Lava, the molten rock extruded onto Earth’s surface, is a destructive natural force. Erupting at temperatures typically ranging from 700 to 1200°C, lava flows present an immense challenge to human intervention. Stopping or redirecting lava is difficult because it requires dissipating the tremendous thermal energy needed for the high-energy liquid to solidify.
Why Stopping Lava is Difficult
The physical properties of lava make interventions challenging. Lava contains a vast amount of thermal energy that must be overcome to force premature solidification. It is also a high-viscosity liquid, roughly 10,000 to 100,000 times more viscous than water, though viscosity varies based on composition and temperature.
Lower-viscosity lavas, like pāhoehoe, flow easily but develop an insulating crust, allowing the fluid core to flow great distances underneath. Higher-viscosity ʻaʻā lava is chunky and blocky; it moves slower but exerts tremendous pressure. Stopping the flow requires contending with the molten rock’s density, its insulating crust, and the massive energy needed to cool it below its solidification point.
Cooling Lava Flows with Water
The most direct intervention method is rapid cooling of the flow front using water. This technique forces the lava to solidify by rapidly extracting heat from the molten rock. The resulting solid crust acts as an insulating dam, slowing or stopping the advance of the material behind it.
This operation requires massive logistics and huge volumes of water delivered directly onto the lava’s surface. During the 1973 Eldfell eruption on Heimaey, Iceland, authorities pumped millions of cubic meters of cold seawater onto the advancing ʻaʻā flow threatening the town’s harbor. The effort escalated quickly, eventually utilizing a dredging boat and thirty-two pumps.
Piping water close to the lava was dangerous, requiring flexible plastic pipes kept cool by the constant flow of seawater inside. This operation successfully slowed the advance and preserved the harbor entrance, demonstrating that forced cooling can be effective. However, this method is primarily feasible in coastal or island locations where a nearly limitless supply of seawater is available.
Using Barriers and Diversion Channels
When stopping the flow is impossible, the goal shifts to redirecting it away from populated areas or critical infrastructure using constructed barriers. These physical obstructions range from simple earthen walls to robust structures made of gabions or loose rock. A barrier’s success depends heavily on the lava’s viscosity, the local topography, and the speed of construction.
Diversion barriers exploit the terrain, guiding the flow into unpopulated areas or pre-dug channels. Experiments in Iceland showed that earthen barriers built from local soil could delay lava advancement by up to sixteen days and redirect it away from vulnerable areas. However, rapidly flowing pāhoehoe lava can accumulate and overtop a barrier, while the pressure from blocky ʻaʻā lava can cause walls to fail.
Authorities have also attempted to use explosives, such as aerial bombing, to breach the solid crust of a lava tube or flow front. The intent is to divert the main lava supply into a new, less-threatening path upslope. This method has a mixed record, as seen during the 1991–1993 Mount Etna eruption. Multiple explosive attempts were necessary before the lava was successfully redirected into an artificial channel, starving the flow that threatened Zafferana Etnea.
Constraints on Large-Scale Interventions
Despite successful examples, large-scale interventions remain rare due to logistical and financial hurdles. A major eruption’s material volume can be staggering, often producing lava for months or years, making sustained efforts costly and impractical. The expense of heavy machinery, materials, and manpower is enormous, and high temperatures and unstable terrain present serious safety problems for workers.
A significant constraint is the ethical and legal complexity of diversion. Changing the flow path inevitably directs lava toward a new location, meaning it is intentionally sent toward another area, potentially impacting unpopulated land, ecosystems, or private property. These decisions require extensive planning, legal approvals, and social consensus, which are difficult to achieve under the pressure of an advancing flow.