(Recasts with gas warning, chemicals removal, USGS update, volcanic smog risk from wind change)
PAHOA, Hawaii, May 10 (Reuters) - Hawaii residents were alerted on Thursday to rising levels of toxic gas from volcanic fissures and geologists warned that new areas east of the erupting Kilauea volcano may be at risk to lava bursting from the ground.
Hawaii County authorities sent a text message to residents in the southeast corner of the Big Island that a wind change would bring rising levels of sulfur dioxide gas, which is fatal if inhaled in large quantities.
"Due to decreasing tradewinds, residents are advised to monitor their sensitivity to increased levels of SO2," the text message sent at 9.22 a.m. said.
Hawaii's governor has warned that mass evacuations may be needed as more fissures open and spew lava and gas in semi-rural residential areas on the east flank of the volcano.
Authorities on Thursday completed the removal of highly flammable chemicals from a nearby power plant in the path of lava.
Geologists said Kilauea may be entering a phase of explosive eruptions not seen in nearly a century that could hurl "ballistic blocks" weighing up to 12 tons for half a mile (800 meters), shoot pebble-sized projectiles for miles and dust downwind towns with volcanic ash and smog.
The Leilani Estates residential area remains in greatest danger, with 15 volcanic fissures so far having destroyed 36 structures, most of them homes, and forcing the evacuation of about 2,000 residents.
But as the eruption progresses, "other areas of the lower East Rift Zone may also be at risk," the Hawaiian Volcano Observatory said in its latest briefing.
Hawaii Governor David Ige has requested federal disaster assistance as he said a mass evacuation of the lower Puna District, where Leilani Estates is located, would be beyond current county and state capabilities.
SURFING IN THE VOG
Dustings of ash were expected downwind of Kilauea on Thursday as summit earthquakes set off rockfalls into the lava lake and sent up plumes of steam, ash and smoke, the USGS said.
Local meteorologists said the change in prevailing winds could send Kilauea's volcanic smog, or vog, northwest to Maui and other islands in Hawaii.
Surfers bobbing in the ocean off Kona on the west side of the Big Island complained of the smog that could be seen in a haze over the coast.
"Does that hat protect against vog?" one surfer was heard quipping to another about the floppy sun hat he was wearing.
In Pahoa, the nearest village to Kilauea, some schools remained closed after the area was hit by a 6.9 magnitude earthquake on Friday, the biggest since 1975.
The closures have added to a sense of disarray and ramped up stress levels, said gallery owner Amedeo Markoff, 49.
"It's like our version of a snow day -- a lava day," joked Markoff.
Magma is draining out of the volcano's sinking lava pool and flowing underground tens of miles (km) eastward before bursting to the surface in and around homes on Kilauea's eastern flank, just a few miles (km) from Pahoa.
(Reporting by Terray Sylvester; Writing and additional reporting by Andrew Hay in New Mexico; Editing by Bill Tarrant and Sandra Maler)