AN EXPERIMENTAL FACILITY FOR THE INVESTIGATION OF MAGMA FRAGMENTATIONBY RAPID DECOMPRESSION

A special experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression. The f acility operates in a regime similar to that of shock tubes and at tem peratures up to 950 degrees C and pressures up to 200 bar. Cylindrical samples (diameter ca. 17 mm, length ca. 50 mm) undergo rapid decompre ssion in a high-temperature, high-pressure section of the facility fol lowing the disruption of a diaphragm separating that section from a lo w-pressure, low-temperature section. Actual vesicular magma samples ha ve been experimentally fragmented at elevated temperatures and pressur es corresponding to those observed during explosive volcanic eruptions and the resulting pyroclastics have been photographically resolved in flight and collected for physical characterization. The results of th ese experiments show that the rapid decompression of highly viscous ve sicular magma can generate pyroclastic ejecta via rapid and complete f ragmentation of magma at high temperature. This new fragmentation faci lity is presented as a tool for experimental volcanology under well-co nstrained conditions.