Geology topics

Christopher F. Waythomas

Publications and source records attributed to Christopher F. Waythomas.

At least 55 records · Page 3Linked to original sources

USGS2014

Significance of a near-source tephra-stratigraphic sequence to the eruptive history of Hayes Volcano, south-central Alaska

Bluffs along the Hayes River valley, 31 km northeast and 40 km downstream from Hayes Volcano, reveal volcanic deposits that shed new light on its eruptive history. Three thick (>10 cm) and five thin (<10 cm) tephra-fall deposits are dacitic in whole rock composition and contain high proportions of amphibole to pyroxene and minor biotite and broadly correlate to Hayes tephra set H defined by earlier investigators. Two basal ages for the tephra-fall sequence of 3,690±30 and 3,750±30 14 C yr B.P. are also consistent with the Hayes tephra set H timeframe. Distinguishing among Hayes tephra set H units is critical because the set is an important time-stratigraphic marker in south-central Alaska and this section provides a new reference section for Hayes tephra set H. Analysis of Fe-Ti oxide grains in the tephras shows promise for identifying individual Hayes deposits. Beneath the dacitic tephra sequence lies an older, poorly sorted tephra (tephra A) that contains dacite and rhyolite lapilli and whose basal age is 4,450±30 14 C yr B.P. Immediately below the tephra-fall sequence (Unit III) lies a series of mass-flow deposits that are rich in rhyodacitic clasts (Unit II). Below Unit II and possibly coeval with it, is a 20–30 m thick pumiceous pyroclastic-flow deposit (Unit I) that extends to the valley floor. Here informally named the Hayes River ignimbrite, this deposit contains pumice clasts of rhyolite with quartz, sanidine, plagioclase, and biotite phenocrysts, an assemblage that is unique among known Quaternary volcanic products of Hayes and other Alaskan volcanoes. Units I, II, and tephra A of Unit III represent at least two previously unrecognized eruptions of Hayes Volcano that occurred prior to ~3,700 yr B.P. No compositionally equivalent distal tephra deposits correlative with Hayes Volcano rhyodacites or rhyolites have yet been identified, perhaps indicating that some of these deposits are pre-Holocene, and were largely removed by glacial ice during the last ice age. More field and analytical work is needed to further refine the eruptive history of Hayes Volcano.

Alaska↗

USGS2012

Preliminary observations of voluminous ice-rich and water-rich lahars generated during the 2009 eruption of Redoubt, Alaska

Redoubt Volcano in south-central Alaska began erupting on March 15, 2009, and by April 4, 2009, had produced at least 20 explosive events that generated plumes of ash and lahars. The 3,108-m high, snow- and -ice-clad stratovolcano has an ice-filled summit crater that is breached to the north. The volcano supports about 4 km 3 of ice and snow and about 1 km 3 of this makes up the Drift glacier on the northern side of the volcano. Explosive eruptions between March 22 and April 4, which included the destruction of at least two lava domes, triggered significant lahars in the Drift River valley on March 23 and April 4 and several smaller lahars between March 24 and March 31. High-flow marks, character of deposits, areas of inundation, and estimates of flow velocity revealed that the lahars on March 23 and April 4 were the largest of the eruption. In the 2-km-wide upper Drift River valley, average flow depths were about 3–5 m. Average peak-flow velocities were likely between 10 and 15 ms -1 , and peak discharges were on the order of 10 4 –10 5 m 3 s -1 . The area inundated by lahars on March 23 was at least 100 km 2 and on April 4 about 125 km 2 . The lahars emplaced on March 23 and April 4 had volumes on the order of 10 7 –10 8 m 3 and were similar in size to the largest lahar of the 1989–90 eruption. The March 23 lahars were primarily flowing slurries of snow and ice entrained from the Drift glacier and seasonal snow and tabular blocks of river ice from the Drift River valley. Only a single, undifferentiated deposit up to 5 m thick was found and contained about 80–95 percent of poorly sorted, massive to imbricate assemblages of snow and ice. The deposit was frozen soon after it was emplaced and later eroded and buried by the April 4 lahar. The lahar of April 4, in contrast, was primarily a hyperconcentrated flow, as interpreted from 1- to 6-m thick deposits of massive to horizontally stratified sand-to-fine-gravel. Rock material in the April 4 lahar deposit is predominantly juvenile andesite. We infer that the lahars generated on March 23 were initiated by a rapid succession of vent-clearing explosions that blasted through about 50–100 m of crater-filling glacier ice and snow, producing a voluminous release of meltwater from the Drift glacier. The resulting flood eroded and entrained snow, fragments of glacier and river ice, and liquid water along its flow path. Small-volume pyroclastic flows, possibly associated with destruction of a small dome or minor eruption-column collapses, may have contributed additional meltwater to the lahar. Meltwater generated by subglacial hydrothermal activity and stored beneath the Drift glacier may have been ejected or released rapidly as well. The April 4 lahar was initiated when hot dome-collapse pyroclastic flows entrained and swiftly melted snow and ice, and incorporated additional rock debris from the Drift glacier. The peak discharge of the April 4 lahar was in the range of 60,000–160,000 m 3 s -1 . For comparison, the largest lahar of the 1989–90 eruption had a peak discharge of about 80,000 m 3 s -1 . Lahars generated by the 2009 eruption led to significant channel aggradation in the lower Drift River valley and caused extensive inundation at an oil storage and transfer facility located there. The April 4, 2009, lahar was 6–30 times larger than the largest meteorological floods known or estimated in the Drift River drainage.

