Mount Katmai Research Articles

Interconnection of magma sources beneath the Katmai volcanic system inferred from seismic tomography and petrology

The Katmai volcanic cluster in Alaska hosts eight volcanoes, most of which are presently active, including Novarupta, whose eruption in 1912 is considered as the largest in the World during the 20th and 21st centuries. We present a new seismic tomographic model for the upper crust beneath the Katmai Group calculated with the use of ∼173,000 arrival times of the P and S waves from ∼11,000 local earthquakes. The resulting distribution of the Vp/Vs ratio reveals a common deep magma source for the entire system below the depth of 5 km centered in the area of Katmai Pass. The tomographic model also demonstrates interconnections of this source with most of the active volcanoes of the group. Based on this model, we propose a possible mechanism for the Novarupta eruption in 1912. The high-Vp/Vs anomaly beneath Novarupta at depths below 5 km may represent a trace of intrusion of low-density volatile-rich rhyolite magma that was made additionally buoyant by vesiculation. This hypothesis is supported by our petrological analysis showing that amphibole is a stable phase in 1912 rhyolite, which require a deeper source than previously suggested. This ascent was accompanied by draining of denser andesite and dacite magmas from under Mount Katmai, which caused the Katmai edifice to collapse. A shallow anomaly of low Vp/Vs ratio right below Novarupta may trace the conduit filled with pumice-type highly porous rocks remained after the eruption.

Reconstruction of recent climate change in Alaska from the Aurora Peak ice core, central Alaska

Abstract. A 180.17 m ice core was drilled at Aurora Peak in the central part of the Alaska Range, Alaska, in 2008 to allow reconstruction of centennial-scale climate change in the northern North Pacific. The 10 m depth temperature in the borehole was −2.2 °C, which corresponded to the annual mean air temperature at the drilling site. In this ice core, there were many melt–refreeze layers due to high temperature and/or strong insolation during summer seasons. We analyzed stable hydrogen isotopes (δD) and chemical species in the ice core. The ice core age was determined by annual counts of δD and seasonal cycles of Na+, and we used reference horizons of tritium peaks in 1963 and 1964, major volcanic eruptions of Mount Spurr in 1992 and Mount Katmai in 1912, and a large forest fire in 2004 as age controls. Here, we show that the chronology of the Aurora Peak ice core from 95.61 m to the top corresponds to the period from 1900 to the summer season of 2008, with a dating error of ± 3 years. We estimated that the mean accumulation rate from 1997 to 2007 (except for 2004) was 2.04 m w.eq. yr-1. Our results suggest that temporal variations in δD and annual accumulation rates are strongly related to shifts in the Pacific Decadal Oscillation index (PDOI). The remarkable increase in annual precipitation since the 1970s has likely been the result of enhanced storm activity associated with shifts in the PDOI during winter in the Gulf of Alaska.

Three-dimensional seismic velocity structure and earthquake relocations at Katmai, Alaska

We invert arrival time data from local earthquakes occurring between September 2004 and May 2009 to determine the three-dimensional (3D) upper crustal seismic structure in the Katmai volcanic region. Waveforms for the study come from the Alaska Volcano Observatory's permanent network of 20 seismic stations in the area (predominantly single-component, short period instruments) plus a densely spaced temporary array of 11 broadband, 3-component stations. The absolute and relative arrival times are used in a double-difference seismic tomography inversion to solve for 3D P- and S-wave velocity models for an area encompassing the main volcanic centers.The relocated hypocenters provide insight into the geometry of seismogenic structures in the area, revealing clustering of events into four distinct zones associated with Martin, Mageik, Trident-Novarupta, and Mount Katmai. The seismic activity extends from about sea level to 2km depth (all depths referenced to mean sea level) beneath Martin, is concentrated near 2km depth beneath Mageik, and lies mainly between 2 and 4km depth below Katmai and Trident-Novarupta. Many new features are apparent within these earthquake clusters. In particular, linear features are visible within all clusters, some associated with swarm activity, including an observation of earthquake migration near Trident in 2008. The final velocity model reveals a possible zone of magma storage beneath Mageik, but there is no clear evidence for magma beneath the Katmai-Novarupta area where the 1912 eruptive activity occurred, suggesting that the storage zone for that eruption may have largely been evacuated, or remnant magma has solidified.

