Electron Mudflow

A sample (sample 8, from Unit 4 in measured section 7) of the Electron Mudflow from the outcrop at measured section 7 contained about 27 percent silt and clay, in which montmorillonite and chlorite together made up about four parts in 10, with the former clay mineral predominating. The remainder of the silt and clay fraction consisted of feldspars and quartz.

Rocks from Mount Rainier volcano made up an average of 41 percent and older rocks of the Cascade Range 59 percent of samples of pebbles collected from the mudflow at five different localities int he lowland (Crandell, 1963b). The most distinctive rock in the mudflow in the lowland is a scoriaceous, black, hypersthene andesite, which occurs as boulders as large as 5 feet in diameter. These are mostly found lying on the surface of the mudflow. The largest boulders, however, are of a reddish-brown breccia derived from the volcano, some of which are at least 35 feet in maximum observable dimension. These large boulders were seen mostly in an area extending from Orting southward for a distance of about 2 miles. This distribution probably is a result of a more gentle gradient in this area than along the valley farther upsteam. The gradient of the mudflow is about 68 feet per mile just downstream form the mountain front at Electron; however, at a point 2 miles south of Orting the valley widens, and the gradient decreases to about 34 feet per mile. The very large masses of breccia probably lodged on the old valley floor in this area as the mudflow spread laterally, thinned, and decreased in velocity.

Hand augering and power augering, in addition to natural exposures, show the mudflow to overlie fluvial sand and gravel deposited by the Puyallup and Carbon Rivers; the mudflow itself is overlain by fluvial sand and silt deposited by overbank floods. A wood fragment obtained from the mudflow near Electron had a radiocarbon age of 530+/-200 years (Crandell, 1963b, P.A51). When corrected, this age is about 600 years.

The Electron Mudflow has been tentatively identified at many places along the Puyallup River valley between Electron and the lower slopes of Mount Rainier. The following unites crop out in a streambank on the north side of the river about 1 mile downstream from the mouth of the Mowich River (measured section 7).

Unit 2 in measured section 7 is the 1,000-year-old lahar described previously; this lahar and the Electron Mudflow lie within a valley eroded into the Round Pass Mudflow by the Puyallup River. The top of the lahars and alluvium within this valley is only a little more than 20 feet above river level, whereas the top of the Round Pass Mudflow in this area is about 100 feet above the river.

near the mouth of St. Andrews Creek, just west of the park boundary, the Electron Mudflow is about 6 feet thick and overlies a sandy lahar more than 15 feet thick. The two deposits form a terrace in the South Puyallup River valley whose top is about 50 feet above river level. Ring counts of recently cut stumps indicate that some trees on the terrace were at least 435 years old. Inside the park two clayey lahars, one of which may be the Electron, overlie the bomb-bearing block-and-ash flow; the three deposits are superposed in a cut (measured section 8) along the West Side Road near the South Puyallup River bridge.

The Electron Mudflow underlies about 14 square miles of the Puyallup River valley in the Puget Sound lowland. In this area its thickness ranges from a few inches to more than 26 feet; it is generally about 15 feet thick from McMillin southward to a point 2 miles south of Orting. If an average thickness of 15 feet is assumed for the area beyond the mountain front, the deposit has a volume of a little more than 200 million cubic yards. An additional amount of unknown area and volume lies between the lowland and its source, 22 miles away, on the west flank of Mount Rainier.

The clay-mineral content of the Electron Mudflow suggests an origin like that proposed for the other clayey mudflows from Mount Rainier; thus, the Electron probably was caused by massive slides of moist, hydrothermally altered rock. These slides may have been triggered by an earthquake or by a volcanic explosion.

The Electron Mudflow may have occurred at a time when the volcano was dormant, for we have found no deposits that indicate an eruption about 600 years ago.