Authors: WARREN HUFF
Abstract: Explosive eruptions from volcanoes are recorded in the stratigraphic record throughout the Phanerozoic. The most visible evidence of these eruptions is generally in the form of preserved tephra layers, and they appear to be concentrated in the stratigraphic record at times of active plate collision and concomitant high stands of sea level. The products of volcanic eruptions are lavas, tephra and gases, and whereas low-silica, anhydrous basaltic magmas are usually erupted in the form of lava flows high-silica, hydrous rhyolitic magmas are commonly explosively erupted as plinian and ultraplinian plumes, and associated pyroclastic flows. Fallout tephras may be preserved in ancient sedimentary sequences as tonsteins, bentonites and K-bentonites, but in many cases they also mix with ambient background sedimentation and thus become simply one of the contributing factors to overall sediment composition. The significance of volcanogenic contributions to mudrocks is easily underestimated since their influence requires detailed study of the fine-grained components by petrographic and X-ray diffraction methods. Volcanogenic sediments react with seawater to produce secondary phases such as zeolites and clay minerals. Typical mudrock clay minerals derived by the alteration of tephra include smectite, illite-smectite and kaolinite, and the most diagnostic non-clay minerals include beta-form quartz, euhedral zircon and apatite, clinopyroxene, amphibole, biotite and sanidine. Further modification of these phases by diagenetic and low-grade metamorphic processes further obscures the nature of their origin.
Keywords: volcanic ash, bentonite, shale, mudstone, clay, siliciclastic, diagenesis
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