volcano hazards at newberry volcano, oregonpaulina peak (back center) forms the highest point on the...

Volcano Hazards at NewberryVolcano, Oregon

By

David R. Sherrod1, Larry G. Mastin2, William E. Scott2, and Steven P. Schilling2

1 U.S. Geological Survey, Hawaii National Park, HI 967182 U.S. Geological Survey, Vancouver, WA 98661

OPEN-FILE REPORT 97-513This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North AmericanStratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S.Government.

1997

U.S. Department of the InteriorU.S. Geological Survey

CONTENTS

Introduction 12anemonehpcinaclovsuodrazaH2snoitpureerutufotediugasiyrotsihcinaclovs'yrrebweN3citlasabebylekiltsomdluowsnoitpureknalF

The caldera would be the site of most rhyolitic eruptions—and other types of3snoitpureevisolpxeylsuoregnad

The presence of lakes may add to the danger of eruptions in the caldera 5The most damaging lahars and floods at Newberry volcano would be limited

5aerakeerCaniluaPehtotSmall to moderate-size earthquakes are commonly associated with volcanic

6ytivitca7noitanozdrazahonacloV7snoitpurecitsalcoryprofenozdrazaH8sdrazaharhpetlanoigeR8keerCaniluaPnosdoolfrosrahalrofenozdrazaH01sesagcinaclovrofenozdrazaH01swolfavalrofsenozdrazaH11ytilibaborpwolfosnoitpureevisolpxeedutingam-egraL21sgninrawdnagnirotinoM21gnidaerrehtrufrofsnoitsegguS

Endnotes 13

ILLUSTRATIONS

tekcopninogerO,onaclovyrrebweNtasdrazahonacloV.1etalP1ytinicivdnaonaclovyrrebweNgniwohspamxednI.1erugiF

Figure 2. Notable volcanic events at Newberry volcano and in central2sraey000,51tsapehtgnirudnogerO

Figure 3. Characteristic volcanic phenomena expected for eruption of4etiloyhrfosemulovetaredomotllams

Figure 4. Map showing annual probability of 1 cm or more of tephra9noitalumucca

Additional paper copies of this report can be purchased from the U.S. Geological Survey,Information Services, Box 25286, Federal Center, Denver, CO 80225

Cover photo: Big Obsidian Flow (center), a 1,300-year-old lava flow, is the youngest product ofNewberry volcano. Paulina Peak (back center) forms the highest point on the rim of NewberryCrater, a large caldera or volcanic depression at the summit of the volcano.

INTRODUCTIONNewberry volcano is a broad shield volcano

located in central Oregon (fig. 1). It has beenbuilt by thousands of eruptions, beginningabout 600,000 years ago. At least 25 vents onthe flanks and summit have been active duringseveral eruptive episodes of the past 10,000years. The most recent eruption 1,300 years agoproduced the Big Obsidian Flow. Thus, thevolcano's long history and recent activityindicate that Newberry will erupt in the future.

The most-visited part of the volcano isNewberry Crater, a volcanic depression or

caldera at the summit of the volcano. Sevencampgrounds, two resorts, six summer homes,and two major lakes (East and Paulina Lakes)are nestled in the caldera. The caldera has beenthe focus of Newberry's volcanic activity for atleast the past 10,000 years. Other eruptionsduring this time have occurred along a rift zoneon the volcano's northwest flank and, to a lesserextent, the south flank.

Many striking volcanic features lie inNewberry National Volcanic Monument, whichis managed by the U.S. Forest Service. Themonument includes the caldera and extendsalong the northwest rift zone to the Deschutes

INTRODUCTION 1

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Fort Rock

Silver Lake

Chemult

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La Pine

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PrinevilleReservoir

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Newberry volcano

extent of Newberrylava flows

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Figure 1. Index map showing Newberry volcano and vicinity.

River. About 30 percent of the area within themonument is covered by volcanic productserupted during the past 10,000 years fromNewberry volcano.

Newberry volcano is presently quiet. Localearthquake activity (seismicity) has beentrifling throughout historic time. Subterraneanheat is still present, as indicated by hot springsin the caldera and high temperaturesencountered during exploratory drilling forgeothermal energy.

This report describes the kinds of hazardousgeologic events that might occur in the future atNewberry volcano. A hazard-zonation map isincluded to show the areas that will most likelybe affected by renewed eruptions. In terms ofour own lifetimes, volcanic events atNewberry are not of day-to-day concernbecause they occur so infrequently; however,

the consequences of some types of eruptionscan be severe. When Newberry volcanobecomes restless, be it tomorrow or many yearsfrom now, the eruptive scenarios describedherein can inform planners, emergencyresponse personnel, and citizens about the kindsand sizes of events to expect.

HAZARDOUS VOLCANICPHENOMENA

Newberry's volcanic history is aguide to future eruptions

Future eruptions at Newberry volcano willprobably resemble those that occurred in thepast 15,000 years (fig. 2). These volcaniceruptions varied widely from relatively quiet

2 VOLCANO HAZARDS AT NEWBERRY VOLCANO, OREGON

Lava flowsDavis Lake area

Tephra fallMount Mazama(Crater Lake)

Lava flowsMt. Bachelor

CalderaEvents at other

volcanoes in central Oregon

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Lava flowsSantiam Pass

Tephra falls and obsidian flowsSouth Sister

Lava flowsMcKenzie Pass area

FlanksEvents at Newberry volcano

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Notable volcanic events in central Oregon during the past 15,000 years

