In this study, unconfined samples of basalt, sintered and bonded perlite, insulating firebricks, and paraffin were subjected to thermal gradients ranging from 15øC/cm to 100øC/cm. Microshocks produced by thermoelastic stress relief were detected by means of miniature accelerometers. Histograms of microshock occurrences have similar shapes for all tests. Activity begins and increasesabruptly following onset of heating or cooling of the sample. It reaches a maximum, and then decays hyperbolically, in approximate correlation with the time dependence of the thermal gradient. There are later swarms of activity apparently initiated by small thermal fluctuations in the sample. The log-log plots of cumulative number of microshoCks versus amplitude were constructed and show kinks or knees. The value of the negative slope b for the linear approximations to these plots ranges from 1.2 to 2.7. These values are greater than those for normal earthquakes series and are similar to those observed for volcanic B-type earthquakes. The effects of a cellular crack pattern, nonuniformity of stress due to the thermal gradient, and sample inhomogeneity may explain these results. shocks were observed. Lamont-Doherty Geological Observatory Contribution No. 1528.In the first part of this paper we describe the model study and our observed results. In the second part we compare our results to those obtained in earthquake, microearthquake, and microfracture studies. TEST SAMPLESTest materials included basalt, insulating firebrick, paraffin, and perlite. Specimens were generally brick size. The smallest sample tested was approximately 12 x 4 x 3 cm. Specimens used in the vacuum chamber tests, as described below, were approximately 15 X 15 X 7.5 cm.The perlite specimens were manufactured from commercial perlite (a crushed and heattreated perlitic obsidian). The perlite grains are vesicular, irregularly shaped, and microfractured. The test samples were made by two methods: (1) by compressing and sintering the loose grains into bricks, and (2) by bonding the grains with sodium silicate.The insulating firebricks are commercial refactory bricks principally composed of silica and alumina, in a 56/40 ratio by volume. The paraffin samples are polycrystalline and not amorphous. The sample of basalt is aphanitic and nonvesicular with nodules of olivine. The basalt sample was obtained from Vulcan's Throne in the Grand Canyon. 4455 4456 WARREN AND LATHAM

The Galápagos mounds sea-floor hydrothermal system is at least 300,000 years old and once produced manganese-poor sediments, which nearly blanketed the area of the present mounds field. Present-day mound deposits are limited manganese-rich exposures, suggesting that the system has changed from rock-to water-dominated and has diminished in intensity with time.

The Jurassic ophiolite at Point Sal, California, contains all the rock types typically found in other ophiolites. The total thickness is just over 3 km, much thinner than seismically determined ocean crust thicknesses of 5-6 km. We measured the seismic velocities of 34 representative watersaturated samples in the laboratory under pressure and at room temperature. The velocity profile is a function of both metamorphic grade and the concentration of mafic minerals. Oceanic layer 2A compressional velocities are not found in the ophiolite. Layer 2B velocities of 4.9-5.4 km s-l are found for greenschist facies metabasalts. A velocity of 6.0 km s-l represents high grade greenschist facies mineral assemblages and amphibole in a dyke and sill complex. Late stage differentiates in the upper plutonic section have layer 3A velocities over 6.0 km s-'. Velocities increase with depth to abov 7.1 km s-l due to the introduction of cumulus olivine and other mafic p k ;es. Olivineclinopyroxene gabbros and troctolites have layer 3B velocities of about 7.4 km s-'. Beneath this, clinopyroxenites and cumulate ultramafics have slightly higher velocities. Serpentinized peridotite (dunite and harzburgite) occurs at the base of the ophiolite and has compressional velocities around 5.5 km s-'. The velocity profile with depth shows prominent gradients in the basaltic and gabbroic layers, but marked discontinuities are seen between the dyke and sill complex and the plutonic section and also between the upper plutonic section and the lower cumulate gabbros and ultramafic rocks. A pronounced velocity inversion, produced by serpentinized dunite underlying 166 J. Nichok et al.the gabbros, has been seismically modelled as a possible in situ characteristic of ocean crust and mantle structure. If a basal low velocity serpentinized layer underlies the ocean crust in some areas, then the Moho is within the mantle and marks the top of unaltered peridotite. Model travel-time curves and seismograms for such a structure show that it would be erroneously interpreted as a thicker ocean crust and that it would yield prominent late reverberations on seismic records.

Four experiments on lunar materials are reported: (i) resonance on glass spheres from the soil; (ii) compressibility of rock 10017; (iii) sound velocities of rocks 10046 and 10017; (iv) sound velocity of the lunar fines. The data overlap and are mutually consistent. The glass beads and rock 10017 have mechanical properties which correspond to terrestrial materials. Results of (iv) are consistent with low seismic travel times in the lunar maria. Results of analysis of the microbreccia (10046) agreed with the soil during the first pressure cycle, but after overpressure the rock changed, and it then resembled rock 10017. Three models of the lunar surface were constructed giving density and velocity profiles.