The USGS also raised the volcano aviation alert level at Hawaii's Kilauea to red - the highest level - which indicates a 'major volcanic eruption is imminent, underway, or suspected with hazardous activity both on the ground and in the air'.
Ash erupts from the Halemaumau crater during ongoing eruptions of the Kilauea volcano near the village of Volcano on Tuesday.
Unsafe levels of sulfur dioxide gas and other emissions prompted state health officials to urge residents to stay indoors or leave the eastern end of Hawaii's Big Island, which has been ravaged by volcanic activity since May 3.
What scientists refer to as "vog" - a combination of volcanic gas and ash reached 12,000 feet into the sky above Kilauea - one of the world's most active volcanoes.
Stunning photos taken at Hawaii's Big Island on Tuesday show golfers coolly hitting the links even as a monstrous ash plume looms behind them.
"We're observing more or less continuous emission of ash now with intermittent, more energetic ash bursts or plumes", Hawaiian Volcano Observatory deputy scientist-in-charge Steve Brantley told reporters.
Smog from Hawaii's Kilauea volcano drifted north up the island chain on Wednesday and communities south of its summit were warned of up to a quarter-inch (0.6 cm) of ashfall as the almost two-week eruption showed no sign of easing.
"At any time, activity may become more explosive, increasing the intensity of ash production and producing ballistic projectiles near the vent", the HVO said in a statement. It can make roads slippery, clog drinking water catchment systems common on the island and cause the failure of electrical power lines in large enough quantities, said USGS chemist David Damby.
But these golfers in Hawaii have rounds to go before they sleep. The eruption has destroyed 25 homes and covered 115 acres in lava.
No serious injuries or deaths have been reported from the eruption.
Shocking images of lava spilling onto roads and 20 fissures expanding in the earth emerged as reports of toxic gas forced residents from their homes.
The explosive, steam-driven eruptions could drive a 20,000-foot (6,100-meter) ash plume out of the crater, hurl boulders the size of small cars up to half a mile (800 meters) and scatter smaller rocks over 12 miles (19 km), the USGS has warned.