A lava dome is a mound-like structure created when thick, pasty lava is slowly extruded from a volcanic vent and piles up around the opening. Lava domes often form near or within the crater of a larger volcano, such as a stratovolcano, but they can also occur as isolated features on a volcanic field. Their presence signifies a style of eruption that is typically slow and effusive, although the resulting structures are inherently unstable and pose significant hazards.
The Role of Viscosity in Lava Dome Formation
The formation of a lava dome is directly controlled by the high viscosity, or “stickiness,” of the magma that feeds it. Viscosity is the measure of a fluid’s resistance to flow, determined primarily by its chemical composition and temperature. Magma that forms domes is characterized by a high silica content, often classifying it as dacite or rhyolite. The silica atoms link together in complex, chain-like structures, which creates immense internal friction and prevents the molten rock from flowing freely.
Because the lava is so resistant to movement, it solidifies almost immediately upon exposure to the cooler surface environment, rather than spreading out. This rapid solidification causes the material to stack up in a steep, dome-like shape right over the vent. Eruptions that build domes are generally slow, with the lava being extruded at a rate that allows it to cool and pile up.
The overall gas content in the magma that forms the dome tends to be relatively low compared to magmas that produce highly explosive eruptions. If the gas pressure were too high, the eruption would likely be violently explosive, shattering the magma into ash rather than allowing it to pile into a dome. The process of dome formation is a delicate balance: the lava is sticky enough to pile up but not so gas-rich that it explodes before reaching the surface.
Distinctive Physical Structures of Lava Domes
Once formed, lava domes have a distinctive morphology defined by their steep sides and a blocky, fractured surface. The dome takes on a generally circular or irregular footprint, sometimes resembling a truncated cone or a hemisphere. The surface is typically covered in large, sharp blocks of solidified lava. As the dome grows, either by internal expansion or by the addition of new lava, the outer solidified crust is constantly fractured and broken. This debris tumbles down the steep flanks to form a blanket of loose rock fragments, known as a talus slope, which surrounds the base.
In some cases, extremely stiff lava is forced upward like a solid plug, creating unstable, tall projections called lava spines. These spines are often short-lived and prone to collapse.
Volcanic Hazards Associated with Dome Collapse
Lava domes are considered dangerous volcanic features because they represent a massive, unstable plug of rock sealing the vent. The primary hazard is the gravitational collapse of the dome’s steep, fractured flanks, which can happen suddenly and without warning. A rapid dome collapse can immediately trigger a pyroclastic flow, a devastating, fast-moving current of superheated gas and volcanic debris. These flows are extremely hot, reaching temperatures up to 1,000°C, and can travel down the slopes at speeds exceeding 100 kilometers per hour. Such flows are capable of destroying everything in their path and have been responsible for major volcanic disasters historically.
A secondary hazard occurs when the hot dome material interacts with groundwater or surface water, which can flash to steam and cause steam-driven explosions known as phreatic explosions. These explosions can hurl blocks of rock and ash over a wide area, adding to the danger posed by the unstable structure.