The Mode of Eruptions and Their Tephra Deposits

Abstract

　In accordance with the physical properties of the magma and the scale of eruption, the types of explosive eruptions are classified as follows: hawaiian, strombolian, vulcanian, sub-plinian, and plinian eruptions for magmatic eruptions; and surtseyan and phreatoplinian for phreatomagmatic eruptions. The mode of transportation of tephra is divided into the three categories of fall, flow, and surge. Each tephra layer has a characteristic depositional structure, produced not only by the mode of eruption but also by the transportation mode. As tephra layers are deposited during active periods, while loam layers accumulate during quiescent periods, a thick tephra sequence consisting of alternating tephra and loam layers will form around an active volcano.

　The preservation or erosion of tephra depends on not only the thickness of the deposits but also the cohesion of the tephra and the environment of deposition. Where thin tephra layers are deposited, they can become difficult to identify as individual tephra layers, due to bioturbation of the soil. In such cases, we have to use different techniques to corelate the tephra not only by the morpho-logical feature of the pumice grains, but also by the petrological properties of the pumice, glass shards and phenocrysts.