Eruptive history of Mount Katmai, Alaska

Mount Katmai has long been recognized for its caldera collapse during the great pyroclastic eruption of 1912 (which vented 10 km away at Novarupta in the Valley of Ten Thousand Smokes), but little has previously been reported about the geology of the remote ice-clad stratovolcano itself. Over several seasons, we reconnoitered all parts of the edifice and sampled most of the lava flows exposed on its flanks and caldera rim. The precipitous inner walls of the 1912 caldera remain too unstable for systematic sampling; so we provide instead a photographic and interpretive record of the wall sequences exposed. In contrast to the several andesite-dacite stratovolcanoes nearby, products of Mount Katmai range from basalt to rhyolite. Before collapse in 1912, there were two overlapping cones with separate vent complexes and craters; their products are here divided into eight sequences of lava flows, agglutinates, and phreatomagmatic ejecta. Latest Pleistocene and Holocene eruptive units include rhyodacite and rhyolite lava flows along the south rim; a major 22.8-ka rhyolitic plinian fall and ignimbrite deposit; a dacite-andesite zoned scoria fall; a thick sheet of dacite agglutinate that filled a paleocrater and draped the west side of the edifice; unglaciated leveed dacite lava flows on the southeast slope; and the Horseshoe Island dacite dome that extruded on the caldera floor after collapse. Pre-collapse volume of the glaciated Katmai edifice was ∼30 km3, and eruptive volume is estimated to have been 57 ± 13 km3. The latter figure includes ∼40 ± 6 km3 for the edifice, 5 ± 2 km3 for off-edifice dacite pyroclastic deposits, and 12 ± 5 km3 for the 22.8-ka rhyolitic pyroclastic deposits. To these can be added 13.5 km3 of magma that erupted at Novarupta in 1912, all or much of which is inferred to have been withdrawn from beneath Mount Katmai. The oldest part of the edifice exposed is a basaltic cone, which gave a 40Ar/39Ar plateau age of 89 ± 25 ka.

Explosive eruptive record in the Katmai region, Alaska Peninsula: an overview

At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin and from Shelikof Strait to Bristol Bay (∼8,500 km2). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy Mountain summit dome ( 6 ka), two tephras (∼3,700 and ∼2,700 14C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time (mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 14C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite ash from Kaguyak’s caldera-forming event (5,800 14C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 14C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500 14C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 14C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available.

Volcanic history and magmatic evolution of Seguam Island, Aleutian Island arc, Alaska

New 40 Ar/ 39 Ar dating coupled with detailed field mapping, stratigraphy, and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of Seguam Island in the Aleutian Island arc, Alaska. Sixty new 40 Ar/ 39 Ar ages from lavas, domes, and pyroclastic deposits were obtained using furnace incremental-heating techniques on replicate samples of whole-rock and ground-mass separates, and they constrain the duration of Pleistocene to Holocene subaerial volcanism to 318 k.y. The 40 Ar/ 39 Ar plateau ages indicate that over 85% of the complex, ∼68 km 3 of material, was erupted almost continuously between 318 ka and 9 ka. At ca. 9 ka, a stratocone on the eastern half of the island partially collapsed producing a 4-km-wide crater. Rhyolitic dome-forming eruptions followed from vents in the newly created crater, and were likely contemporaneous with 8.0 km 3 of basaltic and basaltic andesitic effusions from Pyre Peak, and a 1.4 km 3 basaltic eruption from a monogenetic cone on the far eastern end of the island. Geochemical changes over the last 318 k.y. are subtle. Most notably, the earliest eruptions from 318 to 142 ka produced no andesite, and basalt from this period has larger ranges in Zr/Rb and La/Yb than younger basalts. Small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal-liquid fractionation process that varied only slightly in successive eruptive phases over 318 k.y. We identified minor geochemical changes in magma composition during each of the three main stages of volcanism, but overall the monotonic variations in major- and trace-element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed-system fractional crystallization of basalt. Using the 40 Ar/ 39 Ar geochronology and estimates of individual flow volumes, we calculated a time-averaged eruptive rate at Seguam that is similar to growth rates of other well-dated arc volcanoes in the Cascades and Chilean Southern volcanic zone but less than that of Mount Katmai and Mount Mageik, which are located on the Alaska Peninsula. The eruptive flux at Seguam has been highly variable, fluctuating more than an order of magnitude, from 0.07 km 3 /k.y. during the early history of bimodal volcanism to 1.18 km 3 /k.y. over the past 9 k.y.