Alaska↗

USGS2012

The 2009 eruption of Redoubt Volcano, Alaska

Redoubt Volcano, an ice-covered stratovolcano on the west side of Cook Inlet, erupted in March 2009 after several months of escalating unrest. The 2009 eruption of Redoubt Volcano shares many similarities with eruptions documented most recently at Redoubt in 1966–68 and 1989–90. In each case, the eruptive phase lasted several months, consisted of multiple ashproducing explosions, produced andesitic lava and tephra, removed significant amounts of ice from the summit crater and Drift glacier, generated lahars that inundated the Drift River valley, and culminated with the extrusion of a lava dome in the summit crater. Prior to the 2009 explosive phase of the eruption, precursory seismicity lasted approximately six months with the fi rst weak tremor recorded on September 23, 2008. The first phreatic explosion was recorded on March 15, and the first magmatic explosion occurred seven days later, at 22:34 on March 22. The onset of magmatic explosions was preceded by a strong, shallow swarm of repetitive earthquakes that began about 04:00 on March 20, 2009, less than three days before an explosion. Nineteen major ash-producing explosions generated ash clouds that reached heights between 17,000 ft and 62,000 ft (5.2 and 18.9 km) ASL. During ash fall in Anchorage, the Ted Stevens International Airport was shut down for 20 hours, from ~17:00 on March 28 until 13:00 on March 29. On March 23 and April 4, lahars with fl ow depths to 10 m in the upper Drift River valley inundated parts of the Drift River Terminal (DRT). The explosive phase ended on April 4 with a dome collapse at 05:58. The April 4 ash cloud reached 50,000 ft (15.2 km) and moved swiftly to the southeast, depositing up to 2 mm of ash fall in Homer, Anchor Point, and Seldovia. At least two and possibly three lava domes grew and were destroyed by explosions prior to the final lava dome extrusion that began after the April 4 event. The fi nal lava dome ceased growth by July 1, 2009, with an estimated volume of 72 Mm 3

Alaska↗

USGS2011

Alaska's Pavlof volcano ends 11-year repose

After an 11‐year period of repose, Pavlof volcano on the Alaska Peninsula (Figure 1) began an episode of Strombolian eruption lasting 31 days, from 14 August to 13 September 2007. The eruption began abruptly on 14 August after a minor increase in seismicity the previous day. Nearly continuous lava fountaining, explosions, and lahars caused by minor disruption of the ice and snow cover on the volcano characterized the eruption. The eruption also produced diffuse ash plumes that reached 5–6 kilometers above sea level, but the plumes were too small and did not extend high enough to affect local or regional air travel. Melting of snow and ice on the upper part of the edifice by hot debris avalanches and lava resulted in numerous lahars that entered the sea and inundated a 2×10 6 square meter area on the volcanos southern slope.

