This aerial view of Mount St. Helens immediately after its eruption of May 18, 1980 illustrates the great horseshoe shaped amphitheatre that resulted from the massive avalanche of the north flank that initiated the eruption. Originally, the cone was 2949 m high; after the eruption the highest point on the rim of the amphitheatre was 2560 m, while its floor was at 1800 m. Most of the missing material ended up in the 2.8 cubic kilometer debris avalanche deposit which entered the valley of the Toutle river, but some was reamed away by the violent plinian eruption that ensued. Since 1980, an extrusive dacite lava dome grew within the amphitheatre, but was inactive at the time of writing. (Section 4.4 and Fig. 4.27).

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