Dwight Raymond Crandell

Late Pleistocene Stratigraphy and Chronology in Southwestern British Columbia and Northwestern Washington

Six geologic-climate units are proposed for the late Pleistocene sequence in southwestern British Columbia and northwestern Washington. They include two major units, the Olympia Interglaciation and the Fraser Glaciation, and four subdivisions of the latter—the Evans Creek, Vashon, and Sumas Stades, and the Everson Interstade. The Olympia Interglaciation is a nonglacial episode that started at least 36,000 years B.P. and continued until the advance of Cordilleran glacier ice during the Fraser Glaciation. During the Evans Creek Stade, alpine glaciers formed in the mountains of western Washington and British Columbia while nonglacial sediments were still being deposited in the southern Puget Lowland. Further growth of glaciers in British Columbia resulted in the formation of the Cordilleran ice sheet. This ice entered the northern end of the area after 25,000 years B.P. but did not reach the southern end until after 15,000 years B.P. The Vashon Stade of the Fraser Glaciation began with this advance of Cordilleran ice into the lowlands. It ended with the beginning of marine and glaciomarine conditions there, which commenced in the southern Puget Lowland about 13,500 years B.P. and in the Strait of Georgia about 13,000 years B.P. The episode represented by the marine conditions is called the Everson Interstade and lasted about 2000 years, during which the sea contained much floating ice. The Interstade ended when the land rose with respect to the sea level forcing withdrawal of the sea and the disappearance of floating ice in most of northwestern Washington and southwestern British Columbia; in the eastern part of the Fraser Lowland this event coincided with the advance of a valley glacier during the Sumas Stade.

Catastrophic debris avalanche from ancestral Mount Shasta volcano, California

A debris-avalanche deposit extends 43 km northwestward from the base of Mount Shasta across the floor of Shasta Valley, California, where it covers an area of at least 450 km 2 . The surface of the deposit is dotted with hundreds of mounds, hills, and ridges, all formed of blocks of pyroxene andesite and unconsolidated volcaniclastic deposits derived from an ancestral Mount Shasta. Individual hills are separated by flat-topped laharlike deposits that also form the matrix of the debris avalanche and slope northwestward about 5 m/km. Radiometric ages of rocks in the deposit and of a postavalanche basalt flow indicate that the avalanche occurred between about 300,000 and 360,000 yr ago. An inferred average thickness of the deposit, plus a computed volume of about 4 km 3 for the hills and ridges, indicate an estimated volume of about 26 km 3 , making it the largest known Quaternary landslide on Earth.

Mount St. Helens a decade after the 1980 eruptions: magmatic models, chemical cycles, and a revised hazards assessment

Available geophysical and geologic data provide a simplified model of the current magmatic plumbing system of Mount St. Helens (MSH). This model and new geochemical data are the basis for the revised hazards assessment presented here. The assessment is weighted by the style of eruptions and the chemistry of magmas erupted during the past 500 years, the interval for which the most detailed stratigraphic and geochemical data are available. This interval includes the Kalama (A. D. 1480–1770s?), Goat Rocks (A.D. 1800–1857), and current eruptive periods. In each of these periods, silica content decreased, then increased. The Kalama is a large amplitude chemical cycle (SiO2: 57%–67%), produced by mixing of arc dacite, which is depleted in high field-strength and incompatible elements, with enriched (OIB-like) basalt. The Goat Rocks and current cycles are of small amplitude (SiO2: 61%–64% and 62%–65%) and are related to the fluid dynamics of magma withdrawal from a zoned reservoir. The cyclic behavior is used to forecast future activity. The 1980–1986 chemical cycle, and consequently the current eruptive period, appears to be virtually complete. This inference is supported by the progressively decreasing volumes and volatile contents of magma erupted since 1980, both changes that suggest a decreasing potential for a major explosive eruption in the near future. However, recent changes in seismicity and a series of small gas-release explosions (beginning in late 1989 and accompanied by eruption of a minor fraction of relatively low-silica tephra on 6 January and 5 November 1990) suggest that the current eruptive period may continue to produce small explosions and that a small amount of magma may still be present within the conduit. The gas-release explosions occur without warning and pose a continuing hazard, especially in the crater area. An eruption as large or larger than that of 18 May 1980 (≈0.5 km3 dense-rock equivalent) probably will occur only if magma rises from an inferred deep (≥7 km), relative large (5–7 km3) reservoir. A conservative approach to hazard assessment is to assume that this deep magma is rich in volatiles and capable of erupting explosively to produce voluminous fall deposits and pyroclastic flows. Warning of such an eruption is expectable, however, because magma ascent would probably be accompanied by shallow seismicity that could be detected by the existing seismic-monitoring system. A future large-volume eruption (≥0.1 km3) is virtually certain; the eruptive history of the past 500 years indicates the probability of a large explosive eruption is at least 1% annually. Intervals between large eruptions at Mount St. Helens have varied widely; consequently, we cannot confidently forecast whether the next large eruption will be years decades, or farther in the future. However, we can forecast the types of hazards, and the areas that will be most affected by future large-volume eruptions, as well as hazards associated with the approaching end of the current eruptive period.