Eos, Transactions, American Geophysical Union↗

USGS2011

Review of the origin of the Braid Scarp near the Pebble prospect, southwestern Alaska

A linear geomorphic scarp, referred to as the 'Braid Scarp,' lies about 5 kilometers north of Iliamna Lake, Alaska, and has been identified as a possible seismically active fault. We examined the geomorphology of the area and an 8.5-meter-long excavation across the scarp. We conclude that the scarp was formed by incision of a glacial outwash braid plain into a slightly older outwash plain as ice stagnated in the region during deglaciation 11-15 thousand years ago. We found no evidence for active faulting along the scarp.

Alaska↗

USGS2010

Preliminary slope-stability analysis of Augustine Volcano

Augustine Volcano has been a prolific producer of large debris avalanches during the Holocene. Originating as landslides from the steep upper edifice, these avalanches typically slide into the surrounding ocean. At least one debris avalanche that occurred in 1883 during an eruption initiated a far-traveled tsunami. The possible occurrence of another edifice collapse and ensuing tsunami was a concern during the 2006 eruption of Augustine. To aid in hazard assessments, we have evaluated the slope stability of Augustine's edifice, using a quasi-three-dimensional, geotechnically based slope-stability model implemented in the computer program SCOOPS. We analyzed the effects of topography, variations in rock strength, and earthquake-induced strong ground motion on the relative stability of millions of potential large (>0.1 km 3 volume) slope failures throughout the edifice. Preliminary results from pre-2006 topography provide three insights. First, the predicted stability of all parts of the upper edifice is approximately the same, suggesting an equal likelihood of slope failure, in agreement with geologic observations that debris avalanches have swept all sectors of the volcano. Second, the least stable (by a small amount) sector is on the east flank where a debris avalanche would flow into deeper ocean water and a resulting tsunami would be directed toward the southwestern part of the Kenai Peninsula. Third, most model scenarios predict stable edifice slopes, and only scenarios assuming extensive weak rocks and moderate to strong ground shaking predict potential large collapses. Because other transient triggering mechanisms, such as shallow magma intrusion, may be needed to instigate slope instability, monitoring ground deformation and seismicity could

Alaska↗

USGS2009

A multidisciplinary effort to assign realistic source parameters to models of volcanic ash-cloud transport and dispersion during eruptions

During volcanic eruptions, volcanic ash transport and dispersion models (VATDs) are used to forecast the location and movement of ash clouds over hours to days in order to define hazards to aircraft and to communities downwind. Those models use input parameters, called “eruption source parameters”, such as plume height H , mass eruption rate Ṁ , duration D , and the mass fraction m 63 of erupted debris finer than about 4ϕ or 63 μm, which can remain in the cloud for many hours or days. Observational constraints on the value of such parameters are frequently unavailable in the first minutes or hours after an eruption is detected. Moreover, observed plume height may change during an eruption, requiring rapid assignment of new parameters. This paper reports on a group effort to improve the accuracy of source parameters used by VATDs in the early hours of an eruption. We do so by first compiling a list of eruptions for which these parameters are well constrained, and then using these data to review and update previously studied parameter relationships. We find that the existing scatter in plots of H versus Ṁ yields an uncertainty within the 50% confidence interval of plus or minus a factor of four in eruption rate for a given plume height. This scatter is not clearly attributable to biases in measurement techniques or to well-recognized processes such as elutriation from pyroclastic flows. Sparse data on total grain-size distribution suggest that the mass fraction of fine debris m 63 could vary by nearly two orders of magnitude between small basaltic eruptions (∼ 0.01) and large silicic ones (> 0.5). We classify eleven eruption types; four types each for different sizes of silicic and mafic eruptions; submarine eruptions; “brief” or Vulcanian eruptions; and eruptions that generate co-ignimbrite or co-pyroclastic flow plumes. For each eruption type we assign source parameters. We then assign a characteristic eruption type to each of the world's ∼ 1500 Holocene volcanoes. These eruption types and associated parameters can be used for ash-cloud modeling in the event of an eruption, when no observational constraints on these parameters are available.

