Interactively processed Landsat MSS imagery has been used as an aid to studying the regional geology of approximately 10 800 km2 of terrain at the southeast margin of the Archaean Yilgarn Block in Western Australia. The technique proved successful in extending positions of known lithological contacts and lineaments into poorly exposed, inaccessible areas and in revealing new geological features, especially faults, previously unrecognized at ground level. During this investigation the distribution of granitoids and greenstones was more precisely defined, internal greenstone structures highlighted and three main fault trends were identified: (1) NW–NNW and (2) ENE, both within Archaean shield, and (3) NE–NNE within the transition to adjacent Proterozoic mobile belt. In order for the most information to be extracted from Landsat MSS images it is recommended that, whenever possible, image processing should follow ground-based studies as well as precede them, and that field geologist and Landsat specialist should work at the image processing system together.

The Moine thrust zone of southern Assynt forms part of the northwest margin of the Caledonide belt and has aroused controversy concerning amounts and timing of thrust displacement and sequence of thrust development. Recent mapping shows it to have been a foreland propagating thrust sequence; the uppermost ductile Moine thrust formed first, followed by sequences of imbricates, a major thrust (the Ben More thrust) and then several lower duplex zones. This sequence is clear from new observations that many of the earlier thrusts were folded and/or breached during the development of the underlying structures. A displacement of over 54 km has been estimated for the zone as a whole. An alkaline igneous complex, including the large Borrolan syenite, was intruded during the development of the thrust zone and much of it was carried some 30 km to the west-northwest onto the foreland. Late extensional structures in southern Assynt are an integral part of the Caledonide thrust sequence and probably developed from the collapse of the thrust wedge as it climbed from stronger basement rocks on to a weaker cover sequence on the foreland.

Potassium–argon dating of 44 samples of basalt lavas and pantelleritic ignimbrites provides a provisional chronology for the 1900-metre thick Cenozoic succession of the Suregei–Asille district, northeast of Lake Turkana. Volcanic rocks of the Asille Group range in age from late Oligocene (Chattian) to late Miocene (Tortonian). Data obtained from conventional K–Ar total-rock analysis of basalt samples are appraised statistically to indicate the presence of three periods of enhanced volcanism centred around 27, 17 and 11.5 Ma. Equivalent ages obtained from the pyroclastic pantellerites by conventional K–Ar total-rock analysis and 40Ar/39Ar analysis of sanidine concentrates indicate that basic and acid eruptions were closely spaced in time. Continental tholeiite flood lavas belonging to the Gombe Group are of Pliocene age, but are difficult to date precisely because of their young ages and relatively high atmospheric contamination occurring as a result of the secondary alteration in the ubiquitous glass mesostasis. Those ages involving least atmospheric correction are considered most reliable, and are closely similar for the Chen Alia and Harr formations at around 4.85 Ma, although later flood eruptions may have occurred to the south and west of the Suregei–Asille district at about 3.85 Ma.

The mineralogy and petrology of banded pumice clasts from a mixed-magma non-welded ignimbrite on Tenerife, Canary Islands, is described. Phenocrysts, which frequently show signs of resorption, are grouped into three assemblages in mutual disequilibrium; these assemblages were originally precipitated by alkali basalt magma (assemblage 1, titanaugite + olivine + calcic plagioclase + high-Mg Fe–Ti oxides), trachyandesitic magma (assemblage 2, salite + kaersutite + sodic plagioclase + hauyne + medium-Mg Fe–Ti oxides) and phonolitic magma (assemblage 3, aegirine-augite + biotite + sodic sanidine + sodalite + nepheline + sphene + low-Mg magnetite) respectively. Phenocryst types (e.g. clinopyroxene, feldspar) which are common to two or all assemblages are compositionally distinctive in each. The excess silica content of nepheline in assemblage 3 indicates a temperature of 770 °C for phonolitic magma, while Fe–Ti oxides and clinopyroxene-ilmenite give temperatures of 915 °C and 907 °C respectively for the trachyandesite. Assemblage 1 and 2 phenocrysts are set in mafic glass; assemblage 3 phenocrysts are only found in a matrix of salic (phonolitic) glass. Analyses of the volumetrically dominant phonolitic glass define a compositional trend consistent with fractionation of assemblage 3 phenocrysts. Analyses of the mafic glass scatter about a straight-line trend produced by mixing of phonolitic liquid with homogeneous intermediate (trachyandesitic) magma. These relations are interpreted as the result of two distinct magma mixing events, consequent on the invasion of a stably zoned magma chamber containing phonolitic liquid stratified above denser trachyandesitic magma, by intruding basaltic magma. High Ca content of assemblage 1 plagioclase suggests precipitation of the basaltic phenocrysts under locally high partial pressure of water. The preferred model involves, in chronological sequence: (i) generation of a differentiated phonolitic cap above trachyandesitic magma, through wall crystallization and boundary-layer uprise of phonolite; (ii) pooling, convective heat loss, and partial crystallization of dense, hydrous alkali basalt magma injected into the chamber and forming a liquid layer beneath the trachyandesitic magma; (iii) volatile saturation of the residual basalt liquid through crystallization, consequent buoyant uprise into, and efficient mixing with, trachyandesite; (iv) eruption of chamber contents and partial mixing of the mixed basalt–trachyandesite with phonolite, although the precise relative timing of eruption and the second mixing event is not known.

Geological survey, electron microprobe analyses of clinopyroxenes and isotopic age determinations have revealed that the intrusive dolerites and basic–intermediate lavas of the Shelve Ordovician Inlier are part of a co-magmatic suite which show transitional affinities between tholeiites and alkali basalts. The ages of most of the intrusives are shown to be mid-Ordovician.

Glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of Surtsey volcano. Despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island. The tuff includes abundant accretionary lapilli and tuff vesicles, indicating that it was all deposited subaerially. During the growth of the tuff rings, repeated hydromagmatic explosion cycles began with a series of intermittent tephra-finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours. This nozzle-like continuous activity produced eruption columns 100–250 m in diameter and 500–2000 m in height which probably quarried several hundred metres below the ground surface. The continuous-uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down. During construction of the tuff rings, concentric faults repeatedly downdropped a funnel-like structure (400–800 m in diameter) several hundred metres, thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level. Ring dykes later intruded upward along these faults and fed small lava flows. Heat in the surface tephra probably originated primarily from these shallow intrusions.