Basaltic…? Felsic…? Porphyritic…? Confusion!

Felsite quarry at Mount Ararat (© en.ararat-stone.com)

That’s how I felt at times reading interesting things about volcanoes but had to skip those -ic words. So, I am trying to give you a simple overview on rock types produced by volcanoes… I can already hear the experts groan: omg, “petrology” and “simple” are the two most antagonistic terms in the world! 😉 Nevertheless, I know all too well our typical skip-and-jump motion when it comes to different lavas, just let’s try and achieve a broad understanding about what volcanoes bring up from the depth.

Examining and measuring rock proprties – the main work of geologists and volcanologists.

We are only looking at the IGNEOUS rocks here, these are the “originals” that have come up as magmas from deep down. Everything else, i.e. sediments and metamorphosed rocks, are derived from them during the long history of Earth. And, for the purpose, I am writing only about the EXTRUSIVE of the igneous rocks – the intrusive, or plutonic rocks (that didn’t break through to the surface) may be stuff for another post.

Why are there different magmas in the first place?

Because of the different components of the earth’s mantle / lithosphere / crust that have been brought up under varying conditions. – Deep down, all components of a batch of magma are well blended in a liquid or near-liquid mass. They are just the elements and compounds that will later make up the minerals – and/or escape as gasses. As the magma rises the environment gets cooler and the pressure changes. The components begin to react: new compounds are formed, others get separated, many compounds bond together as minerals.

Further up, certain minerals begin to form crystals in the soup. The remaining liquid minerals still flow around them – until, still higher up, they also reach their own point of crystallization (solidus). Arriving at the surface, the last liquids crystallize rapidly in small grains, or in some cases, with no grains at all as a glassy rock.

Fractional crystallisation in a magma. While rising, and cooling from initially ~1200°C, the magma evolves in composition. Different minerals crystallize from the melt in stages, according to cooling. At the bottom of a magma reservoir forms cumulate rock. The final magma is depleted of the minerals that have already settled underground. (© Woudloper, via Wikipedia)

Thus, minerals that have started forming crystals early had lots of time to grow to a considerable size. Those can later be seen in the solidified rock as more or less complete crystals with the naked eye. The other minerals that started forming chrystals late, and/or cooled quickly, will solidify as a fine-grained mass. Lavas are often fine-grained as they cooled quickly after an eruption.

Let’s summarise: When magma rises, it gets depleted of those minerals that have already crystallized underground by cooling. The final magma is made up of the remaining minerals. Depending on rising time and cooling temperatures, minerals form either evenly shaped crystals or finely grained to powdery masses. The surrounding pressure and temperature conditions compact the minerals to solid rock.

Igneous rocks can be classified according to a great number of characteristics – a huge number, actually. But volcanologists look at two properties in a piece of rock first: texture and composition. Texture is what can be felt and seen without needing a microscope or a laboratory. Composition is the mineralogical and chemical make-up of a rock. Experts may be able to give a first general classification by looking at the visible crystals. But for a proper identification, petrological chemical and optical analysis need to be carried out in a lab.

ROCK TEXTURE

Volcanic bomb of aphanaitic rock texture found by Rob McConnell in the Mojave National Preserve, California, USA. (© Mark A. Wilson, via Wikipedia)

Texture refers to the size, shape and arrangement of crystal grains within a rock. What you can see are the entire or broken larger crystals in a ground mass of powdered or not crystallized minerals. According to the size of crystals they see in a piece of rock volcanologists can make a first assessment of how long it took the magma to rise. They can say if it poured out onto the surface or if it didn’t make it and stayed underground as a pluton. Or if it spent a few thousand years in a magma reservoir before erupting. Different rising and cooling times produce rocks with different textures. Even if the mineralogical composition of the rocks stays the same. Two examples:

Rhyolitic magma, allowed to cool slowly, forms a light-colored, uniformly solid rock called rhyolite (1). If the same lava is filled with bubbles of gas during eruption, it may form the spongy appearing pumice (2). But, should a rhyolitic lava-stream cool very quickly, it can freeze into a glassy substance called obsidian (3).