Journal of Volcanology and Geothermal Research↗

USGS2009

Quaternary science reviews Pacific Basin tsunami hazards associated with mass flows in the Aleutian arc of Alaska

We analyze mass-flow tsunami generation for selected areas within the Aleutian arc of Alaska using results from numerical simulation of hypothetical but plausible mass-flow sources such as submarine landslides and volcanic debris avalanches. The Aleutian arc consists of a chain of volcanic mountains, volcanic islands, and submarine canyons, surrounded by a low-relief continental shelf above about 1000–2000 m water depth. Parts of the arc are fragmented into a series of fault-bounded blocks, tens to hundreds of kilometers in length, and separated from one another by distinctive fault-controlled canyons that are roughly normal to the arc axis. The canyons are natural regions for the accumulation and conveyance of sediment derived from glacial and volcanic processes. The volcanic islands in the region include a number of historically active volcanoes and some possess geological evidence for large-scale sector collapse into the sea. Large scale mass-flow deposits have not been mapped on the seafloor south of the Aleutian Islands, in part because most of the area has never been examined at the resolution required to identify such features, and in part because of the complex nature of erosional and depositional processes. Extensive submarine landslide deposits and debris flows are known on the north side of the arc and are common in similar settings elsewhere and thus they likely exist on the trench slope south of the Aleutian Islands. Because the Aleutian arc is surrounded by deep, open ocean, mass flows of unconsolidated debris that originate either as submarine landslides or as volcanic debris avalanches entering the sea may be potential tsunami sources. To test this hypothesis we present a series of numerical simulations of submarine mass-flow initiated tsunamis from eight different source areas. We consider four submarine mass flows originating in submarine canyons and four flows that evolve from submarine landslides on the trench slope. The flows have lengths that range from 40 to 80 km, maximum thicknesses of 400–800 m, and maximum widths of 10–40 km. We also evaluate tsunami generation by volcanic debris avalanches associated with flank collapse, at four locations (Makushin, Cleveland, Seguam and Yunaska SW volcanoes), which represent large to moderate sized events in this region. We calculate tsunami sources using the numerical model TOPICS and simulate wave propagation across the Pacific using a spherical Boussinesq model, which is a modified version of the public domain code FUNWAVE. Our numerical simulations indicate that geologically plausible mass flows originating in the North Pacific near the Aleutian Islands can indeed generate large local tsunamis as well as large transoceanic tsunamis. These waves may be several meters in elevation at distal locations, such as Japan, Hawaii, and along the North and South American coastlines where they would constitute significant hazards.

Alaska↗

USGS2007

Mid-holocene sector collapse at Mount Spurr Volcano, South-Central Alaska

Radiocarbon-dated volcanic mass-flow deposits on the southeast flank of Mount Spurr in south-central Alaska provide strong evidence for the timing of large-scale destruction of the south flank of the volcano by sector collapse at 4,769^ndash;4,610 yr B.P. The sector collapse created an avalanche caldera and produced an ~1-km3-volume clay-rich debris avalanche that flowed into the glacially scoured Chakachatna River valley, where it transformed into a lahar that extended an unknown distance beyond the debris avalanche. Hydrothermal alteration, an unbuttressed south flank of the volcano, and local structure have been identified as plausible factors contributing to the instability of the edifice. The sector collapse at Mount Spurr is one of the later known large-volume (>1 km,sup>3) flank failures recognized in the Aleutian Arc and one of the few known Alaskan examples of transformation of a debris avalanche into a lahar.

