| Reconstructing a volcano's history:  Deposits  Stratigraphy  Dating  Mapping  | Hazard-zone maps |

Mount Shasta volcano, California Interstate 5 at base of southwest flank

A hazard-zonation map is the best way to show areas most likely to be affected by different types of volcanic activity in the future. When sufficient information about a volcano's eruptive history is available, such maps can also indicate the relative degree of hazard or the expected frequency of occurrence. Typically, scientists prepare one or more maps as part of an overall hazard-assessment report for a specific volcano (see links to online hazard assessments for US volcanoes).

Each volcano hazard-zonation map is unique for several reasons. First, each volcano has significant differences in the type, size, and frequency of past eruptions. A volcano that tends to produce frequent large lahars extending downstream more than 50 km requires a different representation of hazards than does a volcano that produces primarily lava flows that extend less than 5 km from a vent. Second, the shape of each volcano and the surrounding topography vary tremendously. Consequently, the flow direction and extent of hazards vary from one volcano to another. Third, there are different ways to illustrate the aerial extent of volcano hazards with increasing distance from a volcano. A few examples from US volcano hazard assessments are presented below to show different approaches scientists have used to depict where volcano hazards are located and their relative severity or frequency.

Limitations need to be considered

In identifying potential hazard areas based on a volcano's eruptive history, scientists assume that the future behavior of a volcano will be similar to its past behavior in terms of the type, frequency, and magnitude of events. A few shortcomings of this method should be understood by scientists preparing the reports and people using them. First, even when armed with a detailed reconstruction of a volcano's history, which helps to identify the extent of past volcanic activity, scientists cannot predict how far or how much of an area each of the volcano hazards will actually extend during a future eruption. Some of the variables that cannot yet be known in advance of volcanic eruptions include: size of eruption (volume of eruptive products) or of a landslide; an unprecedented event in the history of the volcano could occur duration of eruptive activity extent of topographic changes that occur on an erupting volcano or in nearby valleys as a result of explosive activity or new deposits specific vent location (for example, at summit, on flank, or along a rift zone) for some volcanoes, an abrupt change in chemistry of erupted products (thereby leading to a change in eruptive style) Second, the boundaries between hazard zones are only approximately located and gradational.

Examples of volcano-hazard zones for US volcanoes

Hazard zones based on distance & vent locations: lahars and pyroclastic flows
	Mt. Hood, Oregon	Distance from volcano helps define hazard zones For Mount Hood volcano in Oregon, a proximal and distal hazard zone was identified by scientists in a 1997 hazard assessment. Proximal hazard zones (tan and orange colored areas on map) are areas subject to rapidly moving landslides, pyroclastic flows, and lahars that can reach the hazard boundary in less than 30 minutes. Distal hazard zones (brown areas on map) include areas adjacent to rivers that are pathways for lahars. Location of vent affects hazard zones On the basis of vent location during the next eruption, these hazard zones are subdivided into two additional zones. During the past two eruptive episodes, the vent was located on the south side of the summit at Crater Rock (orange color). Scientists anticipate the vent for the next eruption will most likely be in the same area and affect all or part of the orange area. If the vent is located on the west, north, or east flank of the volcano, however, the hazard zones (tan color) will be located on different sides of Mount Hood. See USGS fact sheet, Mount Hood—History and Hazards of Oregon's Most Recently Active Volcano, for explanation of this map (and a larger map).

Long Valley, California

Uncertain future vent location defines larger zone This hazard map for pyroclastic flows is based on explosive eruptions during the past 10,000 years from vents located in the Long Valley area that are known to have that ejected  up to 1 km3 of magma and generated pyroclastic flows or surges. For each potential vent, the zone extends about 15 km (10 mi) in all directions. The larger zone (lighter color on map) is for future vents located along the Mono-Inyo Craters volcanic chain. Along the west and south sides of the chain, the zone is narrower because pyroclastic flows moving in these directions will be blocked by the high Sierra Nevada. The smaller zone (darker color on map) is for future vents centered along the south moat of the caldera, which is the location of epicenters of many swarms of earthquakes since 1980 and the area of most intense ground movement (deformation). Future pyroclastic flows and surges from a single eruption would affect only a part of the total hazard zones shown on the map. See information from the USGS Long Valley Observatory for more explanation of this map.

Lava-flow hazard zones
	Kilauea Volcano, Hawai`i	Historical eruptions and topography define zones Hazard zones from fluid basalt lava flows on the Island of Hawai`i are based chiefly on the (1) location and frequency of historic and prehistoric eruptions; and (2) topography of the volcanoes. Scientists have prepared a map that divides the five volcanoes of the Island of Hawai`i into zones that are ranked from 1 through 9 based on the relative likelihood of coverage by lava flows. On Kilauea Volcano (left), the most hazardous area (zone 1) includes the summit caldera and two rift zones that extend to the sea. Learn more about this map and how the 9 zones are defined by scientists from the USGS Hawaiian Volcano Observatory.
	Mt. Shasta, California	Distance and location of recent lava flows define zones  Mount Shasta has erupted mostly thick, blocky andesite lava flows from vents located at the summit and on the volcano's flanks. The basaltic andesite flows vary in maximum length from about 6 km for flank flows to about 9 km for flows that originated at the summit. This map shows that the lava-flow hazard decreases with increasing distance from the summit of the volcano. Furthermore, owing to the shape of the volcano and the most probable sites of future vents, sector A covering the north and east flanks is more likely to be affected than sector B. A more detailed explanation of this map is available.

Tephra hazard zones for a single volcano
	Glacier Peak, Washington	Wind direction and frequency and size of explosive eruptions determines hazard zones This map shows annual probability of tephra fall exceeding 0.5 inch thick from an eruption of Glacier Peak. Communities east of the volcano (and other Cascade volcanoes) are more susceptible to tephra fall because the wind is normally from the west. Glacier Peak has produced large tephra eruptions, but not frequently. During an eruption, only a narrow sector of a tephra hazard zone is likely to be affected.

Suggested references for additional information

Crandell, D.R., Hoblitt, R.P., 1986, Lateral blasts at Mount St. Helens and hazard zonation: Bulletin of Volcanology, vol. 48, p. 27-37.

Scott, W.E., 1989, Volcanic-hazards zonation and long-term forecasts: in Tilling, R.I. (ed.), 1989, Volcanic Hazards: American Geophysical Union, p. 25-49.