7.4: Volcanic Hazards

Active Volcanoes in California

In the 2018 update to the USGS national volcanic threat assessment, three California volcanoes were placed in the "very high threat" category. Two of these are located in the Cascades; Mount Shasta is considered the fifth most threatening volcano in the US and Lassen volcanic center is ranked 11^th. The third California volcano in this category is the Long Valley Caldera, located in the Basin and Range Province (see 8.5: Rift-Related Volcanism in Eastern California). The variety of lava compositions and volcano types in the Cascades lead to a variety of hazards associated with a potential volcanic eruption. The sections below describe the various hazards that might occur during the eruption of one of the California Cascade volcanoes.

Lava Flows

While news of lava flowing through neighborhoods and destroying houses is more likely to come from Hawaii than California, the presences of lava flows throughout the Cascade Range and Modoc Plateau imply that lava flows certainly can occur in California as well. Lava flows are streams of molten rock that pour or ooze from an erupting vent. Lava is erupted during either non-explosive activity or explosive lava fountains. The speed at which lava moves across the ground depends on several factors, including the type of lava erupted and its viscosity, the steepness of the ground over which it travels, and the rate of lava production at the vent. Basalt, which is mafic and low-viscosity, can flow tens of kilometers from an erupting vent. These flows are commonly associated with shield volcanoes like Medicine Lake Volcano or Lava field, such as those that exist throughout the eastern Cascades and Modoc Plateau. More viscous andesite flows are more commonly associated with composite volcanoes like Mount Shasta. These flows move only a few kilometers per hour (a couple of feet per second) and rarely extend more than 8 km (5 mi) from their vents. Dacite and rhyolite, which are also commonly found on composite cones, often form steep-sided lava domes, rather than spread-out lava flows.

Pyroclastic Flows

Lava flows and pyroclastic flows are often grouped together for the purposes of volcano hazard mapping because the two hazards encompass a similar area surrounding the volcano. However, these two hazards are very different in style and origin and do not usually occur simultaneously. A single eruption may progress from explosive to effusive, but cannot be both at once. While lava flows consist of lava, pyroclastic flows contain a mixture of materials, including hot lava blocks, pumice, ash, and volcanic gas. They move at very high speed down volcanic slopes, typically following valleys.

Pyroclastic flows form in a variety of ways. During a highly explosive eruption, a column of volcanic material is ejected upwards into the atmosphere. Once the gas in the column cools, the column can become too dense to maintain upward momentum, and can collapse in a pyroclastic flow (Figure \(\PageIndex{1}\)). Small pyroclastic flows of this nature occurred during the May 22, 1915 eruption of Lassen Peak. In some cases, a pyroclastic flow can form in an explosive eruption without first forming a column. Pyroclastic flows can also form from the collapse of lava domes or flows. The fronts of lava flows or domes can become so steep that they collapse due to gravitational force.

Once formed, pyroclastic flows destroy nearly everything in their path. With rock fragments ranging in size from ash to boulders that travel across the ground at speeds typically greater than 80 km per hour (50 mph), pyroclastic flows knock down, shatter, bury, or carry away nearly all objects and structures in their path. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C (390-1300°F), can ignite fires and melt snow and ice. Despite common depictions in movies and literature, most people who directly die from volcanic eruptions, die from pyroclastic flows, not from lava flows.

Figure \(\PageIndex{1}\): A collapsing eruptive column forms pyroclastic flows that surge down the flanks of Mayan Volcano, Philippines during the 1984 eruption. "Pyroclastic flows at Mayon Volcano" by C.G. Newhall, USGS is in the public domain.

Volcanic Ash and Tephra

All explosive volcanic eruptions generate tephra, fragments of rock that are produced when magma or rock is explosively ejected. The largest fragments, blocks and bombs (>64 mm, 2.5 inches diameter), can be expelled with great force but are deposited near the eruptive vent. Lapilli-sized material (6-64 mm, 0.24-2.5 inches diameter) can be carried upward within a volcanic plume and downwind in a volcanic cloud, but fall to the ground as the eruption cloud cools. The smallest material, volcanic ash (<2 mm diameter) is both easily convected upward within the plume and carried downwind for very long distances (Figure \(\PageIndex{2}\); as it falls out of suspension it can potentially affect communities and farmland across hundreds, or even thousands, of square kilometers (Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Ash buries cars and buildings after the 1984 eruption of Rabaul, Papua New Guinea. "Ash buries cars" by the USGS is in the public domain.