Geochronology and eruptive history of the Katmai volcanic cluster, Alaska Peninsula

In the Katmai district of the Alaska Peninsula, K–Ar and 40Ar/ 39Ar ages have been determined for a dozen andesite–dacite stratocones on the arc front and for 11 rear-arc volcanoes, 10 of which are monogenetic. Tied to mapping and stratigraphic studies, our dating emphasized proximal basal lavas that rest on basement rocks, in order to estimate ages of inception of each polygenetic cone. Oldest among arc-front cones is Alagogshak Volcano (690–43 ka), succeeded in the Holocene by the active Mount Martin cone. Mount Mageik consists of four overlapping subedifices, basal lavas of which give ages of 93, 71, and 59 ka, and Holocene. The three small prehistoric cones of Trident Volcano yield ages of 143, 101–58, and 44 ka. Falling Mountain and Mount Cerberus, dacite domes near the 1912 Novarupta vent, are related compositionally to the Trident group and give ages of 70 ka and 114 ka. Mount Katmai, which underwent caldera collapse in 1912, consists of two subedifices that overlapped in space and time, and is the only arc-front center here to include basalt and rhyolite; one cone began by 90 ka, the other by 47 ka. Snowy Mountain also consists of two contiguous cones, which started around 200 and 171 ka, respectively, the younger remaining active into the Holocene. Devils Desk, the only mafic cone on the arc front, was short-lived at about 245 ka. In the rear-arc, (1) Mount Griggs produced mafic-to-silicic andesite in several episodes between 292 ka and the Holocene; (2) the Savonoski River cluster includes a Pliocene dacite dome and five small mafic cones (390–88 ka); (3) Gertrude Creek cone (49.8% SiO 2) yields an age of 500 ka; and (4) the Saddlehorn Creek cluster includes five Pliocene basalt-to-andesite remnants. Eruptive volumes were reconstructed, permitting estimates of average eruption rates for edifice lifetimes. Since the mid Pleistocene, total volume erupted along the arc front here is 210±47 km 3 and in the rear-arc 39±6 km 3, of which Mount Griggs alone accounts for 35±5 km 3. Most productive has been Mount Katmai at 70±18 km 3, yielding a rate of ∼1 km 3/kyr, followed by Mount Mageik (0.33 km 3/kyr) and Mount Griggs (0.3 km 3/kyr since 50 ka).

Multiple Seismogenic Processes for High-Frequency Earthquakes at Katmai National Park, Alaska: Evidence from Stress Tensor Inversions of Fault-Plane Solutions

The volcanological significance of seismicity within Katmai National Park has been debated since the first seismograph was installed in 1963, in part because Katmai seismicity consists almost entirely of high-frequency earthquakes that can be caused by a wide range of processes. I investigate this issue by determining 140 well-constrained first-motion fault-plane solutions for shallow (depth < 9 km) earthquakes occurring between 1995 and 2001 and inverting these solutions for the stress tensor in different regions within the park. Earthquakes removed by several kilometers from the volcanic axis occur in a stress field characterized by horizontally oriented σ1 and σ3 axes, with σ1 rotated slightly (12°) relative to the NUVEL1A subduction vector, indicating that these earthquakes are occurring in response to regional tectonic forces. On the other hand, stress tensors for earthquake clusters beneath several Katmai cluster volcanoes have vertically oriented σ1 axes, indicating that these events are occurring in response to local, not regional, processes. At Martin-Mageik, vertically oriented σ1 is most consistent with failure under edifice loading conditions in conjunction with localized pore pressure increases associated with hydrothermal circulation cells. At Trident-Novarupta, it is consistent with a number of possible models, including occurrence along fractures formed during the 1912 eruption that now serve as horizontal conduits for migrating fluids and/or volatiles from nearby degassing and cooling magma bodies. At Mount Katmai, it is most consistent with continued seismicity along ring-fracture systems created in the 1912 eruption, perhaps enhanced by circulating hydrothermal fluids and/or seepage from the caldera-filling lake. Manuscript received 25 April 2002.