Mount St. Helens volcano: Recent and future behavior

Mount St. Helens volcano in southern Washington has erupted many times during the last 4000 years, usually after brief dormant periods. This behavior pattern. suggests that the volcano, last active in 1857, will erupt again-perhaps within the next few decades. Potential volcanic hazards of several kinds should be considered in planning for land use near the volcano.

Mount St. Helens eruptive behavior during the past 1,500 yr

During the past 1,500 yr Mount St. Helens, Washington, has repeatedly erupted dacite domes, tephra, and pyroclastic flows as well as andesite lava flows and tephra. Two periods of activity prior to 1980, each many decades long, were both initiated by eruptions of volatile-rich dacite which were followed by andesite, then by dacite. A third eruptive period was characterized by the eruption of volatile-poor dacite that formed a dome and minor pyroclastic flows. The prolonged duration of some previous eruptive periods suggests that the current activity could continue for many years. The volatile-rich dacite that has been erupted to date probably will be followed by gas-poor magma, but it cannot yet be predicted whether a more mafic magma will be extruded during the current eruptive period.

Status of correlation of Quaternary stratigraphic units in the western conterminous United States

Abstract Deposits of Quaternary age from the Rocky Mountains to the Pacific Coast in the western conterminous United States represent a great variety of environments. The deposits include those of continental and alpine glaciers, glacial meltwater streams, nonglacial streams, pluvial lakes, marine environments, eolian environments, and masswasting environments. On two charts we have attempted to correlate representative sequences of deposits of many of these environments, based on published sources and recent unpublished investigations. Evidence for correlation is based mainly on stratigraphic sequence, soil characteristics, the amount of subsequent erosion and interlayered volcanic ash beds identifiable as to source. Chronologic control is based on numerous radiocarbon dates, U-series dates on marine fossils, and K-Ar dates on volcanic rocks. The Bishop volcanic ash bed and one of the Pearlette-like volcanic ash beds appear to represent significant regional key horizons, respectively about 700,000 and 600,000 years old. Rock magnetism is shown to suggest the paleomagnetic polarity at the time of rock deposition. Assigned land-mammal ages of included fossils help to put limits on the age of some units.