Big Obsidian FlowPaulina Lake pyroclastic flowtephra falls

East Lake Obsidian Flowstephra falls

Small unnamed obsidian flowsInterlake Obsidian Flow

Central Pumice ConeEast Lake tephra

Lava flowsNorthwest rift system,

including Lava Butte, andflow on south flank

East rim fissure ventand lava flow

Cinder cone or spatter vent

Lava flow

Tephra showersObsidian flow

Pyroclastic flows

Year

sbe

fore

pres

ent

East Lake Fissure

Lava flows of uncertainage but youthful;from Pilpil Butte,

North Kawak Butte,Devils Horn, and The Dome

cinder cones and fissure vents

Figure 2. Notable volcanic events at Newberry volcano and in central Oregon during the past 15,000 years. Dotted linesshow approximate age of events at Newberry volcano; shaded boxes show age of events at other volcanoes. No eruptionshave occurred in the past 1,000 years in this region.

effusion of lava flows to highly explosivedischarge of pumice and ash. The difference ineruptive style stems from the composition of themagma, or molten rock, and the amount ofdissolved gas it contains. At Newberry volcano,the most common magma types are basalt andrhyolite, and each has characteristic eruptivephenomena associated with it.

Flank eruptions would most likelybe basaltic

Basaltic eruptions are well known fromobservations elsewhere, such as at Hawaii,where spectacular fountains of spatter andcinders are associated with lava flows. AtNewberry, basaltic eruptions have occurredrepeatedly on the volcano's flanks and in thecaldera. Typical products of a basaltic eruptionare the 7,000-yr-old cinder cone of Lava Butteand its surrounding lava flows, located 10 km (6mi) south of Bend (fig. 1). Basaltic eruptionscommonly begin with lava fountains that hurlcinders or spatter as far as 1 km (0.6 mi) from thevent. Ejecta are thrown aloft for hundreds to afew thousand meters. Large fragments areexpelled from the vent along ballistictrajectories, like artillery shells. Smallerparticles are carried by wind and convectiveupdrafts. The resulting deposits may be manymeters thick near the vent and build a steep-sided cinder cone, but they generally thin to afew millimeters within 10 km (6 mi) distancedownwind. The chief hazard from ballisticejection is direct impact. Some spatter will behot upon impact and likely will start forest fires.

Lava flows may also issue from cindercones or drain away from spatter ramparts thatare built by lava fountains. Lava flows arestreams of molten rock that move downslopeuntil they cool and solidify. People and animalscan walk or run from lava flows, which onaverage move less than about 500 m per hour(30 ft per minute). But any structures in the flowpath are burned or crushed.

Basaltic magma may erupt from long linearfissures or from pipe-like vents. Excellentexamples of both are found along Newberry'snorthwest rift system, which formed about7,000 years ago. The northwest rift system

traverses the volcano's northern flank for 22 km(14 mi) from Lava Butte to the caldera. EastLake Fissure, on the caldera wall north of EastLake, marks the southern extent of thenorthwest rift system. The rift system includes12 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, lavaflows of this eruptive episode covered morethan 60 km2 (23 square miles).

The caldera would be the site ofmost rhyolitic eruptions–and othertypes of dangerously explosiveeruptions

Rhyolitic eruptions have been restricted tothe caldera during the past 10,000 years.Rhyolitic magma tends to erupt moreexplosively than basaltic magma, owing to theincreased amount of gas commonly trapped init. Gas bubbles in rhyolite cannot easily rise andescape as they can from basalt, and gaspressures may build to much higher levels. Thisis because rhyolite is more viscous (resistant toflowage) than basalt. Gas-rich eruptions aregenerally more explosive and therefore moredangerous than gas-poor eruptions. Someevents expected in a rhyolitic eruption areshown in figure 3.

Explosive volcanic eruptions dischargedebris that is highly fragmented, mainly as aconsequence of gases that froth and disrupt themagma as they expand. Geologists use the term“pyroclastic” (literally, fire-broken) to describethese explosive eruptions and the resultingdeposits. Pyroclastic eruptions present thegreatest threat to lives because of their violenceand the great speed with which the material cansweep out from vents.

During rhyolitic eruptions, gas-chargedmagma and rock along the sides of vents arebroken into fragments, called tephra, that rangein size from large blocks to fine dust. The tephrais jetted into the atmosphere to form clouds thatrise and drift downwind. Larger particles fallclose to the vent, but finer-grained tephra can becarried for tens to hundreds of kilometers.Tephra clouds can create darkness lasting tens

HAZARDOUS VOLCANIC PHENOMENA 3

of minutes to hours, even on sunny days.Deposits of tephra can short-circuit electrictransformers and power lines, especially if thetephra is wet, which makes it highly conductive,cohesive, and heavy. Tephra ingested byengines will clog filters and increase wear.Tephra clouds often generate lightning that mayinterfere with electrical and communicationssystems and start fires. Perhaps mostimportantly, even dilute tephra clouds pose asubstantial hazard to aircraft that fly into them.

In contrast to tephra clouds that ascend intothe atmosphere, other mixtures are denser thanair and flow along the ground surface, driven

by gravity. These mixtures, known aspyroclastic flows, are hot—from 300 to morethan 800° C (570 to >1,470° F). They descenda volcano's flanks at speeds ranging from 10 tomore than 100 m per second (20 to >200 mi perhour). Described figuratively as “glowingavalanches,” these mixtures are sufficientlydense to be funneled into canyons or othertopographically low areas.

If the hot mixture is composed mostly of gaswith a small proportion of rock and ash, itslower density makes its path less governed bytopography. Flows of this type are calledpyroclastic surges. Pyroclastic flows and


pyroclastic surge

tephra cloud

pyroclastic flow

high-altitude wind direction

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tephra falloutfrom cloud

lake

Paulina Lake

Big Obsidian Flow

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pyroclastic-flow deposit

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VERTICAL EXAGGERATION X2

METERS

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Figure 3. Characteristic volcanic phenomena expected for eruption of small to moderate volumes of rhyolite at Newberrycaldera.A, eruptive process. Not shown is a final oozing of degassed magma to form obsidian flow.B, setting today aftersuch a sequence of events 1,300 years ago. During that particular eruption, prevailing winds forced the tephra cloudeastward to blanket the east flank of volcano with thick fall deposit.

surges often occur together. They canincinerate, asphyxiate, bury, and crush objectsand living things in their path. Because of theirhigh speed, pyroclastic flows and surges aredifficult or impossible to escape. Evacuationmust take place before such events occur.

