• Slide 4 of 119: Hannique Ruder, a 65-year-old resident living in the Leilani Estates subdivision, walks past the mound of hardened lava while surveying the neighborhood Friday, May 11, 2018, near Pahoa, Hawaii. Kilauea has destroyed more than 35 structures since it began releasing lava from vents about 25 miles (40 kilometers) east of the summit crater. (AP Photo/Jae C. Hong)
  • Slide 1 of 119: A man swims in the ocean in Hilo, Hawaii, Friday, May 11, 2018. Warnings that Hawaii's Kilauea volcano could shoot boulders and ash out of its summit crater are prompting people to rethink their plans to visit the Big Island. But most of the rest of the island is free of volcanic hazards, and local tourism officials are hoping travelers will recognize the Big Island is ready to welcome them. (AP Photo/Jae C. Hong)
  • Slide 2 of 119: PAHOA, HI - MAY 11: Smoke and volcnic gases rise as lava cools in the Leilani Estates neighborhood, in the aftermath of eruptions and lava flows from the Kilauea volcano on Hawaii's Big Island, on May 11, 2018 in Pahoa, Hawaii. The U.S. Geological Survey said a recent lowering of the lava lake at the volcano's Halemaumau crater "has raised the potential for explosive eruptions" at the volcano. Vog, a haze or smog containing gases, smoke and dust from volcanic eruptions, may eventually spread from the eruptions to other islands in Hawaii. (Photo by Mario Tama/Getty Images)
  • Slide 3 of 119: Ken Gadd, a first-time visitor from Dayton, Ohio, takes pictures of the entrance to Volcanoes National Park, Hawaii, Friday, May 11, 2018. The park is closed due to the threat of an explosive volcanic eruption. Warnings that Hawaii's Kilauea volcano could shoot boulders and ash out of its summit crater are prompting people to rethink their plans to visit the Big Island. (AP Photo/Jae C. Hong)
Slide 1 of 119: A man swims in the ocean in Hilo, Hawaii, Friday, May 11, 2018. Warnings that Hawaii’s Kilauea volcano could shoot boulders and ash out of its summit crater are prompting people to rethink their plans to visit the Big Island. But most of the rest of the island is free of volcanic hazards, and local tourism officials are hoping travelers will recognize the Big Island is ready to welcome them. (AP Photo/Jae C. Hong)

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1/119 SLIDES © Jae C. Hong/AP Photo
A man swims in the ocean on May 11 in Hilo, Hawaii. Warnings that Hawaii’s Kilauea Volcano could shoot boulders and ash out of its summit crater are prompting people to rethink their plans to visit the Big Island. But most of the rest of the island is free of volcanic hazards, and local tourism officials are hoping travelers will recognize the Big Island is ready to welcome them.

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With scientists warning that a steam-driven volcanic eruption could occur virtually without warning, the Hawaii National Guard is prepared to use ground convoys and even helicopters if necessary to pluck hundreds of residents from an isolated southeast corner of Hawaii’s Big Island.

The danger comes from the lava level that is dropping inside the volcano. If it falls below the water table, water will pour onto the lava, generating steam that will likely explode from the summit in a shower or rocks, ash and sulfur dioxide gases.

Hawaii Civil Defense officials says boulders as big as refrigerators could be tossed a half mile, and ash plumes could sore as high as 20,000, spread over a 12-mile area.

“We’ve got all the warning signs we need,” said Steve Brantley, the deputy scientist-in-charge for the U.S. Geological Survey’s Hawaiian Volcano Observatory HVO, the Honolulu Star-Advertiser reports. “There may not be any additional warning before the magma actually starts moving up to the surface.”

“If you are in your car, keep the windows closed,” the alert said. “Ash fallout may cause poor driving conditions, due to limited visibility and slippery driving conditions. Drive with extreme caution, or pull over and park.”

The biggest threat is to residents of the Lower Puna area, a southeast corner of the island, which could get cut off from evacuation routes if fissures cut Route 132 .

a group of people in a forest: Hannique Ruder, a 65-year-old resident living in the Leilani Estates subdivision, walks past the mound of hardened lava while surveying the neighborhood Friday, May 11, 2018, near Pahoa, Hawaii. Kilauea has destroyed more than 35 structures since it began releasing lava from vents about 25 miles east of the summit crater.© Jae C. Hong, AP Hannique Ruder, a 65-year-old resident living in the Leilani Estates subdivision, walks past the mound of hardened lava while surveying the neighborhood Friday, May 11, 2018, near Pahoa, Hawaii. Kilauea has destroyed more than 35 structures since it began releasing lava from vents about 25 miles east of the summit crater.Civil Defense officials said Lower Puna residents in the towns of Kapoho and Kalapana should stay alert for possible volcanic eruption and gas emissions, and “be prepared to evacuate immediately,”

The Hawaii National Guard is prepared, with only 90-minutes notice, to rescue some 2,000 people by ground convoy in troop-carrying vehicles and, if necessary, in Blackhawk or Chinook helicopters.

“We can move 226 people in one convoy. So we could move 226 at once with about an hour and a half notice, and we would drop them off somewhere. The vehicles could come back, and we would just do that round-robin.” Lt. Col. Shawn Tsuha tells KHON-TV.

Kilauea erupted last week, sending 2,200-degree lava bursting through cracks into people’s backyards in the Leilani Estates neighborhood that destroyed 36 structures, including 26 homes. As the magma shifted underground, a magnitude-6.9 earthquake also rocked the Big Island.

President Trump on Friday declared a major disaster on the Big Island. The move will make federal financial assistance available to state and local governments as they repair roads, public parks, schools and water pipes damaged by the eruption.