Lava domes are steep-sided mounds of volcanic rocks formed by extrusion and piling of thick lava around the vent. The high viscosity (resistance to flow) prevents lava from flowing. However, in one type, lava flows a little.
These volcanic domes usually come in various sizes, shapes, and plans. However, most are small. Their surfaces are often blocky, but some are smoother, and the interior may show flow bands and joints.
Here are the common lava dome types you should know:
1. Spiny or Peléan domes
Peléan domes are very steep-sided and have a sharp, craggy spine at their top. Their base has debris from the collapse of the dome’s side.
These domes are named after Mt. Pelée in Martinique, where a famous structure grew on a crater after the 1902 explosive eruption that characterized it. Another example is Santa María or Santiaguito in Guatemala.
Peléan domes have a smooth upper surface and a circular plan view. Also, their cross-sectional profile is smaller.
2. Coulées or axisymmetric domes
Coulées are a type of lava domes with asymmetrical lava distribution around the eruption vent formed when extruded lava flows slightly downslope.
Usually, coulées represent a transition between lava domes and lava flow. Some authors may describe them as a hybrid since they show traits of lava flow and domes.
Where do they form, and how? Coulées are common in eruptions that are on steep slopes. These slopes give the lava shear stress, allowing it to move downward to some degree.
Usually, these domes have irregular outlines, and the outer surface will have Ogives. Ogives are huge or extensive pressure ridges. Also, they often have small blocks on their surface.
Coulées examples include Big Obsidian Flow, Newberry Volcano in Oregon, and Little Glass Mountain on Medicine Lake Volcano in California, USA. Another example is Chao lava in North Mexico.
3. Upheaved plugs
Upheaved plugs, pistons, or plug domes are rare, near-vertical dome types formed from more viscous or stronger lava. They resembles monoliths projecting from the ground and may have a top with country rock.
These plugs form when a section of lava with high yield strength is pushed upward like a piston. It may carry some country rock on its upper side during this process.
Upheaved plugs were seen in the Usu volcano in 1910 and 1943. These plugs were named Meiji Sin-Zan (Roof Mountain) and Showa Sin-Zan (New Roof Mountain).
4. Cryptodomes
Cryptodomes form when magma intrudes near but beneath the surface. This intrusion causes the surface to bulge. A good example was observed in the northern part of Mt. St. Helen in 1980.
One unique feature of this dome type is that molten magma doesn’t reach the surface. Instead, it pushes the surface above it upwards, forming a dome-like bulge.
Lastly, cryptodome literally means hidden or secret dome, as it is invisible. Part of the name comes from the Greek word Kryptos, which means hidden or secret.
5. Low lava dome
These domes are symmetrically formed around the vent and have a subdued profile.
They form from the outward displacement of previously created material as more magma intrudes internally.
6. Tortas
Tortas are circular, flat-topped volcanic domes resembling tortas or cakes. They occur worldwide and erupt in relatively flat places.
During their formation, lava pushes upwards but not far up.
7. Lobate domes
Lobate domes are less steep-sided and have lobes emerging sequentially from the vent.
These lava domes may have small blocks on some portions and a smooth surface on others. Also, their cross-section profile is smaller and plan more irregular.
Examples are domes formed at Mount St. Helens, Katmai, and Pinatubo after the Plinian eruptions. Another example is the 13 domes formed at Redoubt volcano in the 1991 eruption.
8. Cumulo-domes
Cumulo-domes is a general term for low volcanic domes that resemble upturned bowls. They are usually isolated.
Examples are Puy Grand Sarcoui in the Auvergne, France, and tortas (‘cakes’) of the central Andes or Lassen Peak in California, USA. Another is mamelons of Réunion, in the Indian Ocean.
9. Tholoids
These are cumulo-domes within large craters. Their growth is associated with nuée ardente eruptions. An example is Mount Egmont, New Zealand.
10. Platy or endogenous domes
Platy domes have a much lower profile and a blockier surface. Also, they have many transverse ridges on their surface.
Examples of platy domes occur in Mount Merapi in Indonesia, Soufrière at Saint Vincent, and the Grenadines volcano.
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.