Lava entering the ocean creates a distinctive set of hazards that have seriously injured or killed unsuspecting people eager to see up close the interaction of hot lava and cool seawater. Getting too close to an ocean entry, either on land or from the sea, is potentially deadly. Primary hazards include:
Scientists cannot predict the timing or size of a lava delta collapse. They also cannot predict which direction and how far fragments of lava and rock will be hurled on land or seaward during a collapse-triggered explosion. The best way to avoid these hazards is to never walk onto an active lava delta and maintain a safe distance from a delta's leading edge, even when on a boat. Once a new lava delta extends more than a few tens of meters (yards) from the old sea cliff, visitors should stay at least 300 m (330 yd) away from where lava is entering the ocean. This is the maximum distance rocks and spatter have been thrown inland from the older sea cliff during past ocean-entry explosions. Small rock fragments can fall far beyond this distance.
Lava entering the ocean builds a delta on top of unstable lava fragments along the steep submarine slope. As the delta grows seaward and laterally along the shoreline, it may slowly settle or sink as the loose rock debris shifts under the weight of overlying lava flows. This subsidence may allow seawater to get into the lava tube system, which can generate lava bubble bursts and rare littoral lava fountains (see gallery).
All or part of the delta can collapse into the ocean when the underlying debris can no longer support the delta's growing mass or is undercut by a deeper submarine landslide. The collapses occur suddenly or over a period of several hours. Despite a well-posted warning signs in Hawai‘i Volcanoes National Park, a large delta collapse in 1993 caught many people by surprise—a photographer standing on the delta was swept out to sea and lost. More than a dozen other people were injured when they raced to stable ground amidst a shower of hot rocks.
Lava entering the ocean generates different types of explosive interactions with seawater. The largest and most dangerous type of explosion is triggered by a delta collapse. The sudden mixing of seawater, hot rocks, and lava with temperatures about 1,150 degrees Celsius (2,100 degrees Fahrenheit) may cause steam-driven explosions from the collapsed area. Such explosions have hurled hot rocks nearly a meter (yard) in size as far as about 250 m (273 yards) inland from the collapsed delta and scattered rock debris onshore over an area the size of several football fields. These explosions also hurl rocks seaward, probably to similar distances.
Large portions of an active lava delta are extremely hot because of lava flows on its surface, lava tubes beneath the surface, and still-hot solidified flows throughout a delta. A larger than usual ocean wave that sweeps across the surface of a hot delta can quickly reach scalding temperatures. People standing near lava deltas have received second-degree burns from such hot waves and accompanying steam. In 2000, the deaths of two severely burned individuals found near an active coastal lava flow were caused by the inhalation of acidic steam from an ocean entry, according to the medical examiner. Delta collapses can also produce waves both onshore and offshore, which can imperil boats next to a collapsing delta.
A white plume on the edge of an active lava delta marks where lava meets seawater—this plume can cause skin and eye irritation and breathing difficulties and should be avoided. As hot lava boils cool seawater, a series of chemical and physical reactions create a mixture of condensed, acidic steam, hydrochloric acid gas, and tiny shards of volcanic glass. Blown by wind, this plume creates a noticeable downwind haze, known as "laze" (short for lava haze).
During prevailing trade-wind conditions, which exist more than about 80 percent of the time, air flow from mid-morning through late afternoon carries the plume onshore and along the coast. This can cause poor air quality for people downwind of the ocean entry. From nighttime through early morning, trade-wind flow typically blows the laze off shore and out to sea.