CALIFORNIA, (KTXL) — Northern California is home to four volcanos all of which have a threat ranking of high to very high by the United States Geological Survey (USGS), but what does that mean for the security of the communities around them?

The four volcanos are Mount Shasta (220 miles north of Sacramento), Medicine Lake Volcano (294 miles north of Sacramento), Lassen Peak (162 miles north of Sacramento) and Clear Lake (123 miles northwest of Sacramento).

The USGS defines threat as, “the qualitive risk posed by a volcano, when active, to people and property independent of any mitigation efforts or actions.”

Factors such as the type of volcano, the occurrence of unrest, the general frequency of past eruptions and the tendency toward violent eruptions determine the level of threat.

The last major eruption to occur in California was on May 22, 1915, when Lassen Peak threw lava bombs tens of miles from the summit and ash over 300 miles into Nevada.

According to the USGS magma still resides underneath all four of Northern California’s volcanos and they are therefore considered active.

While eruptions can be rather violent and hazardous, the USGS notes that life-threatening and or highly destructive events occur within a few miles of the eruption.

“Generally, hazard severity declines, and the time available to issue warnings increases, with distance from the vent,” the USGS wrote in a 2018 report.

Areas more than 10 miles from a medium to large eruption generally should not experience pyroclastic flows, lava flows or ballistics such s lava bombs.

However, ash fall and floods may cause a “disruption and/or annoyance” to those between 10 miles and 1,000 miles, according to the USGS.

The last documented eruptions at Medicine Lake volcano were 950 and 1,000 years ago forming Little Glass Mountain and Glass Mountain.

Mount Shasta experienced its most significant activity about 3,000 years ago when eruptions broke out at the summit and vents opened in the upper-east flank of the mountain.

While the Clear Lake volcano field has ongoing geothermal activity, its most recent explosive eruption occurred 11,000 years ago. Currently, 18 geothermal power facilities generate about 835 megawatts of electricity from the steam vents and hot springs in the area.

Although these volcano areas have eventful pasts and certainly eventful futures, the USGS believes that eruptions are unlikely to occur in the coming decades.

Within the next 30 years, the USGS expects a 1% chance of eruption at Medicine Lake volcano, a 3.5% chance at Mount Shasta and a 2% chance at Lassen Peak. There is not enough data to determine the possibility of future eruption at Clear Lake Volcano Field.

Overall the USGS predicts a 16% chance of a small-to-moderate-sized eruption in California over the next 30 years. In comparison, there is a 22% chance that a 6.7 magnitude earthquake will specifically hit the San Andreas Fault in the San Francisco Bay Region in the next 30 years.