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Long Valley caldera, a 17‐ by 32‐km elliptical depression on the east front of the Sierra Nevada, formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68‐0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende‐biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende‐biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self‐sealing of near‐surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated north‐west‐trending Sierra Nevada frontal faults that tap hot water at depth.

The central Taupo Volcanic Zone in New Zealand is a region of intense Quaternary silicic volcanism accompanying rapid extension of continental crust. At least 34 caldera-forming ignimbrite eruptions have produced a complex sequence of relatively short-lived, nested, and/or overlapping volcanic centers over 1.6 m.y. Silicic volcanism at Taupo is similar to the Yellowstone system in size, longevity, thermal flux, and magma output rate. However, Taupo contrasts with Yellowstone in the exceptionally high frequency, but small size, of caldera-forming eruptions. This contrast reflects the thin, rifted nature of the crust, which precludes the development of long-term magmatic cycles at Taupo.

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The characteristics of the Geological Survey TRIGA Reactor (GSTR) as a source of fast neutrons for the 40Ar/3 Ar technique of K-Ar dating have been determined using data from more then 45 irradiations in the central thimble (core) facility. The GSTR has a flux over the entire energy spectrum of 1.1 x 1017 n/cm2-MWH and a fast/thermal ratio on the centerline of the central thimble of 117 for fast neutron energies greater than 0.6 MeV. Production of 39Ar is about? x 10~13 mole/gram-percent KgO MWH, and the cross section for the reaction 39K(n, p) 39Ar is 65 ± 4 millibarns for epithermal (> 0.6 MeV) neutrons. Most 4 Ar/ Ar dating applications require about 10-40 hours of irradiation in the GSTR at the maximum continuous power level of 1 MW. The peak neutron flux in the central thimble is 4 cm above the physical centerline, and the verticle flux gradient in the centermost 20 centimeters varies from a small fraction of a percent to a maximum of about 3.5 percent per centimeter. The effect of this gradient can be effectively cancelled by suitable sample encapsulation and the use of a sample holder designed for the purpose. The horizontal flux gradient is less than 0.5 percent over the width of the central thimble. Self-shielding in a solid core of diabase 2.40 cm in diameter and 2.54 cm high is approximately 3 percent from the outside to the center, but self-shielding is probably negligible for the smaller samples usually irradiated for K-Ar dating. Corrections for interfering Ar isotopes produced by neutron reactions with Ca are relatively reproducible with values of 2.64 ± 0.017 x 10~4 for (36Ar/ 37Ar) Ca and 6.73 ± 0.037 x 1Q~4 for (39Ar/37Ar) Ca. The measured values for (40Ar/ 39Ar)ic, however, vary by an order of magnitude. This variability, whose cause is unknown, has been reported from other reactors. The corrections for interfering Ar isotopes can be minimized by using optimization curves for the GSTR to choose the best sample size and irradiation time for a given material. Of more than 100 possible neutron reactions in common rocks and minerals, only 26 need be considered for purposes of radiological safety. The activity produced by these reactions upon irradiation of samples can be conveniently and accurately predicted either by a computer program or from graphs specifically devised for the GSTR.

K‐Ar ages and paleomagnetic data for basalt samples from a new core hole (site E) at the Idaho National Engineering Laboratory (INEL) indicate that the age of the reversed polarity event recorded in Snake River Plain lavas is older than 465±50 ka (1000 years before present) reported previously by Champion et al. (1981). Nine basalt flows, eight with normal polarity and one with reversed polarity, were recognized in the site E core hole. The flows above and below the reversed flow have ages of 491±80 ka and 580±93 ka, respectively. The inclination of the paleomagnetic field direction of the reversed flow at site E agrees with the inclination of reversed flows elsewhere at INEL which have an age of 565±14 ka. These reversed flows were previously thought to be correlative with the Emperor event. We suggest that this polarity event is an older event which we name the Big Lost Reversed Polarity Subchronozone and Subchron. A review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the Brunhes and two besides the Jaramillo in the late Matuyama. These 10 short subchrons begin to indicate the many short events that Cox (1968) hypothesized must exist if polarity interval lengths have a Poisson distribution. These events are true subchrons, not excursions, and may or may not have low associated paleointensities, although low field strengths might explain why the reversal process aborts. The mean sustained polarity interval length since late Matuyama Chron time is 90,000 years. The similarity of this number with the 105‐year period of the Earth's orbital eccentricity suggests anew that linkage between geomagnetic, paleoclimatic, and possible underlying Earth orbital parameters should be evaluated.

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Geologic mapping, K-Ar, and 40 Ar/ 39 Ar age determinations, supplemented by paleomagnetic measurements and geochemical data, are used to quantify the Quaternary volcanic history of the Crater Lake region in order to defi ne processes and conditions that led to voluminous explosive eruptions. The Cascade arc volcano known as Mount Mazama collapsed during its climactic eruption of ~50 km 3 of mainly rhyodacitic magma ~7700 yr ago to form Crater Lake caldera. The Mazama edifi ce was constructed on a Pleistocene silicic lava fi eld, amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for Mount Mazama. Between 420 ka and 35 ka, Mazama produced medium-K andesite and dacite in 2:1 proportion. The edifi ce was built in many episodes; some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region, and some were possibly related to deglaciation following marine oxygen isotope stages (MIS) 12, 10, 8, 6, 5.2, and 2. Magmas as evolved as dacite erupted many times, commonly associated with or following voluminous andesite effusion. Establishment of the climactic magma chamber was under way when the fi rst preclimactic rhyodacites vented ca. 27 ka. The silicic melt volume then grew incrementally at an average rate of 2.5 km 3 k.y. -1 for nearly 20 k.y. The climactic eruption exhausted the rhyodacitic magma and brought up crystal-rich andesitic magma, mafi c cumulate mush, and wall-rock granodiorite. Postcaldera volcanism produced 4 km 3 of andesite during the fi rst 200-500 yr after collapse, followed at ca. 4800 yr B.P. by 0.07 km 3 of rhyodacite. The average eruption rate for all Mazama products was ~0.4 km 3 k.y. -1 , but major edifi ce construction episodes had rates of ~0.8 km 3 k.y. -1 . The longterm eruption rate for regional monogenetic and shield volcanoes was ~0.07 km 3 k.y. -1 , but only ~0.02 km 3 k.y. -1 when the two major shields are excluded. Plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath Mount Mazama is several times that which has been erupted. The eruptive and intrusive history refl ects competition between (1) crystallization driven by degassing and hydrothermal cooling and (2) thermal input from a regional magma fl ux focused at Mazama. Before ca. 30 ka, relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated suffi ciently to sustain voluminous convecting crystal-poor melt. Subsequently, and perhaps not coincidentally, during MIS 2, a large volume of eruptible silicic magma accumulated in the climactic chamber, probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of Mazama. Sediment gravity-flow deposits (Hol.) Talus (Hol. and Pleist.) Landslide deposits (Hol.) Glacial deposits, undivided (Pleist.) Andesite of the E basin (Hol.) Lava Andesite of ...