The activity, “Volcanic Ash Comparison” will help you apply lessons learned from a past Cascade volcanic eruption to understand future hazards.

Query \(\PageIndex{1}\)

Access a detailed description of the image in this activity

Lahars

‘Lahar’ is an Indonesian term that describes a hot or cold mixture of water, volcanic ash, rock fragments, and other debris that flows down the slopes of a volcano and typically enters a river valley. The main fluid medium for a lahar is water, which makes it distinctly different from a pyroclastic flow, in which the fluid medium is gas. Lahars are one of the most concerning volcanic hazards in the Cascades for two main reasons. One is that they can flow further from the volcano than lava flows or pyroclastic flows and the other is that they can occur without an eruption. Lahars occurred during several of the eruptions in the 1914-1917 eruption sequence of Lassen Peak and have occurred on Mount Shasta both with and without an eruption.

To make a lahar, all that is necessary is a large volume of water combined with a large volume of loose volcanic material. Even after thousands of years of quiet slumber, composite volcanoes still have abundant loose volcanic material covering their slopes. Therefore, all that’s needed to produce a lahar is a rapid input of water. Eruptions may trigger lahars by ejecting water from a crater lake melting snow and ice. Many of the composite volcanoes of the Cascades, including Mount Shasta, have year-round ice in the form of glaciers and snow fields, and also have significant snowpack throughout much of the winter and spring. On May 18, 1980, nearly all of the spring snowpack and glaciers of Mount St. Helens melted nearly instantaneously, producing large lahars (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Bridge destroyed by lahar in North Fork Toutle River during eruption of Mount St. Helens, May 18,1980. "Mount St Helens Lahar" by Richard Waitt, USGS is in the public domain.

Lahars can also form when heavy rainfall or rapid snow melt occurs without an eruption. On steep slopes, rainwater or meltwater can easily erode and transport fine-grained, loose volcanic sediment and form a slurry, especially if vegetation has not had time to grow back on recent volcanic deposits. Ice-clad volcanoes are common locations for the initiation of small seasonal lahars, more often referred to as ‘debris flows’ or ‘mudflows’. As Cascade glaciers melt in a warming climate, there is increasing concern over the potential for larger and more frequent non-eruptive debris flows at many Cascade volcanoes, including at Mount Shasta.

Acknowledgments

Parts of the descriptions of volcanic hazards are taken with minimal editing from sources provided by the USGS Volcano Hazards Program, which are in the public domain. Links to the original text can be found in the reference section on this page.

References

1. Ewert, J. W., Diefenbach, A. K., & Ramsey, D. W. (2018). 2018 update to the US Geological Survey national volcanic threat assessment. US Geological Survey. https://pubs.usgs.gov/publication/sir20185140
2. Mangan, M., Ball, J., Wood, N., Jones, J. L., Peters, J., Abdollahian, N., Dinitz, L., Blankenheim, S., Fenton, J., & Pridmore, C. (2019). California’s exposure to volcanic hazards. In Scientific Investigations Report (2018–5159). U.S. Geological Survey. https://doi.org/10.3133/sir20185159
3. U.S. Geological Survey. (n.d.-a). Ashfall is the most widespread and frequent volcanic hazard. Volcano Hazards Program. Retrieved May 29, 2024, from https://www.usgs.gov/programs/VHP/ashfall-most-widespread-and-frequent-volcanic-hazard
4. U.S. Geological Survey. (n.d.-b). Lahars move rapidly down valleys like rivers of concrete. Volcano Hazards Program. Retrieved May 29, 2024, from https://www.usgs.gov/programs/VHP/lahars-move-rapidly-down-valleys-rivers-concrete
5. U.S. Geological Survey. (n.d.-c). Lava flows destroy everything in their path. Volcano Hazards Program. Retrieved May 29, 2024, from https://www.usgs.gov/programs/VHP/lava-flows-destroy-everything-their-path
6. U.S. Geological Survey. (n.d.-d). Pyroclastic flows move fast and destroy everything in their path. Volcano Hazards Program. Retrieved May 29, 2024, from https://www.usgs.gov/programs/VHP/pyroclastic-flows-move-fast-and-destroy-everything-their-path