Texture types

Rocks composed mainly of large crystals have a coarse crystalline texture (called phaneritic). In other rocks, it is often not possible to identify crystals without a microscope. Those have a fine grained texture (called aphanitic). A mixture of those two is called porphyritic texture, where many larger crystals are visible in the finely grained groundmass. There are about a hundred different types of textures used for more specific description of rocks.

Some more textures are:

vesicular: containing voids caused by gases while cooling
vitreous: glassy, without crystals
pyroclastic: rocks consist mainly of erupted rock fragments
brecciated rock: with broken fragments of minerals or rock, cemented by a fine-grained matrix
fiamme: embedded lens-shaped pieces of volcanic ejecta
tuffaceous: rock that contains greater than 50% tuff

Gallery: TEXTURES (Click on first image to begin)

Phaneritic texture: rocks contain mainly crystals visible to the naked eye. Image: Rhyolite. by Damien Mollex

Aphanitic texture: rapid cooling, a uniform, fine grained rock. Image: Quarried basalt.

Porphyritic texture: A magma stays in a reservoir, cooling slowly underground to the point of being a slurry of large crystals and liquid. Then it erupts so that the liquid chills quickly. The result is porphyry, a rock with both phaneritic and aphanitic aspects. Image: Trachyte.

Vesicular texture: Magma typically has some gas in solution. During eruptions, that gas effervesces out of solution, like CO2 bubbling out of a soda. As the magma freezes, these bubbles are preserved as vesicles, and rocks containing them are called vesiclular. Image: Scoria is a highly vesicular volcanic rock. It usually has a basaltic composition, but this is not a defining parameter. It just has to be very porous. Tenerife, Canary Islands. Width of sample is 7 cm. (from sandatlas.org)

Vitreous, glassy texture: These cool almost instantaneously. Obsidian derives from rhyolitic magmas that ascend and cool too quickly to grow any crystals at all. Thus a natural glass, or vitrophyre, is formed. Image Obsidian boulder

Pyroclastic texture: Rocks formed from the deposition of volcanic ash and other tephra. These form when magma erupts as an aerosol of fine particles. Often, ash particles are still slightly sticky when they fall, sticking together to form welded tuff. Lapilli and other fragments are often cemented in. Image: Building stone for the Chapel of St Antony, built in 1715 in Funchal, Madeira. (© Ian G. Stimpson, via blog hypocentre.wordpress.com)

Brecciated texture: A breccia is broken rock that has been cemented back together naturally. Image: Volcanic breccia in Barberton Mountain Land, S. Africa. (© Vitor Pacheco, blog geologicalintroduction.baffl.co.uk).

Fiamme texture: embedded flame- or lens-shaped pieces of volcanic ejecta. It mostly forms from pumice that is still hot and plastic enough to get flattened down by the weight of the rock on top of it. Those flattened pumice are called fiamme. Image: Fiamme in welded tuff. (© Cataclasite, via Wikipedia)

Tuffaceous rock contains greater than 50% tuff, i.e. consolidated volcanic ash. Image: Tuffaceous rhyolitic pyroclastic deposit, with weakened areas (probably from initially hot gases) eroded out over the millenia to form the pits and holes of the wall rock. Banshee Canyon, near Hole in the Wall Campground, Mojave National Preserve, CA. (© Mike Malaska, via Flickr)

ROCK COMPOSITION

The mineral composition of a rock provides important information about the conditions under which it formed. Feldspars, quartz, olivines, pyroxenes, amphiboles, and micas are all important minerals in the formation of almost all igneous rocks, and of course they are basic to the classification of these rocks. But, as almost all rock is made up from those, we need yet another way to group our rocks. This is achieved by looking at their silica content:

Felsic, Mafic, Alcalic Magmas

Note: The “Basic to Acid” terminology was based on the onetime idea that the SiO2 in rocks was precipitated from waters with a high concentration of hydrosilicic acid H4SiO4. Although we now know this is not true, the terms are still very commonly used in literature. Today it is officially “Mafic to Felsic”.

