A phreatic eruption is a steam-driven explosion that occurs when water beneath the ground or at the surface is hated by lava or volcanic rock.
The intense heat of this material hitting the water causes the water to boil, which in turn creates an explosion of steam, water, ash, rock, and lava bombs.
Mount Kilauea has not had a full-blown phreatic eruption yet, but the USGS has warned that one could happen at any moment, as lava from Kilauea’s lava lake continues to drop into the earth.
Scientists have said a phreatic eruption could send ash plumes as far as 12 miles into the air from the summit crater.
The USGS changed the volcano’s warning level to red alert on Tuesday, indicating that a major eruption is imminent and that ash could pose a threat on land as well as in the air for aviation.
A shift in winds is expected to send the ash fall cloud from Kilauea into the southwest region of the erupting volcano summit, including the neighbourhoods of Wood Valley, Pahala, Punaluu, Naalehu, and Hawaiian Oceanview Estates.
An “unhealthy air” advisory has been issued for the community of Pahala, as far as 18 miles from the volcano.
The ash itself is not poisonous, but can cause irritation to airways, particularly for those with pre-existing conditions, and can cause “choking and inability to breath” according to the Hawaiian Volcano Observatory.
However, volcanic smog, or ‘vog’ can be dangerous, with sulphur dioxide emitted by the vents potentially fatal if exposure is excessive.
These latest threats add to the long list of hazards the residents of Big Island have already been met with.
Massive swathes of land have been devoured, 37 structures – most of them homes – have been destroyed, and around 2,000 people have had to flee their homes.
Kilauea has also taken its toll on the areas booming tourism industry, which sees thousands pouring into the area every month to visit the island’s volcano park.
There is also damage to main roads, threatening to create lasting damage in the area.
Lava has burst from the ground in the neighbourhoods of Pahoa, tearing through houses and farmland, threatening state Highway 132, one of the last exit routes from coastal areas.
Road crews have put metal plates over steaming cracks on nearby Highway 130 and reopened it to provide residents an escape route from the neighbourhoods bearing the brunt of this disaster.
There have been no major injuries or deaths reported from the eruption.