Lava is molten rock extruded onto Earth’s surface, typically ranging between 700°C and 1200°C depending on its composition and viscosity. When this superheated liquid encounters water, the interaction is violent and complex. This meeting involves rapid physical forces and significant chemical transformations, resulting in processes that shape geology and create atmospheric hazards.
The Immediate Physical Reaction: Steam Explosions
The instant lava meets water, the immense temperature difference drives an explosive physical reaction. Heat from the lava rapidly transfers to the water, causing it to flash-boil into steam almost instantaneously. Since steam occupies a significantly greater volume than liquid water, this rapid phase change creates an immense and sudden pressure buildup.
When this pressure exceeds the strength of the surrounding rock or the lava itself, a violent explosion occurs, often called a littoral explosion if it happens on a coastline. These blasts fragment the lava and can hurl rock and debris hundreds of meters away, posing a substantial hazard. The depth of the water plays a significant role, as deeper water pressure can initially contain the steam, making shallow-water interactions, particularly where lava is funneled through tubes, far more explosive.
The most energetic explosions happen when water penetrates the lava flow and is quickly trapped. Laboratory experiments show that spontaneous explosions occur when there is sufficient molten rock depth above the mixing point to contain the initial pressure. This physical process explains the dramatic, sudden blasts often observed when lava flows into the ocean or a lake.
Formation of New Rock Structures
The rapid cooling effect of water on lava leads directly to the creation of unique geological formations. As the lava’s outer layer meets the cold water, it quenches almost instantly, forming a glassy, solid crust. The still-molten material inside then breaches this crust, creating new bulbous, interconnected lobes.
This process results in the formation of pillow lava, named for its stacked, pillow-like appearance. This basaltic rock is the most abundant type of volcanic rock found on the ocean floor, forming at mid-ocean ridges and submarine volcanoes. Its glassy outer layer is a direct consequence of the sudden and efficient heat extraction by the water.
In addition to the bulbous masses, the explosive shattering of the supercooled lava produces fragmented, glassy debris called hyaloclastite. This material consists of angular shards formed by the thermal shock and spallation of the lava’s surface. Hyaloclastite often accumulates between the pillow lava masses and is a major component of volcanic deltas built where lava flows into a body of water.
Chemical Hazards: Laze and Acidic Plumes
When lava flows into seawater, the reaction produces a distinct and hazardous chemical plume known as “laze,” a portmanteau of “lava” and “haze.” This process is specific to saltwater, which contains dissolved salts, primarily sodium chloride. The extreme heat causes the seawater to vaporize and its salt to decompose.
The chlorine from the seawater’s salt ions combines with hydrogen to form hydrogen chloride gas. This gas then mixes with the steam plume, creating a cloud containing corrosive hydrochloric acid droplets. The resulting laze plume is highly acidic, with recorded pH levels between 1.5 and 3.5, comparable to diluted battery acid.
The danger of laze is twofold: the inhalation of hydrochloric acid can cause severe irritation to the eyes, skin, and respiratory system, potentially leading to pulmonary edema. Furthermore, the plume carries fine particles of volcanic glass that can be inhaled. This combination of corrosive acid and particulate matter represents a significant public safety risk in areas where lava enters the ocean.