We refer to lavas a lot when discussing volcanic activity – it’s a bit difficult not to! This page summarises the lavas we are encountering on our journey round volcanic regions. It’s work in progress at the moment.
Lava’s main components are silicates or carbonatite; the latter is rare – most lavas are silicates. We start by looking at the main silicate lavas, noting for each both their tectonic settings and well-known examples of volcanoes with these lavas as their dominant one. This is followed by carbonatite lava.
All percentages mentioned throughout are percentage by volume of the rock.
Silicate Lavas
We are structuring this section round the TAS diagram used for geochemical analysis of rocks (TAS standing for total-alkalis vs silica). Here lavas are classified according to their chemical composition, focussing on the silicate, alkali (sodium and potassium oxides) and other mineral content.
GVP, in its description of Holocene volcanoes, includes the major and minor lava types. We have summarised the dominant lava types by tectonic setting from the publicly available volcano list, downloaded as at 8 August 2021.
Basalt / Picro-Basalt lavas dominate intra-plate and rift zone settings, while andesite dominates subduction zone settings.
Andesite / Basaltic Andesite
Andesites are found mostly in subduction zones, being the dominant rock type in island arcs. Well-known andesite / basaltic andesite volcanoes are Whakaari (White Island) in New Zealand, Taal in the Philippines, Krakatau in Indonesia, and, Hekla in Iceland.
Andesite
Andesite is an intermediate rock between basalt and rhyolite, low in alkali metals. The plutonic equivalent is diorite. The texture may be fine grained or porphyritic, containing large phenocrysts. The silica content ranges from 57% to 63%. Less than 20% is quartz and less than 10% feldspathoid. 65% of the feldspathoid is plagioclase.
| Rocks include: | Andesite | Named after the principal phenocryst. |
| Composition: | Sodium-rich plagioclase, Pyroxene or hornblende | Andesine is the typical plagioclase. Plagioclases may vary from anorthite to oligoclase. Pyroxenes include augite, pigeonite or orthopyroxene. |
| Occurrence & Formation | Found in subduction zones and island arcs. Formed by fractional crystallisation of mafic magma, partial melting of the crust, magma mixing of basalt and rhyolite, or, partial melting of metasomatized mantle. | Andesite makes up the average composition of continental crust. |
Basaltic Andesite
Basaltic andesite is a fine-grained igneous rock, intermediate in composition between basalt and andesite with a silica content between 52% and 57% and low alkali content. The feldspathoid and quartz content are less than 10% and 20%, resp.
| Rocks include: | Basaltic Andesite | |
| Composition: | Augite, Plagioclase | |
| Occurrence & Formation | Formation as for andesite. Found in volcanic arcs, back-arc basin and extension of the crust. |
Basalt / Picro-Basalt
Basalt is found in all tectonic settings. Well-known basalt / picro-basalt volcanoes include Mauna Loa and Kilauea in Hawaii, USA, most of Iceland’s active volcanoes such as Askja, Grimsvötn, Eyjafjallajökull and Katla, and Mount Fuji in Japan.
Basalt
Basaltic magma is the main product of decompression melting of the upper mantle. Basalt is erupted at both constructive and destructive plate boundaries, intraplate and in continental break up. The setting in which basalt is erupted determines its characteristics.
Basalt is a fine grained igneous rock with a silica content between 45% to 52% with less than 10% feldspathoid and less than 20% quartz; plagioclase accounts for 65% or more of the feldspar content. Basalts are divided into alkaline basalt and tholeiitic basalt based on their alkali and silica contents.
| Rocks include: | Alkaline Basalt Olivine Tholeiitic Basalt Quartz Tholeiitic Basalt | |
| Composition: | Olivine, pyroxene, plagioclase feldspar | Alkali basalts are silica under saturated, containing nepheline, olivine and pyroxene. Olivine tholeiite basalts are silica saturated, containing olivine and pyroxene. Quartz tholeiitic basalt are silica oversaturated, containing hypersthene. High alumina basalt has a high alumina content and is intermediate between alkaline and tholeiitic basalt; phenocrysts of plagioclase are not present. Boninite is a high magnesium basalt / andesite. |
| Occurrence & Formation | Formed by melting of the upper mantle. Found in: Alkaline Basalt Oceanic Islands Continental Rifting Hotspots Tholeiitic Basalt Mid Ocean Ridges Oceanic Island (predominant in early volcanism) Continental Flood Basalt High Alumina Basalt Volcanic arcs above subduction zones Boninite Back-arc basin |
Picro-Basalt
Picro-Basalt is a high magnesium olivine basalt with visible olivine crystals.
