When molten rock erupts from the Earth and flows across the surface, it is known as lava, a material that maintains an extremely high temperature, typically ranging from 700°C to 1200°C. The interaction between this superheated liquid and water, especially the vast volume of a cold ocean, sets the stage for one of nature’s most dramatic and energetic physical processes. This contact triggers a series of rapid events, transforming both the water and the lava in an instant, leading to explosive fragmentation and the creation of entirely new chemical and geological hazards.
The Immediate Physical Collision and Fragmentation
The immediate consequence of lava meeting water is a violent physical reaction driven by the massive temperature difference. This sudden introduction of intense heat causes the water to flash-boil, instantly converting the liquid into superheated steam. Since water expands up to 1,700 times in volume when vaporized, this rapid transformation results in a powerful, localized steam explosion known as a littoral explosion.
This explosive force mechanically fragments the lava, a process often described as a molten fuel-coolant interaction. The massive pressure wave generated by the expanding steam shatters the molten rock into tiny pieces, which are then ejected high into the air as flying debris. The explosive energy released is directly related to how intimately the lava and water mix before the full flash-boiling occurs.
A second, simultaneous process is thermal shock, which causes the surface of the lava to crack and shatter. As the exterior of the lava is quenched by the cold water, it cools too quickly for an orderly crystal structure to form, creating a glassy, brittle rind. The internal heat and external cooling cause stresses that rapidly fracture this glassy surface into countless small shards.
This brittle fragmentation generates tiny, lightweight pieces of volcanic glass that are carried away by the steam plume. These fragments are sometimes called Pele’s hair or Pele’s tears in Hawaii, referring to the delicate, fiber-like strands and tear-shaped droplets of quenched lava. The combined effect of the steam explosion and thermal shock converts the cohesive lava flow into a chaotic, airborne mixture of superheated gas and fragmented rock.
The Creation of Toxic Steam Plumes (Laze)
When lava flows into the ocean, the violent physical reaction is accompanied by a significant chemical one, creating a hazardous plume known as “laze” (lava + haze). This distinct white cloud is created when the lava’s extreme temperature, which can exceed 1,100°C, causes the seawater to vaporize and decompose. The intense heat liberates chlorine from the dissolved salts, primarily sodium chloride.
The hydrogen ions in the steam then combine with the liberated chloride ions, forming hydrogen chloride gas (HCl). This gas immediately reacts with the water vapor in the plume to create hydrochloric acid droplets. The resulting plume is a corrosive mixture of steam, hydrochloric acid, and tiny particles of volcanic glass, which can possess the stinging properties of dilute battery acid.
Laze poses a direct health hazard, particularly to the respiratory system. Inhaling the hydrochloric acid vapor can cause severe irritation to the eyes, skin, and lungs. Exposure may lead to coughing, labored breathing, and in high concentrations, an accumulation of fluid in the lungs called pulmonary edema.
Furthermore, the hydrochloric acid can precipitate out of the plume as acid rain, with measured pH levels as low as 1.5 to 3.5. This acidic fallout can damage vegetation, corrode metal, and affect water quality in the immediate vicinity. The laze plume can be carried downwind for many miles, extending the zone of potential chemical exposure far beyond the actual ocean entry point.
Geological Outcomes and New Land Formation
Beyond the immediate explosive hazards, the sustained contact between lava and water leaves behind distinctive geological formations. The rapid cooling that causes the initial fragmentation also dictates the shape and structure of the solidified rock. When lava flows underwater, the outer surface solidifies almost instantly, forming a solid, glassy crust.
As the lava continues to flow from the vent, the molten material inside this hardened shell pushes against the crust, causing it to break through and form a new bulbous lobe. This continuous process of budding and inflation creates characteristic structures known as “pillow lava,” which are rounded, stacked formations. Pillow lavas are the most abundant form of volcanic rock on Earth, covering much of the ocean floor, especially along mid-ocean ridges.
Where the lava is highly fragmented by the steam explosions, the resulting fine-grained particles are washed away and deposited in the surrounding area. These fragments, composed primarily of basaltic glass, eventually accumulate to form black sand beaches. This creation of new coastline is a direct result of the lava flow, but the newly formed land is structurally unstable, often consisting of loosely piled lava fragments and solidified rubble that is prone to collapse into the sea.
The formation of new land at the ocean entry point is a process of accretion, where the cooled rock builds up a delta-like structure. This new terrain is geologically young and highly susceptible to both erosion and further explosive events, which can occur as seawater interacts with hot, submerged material.