Katmai volcanic cluster and the great eruption of 1912

In June 1912, the world9s largest twentieth century eruption broke out through flat-lying sedimentary rocks of Jurassic age near the base of Trident volcano on the Alaska Peninsula. The 60 h ash-flow and Plinian eruptive sequence excavated and subsequently backfilled with ejecta a flaring funnel-shaped vent since called Novarupta. The vent is adjacent to a cluster of late Quaternary stratocones and domes that have released about 140 km 3 of magma in the past 150 k.y. Although the 1912 vent is closest to the Trident group and is also close to Mageik and Griggs volcanoes, it was the summit of Mount Katmai, 10 km east of Novarupta, that collapsed during the eruption to form a 5.5 km 3 caldera. Many earthquakes, including 14 in the range M 6−7, took place during and after the eruption, releasing 250 times more seismic energy than the 1991 caldera-forming eruption of the Philippine volcano, Pinatubo. The contrast in seismic behavior may reflect the absence of older caldera faults at Mount Katmai, lack of upward (subsidence opposing) magma flow owing to lateral magma withdrawal in 1912, and the horizontally stratified structure of the thick shale-rich Mesozoic basement. The Katmai caldera compensates for only 40% of the 13 km 3 of 1912 magma erupted, which included 7–8 km 3 of slightly zoned high-silica rhyolite and 4.5 km 3 of crystal-rich dacite that grades continuously into 1 km 3 of crystal-rich andesite. We have now mapped, sampled, and studied the products of all 20 components of the Katmai volcanic cluster. Pyroxene dacite and silicic andesite predominate at all of them, and olivine andesite is also common at Griggs, Katmai, and Trident volcanoes, but basalt and rhyodacite have erupted only at Mount Katmai. Rhyolite erupted only in 1912 and is otherwise absent among Quaternary products of the cluster. Pleistocene products of Mageik and Trident and all products of Griggs are compositionally distinguishable from those of 1912 at Novarupta. Holocene products of Mount Martin and Trident are closer in composition to the andesite-dacite array of 1912, but they reveal consistent differences. The affinity of the 1912 suite is closest with the array of products erupted by the Southwest Katmai cone, the edifice that had produced the only pre-1912 rhyodacite as well as the largest prehistoric Plinian eruption in the cluster. It is doubtful that any 1912 magma had been stored beneath Novarupta or Trident, and there is no evidence that more than one magma chamber erupted in 1912. Despite a compositional gap separating the aphyric rhyolite from the very crystal-rich andesite-dacite continuum, isotopic and chemical affinities linking all the 1912 ejecta and the continuity of all those ejecta in magmatic temperature and oxygen fugacity suggest that the rhyolite originated principally by incremental upward expulsion of interstitial melt from subjacent andesite-dacite mush. A large reservoir of such hot crystal mush is required both as the residue of rhyolitic melt separation and as a proximate heat source to thermally sustain the nearly aphyric condition of the overlying rhyolite. A model is presented for a unitary zoned chamber beneath Mount Katmai.

