Geological study of phreatic eruptions

Abstract

Phreatic (non-juvenile) eruptions are the most common type of magmatic activity on Earth. Here we review the characteristics of phreatic eruptions, which occur when overheated water is rapidly vaporized. Tephra layers produced by phreatic eruptions are composed mainly of clay-rich volcanic ash with variably altered lapilli and volcanic blocks. A single phreatic eruption can last between one hour and one day; however, eruptions may occur successively over a period of years to decades. The total volume of tephra produced by a phreatic eruption is typically 10m, maximum <10 m. Phreatic eruptions may be accompanied by diverse phenomena, including: tephra fallout, ejected rock fragments, low-temperature pyroclastic flows, and syneruptive-spouted type lahars. There are few detailed descriptions of low-temperature (~100°C) pyroclastic flows and syneruptive-spouted type lahars associated with phreatic eruptions. Detailed studies of phreatic phenomena are required, as it remains difficult to identify and reconstruct these processes based on the characteristics of the deposits.