Abstract In the early twentieth century the Earth’s age was unknown and scientific estimates, none of which were based on valid premises, varied typically from a few millions to billions of years. This important question was answered only after more than half a century of innovation in both theory and instrumentation. Critical developments along this path included not only a better understanding of the fundamental properties of matter, but also: (a) the suggestion and first demonstration by Rutherford in 1904 that radioactivity might be used as a geological timekeeper; (b) the development of the first mass analyser and the discovery of isotopes by J. J. Thomson in 1914; (c) the idea by Russell in 1921 that the age of a planetary reservoir like the Earth’s crust might be measured from the relative abundances of a radioactive parent element (uranium) and its daughter product (lead); (d) the development of the idea by Gerling in 1942 that the age of the Earth could be calculated from the isotopic composition of a lead ore of known age; (e) the ideas of Houtermans and Brown in 1947 that the isotopic composition of primordial lead might be found in iron meteorites; and (f) the first calculation by Patterson in 1953 of a valid age for the Earth of 4.55Ga, using the primordial meteoritic lead composition and samples representing the composition of modern Earth lead. The value for the age of the Earth in wide use today was determined by Tera in 1980, who found a value of 4.54 Ga from a clever analysis of the lead isotopic compositions of four ancient conformable lead deposits. Whether this age represents the age of the Earth’s accretion, of core formation, or of the material from which the Earth formed is not yet known, but recent evidence suggests it may approximate the latter.

Abstract This is a progress report on some preliminary 40 Ar/ 39 Ar incremental heating experiments on feldspar separates from four samples of the felsite unit, a complex silicic batholith that intrudes the overlying Franciscan Complex (Late Jurassic to Late Cretaceous) and underlies the Geysers Geothermal Field, northern California (Schriener and Suemnicht, 1981; Thompson, 1989,1991). The felsite unit is only found in the subsurface but it appears to be an elongate body whose axis trends northwest-southeast and whose surface is shallowest in the southeast part of the field (Figure 1). It ranges in composition from granite to granodiorite (Schriener & Suemnicht, 1981; Thompson, 1991). The apparent coincidence of the heat flow anomaly within the Geysers field (Walters and Combs, 1989) with the distribution of felsite within and below the zone of steam production suggests that the felsite unit may be the primary source of heat. Presently available K-Ar ages (0.9 Ma to 2.7 Ma) suggest, however, that the felsite unit may be too old to be the primary source of heat for the present thermal activity. Resolution of this apparent paradox should be of interest for the purposes of both exploration and field management. If the felsite unit is young (<1 Ma), for example, then it should be hot wherever it is found. If the felsite unit is old (>1 Ma), on the other hand, then it may be relatively cold outside of the region of present production. The felsite unit also may be a complex body emplaced over a significant interval of time

Abstract Intraplate volcanism within the Pacific Plate not generated at spreading plate margins is most obvious in Hawaii and the Hawaiian-Emperor volcanic chain. This chain forms a global relief feature of the first order. This chapter consists of five separate sections that summarize the volcanism and geology of Hawaii and the Hawaiian-Emperor chain. Less obvious but probably greater in overall volume are other seamounts and seamount chains scattered across the northern and eastern Pacific basin. Some of these appear to owe their origin to intraplate volcanism, but many probably formed at mid-ocean ridges. Batiza (this volume, Chapter 13) discusses these other, largely submarine, volcanoes. The Island of Hawaii lies at the southeastern end of the Hawaiian-Emperor volcanic chain—a dogleg ridge, largely submarine, stretching nearly 6,000 km across the north Pacific Ocean basin. From Hawaii the chain extends northwestward along the Hawaiian Ridge to a major bend beyond Kure Atoll. North of the bend the chain continues in a northward direction as the submarine ridge of the Emperor Seamounts. Volcanoes are active at the southeast end of the chain and become progressively older to the northwest, reaching ages of 75 to 80 million years at the north end of the Emperor Seamounts. Most of this volcanic chain, with an estimated area of 1,200,000 km 2 , lies beneath the ocean. Only the Hawaiian Islands and a few atolls of the Hawaiian Ridge, totaling some 16,878 km 2 , rise above the sea (Plate 5).

Paleomagnetic and potassium-argon studies support geologic evidence that the lower member of the Bandelier Tuff was deposited 1.4 m.y. ago. The upper member erupted about 1.0 m.y. ago and was followed by caldera collapse which formed the 12- to 14-mile diameter Valles Caldera. Postcaldera activity which resulted in the eruption of rhyolite domes and pyroclastic material, has occurred at about 0.9, 0.7, 0.5, and 0.4 m.y. ago, with later undated eruptions that were estimated at about 0.1 m.y. ago. These data from the Valles Caldera are the basis for the previously published age revision of the Brunhes-Matuyama geomagnetic polarity epoch boundary from 1.0 to 0.7 m.y. ago, and they were used to define the Jaramillo normal polarity event at about 0.9 m.y. ago (Doell and Dalrymple, 1966).

Geologic mapping, paleomagnetic stratigraphy, and potassium-argon dating were used to determine the time and volume relations of tholeiitic and alkalic basalt on Nunivak Island in the Bering Sea near the coast of Alaska. Volcanism on Nunivak Island occurred in distinct episodes separated by quiet intervals that lasted from 1.6 to 0.6 m.y. During the past 6 m.y., tholeiitic basalt was erupted during at least five such episodes, and highly undersaturated alkalic basalt was erupted during at least three episodes. The oldest volcanic rock found on Nunivak Island is an alkalic basalt, succeeded by repeated alternations of tholeiitic and alkalic basalt. During the last episode of tholeiitic volcanism, 130 cu km of basalt erupted during a well-defined interval that lasted from 0.9 to 0.3 m.y. ago. A nearly contemporaneous eruption of alkalic basalt has continued vigorously to historic times and has covered the central part of the island with small cones, flows, and tephra from explosion craters. The volume of alkalic basalt of the latest episode of volcanic activity is from 0.7 to 2.0 percent of the volume of the associated tholeiite. At least one earlier eruption of alkalic basalt occurred in close association with the eruption of tholeiite. As on Hawaii, the highly undersaturated alkalic basalts on Nunivak contain abundant ultramafic inclusions.