In
volcanology, a
lava dome is a roughly circular mound-shaped protrusion resulting from the slow
extrusion of
viscous lava from a
volcano. The
geochemistry of lava domes can vary from
basalt to
rhyolite although most preserved domes tend to have high
silica content.
The characteristic dome shape is attributed to high viscosity that prevents the lava from
flowing very far. This high viscosity can be obtained in two ways: by high levels of
silica in the magma, or by
degassing of
fluid magma. Since viscous
basaltic and
andesitic domes
weather fast and easily break apart by further input of fluid lava, most of the preserved domes have high silica content and consists of rhyolite or
dacite
Lava domes are
dynamic structures that evolve over time undergoing various processes such as growth, collapse, solidification and
erosion.
Lava domes grow by
endogenic dome growth or
exogenic dome growth. The first one implies dome interior expansion to accommodate new lava and the second one refers to superficial piling up of lava.
It is the high viscosity of the lava that prevents it from flowing far from the vent from which it extrudes, creating a dome-like shape of sticky lava that then cools slowly in situ. Domes may reach heights of several hundred meters, and can grow slowly and steadily for months (e.g.
Unzen volcano), years (e.g.
Soufrière Hills volcano), or even centuries (e.g.
Mount Merapi volcano). The sides of these structures are composed of unstable rock
debris.
Due to the intermittent build up of gas
pressure, erupting domes can often experience episodes of
explosive eruption over time. If part of a lava dome collapses while it is still molten, it can produce
pyroclastic flows, one of the most lethal forms of volcanic event. Other hazards associated with lava domes are the destruction of property,
forest fires, and
lahars triggered by pyroclastic flows near
mud,
snow and
ice. Lava domes are one of the principal structural features of many
stratovolcanoes worldwide.
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| The Chaitén Volcano seen from a commercial flight, October 2008. |
Characteristics of lava dome eruptions include shallow, long-period and hybrid
seismicity, which is attributed to excess fluid pressures in the contributing vent chamber. Other characteristics of lava domes include their hemispherical dome shape, cycles of dome growth over long periods, and sudden onsets of violent explosive activity. The average rate of dome growth may be used as a rough indicator of magma supply, but it shows no systematic relationship to the timing or characteristics of lava dome explosions.
