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Figure 1:
Index map showing Newberry volcano and vicinity
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Figure 1:
Index map showing Newberry volcano and vicinity
The most-visited part of the volcano is Newberry Crater, a volcanic depression or caldera at the summit of the volcano. Seven campgrounds, two resorts, six summer homes, and two major lakes (East and Paulina Lakes) are nestled in the caldera. The caldera has been the focus of Newberry's volcanic activity for at least the past 10,000 years. Other eruptions during this time have occurred along a rift zone on the volcano's northwest flank and, to a lesser extent, the south flank.
Many striking volcanic features lie in Newberry National Volcanic Monument, which is managed by the U.S. Forest Service. The monument includes the caldera and extends along the northwest rift zone to the Deschutes River. About 30 percent of the area within the monument is covered by volcanic products erupted during the past 10,000 years from Newberry volcano.
Newberry volcano is presently quiet. Local earthquake activity (seismicity) has been trifling throughout historic time. Subterranean heat is still present, as indicated by hot springs in the caldera and high temperatures encountered during exploratory drilling for geothermal energy.
This report describes the kinds of hazardous geologic events that might occur in the future at Newberry volcano. A hazard-zonation map is included to show the areas that will most likely be affected by renewed eruptions. In terms of our own lifetimes, volcanic events at Newberry are not of day-to-day concern because they occur so infrequently; however, the consequences of some types of eruptions can be severe. When Newberry volcano becomes restless, be it tomorrow or many years from now, the eruptive scenarios described herein can inform planners, emergency response personnel, and citizens about the kinds and sizes of events to expect.
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Figure 2:
Notable volcanic events at Newberry volcano and in central Oregon during the
past 15,000 years. Dotted lines show approximate age of events at Newberry
volcano; shaded boxes show age of events at other volcanoes. No eruptions have
occurred in the past 1,000 years in this region.
Lava flows may also issue from cinder cones or drain away from spatter ramparts that are built by lava fountains. Lava flows are streams of molten rock that move downslope until they cool and solidify. People and animals can walk or run from lava flows, which on average move less than about 500 m per hour (30 ft per minute). But any structures in the flow path are burned or crushed.
Basaltic magma may erupt from long linear fissures or from pipe-like vents. Excellent examples of both are found along Newberry's northwest rift system, which formed about 7,000 years ago. The northwest rift system traverses the volcano's northern flank for 22 km (14 mi) from Lava Butte to the caldera. East Lake Fissure, on the caldera wall north of East Lake, marks the southern extent of the northwest rift system. The rift system includes 12 lava flows that range from 1 to 9 km in length (0.6 to 5.6 mi) and cover areas as great as 24 km2 (6,000 acres or 9 square miles). In total, lava flows of this eruptive episode covered more than 60 km2 (23 square miles).
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Figure 3:
Characteristic volcanic phenomena expected for eruption of small to moderate
volumes of rhyolite at Newberry caldera. A, eruptive process. Not shown
is a final oozing of degassed magma to form obsidian flow. B, setting
today after such a sequence of events 1,300 years ago. During that particular
eruption, prevailing winds forced the tephra cloud eastward to blanket the east
flank of volcano with thick fall deposit.
Explosive volcanic eruptions discharge debris that is highly fragmented, mainly as a consequence of gases that froth and disrupt the magma as they expand. Geologists use the term pyroclastic (literally, fire-broken) to describe these explosive eruptions and the resulting deposits. Pyroclastic eruptions present the greatest threat to lives because of their violence and the great speed with which the material can sweep out from vents.
During rhyolitic eruptions, gas-charged magma and rock along the sides of vents are broken into fragments, called tephra, that range in size from large blocks to fine dust. The tephra is jetted into the atmosphere to form clouds that rise and drift downwind. Larger particles fall close to the vent, but finer-grained tephra can be carried for tens to hundreds of kilometers. Tephra clouds can create darkness lasting tens of minutes to hours, even on sunny days. Deposits of tephra can short-circuit electric transformers and power lines, especially if the tephra is wet, which makes it highly conductive, cohesive, and heavy. Tephra ingested by engines will clog filters and increase wear. Tephra clouds often generate lightning that may interfere with electrical and communications systems and start fires. Perhaps most importantly, even dilute tephra clouds pose a substantial hazard to aircraft that fly into them.
In contrast to tephra clouds that ascend into the atmosphere, other mixtures are denser than air and flow along the ground surface, driven by gravity. These mixtures, known as pyroclastic flows, are hotfrom 300 to more than 800 C (570 to >1,470 degrees F). They descend a volcano's flanks at speeds ranging from 10 to more than 100 m per second (20 to >200 mi per hour). Described figuratively as glowing avalanches, these mixtures are sufficiently dense to be funneled into canyons or other topographically low areas.
If the hot mixture is composed mostly of gas with a small proportion of rock and ash, its lower density makes its path less governed by topography. Flows of this type are called pyroclastic surges. Pyroclastic flows and surges often occur together. They can incinerate, asphyxiate, bury, and crush objects and living things in their path. Because of their high speed, pyroclastic flows and surges are difficult or impossible to escape. Evacuation must take place before such events occur.
Lava flows may also form during rhyolitic eruptions. Rhyolitic lava is so viscous that it typically solidifies without much crystallization, forming volcanic glass called obsidian. Rhyolitic lava may squeeze from the vent to form a steep-sided lava dome. Lava domes and thick lava flows move only meters per day and are not especially hazardous. But the steepened faces may collapse without warning, spawning avalanches of hot volcanic debris that can generate destructive pyroclastic flows and localized clouds of airborne tephra.
The eruptive sequence that culminated in the Big Obsidian Flow 1,300 years ago exemplifies several aspects of a typical rhyolitic eruptive sequence at Newberry volcano. The eruptions began with tephra showers that deposited pumice lumps and dense lava blocks as large as 1 m (3 ft) within the caldera. These tephra deposits, which are thicker than 13 m (43 ft) near the vent, diminish in thickness and grain size downwind. For example, 50 km (30 mi) downwind from the caldera near Brothers, Oregon, these tephra deposits are 25 cm (1 ft) thick and have average grain size of 3 mm (0.1 in.). Newberry tephra can be traced as a fine-grained ash deposit as far east as Idaho.
As the eruption progressed, pyroclastic flows swept downslope from the Big Obsidian vent to Paulina Lake (fig. 3A). The boat ramp at Little Crater Campground is excavated in these pyroclastic-flow deposits, as is the caldera road upslope from Paulina Lake. The flows entered Paulina Lake, perhaps causing secondary steam explosions and displacing water from the lake into Paulina Creek.
The final stage of eruption produced the Big Obsidian Flow itself, a lava flow that moved slowly, probably advancing only a few meters or tens of meters per day as it oozed down an inner caldera wall and ponded on the caldera floor (fig. 3B). The Big Obsidian Flow is about 1.8 km (6,000 ft) long and locally thicker than 20 m (65 ft).
Three kinds of eruptions are expected to occur at Newberry volcano in the future. The most likely type involves explosive pyroclastic eruptions of rhyolitic magma in small to moderate volumes (0.01-1.0 cubic km; 13 million-1300 million cubic yards) from vents in the caldera or just beyond the caldera rim. The caldera is the most likely site for such eruptions, owing to the abundance of rhyolite that has erupted there in the past. Also, the presence of lakes and shallow ground water in the caldera increases the likelihood that eruptions from caldera vents will be explosive. Even basaltic magma can generate strong explosions if erupted through water such as the caldera lakes. The next most likely type of future eruption, and one of lesser potential hazard, is a basaltic eruption from vents on the flanks. These would likely produce lava flows and cinder deposits, also of small to moderate volume. The third type, and fortunately the least likely to occur, is a large explosive eruption from a vent in the caldera that discharges several cubic kilometers or more of magma. Such eruptions include those that created the caldera. A description follows of the hazard zones for each of these eruption types as well as other events that might accompany eruptive activity.