Alaska↗

USGS2003

Preliminary volcano-hazard assessment for Great Sitkin Volcano, Alaska

Great Sitkin Volcano is a composite andesitic stratovolcano on Great Sitkin Island (51°05’ N latitude, 176°25’ W longitude), a small (14 x 16 km), circular volcanic island in the western Aleutian Islands of Alaska. Great Sitkin Island is located about 35 kilometers northeast of the community of Adak on Adak Island and 130 kilometers west of the community of Atka on Atka Island. Great Sitkin Volcano is an active volcano and has erupted at least eight times in the past 250 years (Miller and others, 1998). The most recent eruption in 1974 caused minor ash fall on the flanks of the volcano and resulted in the emplacement of a lava dome in the summit crater. The summit of the composite cone of Great Sitkin Volcano is 1,740 meters above sea level. The active crater is somewhat lower than the summit, and the highest point along its rim is about 1,460 meters above sea level. The crater is about 1,000 meters in diameter and is almost entirely filled by a lava dome emplaced in 1974. An area of active fumaroles, hot springs, and bubbling hot mud is present on the south flank of the volcano at the head of Big Fox Creek (see the map), and smaller ephemeral fumaroles and steam vents are present in the crater and around the crater rim. The flanking slopes of the volcano are gradual to steep and consist of variously weathered and vegetated blocky lava flows that formed during Pleistocene and Holocene eruptions. The modern edifice occupies a caldera structure that truncates an older sequence of lava flows and minor pyroclastic rocks on the east side of the volcano. The eastern sector of the volcano includes the remains of an ancestral volcano that was partially destroyed by a northwest-directed flank collapse. In winter, Great Sitkin Volcano is typically completely snow covered. Should explosive pyroclastic eruptions occur at this time, the snow would be a source of water for volcanic mudflows or lahars. In summer, much of the snowpack melts, leaving only a patchy distribution of snow on the volcano. Glacier ice is no longer present on the volcano or on other parts of Great Sitkin Island as previously reported by Simons and Mathewson (1955). Great Sitkin Island is presently uninhabited and is part of the Alaska Maritime National Wildlife Refuge, managed by the U.S. Fish and Wildlife Service.

Alaska↗

USGS2003

Preliminary geologic map of Great Sitkin Volcano, Alaska

No abstract available.

Alaska↗

USGS2002

Paleoseismology at high latitudes: Seismic disturbance of upper Quaternary deposits along the Castle Mountain fault near Houston, Alaska

Most paleoseismic studies are at low to moderate latitudes. Here we present results from a high-latitude (61°30′ N) trenching study of the Castle Mountain fault in south-central Alaska. This fault is the only one known in the greater Anchorage, Alaska, area with historical seismicity and a Holocene fault scarp. It strikes east-northeast and cuts glacial and postglacial sediments in an area of boreal spruce-birch forest, shrub tundra, and sphagnum bog. The fault has a prominent vegetation lineament on the upthrown, north side of the fault. Nine trenches were logged across the fault in glacial and postglacial deposits, seven along the main trace, and two along a splay. In addition to thrust and strike-slip faulting, important controls on observed relationships in the trenches are the season in which faulting occurred, the physical properties of the sediments, liquefaction, a shallow water table, soil-forming processes, the strength of the modern root mat, and freeze-thaw processes. Some of these processes and physical properties are unique to northern-latitude areas and result in seismic disturbance effects not observed at lower latitudes. The two trenches across the Castle Mountain fault splay exposed a thrust fault and few liquefaction features. Radiocarbon ages of soil organic matter and charcoal within and overlying the fault indicate movement on the fault at ca. 2735 cal. (calendar) yr B.P. and no subsequent movement. In the remaining seven trenches, surface faulting was accompanied by extensive liquefaction and a zone of disruption 3 m or more wide. The presence of numerous liquefaction features at depths of <0.5–1.0 m indicates faulting when the ground was not frozen—i.e., from about April to October. Sandy-matrix till, sand, silt, gravel, and pebbly peat were injected up to the base of the modern soil, but did not penetrate the interlocking spruce-birch root mat. The strength of the root mat prohibited development of a nonvegetated scarp face and colluvial wedge. In only one trench did we observe a discrete fault plane with measurable offset. It lay beneath a 2-m-thick carapace of liquefied sand and silt and displayed a total of 0.9–1.85 m of thrust motion since deposition of the oldest deposits in the trenches at ca. 13,500 yr B.P. We found liquefaction ejecta on paleosols at only one other trench, where there were bluejoint ( Calamagrostis canadensis ) tussocks that lacked an extensive root mat. From crosscutting relationships, we interpret three paleoliquefaction events on the main trace of the Castle Mountain fault: 2145–1870, 1375–1070, and 730–610 cal. yr B.P. These four earthquakes on the Castle Mountain fault in the past ∼2700 yr indicate an average recurrence interval of ∼700 yr. As it has been 600–700 yr since the last significant earthquake, a significant (magnitude 6–7) earthquake in the near future may be likely. Paleoseismic data indicate that the timing and recurrence interval of megathrust earthquakes is similar to the timing and recurrence interval of Castle Mountain fault earthquakes, suggesting a possible link between faulting on the megathrust and on “crustal” structures.