Lava flows may also form during rhyoliticeruptions. Rhyolitic lava is so viscous that ittypically solidifies without muchcrystallization, forming volcanic glass calledobsidian. Rhyolitic lava may squeeze from thevent to form a steep-sided lava dome. Lavadomes and thick lava flows move only metersper day and are not especially hazardous. Butthe steepened faces may collapse withoutwarning, spawning avalanches of hot volcanicdebris that can generate destructive pyroclasticflows and localized clouds of airborne tephra.

The eruptive sequence that culminated inthe Big Obsidian Flow 1,300 years agoexemplifies several aspects of a typical rhyoliticeruptive sequence at Newberry volcano. Theeruptions began with tephra showers thatdeposited pumice lumps and dense lava blocksas large as 1 m (3 ft)within the caldera. Thesetephra deposits, which are thicker than 13 m (43ft) near the vent, diminish in thickness and grainsize 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 (0.1in.). Newberry tephra can be traced as a fine-grained ash deposit as far east as Idaho.

As the eruption progressed, pyroclasticflows swept downslope from the Big Obsidianvent to Paulina Lake (fig. 3A). The boat ramp atLittle Crater Campground is excavated in thesepyroclastic-flow deposits, as is the caldera roadupslope from Paulina Lake. The flows enteredPaulina Lake, perhaps causing secondary steamexplosions and displacing water from the lakeinto Paulina Creek.

The final stage of eruption produced the BigObsidian Flow itself, a lava flow that movedslowly, probably advancing only a few metersor tens of meters per day as it oozed down aninner caldera wall and ponded on the calderafloor (fig. 3B). The Big Obsidian Flow is about1.8 km (6,000 ft) long and locally thicker than20 m (65 ft).

The presence of lakes may add tothe danger of eruptions in thecaldera

The mixing of magma, either basaltic orrhyolitic, with water tends to change thecharacter of the eruption from one of continuousexpulsion of lava or tephra to one involvingdiscrete explosions. The major hazardproduced during such explosions is pyroclasticsurges of tephra, gas, and steam that radiate outfrom the vent at speeds as great as hundreds ofkilometers per hour and temperatures that rangefrom 100° C (212° F) to several hundreddegrees Celsius. Such surges typically extendless than 1 km from an eruptive vent, but somemay reach as far as 10 km (6 mi).

The most damaging lahars andfloods at Newberry volcano wouldbe limited to the Paulina Creek area

Lahars are watery flows of volcanic rocksand mud that surge downstream like rapidlyflowing, soupy concrete. Lahars, also known asmudflows or debris flows, can devastate valleyfloors tens of kilometers from the volcano.Lahars are a major hazard at steep-sided, snow-and ice-clad Cascade volcanoes, but they posemuch less of a threat at Newberry volcano. Thevolcano's slopes are relatively gentle, andalthough it commonly bears a thick seasonalsnowpack, it has no glaciers. Pyroclastic flowsand surges that encounter a snowpack on theouter upper flanks may generate lahars andfloods in some of the numerous small valleysthat crease the volcano's flanks, especially thenortheast flank. Such lahars and floods willspread out and attenuate on the lower flanks orin basins beyond the volcano, such as MillicanValley.

The valley of Paulina Creek, which drainsfrom Paulina Lake through the west rim ofNewberry Crater, is the most likely drainage tocarry damaging lahars and floods. In addition tolahars and floods caused by pyroclastic flowsmelting snow, a lahar could be generated alongPaulina Creek by lake overflow. The naturalbedrock barrier that forms the spillway fromthe lake is stable and unlikely to fail

HAZARDOUS VOLCANIC PHENOMENA 5

catastrophically, but pyroclastic flows enteringthe lake or explosive eruptions in the lake coulddisplace water into Paulina Creek's canyon. Alarge water flow could incorporate enoughdebris from the canyon walls to become a laharor it could remain as sediment-ladenfloodwater.

Geologic evidence suggests at least one suchflood occurred in the recent geologic past, but itsexact origin is uncertain. It may have beentriggered by failure of a 1.5-m-high (5-ft-high)rock ledge at the outlet rather than by aneruption [A] . The flood inundated the entirevalley floor in the reach above Paulina Prairieand probably had a discharge similar to that ofthe flood of record (in 1909) on the DeschutesRiver downstream from its confluence with theLittle Deschutes River (which receives PaulinaCreek flow). On the basis of published reportsfrom other volcanoes around the world, similaror larger floods could accompany futureeruptive activity in Paulina Lake.[B]

Lahars travel faster than water in channelsof similar depth and slope. And because theycarry mostly solid debris, lahars are moredestructive. They destroy bridges, break and fillpipelines, and clog ponds and reservoirs. Theycan also bury roadways, houses, and extensiveareas of agricultural, forest, or grazing land.Lahars or floods from Paulina Lake could reachthe La Pine valley within 30 minutes, so areaslikely to be impacted should be evacuatedbefore an eruption occurs. High ground nearthese areas, such as tops of ridges or buttes, islikely to be safe and may provide suitableemergency refuge.

Small to moderate-size earthquakesare commonly associated withvolcanic activity

Earthquakes occur when rocks breaksuddenly in response to various geologic forces.Magma moving in the Earth's crust may createsufficient force to produce volcanicearthquakes. More common, however, aretectonic earthquakes, which periodically strikeparts of Oregon. These earthquakes, the resultof fault movements driven by regional crustal

stresses, typically have no direct connection tomagma movement. Regardless of type,earthquake size is reported by magnitude, andmany scientists and media describe earthquakesby the well-known Richter magnitude scale.

