Hawaii houses two of the 18 US volcanoes that have been classified as 'very high threat'

There are 161 active and potentially active volcanoes across the country, according to the USGS, which placed them in five categories depending on the hazards they pose to people and infrastructure.

Eighteen volcanoes are in the top category of those deemed to be a "very high threat." That includes Kilauea, considered to be one of the world's most active volcanoes.

Kilauea started erupting in May, causing more than 110 earthquakes and hundreds of evacuations.

Hawaii has six active volcanoes, though only five made the new USGS list and only one other -- Mauna Loa -- appears in the top threat category.

Mauna Loa -- 16th in the "very high threat" category -- is the largest active volcano in the world and has erupted 33 times since its first documented eruption in 1843.

Volcano hazards

In ranking the volcanic threat levels, the USGS weighed "24 factors describing a volcano's hazard potential and exposure of people and property to those hazards (independent of any mitigation efforts or actions)."

The list released this week, an update of a 2005 assessment, is not a forecast of which volcanoes will erupt next but "an indicator of the potential severity of impacts that may result from future eruptions at any given volcano," John Ewert, a USGS volcanologist and the report's chief author, wrote in the report.

This map shows the locations of all US volcanoes with threat category designated by color.

Researchers examined how populated an area is to a known lava flow zone, and how volcanic ash could affect air travel and airports.

Eleven of the 18 "very high threat" volcanoes are in the states of Washington, Oregon and California, the report says. Five are in Alaska, where the high and moderate threat categories dominate, as the volcanoes are near "important population centers," the report adds.

After Kilauea in Hawaii, Washington's Mount St. Helens and Mount Rainier rank two and three on the list. Alaska's Redoubt Volcano and California's Mount Shasta finish the top five.

The most threatening

Mount Rainier's last eruptive period was about 1,000 years ago.

The volcano that may cause the highest threat to the public is Mount Rainier, USGS geologist Angie Diefenbach, a co-author of the assessment, told CNN.

"About 300,000 people are located on the downstream hazard zone near Mount Rainier. Eruptions can be hazardous and go long distances," Diefenbach said.

Diefenbach said the volcano hazards list is not meant to scare the public, but rather help researchers keep a closer watch on volcanoes and prepare for any possible eruptions.

The resulting report considers 39 volcanoes to be high threat; 49 moderate threat; 34 low threat; and 21 very low threat. In the category of very high threat are the following 18: