When did Lassen Peak last erupt?

The most recent eruptive activity at Lassen Peak (California) took place in 1914-1917. This eruptive episode began on May 30, 1914, when a small phreatic eruption occurred at a new vent near the summit of the peak. More than 150 explosions of various sizes occurred during the following year.

By mid-May 1915, the eruption changed in character; lava appeared in the summit crater and subsequently flowed about 100 meters over the west and (probably) east crater walls. Disruption of the sticky lava on the upper east side of Lassen Peak on May 19 resulted in an avalanche of hot rock onto a snowfield. This generated a lahar that travelled more than 18 kilometers down Lost Creek. On May 22, an explosive eruption produced a pyroclastic flow that devastated an area as far as 6 kilometers northeast of the summit.

The eruption also generated lahars that flowed more than 20 kilometers down Lost Creek and floods that went down Hat Creek. A vertical eruption column from the pyroclastic eruption rose to an altitude of more than 9 kilometers above the vent and deposited a lobe of tephra that can be traced as far as 30 kilometers to the east-northeast. The fall of fine ash was reported as far away as Elko, Nevada - more than 500 kilometers east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917.

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Can earthquakes trigger volcanic eruptions?

Sometimes, yes. A few large regional earthquakes (greater than magnitude 6) are considered to be related to a subsequent eruption or to some type of unrest at a nearby volcano. However, volcanoes can only be triggered into eruption by nearby tectonic earthquakes if they are already poised to erupt . This requires two conditions to be met: Enough "...

How many eruptions have there been in the Cascades during the last 4,000 years?

Eruptions in the Cascades have occurred at an average rate of one to two per century during the last 4,000 years. Future eruptions are certain. Learn more: Eruptions in the Cascade Range During the Past 4,000 Years USGS Cascades Volcano Observatory

How dangerous are pyroclastic flows?

A pyroclastic flow is a hot (typically >800 °C, or >1,500 °F ), chaotic mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per second) away from a volcanic vent or collapsing flow front. Pyroclastic flows can be extremely destructive and deadly because of their high temperature and mobility. For example, during the...

Why is it important to monitor volcanoes?

The United States and its territories contain 169 geologically active volcanoes, of which 54 volcanoes are a high threat or very high threat to public safety. Many of these volcanoes have erupted in the recent past and will erupt again in the foreseeable future. As populations increase, areas near volcanoes are being developed and aviation routes...

Can lakes near volcanoes become acidic enough to be dangerous to people and animals?

Yes. Crater lakes atop volcanoes are typically the most acid, with pH values as low as 0.1 (very strong acid). Normal lake waters, in contrast, have relatively neutral pH values near 7.0. The crater lake at El Chichon volcano in Mexico had a pH of 0.5 in 1983 and Mount Pinatubo's crater lake had a pH of 1.9 in 1992. The acid waters of these lakes...

Can volcanic eruptions endanger helicopters and other aircraft?

Yes. Encounters between aircraft and clouds of volcanic ash are a serious concern. Jet engines and other aircraft components are vulnerable to damage by fine, abrasive volcanic ash, which can drift in dangerous concentrations hundreds of miles downwind from an erupting volcano. In the past, many aircraft have accidentally encountered volcanic ash...

How are volcanic gases measured?

Instruments to measure sulfur dioxide and carbon dioxide can be mounted in aircraft to determine the quantity of gas being emitted on a daily basis. Such instruments can also be used in a ground-based mode. An instrument that detects carbon dioxide can be installed on a volcano and configured to send data continuously via radio to an observatory...

How can we tell when a volcano will erupt?

Most volcanoes provide warnings before an eruption. Magmatic eruptions involve the rise of magma toward the surface, which normally generates detectable earthquakes. It can also deform the ground surface and cause anomalous heat flow or changes in the temperature and chemistry of the groundwater and spring waters. Steam-blast eruptions, however,...

Is it dangerous to work on volcanoes? What precautions do scientists take?

Volcanoes are inherently beautiful places where forces of nature combine to produce awesome events and spectacular landscapes. For volcanologists, they're FUN to work on! Safety is, however, always the primary concern because volcanoes can be dangerous places. USGS scientists try hard to understand the risk inherent in any situation, then train...

