May 18, 1980, Mount St. Helens deposits in South Coldwater Creek, Washington

Abstract

Beginning at 0832 Pacific Daylight Time (PDT), Mount St. Helens, Washington, produced a rockslide debris avalanche and a lateral explosion (blast) that generated a laterally directed pyroclastic surge, called here a blast surge. The resulting deposits are thickest in the east‐west trending valley of North Fork Toutle River immediately north of the volcano but are almost as thick in South Coldwater Creek, a small tributary of the Toutle River, in the lee of an east‐west trending ridge (Johnston Ridge) just north of the North Fork Toutle River. The avalanche surmounted Johnston Ridge (300 m relief) and flowed down the valley of South Coldwater Creek. Closely following the avalanche and in part contemporaneously with it, the blast surge left thin (up to ∼0.5 m) deposits on Johnston Ridge and thick (up to ∼ 15 m) deposits in South Coldwater Creek. These deposits have a lower layer (layer A1) which is friable, massive, poorer in fines (particles < 1/16 mm), and more poorly sorted than the upper layer (layer A2). Layer A2 is less friable because it contains more silt‐ and clay‐sized material. Beneath layer A1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface. This zone, designated as layer A0, is a poorly mixed layer containing schlieren of soil, shattered volcanic fragments, and considerable quantities of organic material. Layer A1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge; layer A2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head; layer A0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge. It is concluded that a strongly fines‐depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high‐velocity particulate density current.