GRAIN SIZE AND MOISTURE CONTENT RELEVANCE IN THE THERMAL AND PHYSICAL BEHAVIOR OF MAGMA SEDIMENTS INTERACTIONS, ANALOGUE APPROACH

Magma interactions with near surface sediments are controlled by the grain size distribution and the moisture content available. Specifically, heat transfer from melt through sediment depends on sediment permeability and the presence of silt sized materials. In a series of nine experiments involving ~30 L remelted basalt conducted at the Center for Geological and Climate Hazards, University of Buffalo, different physical and thermal interactions of melt and host were observed.

To understand how water content and grain size influence the heat transfer, remelted basalt at ~1300 °C was poured into a depression atop a sediment cone. The cones varied in composition and moisture: moderately sorted quartz sand (dry), poorly sorted gravel (<5 dry, 14-19% water), small bomb and lapilli scoria (dry), moderately sorted sandy gravel (32% water), poorly sorted sand (7-11, 29-34, 32-40% water), saturated pumice. Thermocouples (5) and moisture sensors (3) were spaced from coordinate system base to the melt-sediment contact at 10, 12, 14, 16 and 18 cm (melt contact). Moisture sensors were distributed at 5 cm (time domain reflectometry), 11 and 13 cm (capacitive sensors)

The rate of temperature change varies with depth, grain size distribution and permeability controlled by the presence of <1 mm particles. Rates at 4 cm below the contact are 0.01, 0.03, 0.06, 0.07, 0.11, 0.11, 0.21, 0.34 and 0.38 °C/sec, for the following sediment/moisture conditions: 34-40% water poorly sorted sand, saturated pumice, 29-34% water poorly sorted sand, 29-34% water poorly sorted gravel, 17-19% water poorly sorted gravel, 0-5% water poorly sorted gravel, 7-11 % water poorly sorted sand, dry quartz sand and dry scoria.

Excavations of sediment piles revealed dehydration zones, color changes, and sometimes newly coherent zones along the contact with the melt. The thickness of the dry zone can be minimal (saturated sediments) to more than the thickness of the overlying pond (~12 cm) in wet sediments. Saturated experiments can transport sediment with steam through the melt in the first minutes after pouring, resulting in minor ejecta (~2 m radially).

These experimental data sets will enable the quantification of combined temperature and moisture conditions at the sediment-melt interface for intrusions and lava flows in the rock record.