Total alkali (N2O + K2O) versus silica (SiO2) content – or “TAS” – classification system of igneous rocks. After Le Maitre (1989).

Thus, igneous rocks can be broadly divided into mafic and felsic types (and anything in between). The division is determined by how much silica they contain. An additional factor is the content of alkali compounds (N2O + K2O). Felsic rocks are light in colour, while mafic rocks are darker, due to their content of dark minerals.

• felsic (or acid) rocks: high silica content, more than 63% SiO2 (e.g. granite and rhyolite)
• intermediate rocks: 52 – 63% SiO2 (e.g. andesite and dacite)
• mafic (or basic) rocks: low silica 45 – 52% and typically high iron-magnesium content (e.g. gabbro and basalt)
• ultramafic (or ultrabasic) rocks: less than 45% SiO2 (e.g. picrite, komatiite)
• alkalic rocks: with 5 – 15% alkali (K2O + Na2O) content, or with a ratio of alkali to silica greater than 1:6. (e.g. phonolite and trachyte)

Depending on their silica content, magmas behave differently:

Felsic magmas (such as rhyolites) are “more evolved” – leaving them less similar to the composition of Earth’s mantle. They are:

• viscous
• often have large quantities of water vapor
• relatively low temperature
• tend to erupt explosively
• rich in lighter elements, such as silica (in SiO2) and sodium (in NaO)
• are produced through lower degrees of mantle/crust melting.

Mafic magmas (such as basaltes) are more “primitive” or closer to the composition of the Earth’s mantle. They are:

• less viscous (i.e. more fluid)
• dry (i.e. little water vapor)
• relatively high temperature
• tend to flow as a liquid after eruptions
• enriched in heavier elements, such as magnesium and iron
• are generally produced through fairly high degree melting (about 10-20%) of the mantle.

GALLERIES

See the following image galleries for some important volcanic rocks. In the descriptions I have omitted the ever repeated statement “It can be fine grained (aphanitic) to coarse (porphyritic) in texture” – because this is true for almost all the rocks, except for vitrious ones. Also, almost all rocks can be any shade of gray or greenish, but have a yellow to red appearance in other places. This would mostly be due to the inner and outer presence of iron oxyde – either as an additional component of the rock, or just “dyed in” by iron-bearing flowing water.

Gallery FELSIC ROCKS
Rhyolite – Dacite – Felsite

Rhyolite is a volcanic rock, of felsic (silicon-rich) composition (typically >69% silica). It may have any color from light gray to dark red or orange. Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcroppings of it often bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts form highly viscous lavas. Slower cooling forms microscopic crystals in the lava and results in textures such as glassy (obsidian), flow foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of volcaniclastic tephra or of ignimbrites. (© Dr. Bruce Martin, on his blog rossway.net/)

Complex flow-banding of Rhyolite lava, apparently on Kozushima Island, near the Izu-Islands in Japan. (Author not found)

Felsic rocks are produced primarily in convergent plate boundaries. Rhyolites tend to form domes or plugs rather than extensive lava flows. Image: Rhyolitic lava dome & lobes of Satsuma Iwo-jima Island in Kikai caldera (© Fumihiko Ikegami, ‏@fikgm, via Twitter)

Dacite is a felsic (or acid) rock intermediate in composition between andesite and rhyolite. It has a silica (SiO2) content between 63-68 wt%. In the hand specimen many of the dacites are grey or pale brown and yellow rocks with white feldspars, and black crystals of biotite and hornblende. Other dacites, especially pyroxene bearing dacites, are darker colored. Dacite tend to form thick blocky lava flows or steep-sided piles of lava (lava domes). Dacitic magma is quite viscous and therefore prone to explosive eruption. Dacitic magma is formed by the subduction of young oceanic crust under a thick felsic continental plate. (image via studyblue.com)

Grey, red, black, altered white/tan, flow-banded dacitic pumice. (Wikipedia)

The probably most famous dacite rock is the whaleback lava dome of Mt. St. Helens that emerged in 2005. Another example is the rock Akrotiri Lighthouse is perched upon – dacitic lavas, overlooking the caldera at Santorini (Greece) (© Kathryn, @kathryns5, via Twitter)

