Microtextural evidence for vesiculated tuff formation in Songaksan tuff ring, Jeju Island, Korea

Abstract

Vesiculated tuff, containing submillimeter-size vesicles, is commonly found in Surtseyan and phreatomagmatic deposits. However, the processes and conditions of vesicle formation and preservation in granular substances have been poorly explored so far in spite of their potential importance in understanding the eruptive and depositional processes of these deposits. This study provides a detailed account of a vesiculated tuff in Songaksan tuff ring, Jeju Island, Korea, together with comparison with non-vesiculated tuffs. The vesiculated tuff, which is interpreted to have been emplaced by a wet pyroclastic surge, is poorer-sorted, poorer in fines, richer in coarser particles, and has lower porosity than the non-vesiculated tuffs. The pores are closed and have round margins surrounded by a fine-grained matrix. On the other hand, non-vesiculated tuffs, emplaced by either pyroclastic surges or fall, are composed of loosely packed ash aggregates and have large interconnected pores. We interpret that the vesicle formation in granular substances is possible only when the pores are water-saturated and airtight. Expansion of trapped gases is also necessary to produce the generally round shape of the vesicles. The gas expansion is possible only when the heat is transferred from the surrounding hotter pyroclasts into the gas bubbles. Vaporization of liquid water and gas exsolution from pyroclasts can be ignored. In order for the heat transfer to take place, the pyroclastic surge needs to be non-isothermal, having different temperatures of the solid and gas phases. The thermal disequilibrium of the surge can be maintained by continual mixing of cold ambient air and thermal expansion of the surge. The pyroclasts deposited from the surge need to increase the temperature of the trapped air/gases by only 3 K. Then, the trapped air/gases can overcome the yield strength of the surrounding wet ash matrix and expand. The essential conditions for vesiculated tuff formation, such as water saturation of pyroclasts and thermal disequilibrium of surge, are closely related with the properties of the diatreme-filling materials and the behavior of pyroclastic surges. Therefore, vesiculated tuffs can provide useful information on the eruptive and transport processes of Surtseyan and phreatomagmatic deposits in addition to the wet nature of tephra.