Proximal Air-Fall Deposits of Eruptions Between May 24 and August 7, 1980 -- Stratigraphy and Field Sedimentology. U.S. Department of the Interior, Geological Survey

During each of the magmatic eruptions of Mount St. Helens on May 25, June 12, and August 7, a vertical eruptive column rose intermittently to altitudes of 12-15 km, from which pumice, lithic fragments, and crystals settled downwind in lobes that generally become thinner and finer away from the volcano. Each ejecta lobe is asymmetric according to several criteria, including (1) the axes of maximum thickness and of maximum pumice size are not midway between the two margins of the lobe, (2) the axis of maximum pumice size does not correspond to the axis of thickness, and (3) the median size of particles grades through several grain-size intervals from one lateral margin to the other. The fining in grain size across the lobe is due to the rotation of wind directions with altitude, so material falling from a high-level airborne plume is winnowed as it falls through transverse low-level winds. Wind directions that rotate clockwise with increasing altitude effect an air-fall lobe whose axis of maximum coarseness is clockwise of the axis of maximum thickness; wind directions that rotate counterclockwise with increasing altitude effect an air-fall lobe whose trend of maximum coarseness is counterclockwise of the axis of maximum thickness. The thickness of air-fall deposits from eruptions on May 25 through August 7 range variously from one-third to one-fortieth that of the May 18 air-fall deposit at a given distance from the volcano. The post-May 18 deposits are an order of magnitude thinner than Mount St. Helens pumice layer T (A.D. 1800) and two orders of magnitude thinner than Mount St. Helens pumice layer Yn (3400 yr B.P.), which is similar in thickness to the most voluminous air-fall deposits of other Cascade Range volcanoes. The maximum size of pumice within the May 18 air-fall lobe is 5-10 times that of the post-May 18 lobes. The overlapping air-fall lobes of May 25, June 12, July 22, and August 7 form a stratigraphic layer that in most places is indivisible into deposits of the separate eruptions

Areal Distribution, Thickness, Mass, Volume, and Grain Size of Air-Fall Ash from the Six Major Eruptions of 1980

The airborne-ash plume front from the Mount St. Helens eruption of May 18 advanced rapidly to the northeast at an average velocity of about 250 km/hr during the first 13 min after eruption. It then traveled to the east-northeast within a high-velocity wind layer at altitudes of 10-13 km at an average velocity of about 100 km/hr over the first 1,000 km. Beyond about 60 km, the thickest ash fall was east of the volcano in Washington, northern Idaho, and western Montana. A distal thickness maximum near Ritzville, Wash., is due to a combination of factors: (1) crude sorting within the vertical eruptive column, (2) eruption of finer ash above the high-velocity wind layer at altitudes of 10-13 km, and (3) settling of ash through and below that layer. Isopach maps for the May 25, June 12, August 7, and October 16-18 eruptions show distal thickness maximums similar to that of May 18. A four-unit tephra stratigraphy formed by the May 18 air fall within proximal areas east of the volcano changes to three units, two units, and one unit at progressively greater distances downwind. Much of the deposits beyond 200 km from the volcano has two units. A lower thin dark lithic ash is inferred to represent products that disintegrated from the volcano\u27s summit in the initial part of the eruption and early juvenile pumice and glass. An upper, thicker, light-gray ash rich in pumice and volcanic-glass shards represents the later voluminous eruption of juvenile magma. The axis of the dark-ash lobe in eastern Washington and norther Idaho is south of the axis of the light-gray ash lobe because the high-velocity wind layer shifted northward during the eruption. The areal distribution of ash on the ground is offset to the north relative to the mapped position of the airborne-ash plume, because the winds below the high-velocity wind layer were more northward. Except for the distal thickness near Ritzville, Wash., mass per area, thickness, and bulk density of the May 18 ash decrease downwind, because larger grains and heavier lithic and crystal grains settled out closer to the volcano than did the lighter pumice and glass shards. A minimum volume of 1.1 km3 of uncompacted tephra is estimated for the May 18 eruption; this volume is equivalent to about 0.20-0.25 km3 of solid rock, assuming an average density of between 2.0 and 2.6 g/cm3 for magma and summit rocks. The estimated total mass from the May 18 eruption is 4.9 x 1014 g, and the average uncompacted bulk density for downwind ash is 0.45 g/cm3. Masses and volumes for the May 24 and June 12 eruptions are an order of magnitude smaller than those of May 18, but average bulk densities are higher (about 1.00 and 1.25), owing to compaction by rain that fell during or shortly after the two eruptions. Volume and mass of the July 22 eruption are two orders of magnitude smaller than those of May 18, and those of the August 7 and October 16-18 eruptions are three orders of magnitude smaller. The eruption of May 18, however, is smaller than five of the last major eruptions of Mount St. Helens in terms of volume of air-fall tephra produced, but probably is intermediate if the directed-blast deposit is included with the air-fall tephra