Eruptions that occur within East or Paulina Lakes or along their shores may produce pyroclastic surges that would spread rapidly outward from the vent. The caldera walls would contain much of the devastation created by these eruptions except along the western caldera rim, which is topographically low. Pyroclastic flows or surges erupted in that area could surmount the caldera rim and descend the west flank.
Any pyroclastic eruptions at Newberry would also produce tephra showers. The caldera and upper flanks are most likely to receive substantial accumulations of tephra (10 cm to several meters, or 4 in. to more than 100 in.), but these sites have few permanent residents. Therefore, risk is minimized by ease of evacuation and sparse development. Downwind sites have more development at risk. Mid- to high-altitude winds in central Oregon blow 80 percent of the time toward the northeast, east, and southeast. Millican or Brothers (fig. 1) are the nearest settlements most likely to be downwind during eruptions from caldera vents. However, they lie sufficiently far from the caldera (30-50 km, 20-30 mi) that tephra from most eruptions would likely accumulate less than a few centimeters (few inches), but could reach 25 cm (1 ft) thick during eruptions like those of 1,300 years ago. Similar thicknesses could fall in Bend or La Pine, but suitable wind directions occur infrequently.
On the basis of eruption frequency during the recent geologic past (fig. 2), we estimate the annual probability of explosive eruptions affecting the caldera and immediately adjacent areas is about 1 in 3,000 (four eruptive periods, one basaltic and three rhyolitic, in 12,000 years). The probability of such an eruption occurring in a 30-year period, the duration of many home mortgages or a human generation, is roughly 30 times the annual probability or 1 in 100. We caution that these probabilities are based solely on the long-term behavior of the volcano. Any signs of increased restlessness at Newberry volcano will increase these probabilities dramatically.
When all Cascade volcanoes are considered, the annual probability that at least 1 cm (0.4 in.) of tephra might accumulate in central Oregon ranges from about 1 chance in 1,000 to 1 in 5,000 (fig. 4). Although 1 cm of ash may seem a trifling accumulation, the recent experience with Mount St. Helens indicates that as little as 0.5 cm of ash (0.2 in.) is sufficient to bring automobile and truck traffic to a crawl and to close businesses for as much as a week or two.
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Figure 4:
Map showing annual probability of 1 cm or more of tephra accumulation in
Washington, Oregon, and northern California from eruptions throughout the
Cascade Range. Probability distribution reflects the frequency of explosive
eruptions at each major volcano, the variability in the thickness of tephra that
could be deposited at various downwind distances, and the variability in wind
direction[E].
The lahar-hazard zone along Paulina Creek encompasses areas that could be inundated by lahars or floods generated by volcanically induced melting of snowpack, by eruptions in Paulina Lake, or by water rapidly displaced when pyroclastic flows enter the lake. We estimate that flows would likely have discharge rates as great as 5,000 cubic meters per second [C]. Such a flow would be contained by Paulina Creek canyon, but if the flow were larger, water would spread as a broad sheet flood across the upland surface west of the caldera rim. There it would either infiltrate or be redistributed among many small channels that lead back into Paulina Creek. This upland area of potential flooding is shown stippled on the hazard zonation map (plate 1).
The downstream reach of Paulina Creek is of greater concern, owing to inhabited sites, highway and railroad routes, and major interstate electric transmission lines and natural gas pipelines in the area north of La Pine. Where Paulina Creek leaves the confines of its canyon, it diminishes in gradient and forms a broad alluvial fan. Lahars could spread across Paulina Prairie and extend north along the flood plain of Paulina Creek to its confluence with the Little Deschutes River. Such lahars or floods could bury or destroy U.S. Highway 97 and tracks of the Burlington Northern-Santa Fe Railway Co.
The 100-year flood plain of the Little Deschutes River downstream from Paulina Creek is also included in the hazard zone for lahars and flooding in the event of volcanically induced surges of water from Paulina Lake. The hazard zone ends at the confluence of the Little Deschutes and Deschutes Rivers [D], but effects of flooding could extend some unknown distance dowstream along the channel and flood plain of the Deschutes River.