Alaska↗

USGS2002

Preliminary volcano-hazard assessment for Kanaga Volcano, Alaska

Kanaga Volcano is a steep-sided, symmetrical, cone-shaped, 1307 meter high, andesitic stratovolcano on the north end of Kanaga Island (51°55’ N latitude, 177°10’ W longitude) in the western Aleutian Islands of Alaska. Kanaga Island is an elongated, low-relief (except for the volcano) island, located about 35 kilometers west of the community of Adak on Adak Island and is part of the Andreanof Islands Group of islands. Kanaga Volcano is one of the 41 historically active volcanoes in Alaska and has erupted numerous times in the past 11,000 years, including at least 10 eruptions in the past 250 years (Miller and others, 1998). The most recent eruption occurred in 1993-95 and caused minor ash fall on Adak Island and produced blocky aa lava flows that reached the sea on the northwest and west sides of the volcano (Neal and others, 1995). The summit of the volcano is characterized by a small, circular crater about 200 meters in diameter and 50-70 meters deep. Several active fumaroles are present in the crater and around the crater rim. The flanking slopes of the volcano are steep (20-30 degrees) and consist mainly of blocky, linear to spoonshaped lava flows that formed during eruptions of late Holocene age (about the past 3,000 years). The modern cone sits within a circular caldera structure that formed by large-scale collapse of a preexisting volcano. Evidence for eruptions of this preexisting volcano mainly consists of lava flows exposed along Kanaton Ridge, indicating that this former volcanic center was predominantly effusive in character. In winter (October-April), Kanaga Volcano may be covered by substantial amounts of snow that would be a source of water for lahars (volcanic mudflows). In summer, much of the snowpack melts, leaving only a patchy distribution of snow on the volcano. Glacier ice is not present on the volcano or on other parts of Kanaga Island. Kanaga Island is uninhabited and is part of the Alaska Maritime National Wildlife Refuge, managed by the U.S. Fish and Wildlife Service. The island is remote and often shrouded by clouds and fog. It can be reached only by boat, helicopter,or amphibiouslanding aircraft.

Alaska↗

USGS2001

Preliminary volcano-hazard assessment for Mount Spurr Volcano, Alaska

Mount Spurr volcano is an ice- and snow-covered stratovolcano complex located in the north-central Cook Inlet region about 100 kilometers west of Anchorage, Alaska. Mount Spurr volcano consists of a breached stratovolcano, a lava dome at the summit of Mount Spurr, and Crater Peak vent, a small stratocone on the south flank of Mount Spurr volcano. Historical eruptions of Crater Peak occurred in 1953 and 1992. These eruptions were relatively small but explosive, and they dispersed volcanic ash over areas of interior, south-central, and southeastern Alaska. Individual ash clouds produced by the 1992 eruption drifted east, north, and south. Within a few days of the eruption, the south-moving ash cloud was detected over the North Atlantic. Pyroclastic flows that descended the south flank of Crater Peak during both historical eruptions initiated volcanic-debris flows or lahars that formed temporary debris dams across the Chakachatna River, the principal drainage south of Crater Peak. Prehistoric eruptions of Crater Peak and Mount Spurr generated clouds of volcanic ash, pyroclastic flows, and lahars that extended to the volcano flanks and beyond. A flank collapse on the southeast side of Mount Spurr generated a large debris avalanche that flowed about 20 kilometers beyond the volcano into the Chakachatna River valley. The debris-avalanche deposit probably formed a large, temporary debris dam across the Chakachatna River. The distribution and thickness of volcanic-ash deposits from Mount Spurr volcano in the Cook Inlet region indicate that volcanic-ash clouds from most prehistoric eruptions were as voluminous as those produced by the 1953 and 1992 eruptions. Clouds of volcanic ash emitted from the active vent, Crater Peak, would be a major hazard to all aircraft using Ted Stevens Anchorage International Airport and other local airports and, depending on wind direction, could drift a considerable distance beyond the volcano. Ash fall from future eruptions could disrupt many types of economic and social activities, including oil and gas operations and shipping activities in the Cook Inlet area. Eruptions of Crater Peak could involve significant amounts of ice and snow that would lead to the formation of large lahars, formation of volcanic debris dams, and downstream flooding. The greatest hazards in order of importance are described below and shown on plate 1.