| Rocks include: | Oceanite | |
| Composition: | Crystals of olivine, augite pyroxene and plagioclase in a ground mass of augite, sodic plagioclase | |
| Occurrence & Formation | Forms with the accumulation of olivine crystals in part of a magma chamber or in a caldera lava lake. Found in hotspots and ocean islands e.g. Mauna Kea, Mauna Loa, Hawaii; Curaçao; and, Piton de la Fournaise, Réunion Island |
Dacite
Dacite occurs predominantly in subduction zones. Well-known dacite volcanoes include Santorini, Greece, and Mount St. Helens, USA.
Dacite is a fine-grained igneous rock, formed by rapid cooling of lava high in silica and low in the alkali oxides. The silica content ranges from 20% to 60%. Plagioclase makes up 65% or more of the feldspar content. It is thought to explain the formation of continental crust in the Archean.
| Rocks include: | Dacite | |
| Composition: | Plagioclase Quartz Alkali feldspar Accessory minerals include: Biotite, Hornblende, Pyroxene (augite or enstatite), Garnet (rare) | Plagioclases range from oligoclase to andesine and labradorite |
| Occurrence & Formation | Formed by fractional crystallisation of more mafic magma or assimilation of sialic continental crust. Found mostly in subduction zones where oceanic crust subducts under continental crust, intra ocean island arcs, active continental margins, continental large igneous provinces and rift zones and ocean spreading centres. |
Foidite
Foidite occurs predominantly in rift zones. Well-known foidite volcanoes include Mount Nyiragongo in the Democratic Republic of the Congo, Fogo in Cape Verde and the West Eifel Volcanic Field in Germany.
Foidite is a fine grained igneous rock with a low silicate content which gives them an unusually low viscosity. These lavas cannot be outrun, unlike other lava types. Lavas erupted from Mount Nyiragongo have reached speeds of 60km – 100km with devastating consequences and temperatures in the order of 1370°C in the higher sections of its fissures.
| Rocks include: | Nephelinite, leucitite, analcite and kalsilite | Rocks are named after the dominant feldspathoid |
| Composition: | Clinopyroxene with a feldspathoid | |
| Occurrence & Formation | Formed by melting of the upper mantle. Found in continental rifts, intra-plate and subduction zones. The source is deeper that than for basanites and alkali basalts |
Phonolite
Phonolite is rare; most occur in intraplate settings. A well-known phonolite volcano is Tenerife, Spain. Mount Erebus in Antarctica has a phonolitic lava lake.
Phonolite is a fine-grained intermediate igneous rock consisting of alkaline feldspar and nepheline and / or other feldspathoids, so named because of the metallic sound produced when an unfractured plate is hit.
| Rocks include: | Phonolite | |
| Composition: | Alkali feldspar (sanidine or anorthoclase), nepheline | Other feldspathoids may be present: Leucite Analcite Aegirine Riebeckite |
| Occurrence & Formation | Formed by low degree of partial melting of highly aluminous rocks of the lower crust and metamorphic rocks. Found in oceanic and intercontinental hotspots. |
Phono-tephrite and Tephri-phonolite
Another rare lava. Examples include Mount Erebus in Antarctica, Mount Vesuvius in Italy, and, Ambitle in Papua New Guinea.
Phono-tephrite and tephri-phonolite are igneous rocks with composition between phonolite and tephrite with 45% to 57% silica content and 7% to 14% alkali oxide content.
Rhyolite
Rhyolite occurs in both subduction zones and rift zones. Well – known examples of rhyolite volcanoes are Yellowstone in the USA, Torfajökull in Iceland and Novarupta in the USA.
Rhyolite is an igneous rock with a high silica content. Granite is the plutonic equivalent. 20% to 60% of the rock is quartz. Alkali feldspar makes up 35% to 90% of the feldspar content. Rhyolite is very viscous so tends to be erupted as pyroclastic rock rather than lava flows.
| Rocks include: | Rhyolite | |
| Composition: | Quartz , Alkali feldspar, Plagioclase . | Sanidine, or less commonly, orthoclase is the feldspar. Oligoclase or andesine is the plagioclase Accessory minerals include Cristobalite, trydimite, biotite, augite, fayalite and hornblende. |
| Occurrence & Formation | Formed from fractional crystallisation of more mafic magmas, assimilation of melted crustal rock or melting of crustal sedimentary rock. Found in any environment where sub alkali basalt is erupted. Most common above subduction zones – island arcs and continental margins. |
Trachyandesite / Basaltic Trachyandesite
Trachyandesite / Basaltic Trachyandesite lavas occur mainly in subduction zones. Well-known volcanoes in this category include El Chicon in Mexico and Stromboli in Italy.