Volcanic earthquakes are commonlysmaller than about magnitude 2.5, roughly thethreshold for felt shaking by observers close tothe event. Swarms of small earthquakes maypersist for weeks to months before eruptions,but little or no damage would occur to buildingsin surrounding communities. Some volcanic-related swarms may include earthquakes aslarge as about magnitude 5. For thecommunities of Bend, La Pine, and Sunriver,shallow earthquakes in the magnitude 4-5 rangethat are located beneath Newberry volcanowould cause walls to rattle or windows anddishes to vibrate. Some items might topple fromshelves, but bookcases and furniture wouldremain intact. The larger earthquakes would befelt by everyone in the area. At night, theshaking would awaken many people, especiallythose living closest to the volcano. Damage tobuildings and utilities would be nil in mostcases. Sustained episodes of magnitude-4earthquakes could crack plaster and damagewalls in older brick or stone buildings near thevolcano.

Tectonic earthquakes occur periodically insouth-central and southeast Oregon, and theyare capable of exceeding the magnitude ofvolcanic earthquakes. An example is theKlamath Falls earthquakes, a swarm that beganin September 1993 with two large earthquakesof magnitude 5.9 and 6.0 that killed two peopleand and caused $7.5 million in propertydamage. Aftershocks as large as magnitude 5.1continued to disturb residents for as much as sixmonths. These earthquakes had no connectionwith volcanic processes.

Newberry volcano lies at the northwestmargin of a broad geographic province knownas the Basin and Range, an area whoselandforms result from earthquake activity.Tectonic earthquakes as large as magnitude 7may strike areas south and east of Newberry.Could such tectonic earthquakes triggereruptions at Newberry? From observations inother earthquake areas, we conclude that


triggering can only occur if the volcanic systemis on the verge of eruption anyway. Statisticallyspeaking, central Oregon residents are far morelikely to feel earthquake shaking than to witnessan eruption in the area.

VOLCANO HAZARD ZONATIONNewberry's long, diverse volcanic history

has produced an array of hazardous eruptions.The accompanying hazard-zonation map (plate1) shows areas that could be affected by variousfuture eruptive phenomena. Not discussed inthis report are nonvolcanic hazards found in allmountainous regions, such as rockfalls oravalanches. Although we show sharpboundaries for hazard zones, the degree ofhazard does not change abruptly at theseboundaries but decreases gradually as distancefrom the volcano increases. Areas immediatelybeyond hazard zones should not be regarded ashazard free, because the boundaries can only beapproximately located. Too many uncertaintiesexist about the source, size, and mobility offuture events to locate zero-hazard zones withconfidence.

Three kinds of eruptions are expected tooccur at Newberry volcano in the future. Themost likely type involves explosive pyroclasticeruptions of rhyolitic magma in small tomoderate volumes (0.01-1.0 km3; 13 million-1300 million cubic yards) from vents in thecaldera or just beyond the caldera rim. Thecaldera is the most likely site for such eruptions,owing to the abundance of rhyolite that haserupted there in the past. Also, the presence oflakes and shallow ground water in the calderaincreases the likelihood that eruptions fromcaldera vents will be explosive. Even basalticmagma can generate strong explosions iferupted through water such as the caldera lakes.The next most likely type of future eruption, andone of lesser potential hazard, is a basalticeruption from vents on the flanks. These wouldlikely produce lava flows and cinder deposits,also of small to moderate volume. The thirdtype, and fortunately the least likely to occur, isa large explosive eruption from a vent in thecaldera that discharges several cubic kilometers

or more of magma. Such eruptions includethose that created the caldera. A descriptionfollows of the hazard zones for each of theseeruption types as well as other events that mightaccompany eruptive activity.

Hazard zone for small to moderate,explosive pyroclastic eruptions inor near the caldera

Hazards expected in this zone are tephrafalls, pyroclastic flows and surges, and ballisticprojectiles. The caldera has repeatedly been thesite of volcanic activity, with rhyolitic eruptiveproducts issuing from seven discrete ventsduring three eruptive periods in the past 7,500years. The great likelihood of explosivepyroclastic events, especially in the initialphases of intracaldera eruptions, warrantsassigning the greatest degree of hazard to thecaldera. On the basis of the distribution of pastvents, pyroclastic eruptions may burst forthfrom vents even as far as 3 km (2 mi) from thecaldera rim, but the probability of rhyolite ventsdiminishes abruptly beyond the caldera rim.The hazard zone for explosive pyroclasticeruptions includes areas exposed to the threat ofpyroclastic flows and surges and thick tephra-fall deposits, such as those erupted 1,300 yearsago from the vent for the Big Obsidian Flow.

Eruptions that occur within East or PaulinaLakes or along their shores may producepyroclastic surges that would spread rapidlyoutward from the vent. The caldera walls wouldcontain much of the devastation created bythese eruptions except along the western calderarim, which is topographically low. Pyroclasticflows or surges erupted in that area couldsurmount the caldera rim and descend the westflank.

Any pyroclastic eruptions at Newberrywould also produce tephra showers. Thecaldera and upper flanks are most likely toreceive substantial accumulations of tephra (10cm to several meters, or 4 in. to more than 100in.), but these sites have few permanentresidents. Therefore, risk is minimized by easeof evacuation and sparse development.Downwind sites have more development at risk.

