Legazpi, Philippines ? Thousands of people moved to temporary shelters after searing gas and debris raced down the slope of the Mayon volcano in the Philippines over the weekend.

Volcanologist Ed Laguerta called it pyroclastic flow, or fast-moving hot gas and rock fragments, that could be the sign of a major eruption to come.

“This is really the dangerous phase,” said Laguerta. “We’re not worried much with lava flows because they’re slow moving, but pyroclastic flows travel at such high velocity and could destroy almost everything in its path.”

On Sunday, Mayon emitted abnormally high levels of sulfuric dioxide and puffing ash at least six times. The volcano near Legazpi is a popular tourist attraction because of its conical shape. It’s one of 22 active volcanoes in the Philippines.