There is also concern that Kilauea volcano could soon send boulders and ash shooting out of its summit crater.
The volcano has sputtered lava for a week, forcing around 2,000 residents to evacuate, and destroying some two dozen homes and threatening a geothermal plant. Fifteen vents are now spread through the Leilani Estates and Lanipuna Gardens neighborhoods.
Hawaii Gov. David Ige said crews at a geothermal energy plant near the lava outbreak accelerated the removal of stored flammable fuel as a precaution on Thursday.
Steaming cracks in the ground, the first sign a fissure may be opening, closed roads in areas to the northeast and southwest of Leilani Estates where scientists believe the next lava geysers may appear.
The plant, which is owned by Ormat Technologies of Reno, Nevada, is across a highway from where lava has been erupting.
High levels of acidic particles in vog "can induce asthma attacks, especially in adolescents, and can also impede the ability of the upper respiratory tract to remove other potentially harmful particles", the USGS says. The coffee farms on the Kona side of the island, which is more than 100 miles (160 kilometers) away from where lava is erupting. As that magma flows from the chamber and out of the fissures on the volcano's flanks, the lava level in the center crater falls. This is an unnerving prospect indeed, because this could set off a series of powerful steam explosions that could shower the surrounding area with 10-ton molten rocks and spew ash as far as 20 miles away. Rather it experiences effusive eruptions, where magma moves along rifts below the surface before rising and pushing up through fissures in the earth. The magma would heat the water and create steam that would push accumulated rocks out in an explosion.
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Quake activity and ground deformation, however, continue and more outbreaks around the Leilani Estates subdivision are likely, according to the Hawaiian Volcano Observatory. The last major one occurred in 1924 and spanned more than 2½ weeks, with more than 50 explosive events.
"We suspect it's a rapid process". "We don't know what's going on underground".
However, the immediate vicinity around the summit, an area controlled by the National Park Service, was to be closed to visitors indefinitely, starting on Thursday night.
The threat of violent explosions has prompted authorities to close the Hawaii Volcanoes National Park beginning Friday and "until further notice", according to the National Park Service.
"It seems pretty safe to me right now but they'd know best", said Cindy Woodd, who was visiting from British Columbia, Canada.
Officials with the agency wouldn't estimate the likelihood of such an explosion, but called it a "distinct possibility". "Life and safety is what's most important".