Mineral-resource assessments in Alaska — background information to accompany maps and reports about the geology and undiscovered-mineral-resource potential of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska Peninsula: J. R. Riehle, S. E. Church, R. L. Detterman & J. W. Miller, US Geological Survey Circular, 1106, 1994, 13 pp

Mineral-resource assessments in Alaska — background information to accompany maps and reports about the geology and undiscovered-mineral-resource potential of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska Peninsula: J. R. Riehle, S. E. Church, R. L. Detterman & J. W. Miller, US Geological Survey Circular, 1106, 1994, 13 pp

Seismicity of the Caldera-Making Eruption of Mount Katmai, Alaska, in 1912

The seismic activity associated with the century's largest eruption and caldera collapse at Mount Katmai, Alaska, during June 1912 was poorly understood and is studied here in detail. The epicenter of the large shock of 10 June was long thought to be 140 km northeast of Mount Katmai, but it is relocated to be 12 km west of Mount Katmai, near the Novarupta dome. Origin times, surface-wave magnitudes Ms , and broadband body-wave magnitudes mB are assigned to 50 new shocks from instrumentally observed data of more than 60 worldwide Milne's and other seismological stations. Of 50 shocks detected, 14 have Ms = 6.0 and greater, and the largest is 7.0. Both the frequency-magnitude relation and the mB − Ms relation apparently exhibit normal appearance for the Alaskan region. The strong earthquakes were concentrated in the nights of 6 and 7 June. It is inferred that the substantial collapse of Mount Katmai took place in these two nights. The total energy released in seismic waves is 7.1 × 1015 J, corresponding to the energy of a single shock with Ms = 7.4. The total seismic moment is estimated to be 1.4 × 1020 Nm from Ms values. This total moment can be interpreted in terms of failures caused by the total volume change of 5 km3. This volume agrees well with the estimated volume of the caldera collapse, which presumably took place in response to transfer of supporting magma from beneath Mount Katmai toward Novarupta.

The timing of caldera collapse at Mount Katmai in response to magma withdrawal toward Novarupta

Several thin layers of nonjuvenile lithic‐rich tephra from Mount Katmai are intercalated with 1912 plinian pumice‐fall deposits that originated at Novarupta 10 km away, demonstrating that caldera collapse at Katmai and eruption at Novarupta were synchronous. Emplaced by phreatic explosions and avalanche‐generated dustclouds, each Katmai‐derived layer is thickest and coarsest at the caldera rim and pinches out within 5–8 km. In contrast, thicknesses and clast sizes of plinian Layers A‐G decrease radially away from Novarupta. The first Katmai‐derived layer lies between Novarupta‐derived Layers B2 and B3, indicating that collapse began after release of ∼6.9 km³ of magma at Novarupta. The first major earthquake (Ms=6.5) at 2356 h on 6 June, ∼11 hours after the eruption began, may have marked the onset of collapse. The Katmai River landslide and several debris‐flow deposits are also sandwiched within the plinian deposits, indicating that they too were emplaced during the eruption.

New perspectives on the eruption of 1912 in the valley of ten thousand smokes, Katmai National Park, Alaska

