Multiple-rind structure in pillow lava as an indicator of shallow water |
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| Authors: | Yosuke Kawachi Ian J Pringle |
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| Institution: | (1) Geology Department, University of Otago, P. O. Box 56, Dunedin, New Zealand;(2) Present address: CSR Ltd, Box 483, 2001 GPO Sydney, NSW, Australia |
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| Abstract: | Multiple-rind structure is common among shallow-water pillows with diameters larger than about 1 m in Oamaru, New Zealand, on the Columbia Plateau (USA), and elsewhere. A rind consists of sideromelane, tachylyte, and tachylytic basalt. A multiple rind is a concentric set of repeated rinds in various forms, e. g., a portion of a broken rind thrust under another part, a series of short and detached subparallel rinds, or a pouch-shaped depression. Transitions and combinations of these three forms are common. Multiple-rind structure develops at any part of the pillow perimeter, but does not cover the pillow completely. It is always accompanied by a rupture in the outermost rind. Up to 13 rinds have been observed, but two to four rinds are most common. The multiple-rind structure is formed by implosion resulting from condensation of exsolved H2O. When H2O condenses, a pressure difference between the interior and exterior of a pillow is created. Above a certain threshold pressure difference, the outer skin of a pillow is torn at weak points, such as radial joints, and thrusts under the neighboring skin, buckles to form a pouch-shaped depression, or produces some variation of these. One set of multiple rinds is thus formed. Further exsolution and condensation of H2O in solidifying pillows may cause development of additional rinds. H2O exsolution and condensation and subsequent implosion are limited to low-pressure environments so that multiple-rind structure is characteristic of shallow-water pillow lava. |
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