VOLCANO HAZARD ZONATION 7

Mid- to high-altitude winds in central Oregonblow 80 percent of the time toward thenortheast, east, and southeast. Millican orBrothers (fig. 1) are the nearest settlements mostlikely to be downwind during eruptions fromcaldera vents. However, they lie sufficiently farfrom the caldera (30-50 km, 20-30 mi) thattephra from most eruptions would likelyaccumulate less than a few centimeters (fewinches), but could reach 25 cm (1 ft) thickduring eruptions like those of 1,300 years ago.Similar thicknesses could fall in Bend or LaPine, but suitable wind directions occurinfrequently.

On the basis of eruption frequency duringthe recent geologic past (fig. 2), we estimate theannual probability of explosive eruptionsaffecting the caldera and immediately adjacentareas is about 1 in 3,000 (four eruptive periods,one basaltic and three rhyolitic, in 12,000years). The probability of such an eruptionoccurring in a 30-year period, the duration ofmany home mortgages or a human generation,is roughly 30 times the annual probability or 1 in100. We caution that these probabilities arebased solely on the long-term behavior of thevolcano. Any signs of increased restlessness atNewberry volcano will increase theseprobabilities dramatically.

Regional tephra hazards

As a group, other Cascade Range volcanoespossess an equally likely chance to thinlyblanket the area with tephra. South Sister andCrater Lake are capable of discharging tephrathat could fall on Bend, La Pine, or other townsin the region. Even Mount St. Helens couldimpact central Oregon. If wind direction hadbeen to the south-southeast on May 18, 1980,Bend would have received 2-4 cm of tephra (1-2in.) from the eruption of Mount St. Helensdespite its location 250 km distant (160 mi).Such wind conditions occur about five percentof the time.

When all Cascade volcanoes are considered,the annual probability that at least 1 cm (0.4 in.)of tephra might accumulate in central Oregonranges from about 1 chance in 1,000 to 1 in5,000 (fig. 4). Although 1 cm of ash may seem a

trifling accumulation, the recent experiencewith Mount St. Helens indicates that as little as0.5 cm of ash (0.2 in.) is sufficient to bringautomobile and truck traffic to a crawl and toclose businesses for as much as a week or two.

Hazard zone for lahars or floods onPaulina Creek

Lahars of greatest concern at Newberryvolcano would be those produced in the PaulinaCreek drainage on the west side of the volcano,where we show a lahar-hazard zone on plate 1.Small valleys on other flanks, especially on thenortheast, could be subject to lahars or floodsinitiated when pyroclastic flows or surges meltpart of a snowpack; forest roads, however, arethe only developments at risk in these areas. Wedon't show a specific lahar-hazard zone in theseareas, but effects of lahars and flooding in theseareas would be greatest along small valleys inthe hazard zone for explosive pyroclasticeruptions and extend downstream into lava-flow hazard zone LA.

The lahar-hazard zone along Paulina Creekencompasses areas that could be inundated bylahars or floods generated by volcanicallyinduced melting of snowpack, by eruptions inPaulina Lake, or by water rapidly displacedwhen pyroclastic flows enter the lake. Weestimate that flows would likely havedischarge rates as great as 5,000 cubic metersper second[C]. Such a flow would be containedby Paulina Creek canyon, but if the flow werelarger, water would spread as a broad sheetflood across the upland surface west of thecaldera rim. There it would either infiltrate orbe redistributed among many small channelsthat lead back into Paulina Creek. This uplandarea of potential flooding is shown stippled onthe hazard zonation map (plate 1).

The downstream reach of Paulina Creek isof greater concern, owing to inhabited sites,highway and railroad routes, and majorinterstate electric transmission lines and naturalgas pipelines in the area north of La Pine.Where Paulina Creek leaves the confines of itscanyon, it diminishes in gradient and forms abroad alluvial fan. Lahars could spread acrossPaulina Prairie and extend north along the flood


plain of Paulina Creek to its confluence with theLittle Deschutes River. Such lahars or floodscould bury or destroy U.S. Highway 97 andtracks of the Burlington Northern-Santa FeRailway Co.

The 100-year flood plain of the LittleDeschutes River downstream from PaulinaCreek is also included in the hazard zone for

lahars and flooding in the event ofvolcanically induced surges of water fromPaulina Lake. The hazard zone ends at theconfluence of the Little Deschutes andDeschutes Rivers [D], but effects of floodingcould extend some unknown distancedowstream along the channel and flood plainof the Deschutes River.


Medicine Lake

Lassen Peak

SRETEMOLIK0020

100 MILES0

less than1 in 10,000

1 in 10,000

1 in 5,000

1 in 1,000

1 in 500

1 in 100

Annual Probability

Mount Baker

Glacier Peak

Mount Rainier

Mount Adams

Mount Hood

Mount Je�erson

Three Sisters

Crater Lake

Mount Shasta

Newberry volcano

Mount St. Helens

Figure 4. Map showing annual probability of 1 cm or more of tephra accumulation in Washington, Oregon, and northernCalifornia from eruptions throughout the Cascade Range. Probability distribution reflects the frequency of explosiveeruptions at each major volcano, the variability in the thickness of tephra that could be deposited at various downwinddistances, and the variability in wind direction [E].

Hazard zone for volcanic gases

Gas presently discharges from hot springs inPaulina and East Lakes and from a gas vent(fumarole) at Lost Lake near the Big ObsidianFlow. Water vapor and carbon dioxide (CO2)are major components in the gas. The gas has anodor of rotten eggs (hydrogen sulfide), but itsnoxious components are currently in very lowconcentrations.

Gas in the caldera is of little consequenceunless the discharge rate were to increasesubstantially. The hazard zone for gases isrestricted to the caldera, owing to the presenceof known gas seeps there and the numeroussmall topographic depressions found upon thecaldera floor. Even with increased gasdischarge, atmospheric circulation wouldprobably be adequate to disperse the gas andreduce the hazard in most settings. Forexample, the broad open basins of Paulina andEast Lakes are sufficiently well ventilated thataccumulation of gases to dangerous levels isunlikely. However, caves and depressions onthe young rugged obsidian flows and elsewherein the caldera are natural sites whereaccumulation of carbon dioxide and other gasesthat are denser than air could become lethal.Artificially created enclosures such asmanholes, excavations, tents, or snowcavespresent the greatest danger of trapping andconcentrating gas sufficient to threaten lives.

