Lava Flows: Rivers of Molten Rock

Lava flows are streams of molten rock that emerge from volcanic vents and travel downhill under the influence of gravity. While they are often the most visually spectacular aspect of volcanic eruptions, lava flows are typically among the least dangerous volcanic hazards to human life, primarily because they usually move slowly enough to allow evacuation. However, they can cause extensive property damage and long-term environmental impacts as they destroy everything in their path and create new landforms.

Types and Characteristics of Lava Flows

The behavior and characteristics of lava flows depend primarily on the composition and temperature of the magma, which determine its viscosity (resistance to flow). Basaltic lavas, which are low in silica content and high in temperature (1000-1200°C), have low viscosity and can travel great distances at relatively high speeds. These flows can reach velocities of 10-30 kilometers per hour on steep slopes, though they typically slow to 1-10 meters per hour as they cool and thicken.

Andesitic and dacitic lavas, which have higher silica content and lower temperatures, are much more viscous and tend to form shorter, thicker flows. These flows move much more slowly, often advancing only a few meters per day, but they can be extremely destructive due to their high temperatures and tendency to bulldoze through obstacles rather than flowing around them.

The surface characteristics of lava flows vary significantly depending on their composition and cooling history. Pahoehoe flows develop smooth, ropy surfaces as they cool slowly and maintain a flexible skin that wrinkles as the underlying lava continues to move. A'a flows develop rough, chunky surfaces covered with sharp, angular blocks called clinker, formed when the outer crust breaks up due to continued movement of the underlying lava.

Flow Dynamics and Behavior

Lava flows follow topographic features, generally moving down the steepest available slopes and following valleys, streams, and other natural channels. However, their behavior can be complex and sometimes unpredictable, particularly when flows encounter obstacles, variations in terrain, or changes in eruption rate.

The leading edge of a lava flow, called the flow front, is typically the thickest and slowest-moving part of the flow. As lava accumulates behind this front, it can break out through the sides of the flow, creating new branches that may take entirely different paths. These breakouts are particularly common when flows encounter obstacles or when the eruption rate changes significantly.

Lava tubes can form when the surface of a flow cools and solidifies while the interior remains molten and continues to flow. These tubes can transport lava for many kilometers with minimal cooling, allowing flows to travel much farther than would otherwise be possible. When lava tubes form, surface evidence of the flow may disappear entirely, making it difficult to track the flow's progress.

The volume and duration of lava flows depend on the eruption rate and the total volume of magma available. Some flows may last only hours or days, while others can continue for months or even years. The 1983-2018 eruption of Kilauea's Pu'u 'O'o vent produced lava flows almost continuously for 35 years, creating over 500 square kilometers of new land.

Impacts and Damage

While lava flows rarely cause direct casualties due to their typically slow movement, they can cause extensive property damage and economic losses. Flows destroy everything in their path through a combination of extreme heat (typically 700-1200°C), mechanical force, and burial under meters of solidified rock. Buildings, roads, utilities, and agricultural areas are completely destroyed and may be buried under flows that can be tens of meters thick.

The economic impact of lava flows can be substantial and long-lasting. The 2018 Lower Puna eruption in Hawaii destroyed over 700 homes and caused an estimated $800 million in damage. Recovery from such events can take years or decades, and some areas may never be rebuilt due to the difficulty and expense of construction on fresh lava rock.

Agricultural impacts can be particularly severe in volcanic regions where fertile volcanic soils support intensive farming. While volcanic soils are eventually enriched by weathered lava, the immediate impact is the complete destruction of crops, topsoil, and agricultural infrastructure. Recovery of agricultural productivity may take years to decades depending on the thickness of the lava flow and local climate conditions.

Environmental impacts include the destruction of ecosystems, alteration of drainage patterns, and changes in local hydrology. However, lava flows also create new habitats and contribute to long-term soil formation, representing part of the natural cycle of destruction and renewal that characterizes volcanic landscapes.

Mitigation and Response

Mitigation of lava flow hazards relies primarily on land-use planning, early warning systems, and emergency preparedness rather than attempts to stop or divert flows. Hazard mapping identifies areas that are likely to be affected by future lava flows based on topography, volcanic history, and eruption modeling. This information can guide development decisions and emergency planning.

Various methods have been attempted to stop or divert lava flows, with limited success. These have included bombing flows to disrupt their channels, building barriers to redirect flows, and cooling flows with water. While some of these methods have achieved temporary diversions, they are generally ineffective against large flows and may simply redirect the hazard to other areas.

Emergency response to lava flow hazards focuses on evacuation, particularly of areas immediately threatened by advancing flows. Modern monitoring techniques including thermal cameras, satellite imagery, and ground-based observations can track flow progress and provide advance warning to threatened communities.

Recovery from lava flow impacts involves clearing accessible flows, rebuilding infrastructure, and adapting to permanently changed landscapes. In some cases, communities may relocate permanently if large areas become inaccessible or unsuitable for reconstruction.