China
Africa
Explore chapters and articles related to this topic
Igneous Petrology and the Nature of Magmas
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Most magmas contain small amounts of water vapor, carbon dioxide, or other gases such as sulfur, chlorine, or fluorine that are typically dissolved in the molten material. When magma is under pressure at depth in Earth, gases are simply minor components and dilutants. However, when magma moves toward Earth’s surface, pressure decreases and the dissolved gases may form bubbles or separate from the magma completely. If the magma chamber is capped and not connected to the surface or if the connection is not large enough, pressure may build up to high levels, eventually resulting in a violent eruption when the pressure is released. As seen in Figure 5.13, The result may be explosive in much the same way that pop may explode out of a bottle if it is opened too quickly.
The “intraorogenic” Svecofennian Herräng mafic dyke swarm in east-central Sweden: age, geochemistry and tectonic significance
Published in GFF, 2020
Åke Johansson, Andreas Karlsson
These melts presumably gathered in a magma chamber at depth, perhaps at the crust-mantle boundary, where different magma pulses could mix and homogenize. Penecontemporaneous with mixing, the magma contained in this magma chamber underwent fractional crystallization, initially probably of Mg-rich olivine, and subsequently of clinopyroxene, which remained as cumulate rocks at depth, while the magma evolved from basaltic to andesitic in composition (cf. fractionation calculation for the Avesta-Östhammar gabbros and diorites in Johansson & Hålenius 2013). More or less well-mixed and fractionated magma then erupted in different portions through narrow fissures into the upper crust where it solidified, although some magma may possibly have reached all the way to the surface and erupted as now-eroded basaltic to andesitic lava.
The geochemistry and petrogenesis of Carnley Volcano, Auckland Islands, SW Pacific
Published in New Zealand Journal of Geology and Geophysics, 2018
John A. Gamble, Chris J. Adams, Paul A. Morris, Richard J. Wysoczanski, Monica Handler, Christian Timm
The volcanic geology of Carnley Volcano is described in Gamble and Adams (1985) and Ritchie and Turnbull (1985). These papers describe the growth and development of Carnley basaltic shield volcano on the eroded Mesozoic basement of the Campbell Plateau. In the case of the Auckland Islands, basement comprises a ∼95 Ma Cretaceous granitoid (Denison and Coombs 1977; Adams 1983; Pickett and Wasserburg 1989; Adams et al. 2017), that is exposed on Musgrave Peninsula in the core of Carnley Harbour (Gamble and Adams 1985). Here, it is intersected by an intense plexus of dikes and sills related to Neogene volcanism (cf. Gamble and Adams 1985, Figures 5 and 6). Early stages of volcanism were explosive and possibly subaqueous with fine ash and pillowed lavas abundant in the basal succession on Musgrave Peninsula which is intercalated with thin carbonate sedimentary deposits (Ritchie and Turnbull 1985) whose fossil assemblages indicate late Oligocene – Miocene ages. Overlying lava flows and rarely preserved pyroclastic rocks are largely subaerial and individual lava flows and packages of flows can be traced over kilometre distances, with some flows up to 15 or 20 m thick. Potassium–Argon age dating of lava samples from Musgrave Peninsula are reported in Adams (1983) and yield ages between 19 and 26 Ma in close agreement with the faunal assemblages. In the lower part of the succession, trachytes and rhyolites are more common than higher in the succession, where mafic rocks (basalts and trachy-basalts) are predominant. Nevertheless, the felsic rocks comprise <5% of the volume exposed. The entire suite is intersected by a NNE-SSW trending dike swarm (Gamble and Adams 1985, see Figure 4 for detailed map of Carnley Harbour region and Figures 5, 6, 10 and 12 for field photography) whose compositions span those of the lava flows. In the eroded interior of Carnley Harbour, on McClure Head and Circular Head, an intrusive complex of coarse grained, biotite-bearing, olivine gabbro and intimately associated minor felsic rocks (Gamble and Adams 1985) has been emplaced into the base of the volcanic series. These rocks are genetically related to the overlying volcanic suite and appear to represent the cooled and crystallised contents of a shallow magma chamber.