Some readers may be familiar with rareevents in which volcanic lakes trap carbondioxide in their lower levels for several yearsand then release the gas catastrophically. In1986, 1,700 people living near Lake Nyos,Cameroon, were asphyxiated in this manner.Fortunately, such an event is highly improbableat Newberry caldera. Paulina and East Lakesare not deep enough and their water mixes toowell during the year to accumulate sufficientcarbon dioxide to produce a deadly gas release.

Hazard zones for lava flows ofbasaltic flank eruptions

Renewed flank eruptions would producecinder cones or fissure vents and lava flows.Eruptions would probably include several lava

flows, possibly from more than one vent, duringa time interval we call an eruptive period. Suchperiods might range in duration from weeks to adecade. We define two lava-flow hazard zonesfor Newberry on the basis of likelihood of futurelava flows within each zone. Lava-flow hazardzone LA encompasses the area more likely to bethe site of flank vents or to be covered by lava,including the caldera. Zone LB includes twomain areas: (1) areas on the lower flanks ofNewberry that have relatively few flank ventsand are chiefly covered by large lava flows fromvents farther upslope and (2) lava flows fromvents elsewhere in the Cascade Range or Basinand Range. Particular sites that might beaffected within each zone cannot be specified inadvance. Once precursory activity or a lava-flow eruption begins, scientists can betterdefine areas likely to be affected.

The outer boundary of lava-flow hazardzone LA is determined by encircling the part ofthe volcano with greatest density of vents asdetermined by geologic mapping. As shown onthe hazard-zonation map, the outline of zone LAbroadly defines the elongate shape of Newberryvolcano itself, consistent with the idea that thevolcano has grown by the repeated eruption oflava from vents preferentially located on thenorth and south flanks and in the summit region.Indeed, the topographic contour lines maythemselves be thought of as probabilisticcontours, with likelihood of eruption increasingat higher elevations on the volcano. The caldera,which originated by repeated collapse, is anobvious exception to this concept of linkingelevation and eruption probability.

The probability that a flank eruption willaffect a given area in zone LA can be estimatedonly approximately because the frequency ofsuch eruptions prior to the last ones about 7,000years ago are so poorly known. We infer that theannual probability of a flank eruption occurringin zone LA is roughly 1 in 5,000 to 1 in 10,000.But because lava flows of a flank eruptiveperiod would cover only part of zone LA, theannual probability of a given point in the zonebeing covered by a lava flow is less than 1 in10,000, perhaps substantially less. Within zoneLA the probability would be somewhat highernear the caldera and along rift zones and


somewhat lower at the outer boundary. Again,we caution that these probabilities are basedsolely on the long-term behavior of thevolcano. Any signs of increased restlessness atNewberry volcano will increase theseprobabilities dramatically.

Another way to estimate a probability is toconsider the results from deep drilling midwayalong the north and south flanks. These holes,located at roughly the 1,700-m elevation (5,600ft) on the volcano (see plate 1), indicate that lavaflows about 600 m (2,000 ft) in total thicknesshave been emplaced during the past 600,000years. From studies at Newberry and otherCascade volcanoes and volcanic fields, weestimate that 10-20 m (33-66 ft) is arepresentative range for the average thicknessof a field of lava flows that would accumulateduring an eruptive period. Therefore, the 600 mof material in a drillhole would record 30-60eruptive periods, or an average frequency ofburial of that point of once every 10,000-20,000years at the middle elevations of the volcano.Such frequencies represent annual probabilities(1 in 10,000 to 1 in 20,000) that are similar tothose estimated above.

Lava-flow hazard zone LB encompasses theentire hazard-map area beyond zone LA. ZoneLB includes areas on the lower flanks anddownslope from Newberry volcano andelsewhere in the region that have been affectedby lava flows less frequently than areas in zoneLA. Sources for flows include Newberryvolcano or, toward the edges of the map area,other volcanoes in the Cascade Range or centralOregon. We estimate that the annualprobability of an eruption in this zone or of lavaflows invading the zone from vents in zone LAis roughly 1 in 100,000, or less, on the basis ofthe frequency of lava-flow coverage in the pastone million years and the few, widely scatteredvents in the region.

Could eruptions occur in Bend? Could LaPine witness the growth of a small cinder cone?Could lava flows reach the Fort Rock PostOffice? The answer to all these questions is yes,but the probability is exceedingly small. PilotButte and a handful of other small vents haveerupted during the past 500,000 years withinwhat is now the city of Bend. Lava flows that

erupted from the flanks of Newberry volcanoonce progressed across the plain north ofBend,reaching 25 km (16 mi) beyond Redmond(fig. 1). Geologically, central Oregon is avolcanic terrane, and volcanic activity can beexpected in the future. Fortunately for ourhomes and businesses, eruptions recurinfrequently in these more developed areas.

Hazards from large-magnitudeexplosive eruptions of lowprobability

How large an eruption is possible atNewberry volcano? The worst-case scenario isfor a large-magnitude explosive eruption oreven another caldera-forming eruption, the veryprocess that has occurred at least twice in thepast 600,000 years to form Newberry Crater.Such a low frequency of occurrence suggeststhat the annual probability of another such eventis no greater than 1 in 100,000. Another famousexample of a caldera-forming eruption createdCrater Lake, Oregon, about 100 km (60 mi)southwest of Newberry volcano. A caldera-forming eruption would include violentshowers of pumice and ash that could continuefor days and deposit several meters of tephra onthe volcano. Devastating pyroclastic flowscould sweep out for 50 km (30 mi) from thevolcano. Today, however, the volcano showsno signs of the volcanic buildup that wouldprecede such a devastating eruption. Althoughpreparing for an event of such small probabilityis unreasonable, we should nonethelessunderstand the events that would occur duringthe maximum credible event.

