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Igneous activity and landforms
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
Mantle plumes occur not only along zones of spreading, but also within continental and oceanic plates. They are varied both in size and magmatic production, have little obvious spatial organization at a global scale, but are clearly associated with local updoming and excessive crustal heatflow. Tholeiite plumes occur more exclusively in the ocean basins, one of the most notable examples of which are the Hawaiian Islands which have existed for the past 70 million years, apparently tapping magma chambers which extend from depths of more than 60 km to within 8 km of the surface. A persistent mantle hot spot may exert a continued effect over successive portions of a lithospheric plate as it moves above it, giving rise to a line of volcanic peaks decreasing in age in the direction of the centre of spreading. In the Pacific Ocean these plumes have formed many seamounts and guyots (See Figures 6.3 and 6.15) which appear to be older in the west and younger in the east, supporting their place in the general scheme of sea floor spreading. Those which are truncated, submerged and coral-capped give further support to Darwin’s theory of coral-atoll formation by successive volcanic action, marine erosion, subsidence and coral growth (see Section 15.6.1). Plumes of alkali basalt are widespread in both continental and oceanic locations, occurring with latite, trachyte and phonolite. In the case of Easter Island even associated rhyolites are found. In other instances early-stage eruptions of tholeiite are followed by alkali basalt as a result of magma differentiation and even by andesites, presumably developed by the anatexis of shallow crustal rocks. The pronounced tendency for tholeiitic plumes in marginal sea basins has suggested to some authorities that these belts may be subjected to tension and spreading.
Principles of Petrological Analysis of Igneous Rocks
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
The K–Na moderately alkaline series (alkali basalt) consists of high-Ti and low-Ti varieties. The first one is composed of moderately alkaline picrobasalt (ankaramite), olivine alkali basalt, alkali basalt, hawaiite, trachyte and moderately alkaline and alkaline dacite and rhyolite. Rocks of andesitic composition (trachyandesite, benmoreite) are subordinate, as in the tholeiitic series, resulting in a bimodality characteristic of the volcanic activity of ocean islands and continental rift zones. The low-Ti series is characteristic of active plate margins.
The Lord Howe Volcanic Complex, Australia: its geochemistry and origins
Published in New Zealand Journal of Geology and Geophysics, 2022
Megan L. Williams, Brian G. Jones
Both samples located near the junction of the Roach Island Tuff and the North Ridge Basalt (DR135 and DR137) are porphyritic and vesicular, with microcrystalline groundmass that shows subtle trachytic texture. Overall phenocryst abundances are similar in each sample, however, DR137 contains olivine phenocrysts only whereas DR135 has plagioclase and clinopyroxene phenocrysts in addition to olivine. Vesicle abundance volumes are also similar, however, vesicles in DR137 are smaller and less distorted than those in DR135. Petrographically, both samples can be classified broadly as alkali basalts. Furthermore, DR137 is consistent with picrate. Their exact stratigraphic context is unknown, but the absence of features indicative of explosive volcanism indicates their origins are probably lower flows within the North Ridge Basalt.