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Dangers of Lava Domes

While lava dome formation may not seem like a major hazard because the average person can outrun them, there collapse is one of the most hazardous volcanic phenomena in the world.

Lava domes form when viscous magma erupts effusively onto the surface. This viscous material then piles up on the surface or at the volcanic edifice. The shape or structure of these features is controlled by rheological properties and mass eruption rate. At one end, volatiles stay in the melt, limited crystallization creates pancake domes or obsidian flows. On the other, very slow ascent rates favor degassing and crystallization creating a high viscosity and non-Newtonian rheologies. The lava often solidifies as a rigid spine. Think, Mt St Helens lava spine in 1983 or the 2005 whaleback. Domes may be subdivided into different categories on the basis of morphology, surface texture, and eruptive style. Steep sided domes are referred to as Pelean or spiny dome. These are circular in plane view and have a relatively smooth upper surfaces punctuated by tall vertical spines. Other domes can be less steep and are composed of a series of distinct lobes the emerge sequentially from a vent. A third type are platy domes which are characterized by a blockier carapace, fewer and shallower fractures, and many transverse surface ridges. Finally, domes that have nearly level upper surfaces, highly irregular outline and surfaces covered in small blocks and linearly extensive surface corrugation are called axisymmetric domes because their shapes tend to be controlled by local topography. Coulee is a term saved for an extrusion of lava that has aspects of a dome and a flow. basically a lava dome that has experienced some flow away from the vent.

The hazardous part about these volcanic features is what happens when they collapse. Lava domes collapse under two main mechanisms: superficial explosion or gravitational failure. Superficial eruptions occur when the pore pressure within the dome exceeds the tensile strength of the out carapace of the dome. Overpressurization can occur two ways: formation of zones of highly concentrated water due to rapid microlite crystallization caused by undercooling during eruption and continued degassing of the conduit. When the collapse occurs, it creates something called a block and ash flow. Block-and-ash flows commonly extend up to 10 km from their source at speeds up to 100 km/h and are accompanied by ash cloud surges, which reach a wider areal distribution compared to the valley-confined block-and-ash flows. A seared zone where little or no ash is deposited but vegetation is burned fringes the ash cloud surge deposits. The hot ash-cloud surges can knock down trees and destroy houses. They are the most dangerous aspects of block-and-ash flow activity, because they are less controlled by the topography and can detach from the channelized block and ash flows. Block-and-ash flow deposits differ from pumiceous ignimbrites by containing little fine ash but a large fraction of dense to moderately vesicular, rarely pumiceous, blocks up to several meters in diameter that are derived from the juvenile source dome and possibly from older dome remnants.

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