Felsite (or felstone) contains a very high 68-75% SiO2 depending on the amount of quartz in it. It is a very fine grained light colored felsic rock, usually white through light gray, or reddish to tan. The mass of the rock consists of a matrix of particularly quartz, sodium and potassium feldspar, often with phenocrysts of quartz. This rock is typically of extrusive origin, formed by compaction of fine volcanic ash, and may be found in association with obsidian and rhyolite. Image: Dendritic pyrolusite formations on felsite, from Elba, Italy. (Photograph taken at the Natural History Museum, London by Aram Dulyan)

In the industry felsites are used as acid-proof material and as a hardening additive for cement. Felsite is also used as a decorative stone, but this is limited for the difficulty of mechanical tooling. Image: Different colored pieces of felsites in a quarry at Mount Ararat, in the triangle Turkey-Armenia-Iran. (© en.ararat-stone.com)

Evidence of past volcanic activity: this is part of a spectacular white ring-dyke of felsite at Campa, Islay, Scotland. (Source: “Islay”, by Norman S. Newton).

Gallery: INTERMEDIATE and ALCALIC ROCKS
Trachyte – Andesite – Phonolite

The trachyte is a gray-bluish volcanic rock, but may be yellowish or brownish. It forms a viscous lava, giving the volcano an explosive character. The macrophotography (x1 scale) shows distinctly isolated macro-crystals of white feldspar and black amphibole (rods). These crystals are embedded in a gray-bluish groundmass of non-crystallized appearance. Under the microscope, this would show many microcrystals, the microliths. Chemically, trachyte contains 60 to 65% silica content; less SiO2 than rhyolite and more (Na2O plus K2O) than dacite. Glassy forms of trachyte (obsidian) are rare but have been found in Iceland, and pumiceous varieties are known in Tenerife and elsewhere. Trachyte (© Georges Grousset, via Banque nationale de photos en SVT)

Trachyte2 – Trachytes are well represented among the volcanic rocks of Europe. In Britain they occur in the Isle of Skye as lava flows, dikes or intrusions. They are also very common in continental Europe, as in the Rhine district and the Eifel, in Auvergne and in Bohemia. In the neighborhood of Rome, Naples and the island of Ischia trachytic lavas and tuffs are common. In the United States, trachytes crop out extensively in the Davis Mountains, Chisos Mountains, and Big Bend Ranch State Park in the Big Bend (Texas) region, as well as southern Nevada and South Dakota (Black Hills). In Iceland, the Azores, Tenerife and Ascension there are recent trachytic lavas, and rocks of this kind occur also in Australia (New South Wales and Queensland), East Africa, Madagascar, Yemen and in many other countries. Image: Columnar trachyte rock outcrop at Double Head near Rosslyn in Queensland, Australia. This part of Double Head is known locally as “Fan Rock”. (© Michael Zimmer, via Wikimedia)

Trachyte plug and dome. Location: Maui, Puu Koai Ukumehame (© Forest & Kim Starr, via Wikimedia)

Andesite is the intermediate type between basalt and dacite, and ranges from 57 to 63% silicon dioxide (silica, SiO2). Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. Also, the average composition of the continental crust is andesitic.

Andesite Boulder

Andesitic Wolf Rock, in the Oregon Cascades, is Oregon’s largest monolith. It is the remains (plug) of an ancient volcano. (© YocumRidge, via cascadeclimbers.com/forum)

Phonolite (also clinkstone in Engl.) is a rare rock of intermediate composition (49-63 wt% silica), so it is silica undersaturated. It may occur after certain tectonic processes like intracontinental hotspot volcanism. Phonolites are products of low degree partial melts. Image: Phonolite, Cripple Creek, Colorado. (© Michael P. Klimetz)

Hoodoo Mountain, a potentially active flat-topped stratovolcano in the Stikine Country of northwestern British Columbia, Canada. Hoodoo is a well-exposed example of peralkaline phonolitic ice-contact and subglacial volcanism. Image: The north side of Hoodoo Mountain features lava flows that were likely extruded under the glacier, giving them this interesting formation. (© Here fishy, via Wikipedia)

Bořeň (539 m) is a phonolite hill two kilometres south of Bílina in the Czech Republic. It is a structure similar to the Devils Tower in Wyoming, and is the largest phonolite rock of its kind in Europe.