A question commonly asked is whetherNewberry volcano could produce an eventsimilar to the large lateral blast that devastatedmore than 500 km2 (200 sq. miles) when MountSt. Helens erupted in May 1980. Prior toeruption, a large landslide slipped from thenorth side of Mount St. Helens after magma hadaccumulated in the volcano's throat. The effectwas to abruptly uncork a pressurized mixture ofmagma and gas, freeing it to surge across thelandscape. At Newberry volcano, such a lateralblast 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 atNewberry volcano would be buttressed by asubstantial mass of rock. Small slope failureand associated blasts could conceivably beassociated with eruptions near but slightlybeyond the caldera walls. The resulting hazardswould be confined to the hazard zone forexplosive eruptions.

MONITORING AND WARNINGS

Future eruptions at Newberry volcano willbe preceded by premonitory activity.Earthquakes associated with rising magma mostlikely will give days or weeks of advancewarning. Changes in the composition,temperature, or volume of volcanic gasesemanating from hot springs and fumarolesmight also indicate that an eruption is about tooccur. Increased gas discharge could lead totree kills as observed recently at MammothMountain, California, providing anotherindication that volcanic gas concentrations wereincreasing to dangerous levels. When any ofthese events are recognized, emergency-management agencies would be contactedimmediately and the level of monitoring wouldbe increased.

Newberry volcano is monitored by the U.S.Geological Survey (USGS). A regionalnetwork of seismometers for measuringearthquakes is operated jointly by the USGSand the Geophysics Program at the Universityof Washington. The USGS conducts periodicleveling surveys across the volcano to assess thevolcano's elevation profile. The levelingstations will be remeasured in the event of futureearthquake swarms to look for changes that mayindicate the volcano is swelling in response tomagma injection. Hot-spring gases and calderalake waters are sampled intermittently. GivenNewberry's inactivity, this level of monitoringis appropriate and economical.

At Newberry volcano, much of the area inhazard zones lies within the Deschutes NationalForest. The near-absence of people living in thehigher-hazard areas on the upper flanks of the

volcano simplifies the often complex economicand social aspects of hazard management.Distal parts of the lahar hazard zone on the westflank are already managed as flood plains alongPaulina Creek and the Little Deschutes andDeschutes Rivers. Areas subject to lahars thataren't in these flood plains are limited in size butinclude several subdivisions north of La Pine.People living in these areas at some distancefrom urban centers need to know about volcanohazards and be prepared to make informeddecisions on their own. Planning is prudentbecause once an emergency begins, publicresources may be overwhelmed, and citizensmay need to provide for themselves.

SUGGESTIONS FOR FURTHERREADING

Volcano hazards in general

Blong, R.J., 1984, Volcanic hazards-—a sourcebook onthe effects of eruptions: Orlando, Fla., AcademicPress, 424 p.• Probably the most complete reference on volcanic

hazards-—including the effects on people,infrastructure, and economic activity. Manyexamples from specific volcanoes, but not overlytechnical in its presentation.

Casadevall, T.J. (ed.), 1994, Volcanic ash and aviationsafety: Proceedings of the First InternationalSymposium on Volcanic Ash and Aviation Safety:U.S. Geological Survey Bulletin 2047, 450 p.• Several near-tragic encounters between jet

aircraft and ash during the past two decades haveled to an improved protocol for avoiding ash or toescape safely whenever an ash plume isencountered accidentally. This report containsnumerous articles that explain the effects of ashon aircraft and provide advice to pilots.

Warrick, R.A, and six other authors, 1981, Fourcommunities under ash: after Mount St. Helens:Boulder, Colo., University of Colorado Institute ofBehavioral Science Monograph No. 34, 143 p.• Written in a clear, nontechnical style, this report

compares the effects of ash on transportation,public facilities, and businesses in four townslocated at increasing distance downwind fromMount St. Helens. The startling conclusion is thateven thin ash deposits can cripple a town or city,


and 0.5 inch or more of ash creates a hazard withimpacts lasting for weeks or months. This shortbook is of special interest to city and countyelected officers, city managers, fire and policestaff, hospital administrators, or anyone involvedin emergency preparedness planning.

Geology and eruptive history ofNewberry volcano

Chitwood, L.A., 1990, Newberry,in Wood, C.A., andKienle, Jürgen, eds., Volcanoes of North America:United States and Canada, Cambridge, Mass.,Cambridge University Press, 354 p.• A concise summary of Newberry's volcanic

history is found on p. 200-202. Available in manylibraries or from local booksellers.

Jensen, R.A., 1988, Roadside guide to the geology ofNewberry volcano: Bend, Oreg., CenOreGeoPub(20180 Briggs Road, Bend, OR 97701), 75 p.• A friendly, descriptive road log for geologic field

trips around Newberry volcano and intoNewberry Crater.

MacLeod, N.S., Sherrod, D.R., Chitwood, L.A., andJensen, R.A., 1995, Geologic map of Newberryvolcano, Deschutes, Klamath, and Lake Counties,Oregon: U.S. Geological Survey MiscellaneousInvestigations Map I-2455, scales 1:62,500 and1:24,000.• A full-color geologic map and pamphlet

explaining many aspects of the geology atNewberry volcano. Available from stores locally,from Oregon Department of Geology and MineralIndustries (ph: 503-731-4100), or from USGSDistribution Center (ph: 303-202-4693). TheOregon Geology store may have the best price ifordering fewer than four maps.

