The eruptive period was climaxed by the building of the volcano’s present summit cone, which is at least 1,000 feet high and 1 mile across at its base. Although dwarfed by the tremendous bulk of Mount Rainier, it is a little larger than the cone of the well-known Mexican volcano Parícutin that appeared in 1943 and erupted until 1952. Mount Rainier’s present summit cone was built within a broad depression at the top of the main volcano that had been formed nearly 4,000 years earlier ([fig. 11]; see [p. 40]). The cone consists of a series of thin black lava flows, and its top is indented by two overlapping craters. Rocks around the craters are still warm in places, and steam vents melt caves in the summit icecap. The first climbers who reached the top of the mountain, in 1870, spent the night in one of these caves, as have many benighted climbers since.
The snow-covered lava cone lies in a depression 1¼ miles wide at the summit of the volcano. The cone was probably built about 2,000 years ago. Liberty Cap is to the left and Point Success is to the right. The cliffs below and to the right of Liberty Cap enclose Sunset Amphitheater. (Fig. 11)
Even though the lava flows that formed the summit cone were relatively short, their eruption greatly affected some valleys at the base of the volcano. The hot lava melted snow and ice at the volcano’s summit, causing floods that rushed down the east and south sides. When the floods reached the valley floors, they picked up great quantities of loose rock debris and carried it downstream, sometimes forming mudflows. The resulting flood and mudflow deposits raised the floors of the White and Nisqually River valleys as much as 80 feet higher than they are today. These valley floors, as well as several others, then became broad wastes of bare sand and gravel that extended beyond the park boundaries. Later, the rivers cut down to their present levels, but they left remnants of the flood and mudflow deposits as terraces or benches along the sides of the valleys. You can camp on such a terrace in the Nisqually River valley at the Cougar Rock campground. The White River campground occupies a similar terrace in the White River valley.
When did Mount Rainier erupt last? The most recent pumice eruption was just a little over a century ago. However, between 1820 and 1894, observers reported at least 14 eruptions. Some of these may have been just large dust clouds, caused by rockfalls, that were mistaken for clouds of newly erupted ash. Other clouds may have been from genuine eruptions that left no recognizable deposits. D. R. Mullineaux has found that at least one eruption of that era did spread pumice over an area east of the volcano between Burroughs Mountain and Indian Bar to a distance of at least 6 miles from the crater. Pieces of the pumice, layer X, are light brownish gray and as large as 2 inches across. We find only scattered fragments of the pumice, and nowhere are they in a continuous layer. Where the X pumice is directly on top of layer C, we cannot tell them apart. The best areas for us to study the younger pumice, therefore, are glacial moraines formed within the last 150 years, because no pumice other than layer X is present on the moraines. Fortunately, R. S. Sigafoos and E. L. Hendricks of the U.S. Geological Survey have determined the ages of the moraines by counting the growth rings of trees on them. Their studies show that the pumice was erupted between about 1820 and 1854.
Captain John Frémont, an early explorer of the Oregon Territory, recorded that Mount Rainier was erupting in November 1843, but his journals give no details. Others have reported eruptions in 1820, 1846, 1854, and 1858. Pumice layer X probably was erupted during one or more of these times, but we do not know exactly when.
And will Mount Rainier erupt again? We think that it will, but we now have no sure way of predicting the time, the kind, or the scale of future eruptions.
Why Glaciers?
We frequently hear the question: “Why are there glaciers on Mount Rainier?” A glacier forms wherever snowfall repeatedly exceeds melting over a period of years. Above 6,500-7,000 feet on Mount Rainier, more than 50 feet of snow falls each winter, and not all of it melts before the next winter. The survival of this snow from one year to the next depends partly on the cooler temperatures at the higher altitudes, and perhaps also on the somewhat deeper snowfalls there.