New data extend our understanding of the 1912 eruption, its backfilled vent complex at Novarupta, and magma-storage systems beneath adjacent stratovolcanoes. Initial Plinian rhyolite fallout is confined to a narrow downwind sector, and its maximum thickness may occur as far as 13 km from source. In contrast, the partly contemporaneous rhyolite-rich ash flows underwent relatively low-energy emplacement, their generation evidently being decoupled from the high column. Flow veneers 1–13 m thick on near-vent ridge crests exhibit a general rhyolite-to-andesite sequence like that of the much thicker valley-confined ignimbrite into which they merge downslope. Lithics in both the initial Plinian and the ignimbrite are predominantly fragments of the Jurassic Naknek Formation, which extends from the surface to a depth of ca. 1500 m. Absence of lithics from the underlying sedimentary section limits to 100 m thick near source and 10 m thick 3 km away, which dip back into an inner vent <0.5 km wide, nested inside the earlier vent funnel of the ignimbrite. The dacite fallout is poor in Naknek lithics but contains abundant fragments of vitrophyre, most of which was vent-filling, densely welded tuff reejected during later phases of the 3-day eruption. Adjacent to the inner vent, a 225-m-high asymmetrical accumulation of coarse near-vent ejecta is stratigraphically continuous with the regional dacite fallout. Distensional faulting of its crest may reflect spreading related to compaction and welding. Nearby andesite-dacite stratovolcanoes, i.e., Martin, Mageik, Trident, and Katmai, display at least 12 vents that define a linear volcanic front trending N65°E. The 1912 vent and adjacent dacite domes are disposed parallel to the front and ca. 4 km behind it. Mount Griggs, 10 km behind the front, is more potassic than other centers, taps isotopically more depleted source materials, and reflects a wholly independent magmatic plumbing system. Geochemical differences among the stratovolcanoes, characteristically small eruptive volumes ( < 0.1 to 0.4 km3), and the dominance of andesite and low-SiO2 dacite suggest complex crustal reservoirs, not large integrated magma chambers. Linear fractures just outside the 1912 vent strike nearly normal to the volcanic front and may reflect dike transport of magma previously stored beneath Trident 3–5 km away. Caldera collapse at Mount Katmai may have taken place in response to hydraulic transfer of Katmai magma toward Novarupta via reservoir components beneath Trident. The voluminous 1912 eruption (12–15 km3 DRE) was also unusual in producing high-silica rhyolite (6–9 km3 DRE), a composition rare in this arc and on volcanic fronts in general. Isotopic data indicate that rhyolite genesis involved little assimilation of sedimentary rocks, pre-Tertiary plutonic rocks, or hydrothermally altered rocks of any age. Trace-element data suggest nonetheless that the rhyolite contains a nontrivial crustal contribution, most likely partial melts of Late Cenozoic arc-intrusive rocks. Because the three compositions (77%, 66−64.5%, and 61.5−58.5% SiO2) that intermingled in 1912 vented both concurrently and repeatedly (after eruptive pauses hours in duration), the compositional gaps between them must have been intrinsic to the reservoir, not merely effects of withdrawal dynamics.

Effects of Volcanic Ashfalls on Chemical and Sediment Characteristics of Two Alaskan Lakes

Cores from two lakes on Afognak Island, 150 km southeast of the Aleutian volcanic arc, indicated that several ashfalls have formed continuous layers over the lake basins, at least partially returning the lakes to a juvenile state with respect to their sediments. Abundant diatom remains were found in the sediments above each ash layer. During several hundred years after each ashfall, the transition to mature conditions followed a pattern characterized by decreasing diatom abundance and gradual increases in organic matter and clay-size inorganic particulates. High silica concentrations occurred in lakes within the area covered by the ashfall of the 1912 Mount Katmai eruption.

Seismic Body Waves Observed in the Vicinity of Mount Katmai, Alaska, and Evidence for the Existence of Molten Chambers

A microearthquake study carried out in the vicinity of Mount Katmai, Alaska, revealed disappearence of shear waves along certain paths crossing the volcanic range. A close geographical correlation of these paths and active volcanoes strongly suggests that magma or zones of partial melting exist and are responsible for S-wave shadowing. Three shallow (< 10 km) and one deeper (about 20 to 30 km) magma chambers are proposed. Additional evidence for the magma chamber is provided by the high attenuation of high-frequency P waves. From this attenuation an estimate of viscosity in a magma chamber of 108 cgs units is made.

Gravity traverses in the Valley of Ten Thousand Smokes, Katmai National Monument, Alaska

Three structural sections across the pyroclastic flow of the 1912 Mount Katmai eruption in the Valley of Ten Thousand Smokes were derived drom gravity data. Minimum average ash flow thicknesses of 40, 45. and 30 meters were deduced for profiles across the main, the south, and the southeast branches of the valley, respectively. The total volume of the ash flow is estimated to be 3.8 to 4.7 km3.

