Geological Survey Bulletin 1288 Surficial Geology of Mount Rainier National Park Washington

Thin layers of pumice erupted by Mount Rainier and by other volcanoes in the Cascade Range are the most wide spread surficial deposits in the park. Although these deposits are not represented on the geologic map because of their thinness, the distribution of five layers erupted by Mount Rainier is shown in figure 2.

DISTRIBUTION OF PUMICE DEPOSITS in Mount Rainier National Park. Only the pumice layers erupted by Mount Rainier within the last 10,000 years are shown. The extent of layer X is based on the recognition of the pumice at certain localities, regardless of its thickness; other layers are shown wherever they are at least 1 inch thick. Letters represent the following localities: C, Cougar Rock campground; I, Ipsut Creek camp ground; L, Longmire; M, Mowich Lake; O, Ohanapecosh campground; P, Paradise Park; S, summit crater; T, Tipsoo Lake; W, White River campground; and Y, Yakima Park. Based on studies by D. R. Mullineaux. (Fig. 2)

The ages of most of the pumice deposits are known within a few hundred years because wood buried with them can be dated by determining the proportion of radioactive carbon the wood contains. Other surficial deposits that are interbedded with pumice layers of known age can be dated approximately. Elsewhere, the age of a surficial deposit, such as a moraine, can be limited by the presence or absence of a certain pumice layer on top of it (see sketches at the bottom of the map explanation [plate 1]) — if the pumice is present, the deposit must be older; if the pumice layer is not on top, the deposit is younger than the pumice. This method of dating surficial deposits, of course, can be used only in areas covered by the pumice layer in question.

The most distinctive and widespread pumice deposits at Mount Rainier—and those most useful for purposes of dating other surficial deposits—were erupted by two other volcanoes and brought to the park by southwesterly winds. Layer O (table 1) is about 2 inches thick and blankets the entire park and adjoining region. It can be readily recognized by a distinctive yellowish-orange color (fig. 3) and a flourlike texture. When it is studied under the microscope, its characteristics are found to correspond precisely to those of the pumice that was erupted by Mount Mazama volcano at the site of Crater Lake, Oreg., about 6,600 years ago. The extreme fineness of layer O can be attributed to the distance of 250 miles that separates Mount Rainier from Crater Lake.

RECENT PUMICE DEPOSITS on the floor of a cirque a quarter of a mile southeast of Sluiskin Falls. The yellow bed at the bottom is layer O, which was erupted at the site of Crater Lake, Ore., about 6,600 years ago. The yellowish-brown pumice a few inches above layer O is layer D, which was erupted by Mount Rainier more than 5,800 years ago, and the light-yellowish-brown pumice bed at the top of the outcrop is layer Y, which originated at Mount St. Helens volcano between 3,300 and 4,000 years ago. (Fig. 3)

Pumice layers Y and W originated at Mount St. Helens volcano, which is about 50 miles southwest of Mount Rainier. Both layers cover lobate areas that extend far northeastward from their source. The centerline of the lobe of layer Y lies near the west edge of the park, where the pumice is commonly as much as 18 inches thick. The thickness of layer Y decreases to the east, and the layer is thin or absent in the southeast corner of the park. Layer Y is light yellowish brown (fig. 3), which helps to distinguish it from layer W, which is white. The center of the lobe of layer W lies near the east boundary of the park, where the pumice is about 2 inches thick.

The thickest and most widespread pumice from Mount Rainier is layer C, which is found throughout the eastern and northeastern parts of the park. Layer X, which represents Mount Rainier's most recent known eruption, was formed about the middle of the last century. It is found only in a small area close to the volcano.