| Abstract: | Simultaneous measurements of compressional and shear wave velocities, Vp and Vs, in acidic and basic igneous rocks and volcanic glasses, were made up to 900°C and at 10–20 kbar.The effects of pressure and temperature on Vp and Vs in glasses and glassy rocks change at about 600°C, presumably the glass transition temperature. These effects are directly related to the silica content in the samples.
and
for obsidian are negative at room temperature and 245°C, but are positive at 655°C. The velocity—pressure relations for obsidian display an obvious hysteresis phenomena.
for basalt glass is slightly negative, but
is positive for usual substances at room temperature,
and
for obsidian and glassy andesite are positive up to about 600°C but are negative above that temperature. However, for basalt glass as well as other crystalline rocks,
and
are negative at all temperatures. Glass once heated above the glass transition temperature Tg under pressure P1 retains the memory of pressure P1 after it is cooled down below Tg and while subjected to another pressure P2. An abrupt shift of the velocities correlating to pressure P2 occurs when the glass is again heated to Tg. Vp−T and Vs−T relations for obsidian, glassy andesite, and basalt glass clearly exhibit this pressure memory. |