Microearthquake study of Mount Katmai and vicinity, Alaska

A seismic investigation was made during the summer of 1965 in Katmai National Monument, Alaska. During 39 days of nearly continuous recording with a high-gain high-frequency tripartite array of seismometers, 1800 events were recorded. The majority of events were shallower than 10 kin, although some occurred at depths up to 150 kin. Most of the events were associated with the volcanic axis that trends northeast-southwest along the Alaska Peninsula. Although there are fifteen volcanic vents in the Katmai area, historic volcanic activity, ranging from minor steaming to major explosive eruptions has been limited to seven peaks. Seismic activity was generally greatest near the more recently active volcanos and was quite low near the volcanos with no record of historic eruptions. Two exceptions to this general tendency, however, were near Snowy and Fourpeaked Mountains. Most of the events in these two areas were attributed to short-term increases of seismicity in aftershock or swarm-type sequences. The b value in a log-log plot of frequency versus amplitude for all shocks was 1.4, which is intermediate between that of tectonic earthquakes (b -- 0.5 to 1.0) and that of volcanic earthquakes (b = 2 to 3). This b value plus the nature of the earthquakes observed imply a mixture of tectonic and volcanic shocks in a highly heterogeneous structure with locally concentrated stresses. A comparison of seismograms recorded at Brooks, Overlook, and Katmai Canyon shows a disappearance of the $ phase along a path crossing the volcanic range. A study of specific events suggests this is due to a shadowing effect by a magma chamber rather than to a source mechanism. A slight increase of seismicity was observed in early August near Mounts Trident and Martin, coinciding approximately with evidence for a minor eruption in that region. Detailed and confirming evidence of an eruption was limited because of remoteness and unfavorable weather conditions. A normalized count of seismic events in early August indicates that an increase of seismicity associated with a major eruption could be comparable to that observed at other volcanos of the explosive type and suggests that continuous seismic monitoring of the Katmai area would be of considerable value.

The Knife Creek Glaciers of Katmai National Monument, Alaska

AbstractA group of previously unmapped glaciers on the slopes of Trident and Katmai Volcanoes, southwestern Alaska, was studied for information on the abnormal regimen produced by effects of the 1912 eruption of Mount Katmai. During the eruption an area of 5 square miles (13 km.2) of the summit above regional snow line was destroyed, and the beheaded glaciers were buried under a blanket of pumice and ash. Field data are presented to indicate that the terminus of Fourth Glacier has been essentially stationary since the 1912 eruption. Upper portions of the glacier have thinned, exposing the rim of the caldera which 40 years ago was partly ringed with fringing ice cliffs. Permafrost has developed to within a few feet of the surface of the pumice mantle. Under prevailing climatic conditions Fourth Glacier may be preserved in its present static condition for an indefinite period.

Incipient Glacier Development within Katmai Caldera, Alaska

Abstract An unusual opportunity for the study of glaciers in the process of development is afforded in Katmaicaldera in south-western Alaska. A violent eruption in 1912 destroyed the summit of glacier-clad Mount Katmai, creating a caldera 4 km. wide and 800 m. deep. Ice cliffs produced by beheading of the glaciers have since thinned and shrunk away from the rim of the caldera, except in the south-west. There, local reversal of direction of movement has resulted in an ice fall which descends part way down the crater wall. In the past thirty years two small glaciers have formed, near 1525 m. above sea level, within the caldera on large masses of slumped wall-rock below the north and south rims respectively. Elsewhere the sheer walls of the crater descend so steeply to the level of the caldera lake that permanent snowbanks cannot accumulate. The lake, which continues to rise at a rate of more than five meters per year, is at present the primary deterring factor in glacier development in the caldera.

The Knife Creek Glaciers of Katmai National Monument, Alaska

AbstractA group of previously unmapped glaciers on the slopes of Trident and Katmai Volcanoes, southwestern Alaska, was studied for information on the abnormal regimen produced by effects of the 1912 eruption of Mount Katmai. During the eruption an area of 5 square miles (13 km.2) of the summit above regional snow line was destroyed, and the beheaded glaciers were buried under a blanket of pumice and ash. Field data are presented to indicate that the terminus of Fourth Glacier has been essentially stationary since the 1912 eruption. Upper portions of the glacier have thinned, exposing the rim of the caldera which 40 years ago was partly ringed with fringing ice cliffs. Permafrost has developed to within a few feet of the surface of the pumice mantle. Under prevailing climatic conditions Fourth Glacier may be preserved in its present static condition for an indefinite period.