California’s last volcanic eruption was 104 years ago.

Mount Lassen — some 200-plus miles northeast — was awakened from its slumber on May 14, 1915.

It did so in what is known as a phreatic event.

Basically, it was a blowout that happened when groundwater and rising magna collided.

The intense heat boils the water.

If hot enough the water flashes straight to steam.

In doing so, it creates an explosion.

Nearly 200 such explosions happened in the ensuing months.

During that time, the crater widened to close to 1,000 feet.

Observers reported the crater started glowing on May 14, 1915 as lava pushed upward creating a large dome.

Then, on May 19, the “main event” happened and lasted for four days.

A new crater opened with an explosion, breaking the dome.

The lava combined with a 30-foot snowpack.

It was the start of a chain of events that ended up causing a massive fish kill in the Pit River and wiping out ranch houses after the snow and lava had traveled roughly four miles downhill and turned into a massive mudflow that leveled trees over an 11-mile route.

It was capped on May 22 with a scene people usually associate with volcanoes — a high column of ash that formed a mushroom-like cloud shooting thousands of feet heavenward as a new crater opened near the 10,457-foot summit.

The ash cloud was seen as far away as Sacramento while ash showered down on areas of Nevada 300 miles to the east.

Afterwards, the phreatic explosions continued for another two years before finally stopping.

California — as evidenced by Lassen — is young geologically. It is marred by quakes, granite carving by recent glacial ages, and weather that can bring epic snow, major winds, and sun-drenched days that further help to slowly but surely alter the landscape.

Lassen is the southernmost volcano in the Cascades that stretch into British Columbia.

Among the other volcanoes is Mount St. Helens that erupted 43 years ago.

The Cascades are part of the Pacific Ring of Fire — volcanoes that border and can be found within the world’s largest ocean.

Lassen’s large dome — one of the largest in the world — was formed 27,000 years ago.

Lassen’s eruption 104 years ago wasn’t even a fraction of a micro-second on the geological clock. Its actual formation on the same scale occurred a second or so ago.

It’s against that yardstick, one weighs the updated United States Geological Survey of active volcanoes in California that was published in 2018.

‘Very high threat’ volcano

located along Highway 120

Take Highway 120 less than 200 miles east and you will be near the edge of one of 18 volcanoes in the United States that the U.S. Geological Society has designated as posing a “very high threat.”

The Long Valley Caldera — essentially a massive depression left after the eruption or evacuation of a magma chamber — is one of the largest on earth. It is 20 miles long, 11 miles wide and up to 3,000 feet.

It’s located east of Mammoth Mountain and June Lake and south of Mono Lake where Highway 120 passes by its southern shore.

Yellowstone Park is also on top of a caldera.

Calderas are sometimes referred to as super-volcanoes.

The “very high threat” does not mean the volcanoes on the list pose an immediate risk of erupting. Instead the list — updated five years ago for the first time since 2005 — identified volcanoes based on recent activity typically measured in hundreds or thousands of years, the frequency of past eruptions, and seismic activity that justify closer study.

Geological Survey experts cautioned that the threat rankings aren’t about what will blow next, but the potential severity of the damage.

The score is based on the type of volcano, how explosive it can be, how recently it has been active, how frequently it erupts, if there has been seismic activity, how many people live nearby, if evacuations have happened in the past and if eruptions disrupt air traffic.

The Long Valley Caldera was formed 760,000 years ago.

The most significant activity was a strong swarm of earthquakes in May 1980 that involved four quakes at or slightly above 6.0 on the Richter Scale that hit in the southern region of the caldera that created a 10-inch, dome-shaped uplift on the caldera floor.

Long Valley is rated as the 18th at the bottom of the U.S. Geological Survey’s volcanoes deemed “very high threat.”

Just to the north of the Long Valley Caldera is the Mono-Inyo Craters that Highway 120 passes through on its way east from Highway 395 to its terminus in Benton at U.S. Highway 6.

The southern part of that chain had an explosive eruption 600 years ago. The latest on the northern part of the chain — an eruption on Pahoa Island in Mono Lake — occurred 300 years ago. Mono Craters due to the type of volcanic activity is not considered a major threat.

The highest California on the “very high threat” is Mt. Shasta at No. 5.

The only other volcano to make the list is Mt. Lassen volcanic center at No. 11.

The two volcanoes — especially Mt. Shasta — are what comes to most people’s minds when they think of a typical volcano in terms of its mountain peak-like formation and how it erupts.

Shasta last erupted in 1786 while Lassen last erupted in 1917.

Up until a few years ago, it was believed Mt Shasta had erupted in 1786 based on what a French explorer believed was an ash cloud he saw in the general area from the edge of the Pacific Ocean.

Volcanologists over the years haven’t been able to find evidence of an eruption in 1786.

They concluded it was a smoke plume from a massive grassland fire that used to burn for months in the Great Central Valley.

There are five active volcanoes in California.

Lassen was the last volcanic eruption in the continental United States until Mount St. Helens in the State of Washington erupted in 1980.

That 1980 eruption killed 57 people, partially collapsed the volcano, caused $1.1 billion in property damages and spread ash across 11 states.

Hawaii’s Kilauea, which has been erupting this year, tops the list. The others in the top five besides Mt. Shasta are Mount St. Helens and Mount Rainier in Washington, and Alaska’s Redoubt Volcano. Eleven of the 18 very high threat volcanoes are in Oregon, Washington and California.

The other volcanoes on the list besides the top five, Long Valley, and Lassen are Mt. Hood, Three Sisters and Crater Lake in Oregon; Akutan Island, Makushin, Mt. Spurr and Augustine in Alaska; Mt. Baker and Glacier Peak in Washington; and Mauna Loa in Hawaii.