. Sometimes, they may show banding or be semi-vesicular. Also, they will hardly flow more than 10 kilometers due to the high viscosity.

Besides blocky lava flows, some evolved and viscous andesitic flows like those of Soufriere Hills volcano in Montserrat (1995) formed an andesitic dome, not a tabular flow. However, domes are commonly in dacitic to rhyolitic lava eruptions.

Lastly, a rare Hawaiian-style eruption, which is non-explosive, may occur. For instance, a andesitic Hawaiian-style with 57-60% SiO₂ was reported from Kilauea Volcano as a part of a fissure eruption during the 2018 eruption.

Where are andesite rocks found or occur?

Andesite rocks occur mostly at convergent boundaries, especially in subduction zones above island arcs and continental margins. A common example is along the Ring of Fire. In these areas, the oceanic plate subducts below the overriding continental plate.

However, andesite rocks can also occur in continental rifts, hotspots, and rarely on mid-ocean ridges or even oceanic hotspots like the Mid-Atlantic ridge in Iceland.

On the continental volcanic arcs, andesite occurs in the Andes (South America), Cascade Range Alaskan peninsula, and Cook Inlet (North America) and Central America volcanic arcs.

On the other hand, the island arcs where this rock occurs are in the western or southwestern Pacific, and smaller ones are in the Southern Atlantic and Caribbean. These arcs are in Indonesia, Japan, Papua Guinea, Tonga, the Philippines, and the Aleutian Islands in the US.

Besides being on Earth, this rock has also been found to occur in extraterrestrial bodies like Martian meteorites.

Lastly, andesitic rock occurs as surface emplacement (from eruptions) as tuff, breccia, conglomerates, and other pyroclasts or as blocky lava flows. Only a small emplacement happens in shallow dikes or plugs.

What is andesite used for?

Andesite is a durable, hard rock with a Mohs hardness of 6-7 and high compressive strength, making it ideal for various uses.

Some uses of andesite rocks include building stones, making aggregate, sculptures, and other decorative or ornamental accessories. Also, they make perfect landscaping stones and are rough, making them ideal for slip-resistant paving or flooring tiles or bricks.

As a building stone, some notable andesite stonemasonry structures include Sacsayhuamáncitadel in Peru, Borobudurin Indonesia, Gate of the Sun in Bolivia, and Templo Mayorruins in Mexico.

On the other hand, crushed andesite aggregate uses include building houses, roads, bridges, seaports, airports, or as fill. However, its high silica makes it not ideal for making concrete.

Lastly, porphyritic andesite is associated with copper deposits and often has andesine, a valuable gemstone.

Frequently asked questions