How often does Mount Shasta erupt?

USGS scientists are currently working on this question. Mount Shasta doesn’t erupt on a regular timescale. Research indicates that the volcano erupts episodically with ten or more eruptions occurring in short (500-2,000 year) time periods separated by long intervals (3,000-5,000 years) with few or no eruptions. Evidence suggests that magma most...

Which volcanoes in the conterminous United States have erupted since the Nation was founded?

Excluding steam eruptions, these volcanoes have shown activity: Mount St. Helens, Washington - Eruptions and/or lava dome growth occurred in the late 1700s, 1800-1857, 1980-1986, and 2004-2007. Lassen Peak, California - A series of steam blasts began on May 30, 1914. An eruption occurred 12 months later on May 21, 1915. Minor activity continued...

What kind of school training do you need to become a volcanologist?

There are many paths to becoming a volcanologist. Most include a college or graduate school education in a scientific or technical field, but the range of specialties is very large. Training in geology, geophysics, geochemistry, biology, biochemistry, mathematics, statistics, engineering, atmospheric science, remote sensing, and related fields can...
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Date published: May 16, 2017


Which sounds more dangerous, lava or mud? The answer may surprise you...

Date published: October 5, 2015

EarthWord: Fumarole

Fumaroles are openings in the earth’s surface that emit steam and volcanic gases, such as sulfur dioxide and carbon dioxide. They can occur as holes, cracks, or fissures near active volcanoes or in areas where magma has risen into the earth’s crust without erupting. A fumarole can vent for centuries or quickly go extinct, depending on the longevity of its heat source.

Date published: May 21, 2015

Eruptions of Lassen Peak, California, 1914 to 1917 — A Centennial Commemoration

2015 marks the 100th anniversary of the May 22, 1915 explosive volcanic eruption of Lassen Peak in northern California. 

Date published: February 9, 2012

Say Hello to CalVO: USGS California Volcano Observatory Opens

The U.S. Geological Survey announces the establishment of the USGS California Volcano Observatory, or CalVO, headquartered within existing USGS facilities in Menlo Park, Calif. Establishing CalVO will increase awareness of and resiliency to the volcano threats in California, many of which pose significant threats to the economy and well being of the state and its inhabitants.

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July 28, 2016

July Public Lecture — USGS CalVO: It's not just earthquake country!

Volcanic eruptions occur int he State about as frequently as the large San Andreas Fault Zone earthquakes. California's "watch list" volcanoes are dispersed throughout the State and future eruptions are inevitable—the likelihood of renewed volcanism is on the order of one in a few hundred to one in a few thousand annually.

With Margaret Mangan, Scientist-in-Charge,

Lassen Peak Eruption
April 18, 2016

Lassen Peak Eruption

Image: Collecting Gas Sample at a Fumarole
March 15, 2016

Collecting Gas Sample at a Fumarole

USGS geologist Deborah Bergfeld collects a gas sample from a superheated (hotter than the boiling point) fumarole in Little Hot Springs Valley at Lassen Volcanic National Park.

April 23, 2015

PubTalk 04/2015—"Fearfully Grand" Eruptions: Lassen Peak, CA, 1914-17

Title: A Sight "Fearfully Grand" Eruptions of Lassen Peak, California, 1914 to1917

  • A summary of the eruptions and their effects
  • Illustrated with historical photographs
January 26, 2012

PubTalk 1/2012 — Lassen Volcanic National Park

--a wonderland of volcanoes and thermal features

By Patrick Muffler, Geologist Emeritus


  • Lassen Peak, the southernmost active volcano in the Cascade Range,explosively erupted in 1915, devastating nearby areas and raining volcanic ash as far away as 200 miles to the east
  • Lassen National Park, in addition to
Image: Finding the Fumarole Vent
August 11, 1992

Finding the Fumarole Vent

USGS hydrologist Michael Sorey tries to locate the steam vent at a fumarole in the Bumpass Hell area in Lassen Volcanic National Park.

Image: Sampling a Fumarole
July 31, 1990

Sampling a Fumarole

USGS geochemist Cathy Janik (left) and Iceland Geosurvey chemist Jón Örn Bjarnason (right) collect a gas sample from a fumarole in Lassen Volcanic National Park.