The May 18, 1980 eruption of Mt. St. Helens gives us much information concerning identification, mode of eruption, transportation and deposition of volcanic dry avalanches. Precursor events including swarm of volcanic earthquakes, small phreatic explosions, bulging of volcanic edifice at northern part and growth of normal fault scarps at the top preceded within 2 months duration. Unstable portion of the volcanic edifice was detached 7-26 seconds after the great volcanic earthquake of M=5.1 and had collapsed towards the northern slope, filling the Toutle River valley until the area 28 km away from the source. The deposit covers an area of 60 km
2. Mean thickness is 45 m (VOIGHT et al., 1981). Surface of the deposit is topographically characterised with hummockey reriefs, spiracles, natural levees and depositional ramps. Volcanic edifice exposed along the inner wall of the new crater shows fresh and extensive shattered structure. Shattered megablocks are abundant in the main depositional area. Fine-grained tuffaceous layers are found between two adjacent megablocks. Size and amount of megablocks decrease towards the distal end of the deposit. Matrix materials consisting of mixture of clasts derived from several megablocks of different lithology and pelets of soil, gravel and wood fragments captured during transportation, becomes abundant towards the downflow area. These textural evidences suggest the shattering of volcanic edifice prior to the collapse, and continuous decomposition of megablocks towards the distal end of the deposit.
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