Lava domes are rounded, often steep-sided mounds or spines formed from thick or viscous lava extrusion from volcanic vents. This lava cools around the vent since it is too thick to flow far.
Depending on the lava type they form from, they may be bulging dome-shaped (half-sphere), spine, plug, pancake, or irregular mounds. These mounds may be circular, elliptical, or with irregular outlines.
Usually, lava domes are a few meters to several kilometers in diameter and a few meters to over a kilometer in height. However, most are relatively small.
Also, they don’t have a crater and represent about 6% of total volcanic eruptions.
How do they occur? Usually, lava domes can occur alone or in linear or curved chains, some 20 km long on Earth and 150 km on Venus.
Surface and interior appearance
Lava domes can be uniform or vary in texture and color. Most have aphanitic or fine-grained texture and may have vesicles or voids left as volatiles escape.
However, very thick ones can have a coarse-grained interior due to slower cooling rates.
On the contrary, those that cool fast may form obsidian. Obsidian is a natural volcanic glass, i.e., an amorphous solid that didn’t crystallize.
1. Surface
The surface of lava domes is blocky, resembling blocky lava flows. It has up to 5 meter-sized angular to slightly smooth blocks. However, some have a smooth or cracked surface.
Also, this surface may have explosion pits formed far from the vent. Such disrupts flow.
Lastly, some can have surface ridges from compressions parallel to the flow and crease structure.
2. Interior
A cross-section of a young lava dome may show subparallel flow bands and joints.
Flow bands can be large and visible in less than a millimeter scale. These bands may vary in texture, vesicularity, crystallinity, and chemical composition. Chemical variation may indicate the mingling of magma during extrusion.
A good example of flow banding is the Aliso lava dome in southern Arizona.
On the other hand, joints may be columnar, polygonal, or have a sheetlike joint pattern. Also, some may show concentric or onion-skin foliation outward or a fanlike pattern, platy or subparallel.
These joints form from cooling thermal stress.
Types
There are various types of lava domes. Examples are Peléan, upheaved plugs, and Cryptodomes. More examples include comulo-dome, platy, lobate, or coulees.
Lava dome composition
Lava domes usually form from any highly viscous lava with intermediate to felsic or acidic composition. This lava may be rhyolitic, dacitic, trachytic, or andesitic.
Felsic or acidic magmas are high in silica and light-colored elements or minerals. These elements are silicon, oxygen, sodium, and potassium, while minerals are feldspar, quartz, and muscovite.
Intermediate has a composition between felsic and mafic. Mafic magmas are low in silica, light-colored elements, or minerals and high in iron and magnesium. Such magmas are less viscous and will form shield volcanoes.
For specific examples, Inyo Lava domes are rhyolitic, Mount Unzen in Japan erupted dacitic, while Obsidian Dome has pumice and rhyolite.
However, basaltic lava domes are known to occur in some mid-ocean ridges or sea spreading centers and seamounts.
How do lava domes form?
Lava domes form when degassed or low volatile, highly viscous lava extrudes and piles around the vent without flowing away.
The high viscosity of this thick, pasty lava prevents it from flowing or slipping. Therefore, it will build steep-sided spines, domes, or mounds around the vent.
Most lava domes form from effusive eruptions involving degassed or low-volatile viscous silicic to intermediate lava. Effusive rates can be less than 1 m3 to more than 100 m3.
However, it may form after explosive eruptions when most gases have escaped. These kinds form towards the end of an eruption.
During formation, these domes grow in two ways: endogenous or exogenous. In endogenous growth, the dome expands as more lava intrudes interior parts without reaching the surface,
On the other hand, exogenous growth involves the formation of successive lava lobes at the vent’s summit. These lobes are discrete and pile on top of each other.
These growths can be sudden or episodic or take months or even years. Some are usually repetitive, such as seen in Redoubt Shield Volcano in Alaska, USA, and Chaitén Volcano in Chile.
For instance, Mt. St. Helens formed a volcanic dome during the 1980 eruption. This dome grew in 1983, increasing its height by 200 feet. Again, after 18 years of quietness, its eruption began in 2004, and a new dome was developed.
Lastly, the high viscosity and associated energy cause the surface of the domes to fracture and form a blocky surface.
Explosion and collapse or destruction
Actively growing lava domes formed may explode or collapse. These can partially or fully destroy the dome.
Collapse and explosion may occur when the dome is unstable or has plugs that trap gas, more like a champagne cork. Internal pressure buildup can then blow the plug, causing an explosion.
Lava dome explosion and collapse will form pyroclastic flows or currents downslope. These avalanches of hot, fragmented materials, including blocks, ash, and gases, are hazardous.
Pyroclastic flows form from decompression and fragmentation of lava due to an explosion or collapse. They represent some of the dangerous volcanic hazards. We will see more under hazards.
Also, the outer layers or part of the steep-sided slopes may crumble and fall as the dome expands. Such may produce small pyroclastic flows with large blocks.
Lastly, dome destruction may happen if part of it breaks. This can cause landslides.
Notable examples
Lava domes commonly form viscous lava eruptions worldwide. Usually, they form on any calderas or craters of stratovolcanoes.
Some notable lava domes in the USA are Lassen Peak (one of the largest in the world), Obsidian Dome, Black Butte, Inyo Lava domes, and Chaos Crags, which has a row of domes, all in California. Others are at Mt. St Hellen in Washington, Wizard Island in Oregon, and Novarupta in Alaska.
Elsewhere, they occur in Tate-iwa in Japan, Mount Merapi in Indonesia, and Soufrière in Montserrat, West Indies. Others are Santiaguito and Galeras in Guatemala, Sollipulli in Chile, and Tata Sabaya in Bolivia.
Hazards
Since they grow or advance slowly, lava domes don’t pose a burial risk to nearby populations.
However, explosions or collapse of hot, molten, weakly consolidated lava can result in deadly 1) pyroclastic flows or 2) glowing dust and ashes known as nuée ardentes. Such will cause the death of people and animals and damage properties or infrastructure.
Hazards from the formation, collapse, and destruction of lava domes represent some of the deadliest volcano eruption hazards.
Some recent fatalities include 44 people at Mount Unzen in Japan in 1991 and at Mount Merapi in Indonesia in 1994, which claimed 66 lives.
Also, Soufriere Hills in Montserrat claimed 19 lives in 1997. Furthermore, the collapse of Soufrière Hills Volcano in 1995 made about half of Montserrat inhabitable.
Others are Mount Pelée in Martinique (1902), 29,025 fatalities; Soufrière (1902), 1,600 deaths; Santiaguito in Guatemala (1929), 1,000 fatalities; and Merapi (1954), 64 deaths (1976) 28 fatalities.
Frequently Asked Questions (FAQs)
Lava domes are small mounds of thick, steep-sided piles of volcanic rocks formed from viscous lava flow emplaced around the vent. In contrast, shield volcanoes are low-profile volcanoes with thin lava formed from highly fluid lava.
References
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical geology (15th ed.). McGraw-Hill Education.
- Huff, W.D & Owen, L.A. (2013). Volcanic Landforms and Hazards. In Shroder, J. (Ed). Treatise on geomorphology (Vol. 4 pp. 160-161). (2013). Elsevier.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Huggett, R. J. (2011). Fundamentals of Geomorphology (3rd ed.). Routledge.