ENDNOTES[A] The banks of upper Paulina Creek werestripped of Mazama ash and bedrock channels wereeroded sometime after about 7,500 years ago(Jensen, R.A., and Chitwood, L.A., 1996, Evidencefor recent uplift of caldera floor, Newberry volcano,Oregon [abs.]: Eos [American Geophysical UnionTransactions], v. 77, no. 46, p. F792). Crosssectional areas of the flood channel, determined byfinding the height above creek floor at whichMazama ash is preserved, range from 90 to 230 sq.meters (1,000-2,500 sq. ft) (L.A. Chitwood, written

commun., 1996). The flow volume and velocity weuse in our worst-case flooding analysis [endnote B]requires cross sectional area of 500 sq. meters.

[B] Few published examples are available forvolcanoes that have displaced water from theirsummit lakes during eruptions. A large eruptioninvolving much nonmagmatic debris at RuapehuVolcano, New Zealand, in 1975 ejected about 23percent of the water from that lake (roughly 1.6million cubic meters (m3) of water and lake-floorsediment) in a few hours (Nairn, I.A., Wood, C.P.,and Hewson, C.A.Y., 1979, Phreatic eruptions ofRuapehu: April 1975: New Zealand Journal ofGeology and Geophysics, v. 22, no. 2, p. 155-173.).Most of the water was thought to be ejected duringsurges, and most washed back into the lakefollowing each explosion. The eruptions were notobserved because they occurred at night, but theyproduced floods with maximum estimateddischarges as great as 5,000 m3 per second at gagingstations 5-6 km (3-4 mi) downstream in severaldrainages. Average flow velocities of thefloodwater ranged from about 5.5 m per second to 12m per second (11 to 27 mi per hour), with highervalues close to the volcano in those drainages havinghigher discharges.

The explosions took place during a heavyrainstorm with strong winds. It was surmised thatthe winds directed a disproportionate amount ofwater into those drainages where higher dischargeswere recorded. The snow pack on the volcano waslow, so snowmelt contributed little to the flooding.

The Ruapehu lake is only 500 m in diameter,compared to Paulina Lake's 1,300-m diameter(1,640 ft compared to 4,260 ft diameter). PaulinaLake's larger size might pose a larger hazard, but itslarger volume and greater depth would absorbsubstantially more of the energy released during theexplosion, perhaps actually reducing the amount ofwater expelled.

[C] Maximum discharge on the upper part ofPaulina Creek would probably be on the order of afew thousand cubic meters per second, using theRuapehu example described in note B. To calculateinundation levels on Paulina Creek, a dischargethroughout the channel of ~5000 m3 per second(180,000 cfs) and an average velocity of 10 m persecond was assumed (on basis of measurements atRuapehu).

A flow of 5,000 m3 per second with a velocityof 10 m per second would raise the stream levelsuch that the inundated cross-sectional area equals

ENDNOTES 13

500 m2. Using 1:24,000-scale topographic maps,the level of inundation required for the flow tooccupy 500 m2 was calculated at stream profilesevery 400 mor sodownPaulinaCreek fromPaulinaLake to Paulina Prairie. The inundation remainsentirely within thePaulinaCreek canyon.

Discharge could either decrease, remainconstant, or increase with distance downstream,depending on whether this flood represented abriefsurge from the lake, whether it was prolonged,whether it bulked up to a debris flow along thestreamcourse,or whether asnowpack waspresent inthe overflow area. Presuming that dischargeremainsnear-constant downstream asfar asPaulinaPrairie, then the flood behaves roughly as asteady-statewater flood or an impulsive lahar.For readersinterested in comparing theseflow rateswith the maximum discharge resulting solely fromweather-related flooding on central Oregon streamsand rivers, we recommend the following reference:Moffatt, R.L., Wellman, R.E., and Gordon, J.M.,1990, Statistical summaries of streamflow data inOregon: Volume 1—Monthly and annualstreamflow, and flow-duration values: U.S.Geological Survey Open-FileReport 90-118, 413p.Someof thisdatafor theDeschutesRiver basinmaybeaccessed electronically on theWorld WideWebusing the following Uniform ResourceLocator:

http://wwworegon.wr.usgs.gov/data_dir/mans_dir/actv94.html#HDR12

[D] From PaulinaPrairiedownstream to theLittleDeschutes River, the stream valley is too broad to

determine inundation levels without far moreprecisecontouringof thegroundsurfacethanthe20-ft contour interval shown on published maps. Also,modelingtheflow intheabsenceof confiningvalleywalls is problematic. Consequently, the laharhazard-zone boundaries have their greatestsubjectivity along this stretch of stream.Downstream from Paulina Prairie, the hazard zoneis drawn to encompass the flood plain.

From Paulina Prairie downstream, dischargewould decrease for lahars, owing to deposition ofentrained material. For impulsivefloods, dischargewould also decrease downstream as the peak floodheight attenuates with distance.

[E] Tephra hazard zones generated by computerprogramdevelopedby R.P.Hoblitt (U.S.GeologicalSurvey, Cascades Volcano Observatory, writtencommun., 1996).

[F] We are indebted to Norm MacLeod (U.S.Geological Survey, ret.) and Larry Chitwood (U.S.Forest Service, Deschutes National Forest), whosereviews of the manuscript both improved its finalpresentation and sharpened our thinking about theprobability andextent of hazardzones. DavidLeslie(Deschutes County Community DevelopmentDepartment) provided advice and maps for floodplains and downstream flooding hazards. BobJensen(DeschutesNational Forest) hascontinuedtoprobe and describe the geologic mysteries ofNewberry volcano,someof whichhave abearingonour interpretation of hazards.

14 VOLCAN O HAZARD S AT NEWBERRY VOLCANO , OREGON

volcano hazards at newberry volcano, oregonpaulina peak (back center) forms the highest point on the...

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