ESM Appendix – Direct quotes from historic studies on HTCDs

  1. Cole (1885): “When cut across, their cavities are seen to be divided into chambers by dome-like lamellae, one above the other, as if produced by successive expansions of the gas. In some cases the solid part of the "Lithophyse" is in a loose and powdery condition.”
  2. Cole (1885): “We are left, then, with two possible explanations of the hollow nodules found in lavas. Their outer and solid portion may safely be ascribed to spherulitic segregation from the matrix; but the inner cavity may be either the result of weathering upon a complete spherulite, or a sham-vesicle that has acted, like an included crystal or foreign body, as a centre of devitrification during cooling.”
  3. Iddings (1887): ”…are of aqueo-igneous origin, and have been produced by the action of the absorbed gases upon the molten glass from which they were liberated during the crystallization consequent upon cooling.”
  4. Iddings (1887): “In the still plastic glass from a center of crystallization a multitude of incipient microlites of feldspar radiated through a sphere of glass. As these anhydrous microlites increased in size the nature of the cementing paste was changed. Being impoverished of alumina and alkalies, it became more siliceous and relatively more hydrated. A point was reached where the absorbed vapors could no longer be retained in combination, but were released in innumerable bubbles which were either uniformly disseminated through the paste or aggregated into larger bubbles. The gas thus liberated acted as superheated steam and eventually produced the separation and crystallization of all the elements of the original sphere of plastic glass. Before the final crystallization of the paste when then hydrated glass gave up its combined vapors and became anhydrous, it shrank in consequence of the reduction of volume and produced in some cases cracks so frequently observed. That these cracks were formed before the final crystallization of the cementing paste is shown by the deposition on their walls of crystals of quartz, tridymite and fayalite.”
  5. Wright (1915), citing (and somewhat over interpreting) Iddings (1887): “that the cavities were formed by a kind of uniform tension in the viscous, cooling, and contracting magma (just as joints are formed in a later stage of cooling), and that at such points crystallization began and was accompanied by escape of gas into the cavity.”
  6. Ross (1941): “While still hot, local centers of crystallization were set up, around which spherulitic masses of intergrowncristobalite and feldspar were formed. The formation of these anhydrous minerals released volatiles originally in solution in the glass. The gradual collection of volatiles exerted a pressure, which combined with the cooling shrinkage of the enclosing material, forced the walls of the cavity outward, expansion being by rupture along symmetrically arranged planes.”
  7. Ross (1941):“However, when the cavity is large and especially where it develops in a highly viscous or almost solid glass, the force required for spherical expansion is very great. In such a medium, expansion by rupture or tearing will require very much less energy, and the rupture planes will tend to symmetrically enclose the cavity developed.”
  8. Bryan (1934) stated that the origin of the cavities is “…based on hydrostatic tension …. The latter explanation is almost universally accepted at the present day. This is due to the convincing nature of Iddings (1899) classic work on the spherulitic structures of the Obsidian Cliff.”
  9. Bryan (1963)“…concerns Ross’ (1941)suggested causes of expansion. His primary cause, namely, self-inflation, is that which had already been advocated for the expansion of the Queensland specimens…, but his secondary cause, tension due to contraction of the enclosing glass, is not locally applicable for here expansion took place in a freely flowing lava-stream.”