The tectonic causes of ignimbrite and pumice tuff deposition and of subsequent basalto-andesitic volcanism

Abstract

The majority of investigators of ignimbrite formations, including welded tufts and related "pumice tufts, at present seem to agree on their deposition as pyroclastic material out of incandescent ash flows, which held the glass-shard and crystal material, together with a certain quantity of foreign-rock substances, in suspension in a gas medium over frequently considerable distances after they issued from the eruption vents. This view, already expressed by MARSHALL (1935), again finds its way in a very careful petrographic study of ignimbrite samples from all over the world recently published by Ross and SMITH (1961), in which all subsequent stages of the welding of glass shards and pumice fragments and of their devitrification are shown in a great number of photographs. A similar opinion is given by Bovo (1961) concerning the origin of the Yellowstone Tufts, constituting the basal part of the Yellowstone Park rhyolite formations ,although this author suggests that a minor portion at least of this rhyolite material erupted as rhyolite froth loosing its vesiculation through collapse, after which it continued to move as flowing lava (the Canyon Flow at the upper end of the Yellowstone Canyon). In spite of much petrographic counter-evidence, a few authors still quite recently expressed as their view that ignimbrites ought to be regarded as true lavas. VLODAVETZ (1957), for instance, envisages the possibility of three different modes of extrusion of his tutf-lavas: 1) after previous differentiation of the ignimbrite magma, followed by subsequent intrusion of a fluid, less acid lava into the more acid and already partly or entirely consolidated variety; 2) as a banded lava; 3) as a lava froth resembling milk. The lenticular glass bodies