The nature and possible origin of the pillow lavas and hyaloclastite breccias of King Island, Australia

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Abstract
Basaltic pillow lavas and breccias of possible Cambrian age are exposed in the south-east of King Island. Pillow flows and massive flows generally occur at the base of the succession and are overlain by hyaloclastite breccias. Studies of the cooling of pillows show that they must develop their globular form almost immediately the lava contacts the cold medium. At this stage surface tension could be important in determining the spherical shape if the magma was projected or was moving rapidly. Because of its high latent heat of vaporization, water is a far more efficient coolant than air and the cooling in water may be rapid enough to 'freeze' the pillow shape before it becomes unstable. The hyaloclastite breccias consist of whole pillows, fragments of pillows, or parts of thin lava flows, within a tuff matrix of devitrified and metasomatized micro-globular particles and shard-like fragments. The components of the matrix were not derived from pillow or flow margins but formed by micro-globulation and shattering. Two rock-types are represented: the spilite or albite-diopside type which forms massive or pillowy flows, and the picritic type (chlorite pseudomorphing olivine) which forms the hyaloclastite breccias and similar rocks. Most of the volcanic activity was sub-aqueous, and judging by the proximity of tillite and varve-like rocks, may have taken place under or in ice.