INFLUENCES OF SEDIMENT PROPERTIES AND WATER AVAILABILITY IN THE PRESERVATION OF MAGMA-SEDIMENT INTERACTIONS

The interaction of magma and wet sediment can produce a range of products including steam, baked zones, peperites and even phreatomagmatic explosions. To evaluate the influence of sediment properties such as porosity, permeability and water content on these interactions we conducted a series of lava pond experiments using remelted basalt (1300° C) at the University at Buffalo.

Six sediment conditions were tested: lapilli pumice (saturated), poorly sorted sand (7-11%, 29-34% and 34-40% moisture content), poorly sorted gravel (32%), and very poorly sorted gravel (17-19%). Sediments were used to build cones with a ~60 cm wide and 11 cm deep depression on top, where basalt was poured. Two different water tables were tested: 1. Stable: cone built inside of a plastic pool (Pumice and Poorly sorted sand), 2. Dynamic: free-standing cone (Poorly sorted gravel, Very poorly sorted gravel, Very poorly sorted sand). Five thermocouples and one moisture sensor were positioned in the cone beneath the pond to measure sediment temperature and water content change during the experiments. Thermocouples were distributed from the cone base to the melt contact at 0,2,4,6 and 8 cm bellow the sediment contact and moisture sensors at 13 cm (Time Domain Reflectometry).

The depth of penetration of heat into the sediment, changes to the sediment, path of steam and geometry of the melt-sediment interface are found to be dependent on both moisture content and fines content. Materials with >7 wt% fine (< 63µm) with permeabilities of 1.79 e-4 (m/s) produce interactions between the sediment contact surface and the magma. When moisture content is >29% steam travels through the melt pond, producing ejecta (~2 m from pond) and trapping sediment in vertical to convolute pathways in the melt. When the moisture content is <29% steam travels through the sediment and produces baked areas (up to 10 cm), including a coherent layer right under the contact. Materials with <7% of fine materials, permeabilities of 0.025 (m/s) and moisture content >29% the steam travels through sediment and produces baked areas (12 cm) or baked clast without coherent layers.

The experimental data sets are the first quantification of combined temperature, moisture and sediment conditions in magma sediment interactions for intrusions and lava flows in the rock record.