Gas presently discharges from hot springs in Paulina and East Lakes and from a gas vent (fumarole) at Lost Lake near the Big Obsidian Flow. Water vapor and carbon dioxide (CO2) are major components in the gas. The gas has an odor of rotten eggs (hydrogen sulfide), but its noxious components are currently in very low concentrations.
Gas in the caldera is of little consequence unless the discharge rate were to increase substantially. The hazard zone for gases is restricted to the caldera, owing to the presence of known gas seeps there and the numerous small topographic depressions found upon the caldera floor. Even with increased gas discharge, atmospheric circulation would probably be adequate to disperse the gas and reduce the hazard in most settings. For example, the broad open basins of Paulina and East Lakes are sufficiently well ventilated that accumulation of gases to dangerous levels is unlikely. However, caves and depressions on the young rugged obsidian flows and elsewhere in the caldera are natural sites where accumulation of carbon dioxide and other gases that are denser than air could become lethal. Artificially created enclosures such as manholes, excavations, tents, or snowcaves present the greatest danger of trapping and concentrating gas sufficient to threaten lives.
Some readers may be familiar with rare events in which volcanic lakes trap carbon dioxide in their lower levels for several years and then release the gas catastrophically. In 1986, 1,700 people living near Lake Nyos, Cameroon, were asphyxiated in this manner. Fortunately, such an event is highly improbable at Newberry caldera. Paulina and East Lakes are not deep enough and their water mixes too well during the year to accumulate sufficient carbon dioxide to produce a deadly gas release.
The outer boundary of lava-flow hazard zone LA is determined by encircling the part of the volcano with greatest density of vents as determined by geologic mapping. As shown on the hazard-zonation map, the outline of zone LA broadly defines the elongate shape of Newberry volcano itself, consistent with the idea that the volcano has grown by the repeated eruption of lava from vents preferentially located on the north and south flanks and in the summit region. Indeed, the topographic contour lines may themselves be thought of as probabilistic contours, with likelihood of eruption increasing at higher elevations on the volcano. The caldera, which originated by repeated collapse, is an obvious exception to this concept of linking elevation and eruption probability.
The probability that a flank eruption will affect a given area in zone LA can be estimated only approximately because the frequency of such eruptions prior to the last ones about 7,000 years ago are so poorly known. We infer that the annual probability of a flank eruption occurring in zone LA is roughly 1 in 5,000 to 1 in 10,000. But because lava flows of a flank eruptive period would cover only part of zone LA, the annual probability of a given point in the zone being covered by a lava flow is less than 1 in 10,000, perhaps substantially less. Within zone LA the probability would be somewhat higher near the caldera and along rift zones and somewhat lower at the outer boundary. Again, we caution that these probabilities are based solely on the long-term behavior of the volcano. Any signs of increased restlessness at Newberry volcano will increase these probabilities dramatically.
Another way to estimate a probability is to consider the results from deep drilling midway along the north and south flanks. These holes, located at roughly the 1,700-m elevation (5,600 ft) on the volcano (see plate 1), indicate that lava flows about 600 m (2,000 ft) in total thickness have been emplaced during the past 600,000 years. From studies at Newberry and other Cascade volcanoes and volcanic fields, we estimate that 10-20 m (33-66 ft) is a representative range for the average thickness of a field of lava flows that would accumulate during an eruptive period. Therefore, the 600 m of material in a drillhole would record 30-60 eruptive periods, or an average frequency of burial of that point of once every 10,000-20,000 years at the middle elevations of the volcano. Such frequencies represent annual probabilities (1 in 10,000 to 1 in 20,000) that are similar to those estimated above.