Recent Lahars from Mount St. Helens, Washington

Late Recent eruptions of Mount St. Helens volcano in the Cascade Range of southern Washington have caused numerous lahars; some extend more than 40 miles down valleys west of the volcano. These lahars typically consist of poorly sorted and unstratified detritus derived almost wholly from the volcano. The lahars and inter-bedded fluvial gravel commonly form valley fills; such a fill in the valley of the North Fork of the Toutle River contains at least three lahars. Lahars from Mount St. Helens are composed mainly of nonvesicular rock fragments; thus, their great distance of transport is attributed to mobility provided by water rather than by gas emitted from particles in the matrix. Near the volcano, some of the lahars contain charred wood, proving that they were hot and resulted directly from eruptions. Wood from within a lahar in the fill in the North Fork of the Toutle River valley has a radiocarbon age of about 2000 years. The lahar consists of material derived from a Mount St. Helens volcano that existed before the present cone was built; thus, the present cone must be less than 2000 yearsold. In addition, interpretation of soil profiles younger than the dated wood suggests that the modern cone may have been built within the last thousand Years.

Pre-Olympia Pleistocene Stratigraphy and Chronology in the Central Puget Lowland, Washington

Drifts of two pre-Olympia glaciations separated by nonglacial sediments are widespread in the central Puget Lowland of western Washington. The Double Bluff Drift (older) and Possession Drift represent advances of the Puget lobe of the Cordilleran ice sheet more than 40,000 years ago. The nonglacial Whidbey Formation between the drifts was formed in streams and lakes. During its deposition, climate was initially cool and moist, as inferred from pollen in peat beds, but subsequently it became much like that of the present in the lowland. The Possession Drift is tentatively correlated with glacial deposits of Salmon Springs age in the southern part of the lowland. The Whidbey Formation may correlate with nonglacial deposits between two Salmon Springs Drifts or with the Puyallup Formation.

Lateral blasts at Mount St. Helens and hazard zonation

Lateral blasts at andesitic and dacitic volcanoes can produce a variety of direct hazards, including ballistic projectiles which can be thrown to distances of at least 10 km and pyroclastic density flows which can travel at high speed to distances of more than 30 km. Indirect effect that may accompany such explosions include wind-borne ash, pyroclastic flows formed by the remobilization of rock debris thrown onto sloping ground, and lahars.Two lateral blasts occurred at a lava dome on the north flank of Mount St. Helens about 1200 years ago; the more energetic of these threw rock debris northeastward across a sector of about 30° to a distance of at least 10 km. The ballistic debris fell onto an area estimated to be 50 km2, and wind-transported ash and lapilli derived from the lateral-blast cloud fell on an additional lobate area of at least 200 km2. In contrast, the vastly larger lateral blast of May 18, 1980, created a devastating pyroclastic density flow that covered a sector of as much as 180°, reached a maximum distance of 28 km, and within a few minutes directly affected an area of about 550 km2. The May 18 lateral blast resulted from the sudden, landslide-induced depressurization of a dacite cryptodome and the hydrothermal system that surrounded it within the volcano.We propose that lateral-blast hazard assessments for lava domes include an adjoining hazard zone with a radius of at least 10 km. Although a lateral blast can occur on any side of a dome, the sector directly affected by any one blast probably will be less than 180°. Nevertheless, a circular hazard zone centered on the dome is suggested because of the difficulty of predicting the direction of a lateral blast.For the purpose of long-term land-use planning, a hazard assessment for lateral blasts caused by explosions of magma bodies or pressurized hydrothermal systems within a symmetrical volcano could designate a circular potential hazard area with a radius of 35 km centered on the volcano. For short-term hazard assessments, if seismicity and deformation indicate that magma is moving toward the flank of a volcano, it should be recognized that a landslide could lead to the sudden unloading of a magmatic or hydrothermal system and thereby cause a catastrophic lateral blast. A hazard assessment should assume that a lateral blast could directly affect an area at least 180° wide to a distance of 35 km from the site of the explosion, irrespective of topography.

Time-Lapse Motion Picture Technique Applied to the Study of Geological Processes

Light-weight, battery-operated timers were built and coupled to 16-mm motion-picture cameras having apertures controlled by photoelectric cells. The cameras were placed adjacent to Emmons Glacier on Mount Rainier. The film obtained confirms the view that exterior time-lapse photography can be applied to the study of slow-acting geologic processes.