Gallery: MAFIC and ULTRAMAFIC ROCKS
Basalt – Basanite – Komatiite – Picrite
Note: Volcanic ultramafic rocks are rare. Sub-volcanic ultramafic rocks may persist longer in maars and dykes, but are also rare. Because these magmas require the most heat to form, it is inferred that the Earth has cooled down since 2 Ga so that these magmas can no longer form.

Basalt is a dark, fine-grained igneous rock with generally 45-55% silica (SiO2). It is mostly very liquid and feeds effusive hawaiian/strombolian style eruptions. Due to its low viscosity, it often builds shield volcanoes and/or cinder cones. Basalt underlies more of Earth’s surface than any other rock type; most areas within ocean basins are underlain by basalt. Although basalt is much less common on the continents, lava flows and flood basalts underlie huge areas, making up several percent of Earth’s land surface.(© James St. John, via Flickr)

Also basalt: Lava tubes at Piton de la Furnaise on Réunion Island. See 1 franc coin for scale.

Devil’s Canyon, south-eastern Washington, USA. The canyon is cut through the flood basalts of the Columbia River Basalt Group. (© paul on hiketricities.com)

Basanite is a black-gray volcanic (extrusive) rock which occurs in sheets and flows. It is similar to basalt but is much lower in silica (42 to 45% SiO2) and high in alkalis (3 to 5.5% Na2O and K2O). The olivine content is higher than 10 wt%. Basanite magmas were often generated by direct partial melting of the mantle. (image from: sandatlas.org)

Basanite with olivine from the top of Gibret, an acient volcanic complex in southern France. (collection & © Daniel Marmet)

Basanite is the main material of dykes and domes in the volcanic region Westeifel in Germany. See person on the right edge for scale. (© WMF on magmatism.blogspot.de/)

Komatiite – The rare komatiites are significant in that most of them are older than about 2 billion years. Some geologists postulate that the oceanic crust on early Earth may have actually been komatiite rather than basalt as it is today. The youngest komatiites (at ~90 Ma) are from the island of Gorgona/Colombia. Komatiites are denser and contain much more magnesium and iron than basalts. A typical komatiite has 18 %wt magnesium, roughly double that of a typical basalt. Such a composition means that komatiite magmas are very hot, erupting at temperatures between 1300-1600°C. It is also very similar to the mantle, so if we were able to melt a good part of a mantle probe (maybe 50 to 60 percent of it!), we’d get a komatiite composition. – Left image: Komatiite with spinifex textures – between the Spinifex grass, which the textures are named after. Right image: Photomicrograph of a small, thin section of komatiite lava. The “spinifex texture” is a hallmark and considered unequivocal evidence of their ancient origin as molten rock that was extruded from deep in the Earth. (© Frank Beunk; and Igor Puchtel, UMD)

Picrite basalt, or picrobasalt, is dark with yellow-green olivine phenocrysts (20 – 50%) and black pyroxene, mostly augite. The olivine-rich picrites that occur with other more common basalts of Hawaiian volcanoes are the result of accumulation of olivine crystals either in a portion of the magma chamber or in a caldera lava lake. Picrite basalts are commonly erupted at oceanic island volcanoes such as Mauna Kea and Mauna Loa in Hawaiʻi, Curaçao and the Piton de la Fournaise on Réunion Island. Image: Picrite basalt (formerly called oceanite) found in the Grand Brûlé (SE slopes of the Piton de la Fournaise) on Réunion Island. (© David Monniaux, via Wikipedia)

