K–Ar and 40Ar/39Ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the Snake River Plain (Idaho), Mount Adams (Washington), and Crater Lake (Oregon). The K–Ar ages were calculated from Ar measurements made by isotope dilution and K2O measurements by flame photometry. The 40Ar/39Ar ages are incremental-heating experiments using a low-blank resistance-heated furnace. The results indicate that high-quality ages can be measured on young, mafic volcanic rocks using either the K–Ar or the 40Ar/39Ar technique. The precision of an 40Ar/39Ar plateau age generally is better than the precision of a K–Ar age because the plateau age is calculated by pooling the ages of several gas increments. The precision of a plateau age generally is better than the precision of an isotope correlation (isochron) age for the same sample. For one sample the intercept of the isochron yielded an 40Ar/36Ar value significantly different from the atmospheric value of 295.5. Recalculation of increment ages using the isochron intercept for the composition of nonradiogenic Ar in the sample resulted in much better agreement of ages for this sample. The results of this study also indicate that, given suitable material and modern equipment, precise K–Ar and 40Ar/39Ar ages can be measured on volcanic rocks as young as the latest Pleistocene, and perhaps even the Holocene.

Potassium–argon (K–Ar) ages on whole rock samples have been measured on lavas from the subaerial Samoa Islands, which form a broadly linear volcanic chain that extends from the ESE to the WNW for about 360 km. The Manu'a Islands near the southeast limit of the chain exhibit youthful ages, with most <0.4 Ma, in keeping with the geological observations. Tutuila consists of several volcanoes, and previous work yielded a mean K–Ar age of 1.26 ± 0.15 Ma for the shield-building volcanism. Upolu, to the WNW of Tutuila, gives a mean age of 2.15 ± 0.35 Ma for the shield-building phase, represented by the Fagaloa Volcanics, with much of the island covered by significantly younger volcanic rocks. Savai'i, further to the WNW, is dominated by youthful volcanism, extending into historic times. In a restricted area, adjacent to the NE coast of Savai'i, previously thought to have volcanic rocks correlating with the Fagaloa Volcanics of Upolu, the ages are much younger than those on Upolu, lying between 0.32 and 0.42 Ma. Considering only the subaerial volcanism from Ta'u to Upolu, but also including Vailulu'u, the volcanism has migrated in a systematic ESE direction at 130 ± 8 mm a−1 over 300 km in the last 2.2 Ma. This rate is nearly twice that obtained from GPS measurements of Pacific Plate motion of 72 mm a−1 at N64°W in this area. However, if the much older age of shield-building volcanism from the submarine foundations of Savai'i is included, the regression yields a volcanic migration rate of 72 ± 14 mm a−1, in keeping with the measured GPS rate and consistent with a hotspot origin for the island chain. This suggests that the volcanic migration rates determined from the age of subaerial volcanism can be considerably overestimated, and this is now evident in other Pacific Ocean island chains. Clearly, the ages of the main shield-building volcanism from subaerial volcanism are minima, and if the older submarine lavas can be measured, these may yield a migration rate more in keeping with current plate motions.

The northern Kenya Rift is bounded on the west by uplands of Turkana which comprise horst-like blocks that include metamorphic basement rocks, locally overlain unconformably by the Cretaceous Lubur Sandstone, in turn overlain by predominantly volcanic sequences in which relatively thin sedimentary packages occur. Amphibolite facies crystalline rocks of the basement yield Early Palaeozoic K–Ar cooling ages reflecting the Pan-African Orogeny. Volcanism in Turkana was initiated through voluminous eruptions of transitional tholeiitic basalts commencing about 36 Ma ago in the Late Eocene, with some evidence for concomitant rhyolitic volcanism. Volcanism became dominantly rhyolitic in the interval from about 27 to 23 Ma ago, but remained bimodal as basaltic lavas are also known from this period. From about 19 to 15 Ma or younger, basaltic volcanism again dominated, often alkaline in nature, with thin but significant sedimentary sequences interleaved that have yielded important vertebrate faunal assemblages. Parallels exist between the volcanic history recorded in Turkana and that found in the Nabwal Hills east of Lake Turkana. In the southern Turkana region, oil exploration by seismic methods and deep drill holes has shown the existence of northerly-trending half-graben with up to 7 km of fill, and that these developed from at least Oligocene and possibly Late Eocene times. This suggests that the widespread basaltic volcanism at about 36 Ma ago (Late Eocene) heralds an earlier initiation of the Kenya Rift in northern Kenya than most workers have previously suggested.

The Dalradian terrane of Connemara was thrust southsoutheastwards about 460 Ma ago (Rb–Sr and K–Ar ages). It rides on a major thrust of post-D age over mylonitised acidic volcanic rocks of putative lower Ordovician age and contains a number of thrusts of similar age. Several major S- to SE-directed thrusts also limit the southeastern margin of the Dalradian rocks in Mayo and Tyrone. It is suggested by analogy with Ireland that during mid-Ordovician times the Highland Boundary fault in Scotland could have been a thrust zone which carried the Scottish Dalradian rocks over a lower Ordovician basement now represented only as fragments in the Highland Border Complex.

The K–Ar ages of 117 clay concentrates from samples associated with mineral deposits in Ireland indicate that most, if not all of the major ore deposits were formed during Carboniferous times or earlier. The Avoca orebodies are pre-400 Ma in age. Many of the vein deposits traversing Palaeozoic rocks were either formed or rejuvenated during the Hercynian orogeny c. 300 Ma ago. The SW Ireland Cu–Ba ores are 290 Ma old, and the major, frequently stratiform, base metal deposits stratabound in the Lower Carboniferous were, at least largely, formed during the Carboniferous period. In the case of the Navan orebody, an early Carboniferous age is indicated. Previous models invoking major mineralisations during Mesozoic or Tertiary times are rendered invalid. However, in some areas there appears to have been hydrothermal activity during the Triassic and possibly the Permian. The Triassic event is thought to be genetically related to coeval hydrothermal avtivity found throughout the N Atlantic regions. The data suggest that probably <10 km of cover has been removed from the majority of Ireland since Lower Carboniferous times. K–Ar dating of clays is shown to be a powerful method for constraining the ages of oredeposits.