Roy A. Bailey

Physical geology and eruptive history of the Matahina Ignimbrite, Taupo Volcanic Zone, North Island, New Zealand

Abstract The Matahina Ignimbrite is a 280 ka ash‐flow sheet that erupted from Haroharo Caldera in the Okataina Volcanic Centre, northern Taupo Volcanic Zone, North Island, New Zealand. The ignimbrite underlies a 2000 km2 area mainly east of the caldera, ranges in thickness from 5 to 200 m, and has a outflow volume of c. 120 km3, equivalent to c. 75 km3 of magma. It is a multiple‐flow, compound cooling unit consisting of a basal tephra (fallout) member and three ash‐flow members, designated lower, middle, and upper, that record three eruptive pulses separated by brief time intervals, estimated from cooling and compaction noddling to range from 20 to 60 days. Distribution of coarse lithic clasts, together with local interbedded co‐ignimbrite lag breccias and tephra layers east of the Puhipuhi Easin, confirm Haroharo Caldera as the eruptive source. Over most of its extent on the Kaingaroa Plateau, the outflow s leet thickens eastward away from its source and attains its greatest thickness in the elongate, no...

Short Reversal of the Palaeomagnetic Field about 280 000 Years Ago at Long Valley, California

A reversal of the palaeomagnetic field is recorded in exposed lake sediments at Long Valley and Mono Basin in east-central California. The reversal is estimated to be several thousand years long and 280 000 years old. The chronology is based on correlation of volcanic ash beds at Long Valley and Mono Basin with ones at Summer Lake, Oregon, and correlation of ash beds at Summer Lake with ash beds in a core at Tulelake, California, where age control is provided by tephrochronology and magnetostratigraphy.

Volcano hazards program in the United States

Abstract Volcano monitoring and volcanic-hazards studies have received greatly increased attention in the United States in the past few years. Before 1980, the Volcanic Hazards Program was primarily focused on the active volcanoes of Kilauea and Mauna Loa, Hawaii, which have been monitored continuously since 1912 by the Hawaiian Volcano Observatory. After the reawakening and catastrophic eruption of Mount St. Helens in 1980, the program was substantially expanded as the government and general public became aware of the potential for eruptions and associated hazards within the conterminous United States. Integrated components of the expanded program include: volcanic-hazards assessment; volcano monitoring; fundamental research; and, in concert with federal, state, and local authorities, emergency-response planning. In 1980 the David A. Johnston Cascades Volcano Observatory was established in Vancouver, Washington, to systematically monitor the continuing activity of Mount St. Helens, and to acquire baseline data for monitoring the other, presently quiescent, but potentially dangerous Cascade volcanoes in the Pacific Northwest. Since June 1980, all of the eruptions of Mount St. Helens have been predicted successfully on the basis of seismic and geodetic monitoring. The largest volcanic eruptions, but the least probable statistically, that pose a threat to western conterminous United States are those from the large Pleistocene-Holocene volcanic systems, such as Long Valley caldera (California) and Yellowstone caldera (Wyoming), which are underlain by large magma chambers still potentially capable of producing catastrophic caldera-forming eruptions. In order to become better prepared for possible future hazards associated with such historically unpecedented events, detailed studies of these, and similar, large volcanic systems should be intensified to gain better insight into caldera-forming processes and to recognize, if possible, the precursors of caldera-forming eruptions.

The diamicton at Deadman Pass, central Sierra Nevada, California: A residual lag and colluvial deposit, not a 3 Ma glacial till

A diamicton exposed at Deadman Pass in the central Sierra Nevada has been previously described as glacial till and dated at about 3 Ma. If till, the deposit would document an exceptionally old and previously unrecognized glaciation in the Sierra Nevada. The age and glacial origin of the diamicton at Deadman Pass has been widely cited in the geologic literature. Recent work, however, demonstrates that the diamicton is a residual lag and colluvial deposit formed by weathering of poorly consolidated Pliocene pyroclastic rocks that are unusually rich in coarse lithic basement clasts, including granitic and metamorphic rock types. Evidence that the diamicton at Deadman Pass is not till includes the following: (1) distribution of the diamicton is limited to areas underlain by the distinctive clast-rich lower pyroclastic member of the quartz latite of San Joaquin Ridge, (2) clasts in the diamicton and in the lower pyroclastic member are identical, (3) clast lithologies in the diamicton reflect nearby sources, (4) glacial deposits are absent in well-exposed sections of the lower pyroclastic member, and (5) formation of diamicton from present-day weathering and mass wasting of outcrops of the lower pyroclastic member can be observed locally.

California's restless giant: the Long Valley Caldera

S have monitored geologic unrest in the Long Valley, California, area since 1980. In that year, following a swarm of strong earthquakes, they discovered that the central part of the Long Valley Caldera had begun actively rising. Unrest in the area persists today. The U.S. Geological Survey (USGS) continues to provide the public and civil authorities with current information on the volcanic hazard at Long Valley and is prepared to give timely warnings of any impending eruption.