Gallery: CARBONATITES

Carbonatites are predominantly composed of carbonate minerals. They are rare, peculiar igneous rocks formed by unusual processes and from unusual source rocks. The origin of carbonatite magma is obscure. Most carbonatites occur close to intrusions of alkaline rocks (those rich in potassium or sodium relative to their silica contents) or to the ultramafic igneous rocks. These associations suggest a common derivation, but details of the way that carbonatite magmas might concentrate geochemically scarce metals remain conjectural. There are about 330 known occurrences of carbonatites worldwide. They often occur as minor members of larger intrusions of silicate igneous rocks, commonly referred to as ‘carbonatite complexes’. Most are calcite carbonatites. Carbonatites are, almost exclusively, associated with continental rift-related tectonic settings. In historical time only one volcano actively erupting carbonatite to the surface is known to us, Oldoinyo Lengai in Tanzania. Image: Extrusive calciocarbonatite (calciocarbonatitic lapillistone; welded calciocarbonatite, 7.4 cm across), erupted during the Miocene from Germany’s extinct Kaiserstuhl Volcano. This carbonatite is special: it was the first ancient carbonatite ever that demonstrated to have originally been erupted onto the surface. (© James St. John, via Flickr)

Carbonatite, Oldoinyo Lengai, Tanzania: Due to its low temperature (just over 500°C) the molten lava appears black in sunlight, rather than having the red glow common to most lavas. It is also much more fluid than silicate lavas, often less viscous than water. The sodium and potassium carbonate minerals of the lavas erupted at Oldoinyo Lengai are unstable at the Earth’s surface and susceptible to rapid weathering, quickly turning from black to grey in colour. The resulting volcanic landscape is different from any other in the world. (© Thomas Kraft, Kufstein, via Wikimedia)

Carbonatite3 – Carbonatites have been found on all continents; they also range widely in age, from quite recent deposits in the East African Rift Valley to South African deposits dating from the early Proterozoic Eon. Many carbonatites are mined. Among the most important sources are Mountain Pass, California, U.S., a major source of rare earths; the Loolekop Complex, Palabora, South Africa, mined for copper and apatite, plus by-products of gold, silver, and other metals; Jacupiranga, Brazil, a major resource of rare earths; Oka, Quebec, Canada, a niobium-rich body; and the Kola Peninsula of Russia, mined for apatite, magnetite, and rare earths. Image: Oldoinyo Lengai, Tanzania: While most lavas are rich in silicate minerals, Oldoinyo Lengai produced natrocarbonatitic and nephelinite tephra during the Holocene. They are rich in the rare sodium and potassium carbonates, nyerereite and gregoryite. Due to this unusual composition, the lava erupts at relatively low temperatures of approximately 510 °C (950 °F). Oldoinyo Lengai’s uniqueness was not recognized before Dawson’s work in 1962. (© Tobias Fischer, University of New Mexico)

Freshly erupted carbonatite lavas are pitch black before they cool and oxidize into their whitish appearance. (Image © Dr Richard Roscoe, photovolcanica.com)

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Not only make the rocks things difficult for us when the same rock type displays different textures. Or when different rock types have the same chemical composition. They find yet another way to fox us: different rocks with different textures can form very similar structures. Following the physical laws, almost all lavas can form the well known (more or less) hexagonal columns, for example. Like these two:

(1) Inharan peak, a 1732 m high trachytic volcanic plug in Algeria dominating the town of Tamanrasset. (© Jacques Janin), and (2) the phonolitic Devils Tower in Wyoming, U.S.

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Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.

Enjoy! – GRANYIA

SOURCES & FURTHER READING

– Wikipedia: Igneous rock
– Sandatlas
– Wikipedia: Rock microstructure
– General Classification of Igneous Rocks (2011)
– A Browser Flow Chart […] of Igneous Rocks
– Roches et Minéraux du Languedoc Roussillon
– Geowords (Hugh Rance’s geological website)
– Erik Klemetti: Word of the Day: Dacite
– Magmatismus und Geodynamik Deutschlands (German)
– Virtual Geology Museum Gallery C
– Geo images on Flickr – w/ explanations, by James St. John
– Classification of Igneous rock
– Igneous Rocks and Igneous Processes
– The Lava That Doesn’t Erupt Anymore (E. Klemetti)
– Geological Introduction -2- Igneous Rocks (Blog)

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