Rootless cones, pseudocraters, or secondary vents describe craters, vents, or volcanic landforms without a conduit connecting them to magma chambers beneath the surface.
These volcanic landforms were first recognized in Iceland on lava flowing over ice or water-rich sediments. However, the first observation occurred at Eyjafjallajökull 2010 in the same country.
Rootless cones or vents include littoral cones, hornitos, and explosion pits. Usually, they are smaller than cinder or spatter cones. However, they differ from these two as they lack a conduit to underground magma chambers.
How do rootless vents or cones form?
Rootless cones form in two ways. The first is via a phreatomagmatic eruption that occurs when magma interacts with water, ice, or wet sediments. This is how littoral cones and some explosion pits form.
Factors that control littoral cone size are water and lava availability. Water availability and high discharge rates result in larger ones. In contrast, limited lava supply and water will create smaller ones.
The other way involves pahoehoe, or tube-fed lava flows. These rootless cones form when lava squeezes and spatters from cracks on top of lava tubes. Spattered lava will pile up to create hornitos.
Types of rootless cones and craters
There are three main types of rootless craters and cones: littoral cones, hornitos, and explosion pits.
Depending on the type, these volcanic landforms may have scoria, ash, volcanic bombs, lapilli, and spatter.
Their size ranges from small to about a hundred meters tall and several hundred meters in diameter at the base, and most involve basaltic and other low-viscosity mafic magma.
Here is more on each type:
1. Littoral cones
Littoral cones are small, conical-shaped, rootless hydrovolcanic landforms. They form when flowing lava interacts with water, ice, or wet sediments.
This interaction causes a phreatomagmatic eruption, i.e., it makes water flash to steam, causing a steam explosion that will eject steam, water, and pyroclasts like ash, lapilli, and volcanic bombs.
Lastly, littoral cones occur in the lava flow field, especially on shores, seas, oceans, or other water bodies. Also, some may form where lava flows over wet sediments or ice.
2. Hornitos
Hornitos are small, conical, or vertical pipe-like structures built above lava tubes. They have no roots and are associated with tube-fed pahoehoe lava flow fields.
The word hornito is Spanish and means little ovens. These cones are often less than 10 meters high and tens of meters wide. However, larger ones exist, such as in Wudalianchi National Park, China.
Lastly, hornitos form when magmatic pressure forces and spatter magma from cracks on top of lava tubes. These magmas will accumulate and solidify, forming hornitos.
3. Explosion pits
Explosion pits are rootless craters that don’t have ramparts or raised walls on their side. It is possible that their walls may have been eroded or buried by subsequent landforms.
Examples
Examples of rootless cones and vents include Skútustaðagígar pseudocraters near Lake Mývatn, Rauðhólar, Landbrotshólar, and Álftaversgígar in Iceland.
Also, they occur in Hawaii and Colombia River Flood Basalts in the USA and Galapagos Island.
Rootless pitted cones on Mars
The Ascraeus and Olympus Mons on Mars are probably rootless cones, considering their resemblance to those on Earth. However, it is difficult to determine if they are the vents like primary vents or roots.
References
- Németh, K. (2015). Hornito. In Hargitai, H. & Kereszturi, Á. (ed) Encyclopedia of planetary landforms (2nd ed. pp. 940-943). Springer.
- Boreham, F., Cashman, K., Rust, A., & Höskuldsson, Á. (2018). Linking lava flow morphology, water availability, and rootless cone formation on the younger Laxá Lava, Ne Iceland. Journal of Volcanology and Geothermal Research, 364, 1–19. https://doi.org/10.1016/j.jvolgeores.2018.08.019