Trachyandesite
Trachyandesite is an igneous rock with a composition between trachyte and andesite, a silica content of c 58% and alkali oxide content of 9%.
| Rocks include: | Benmoreite Latite Tristanite | |
| Composition: | Sodic plagioclase, Alkali feldspar, Augite. | Plagioclase is usually andesine. Alkali feldspar makes up more than 10%. Accessory minerals include amphibole, biotite, pyroxene, apatite. Small amounts of nepheline may be present. Magmas have a high sulphur content from anhydrite phenocrysts. |
| Occurrence & Formation | Formed from the fractional crystallisation of alkali basaltic magmas. Found in continental rift valleys, hot spots and late eruptions of ocean islands. |
Basaltic trachyandesite
Basaltic trachyandesite is an intermediate rock between trachybasalt and trachyandesite, includes mugearite and shoshonite.
Trachybasalt / Tephrite – Basanite
Trachybasalt / Tephrite – Basanite occur in all settings with about half of them in intraplate settings. Well-known examples include Mount Etna in Italy and Mount Tambora in Indonesia.
Trachybasalt
Trachybasalt is a fine-grained igneous rock intermediate between trachyte and basalt with an alkaline oxide content between 5% and 7%.
| Rocks include: | Hawaiite Potassic trachybasalt Anorthoclase basalt | |
| Composition: | Calcic plagioclase, augite, alkali feldspar. | Accessory minerals include olivine, clinopyroxene, leucite or analcime. |
| Occurrence & Formation | Found at intra-plate or constructive margins |
Tephrite
Tephrite is an igneous rock with aphanitic to porphyritic texture.
| Rocks include: | Tephrite | |
| Composition: | Calcic plagioclase, augite, nepheline. | Olivine may be present; to be classed as tephrite, the olivine content is less than 10%. Accessory minerals include pyroxenes. |
| Occurrence & Formation | Found in oceanic hotspots, e.g Pico Teide, Tenerife |
Basanite
Basanite is a fine-grained igneous rock that is low in silica and enriched alkali material.
| Rocks include: | Basanite | |
| Composition: | Calcic plagioclase, augite, nepheline, olivine | More than 10% olivine. |
| Occurrence & Formation | Characteristic of late alkaline phase in the rejuvenation of volcanic islands 3 to 5 million years after the shield building phase. Also seen in early pre-shield volcanism. Found in oceanic hot spots and rift zones. |
Trachyte / Trachydacite
Trachyte / Trachydacite volcanoes occur in mainly intraplate or rift zones. Well-known examples include Campi Flegrei in Italy and Rungwe in Tanzania.
Trachyte is a fine-grained igneous rock, often porphyritic, with a silica content between 60% and 65% and alkali oxide of more than 7%. Trachydacite has a quartz content of more than 20%
| Rocks include: | Trachyte Trachydacite | |
| Composition: | Alkali feldspar Plagioclase, quartz or felspathoid may be present. Accessory minerals include biotite, clinopyroxene and olivine. | If quartz is present, the lava is referred to as quartz trachyte. |
| Occurrence & Formation | Formed from the fractional crystallisation of alkaline basaltic magma. Common when alkali basalts erupt. Found in continental rift / hotspot and the late stages of ocean island volcanism. |
Carbonatite Lava
Carbonatite is an igneous rock made up of carbonate minerals which have crystralised from carbonate melts. The carbonates are calcite, dolomite, siderite, ankerite and sodium carbonate minerals such as nyerereite.
Carbonatite lavas occur at continental rifts and less often in orogenic zones (Himalayan thrust belt in Pakistan); they have also been found at Fuerteventura, Cape Verde Islands and the Kerguelen archipelago.
The only known active carbonatite volcano is Ol Doinyo Lengai, Tanzania, which erupts natrocarbonatite lava and tephra. Natrocarbonatite has a very low viscosity and erupts at c. 590°C.
Armchair Volcanologist
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Sources and Further Reading
R. Gill, “Igneous Rocks and Processes, A Practical Guide”, Wiley-Blackwell, 2010
Global Volcanism Program, Smithsonian Institution: https://volcano.si.edu
Wikipedia:
An Armchair Volcanologist 
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