Lava-flow hazard zone LB encompasses the entire hazard-map area beyond zone LA. Zone LB includes areas on the lower flanks and downslope from Newberry volcano and elsewhere in the region that have been affected by lava flows less frequently than areas in zone LA. Sources for flows include Newberry volcano or, toward the edges of the map area, other volcanoes in the Cascade Range or central Oregon. We estimate that the annual probability of an eruption in this zone or of lava flows invading the zone from vents in zone LA is roughly 1 in 100,000, or less, on the basis of the frequency of lava-flow coverage in the past one million years and the few, widely scattered vents in the region.
Could eruptions occur in Bend? Could La Pine witness the growth of a small cinder cone? Could lava flows reach the Fort Rock Post Office? The answer to all these questions is yes, but the probability is exceedingly small. Pilot Butte and a handful of other small vents have erupted during the past 500,000 years within what is now the city of Bend. Lava flows that erupted from the flanks of Newberry volcano once progressed across the plain north of Bend, reaching 25 km (16 mi) beyond Redmond (fig. 1). Geologically, central Oregon is a volcanic terrane, and volcanic activity can be expected in the future. Fortunately for our homes and businesses, eruptions recur infrequently in these more developed areas.
A question commonly asked is whether Newberry volcano could produce an event similar to the large lateral blast that devastated more than 500 square km (200 sqare miles) when Mount St. Helens erupted in May 1980. Prior to eruption, a large landslide slipped from the north side of Mount St. Helens after magma had accumulated in the volcano's throat. The effect was to abruptly uncork a pressurized mixture of magma and gas, freeing it to surge across the landscape. At Newberry volcano, such a lateral blast is unlikely. Newberry is broad and gently sloping, not a steep-sided cone like Mount St. Helens or other Cascade composite volcanoes. Magma rising into the shallow crust at Newberry volcano would be buttressed by a substantial mass of rock. Small slope failure and associated blasts could conceivably be associated with eruptions near but slightly beyond the caldera walls. The resulting hazards would be confined to the hazard zone for explosive eruptions.
Newberry volcano is monitored by the U.S. Geological Survey (USGS). A regional network of seismometers for measuring earthquakes is operated jointly by the USGS and the Geophysics Program at the University of Washington. The USGS conducts periodic leveling surveys across the volcano to assess the volcano's elevation profile. The leveling stations will be remeasured in the event of future earthquake swarms to look for changes that may indicate the volcano is swelling in response to magma injection. Hot-spring gases and caldera lake waters are sampled intermittently. Given Newberry's inactivity, this level of monitoring is appropriate and economical.
At Newberry volcano, much of the area in hazard zones lies within the Deschutes National Forest. The near-absence of people living in the higher-hazard areas on the upper flanks of the volcano simplifies the often complex economic and social aspects of hazard management. Distal parts of the lahar hazard zone on the west flank are already managed as flood plains along Paulina Creek and the Little Deschutes and Deschutes Rivers. Areas subject to lahars that aren't in these flood plains are limited in size but include several subdivisions north of La Pine. People living in these areas at some distance from urban centers need to know about volcano hazards and be prepared to make informed decisions on their own. Planning is prudent because once an emergency begins, public resources may be overwhelmed, and citizens may need to provide for themselves.
Casadevall, T.J. (ed.), 1994, Volcanic ash and aviation safety: Proceedings of the First International Symposium on Volcanic Ash and Aviation Safety: U.S. Geological Survey Bulletin 2047, 450 p.
Warrick, R.A, and six other authors, 1981, Four communities under ash: after Mount St. Helens: Boulder, Colo., University of Colorado Institute of Behavioral Science Monograph No. 34, 143 p.
Jensen, R.A., 1988, Roadside guide to the geology of Newberry volcano: Bend, Oreg., CenOreGeoPub (20180 Briggs Road, Bend, OR 97701), 75 p.
MacLeod, N.S., Sherrod, D.R., Chitwood, L.A., and Jensen, R.A., 1995, Geologic map of Newberry volcano, Deschutes, Klamath, and Lake Counties, Oregon: U.S. Geological Survey Miscellaneous Investigations Map I-2455, scales 1:62,500 and 1:24,000.