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Mauna Loa, the world's largest active volcano, has erupted for the first time in nearly four decades, and civil defense officials Monday warned residents on Hawaii's Big Island to stay alert even though there doesn't appear to be any immediate danger.

"Lava flows are not threatening any downslope communities," the U.S. Geological Survey said in its most recent update.

Still, the agency warned, "the early stages of a Mauna Loa rift zone eruption can be very dynamic, and the location and advance of lava flows can change rapidly."

The eruptions began late Sunday after a series of fairly large earthquakes, said Ken Hon, the scientist-in-charge at the Hawaiian Volcano Observatory.

“Typically, Mauna Loa eruptions start off with the heaviest volume first,” Hon told The Associated Press. “After a few days, it starts to calm down a little bit.”

It could take the lava weeks or months to reach the county seat, Hilo, or other towns on the east side of the island, scientists said. But the 200,000 people potentially in harm's way are more than double the population 38 years ago when Mauna Loa last erupted.

“We don’t want to try and second-guess the volcano,” Hon said. “We have to let it actually show us what it’s going to do and then we inform people of what is happening ASAP.”