Basaltic…? Felsic…? Porphyritic…? Confusion!

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

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.

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)

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)

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:

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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).

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)

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).

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

Gallery: INTERMEDIATE and ALCALIC ROCKS
Trachyte – Andesite – Phonolite

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.

Gallery: CARBONATITES

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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.

(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)