Alaska↗

USGS2000

Preliminary volcano-hazard assessment for Aniakchak Volcano, Alaska

Aniakchak is an active volcano located on the Alaska Peninsula 670 kilometers southwest of Anchorage. The volcano consists of a dramatic, 10-kilometer-diameter, 0.5 to 1.0-kilometer-deep caldera that formed during a catastrophic eruption 3,500 years ago. Since then, at least a dozen separate vents within the caldera have erupted, often explosively, to produce lava flows and widespread tephra (ash) deposits. The most recent eruption at Aniakchak occurred in 1931 and was one of the largest explosive eruptions in Alaska in the last 100 years. Although Aniakchak volcano presently shows no signs of unrest, explosive and nonexplosive eruptions will occur in the future. Awareness of the hazards posed by future eruptions is a key factor in minimizing impact.

Alaska↗

USGS2000

Landslides and tsunamis

The study of tsunamis has been shifting away from theoretical modeling of tsunami source, wave propagation and runup toward multidisciplinary investigations, with an emphasis on field studies. This collection of papers highlights the many approaches being utilized to study landslides and tsunamis.

Book↗

USGS1999

Preliminary volcano-hazard assessment for Iliamna Volcano, Alaska

Iliamna Volcano is a 3,053-meter-high, ice- and snow-covered stratovolcano in the southwestern Cook Inlet region about 225 kilometers southwest of Anchorage and about 100 kilometers northwest of Homer. Historical eruptions of Iliamna Volcano have not been positively documented; however, the volcano regularly emits steam and gas, and small, shallow earthquakes are often detected beneath the summit area. The most recent eruptions of the volcano occurred about 300 years ago, and possibly as recently as 90-140 years ago. Prehistoric eruptions have generated plumes of volcanic ash, pyroclastic flows, and lahars that extended to the volcano flanks and beyond. Rock avalanches from the summit area have occurred numerous times in the past. These avalanches flowed several kilometers down the flanks and at least two large avalanches transformed to cohesive lahars. The number and distribution of known volcanic ash deposits from Iliamna Volcano indicate that volcanic ash clouds from prehistoric eruptions were significantly less voluminous and probably less common relative to ash clouds generated by eruptions of other Cook Inlet volcanoes. Plumes of volcanic ash from Iliamna Volcano would be a major hazard to jet aircraft using Anchorage International Airport and other local airports, and depending on wind direction, could drift at least as far as the Kenai Peninsula and beyond. Ashfall from future eruptions could disrupt oil and gas operations and shipping activities in Cook Inlet. Because Iliamna Volcano has not erupted for several hundred years, a future eruption could involve significant amounts of ice and snow that could lead to the formation of large lahars and downstream flooding. The greatest hazards in order of importance are described below and shown on plate 1.

Alaska↗

USGS1998

Tsunami generation by pyroclastic flow during the 3500-year B.P. caldera-forming eruption of Aniakchak Volcano, Alaska

A discontinuous pumiceous sand, a few centimeters to tens of centimeters thick, is located up to 15 m above mean high tide within Holocene peat along the northern Bristol Bay coastline of Alaska. The bed consists of fine-to-coarse, poorly to moderately well-sorted, pumice-bearing sand near the top of a 2-m-thick peat sequence. The sand bed contains rip-up clasts of peat and tephra and is unique in the peat sequence. Major element compositions of juvenile glass from the deposit and radiocarbon dating of enclosing peat support correlation of the pumiceous sand with the caldera-forming eruption of Aniakchak Volcano. The distribution of the sand and its sedimentary characteristics are consistent with emplacement by tsunami. The pumiceous sand most likely represents redeposition by tsunami of climactic fallout tephra and beach sand during the approximately 3.5 ka Aniakchak caldera-forming eruption on the Alaska Peninsula. We propose that a tsunami was generated by the sudden entrance of a rapidly moving, voluminous pyroclastic flow from Aniakchak into Bristol Bay. A seismic trigger for the tsunami is unlikely, because tectonic structures suitable for tsunami generation are present only south of the Alaska Peninsula. The pumiceous sand in coastal peat of northern Bristol Bay is the first documented geologic evidence of a tsunami initiated by a volcanic eruption in Alaska.

Alaska↗