Post-Mazama (7 KA) Faulting Beneath Upper Klamath Lake, Oregon

High-resolution seismic-reflection profiles (3.5 kHz) show that a distinctive, widespread reflection occurs in the sediments beneath Upper Klamath Lake, Oregon. Coring reveals that this reflection is formed by Mazama tephra (MT), about 7 ka in age. The MT horizon is faulted in many places and locally displaced by as much as 3.1 m. Differential displacement of multiple horizons indicates recurrent fault movement, perhaps three episodes since deposition of the Mazama. The pattern of faulting indicates northeast–southwest extension beneath the lake basin

Hanslowe, Kurt Loewus

Memorial Statement for Professor Kurt Loewus Hanslowe who died in 1983. The memorial statements contained herein were prepared by the Office of the Dean of the University Faculty of Cornell University to honor its faculty for their service to the university

Age of the Mono Lake excursion and associated tephra

The Mono Lake excursion (MLE) is an important time marker that has been found in lake and marine sediments across much of the Northern Hemisphere. Dating of this event at its type locality, the Mono Basin of California, has yielded controversial results with the most recent effort concluding that the MLE may actually be the Laschamp excursion (Earth Planet. Sci. Lett. 197 (2002) 151). We show that a volcanic tephra (Ash #15) that occurs near the midpoint of the MLE has a date (not corrected for reservoir effect) of 28,620 ± 300 14C yr BP (~32,400 GISP2 yr BP) in the Pyramid Lake Basin of Nevada. Given the location of Ash #15 and the duration of the MLE in the Mono Basin, the event occurred between 31,500 and 33,300 GISP2 yr BP, an age range consistent with the position and age of the uppermost of two paleointensity minima in the NAPIS-75 stack that has been associated with the MLE (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009). The lower paleointensity minimum in the NAPIS-75 stackis considered to be the Laschamp excursion (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009)

Chronology of sediment deposition in Upper Klamath Lake, Oregon

A combination of tephrochronology and 14C, 210Pb, and 137Cs measurements provides a robust chronology for sedimentation in Upper Klamath Lake during the last 45 000 years. Mixing of surficial sediments and possible mobility of the radio-isotopes limit the usefulness of the 137Cs and 210Pb data, but 210Pb profiles provide reasonable average sediment accumulation rates for the last 100–150 years. Radiocarbon ages near the top of the core are somewhat erratic and are too old, probably as a result of detrital organic carbon, which may have become a more common component in recent times as surrounding marshes were drained. Below the tops of the cores, radiocarbon ages in the center of the basin appear to be about 400 years too old, while those on the margin appear to be accurate, based on comparisons with tephra layers of known age. Taken together, the data can be combined into reasonable age models for each site. Sediments have accumulated at site K1, near the center of the basin, about 2 times faster than at site CM2, on the margin of the lake. The rates are about 0.10 and 0.05 cm/yr, respectively. The chronological data also indicate that accumulation rates were slower during the early to middle Holocene than during the late Holocene, consistent with increasing wetness in the late Holocene