Subglacial volcanics — On the formation of acid hyaloclastites

Abstract

Several rhyolitic volcanic accumulations of subglacial origin in Iceland consist of two principal components: (1) hyaloclastite; and (2) ellipsoidal to irregular lobes averaging about 7 m in diameter. Two different types of hyaloclastite, reflecting major changes in the style of subglacial eruptions, occur either as separate layers or in intimate association. A pumice-bearing type is thought to have resulted from explosive events during an eruption. The second type, characterized by fragments of obsidian and lithic rhyolite, is genetically related to the lobes. The lobes generally consist (from the margin inwards) of an obsidian rind, a zone of flow-banded/flow-folded pumiceous material, and a central zone of columnar rhyolite. They are thought to represent remnants of disintegrated subglacial (subaquatic) rhyolitic flows or intrusions into the water-logged pumice-bearing hyaloclastite. Due to the rapid chilling of subglacial flows, dome-shaped bodies representing the initial stage of a growing lobe, develop on flow units. When the internal pressure of the lobe due to inflow of magma exceeds the tensional strength of the obsidian rind the latter shatters, and new lava is exposed to the quenching medium. As long as this process of build-up/ sudden release of pressure can take place during a relatively quiet phase of an eruption, the second type of acid hyaloclastite will form.