| Common range of thickness in park | ||||||
|---|---|---|---|---|---|---|
| Pumice layer | West side (inches) | East side (inches) | Common range in diameter of pumice fragments (inches) | Color | Source | Approximate age in 1968, or limiting dates (years ago) |
| X | Absent | [1] | ¼-2 | Light olive gray | Mount Rainier. | 100-150 |
| W | 0-1 | 1-3 | Medium sand | White | Mount St. Helens. | [2]450 |
| C | Absent | 1-8 | ¼-8 | Brown | Mount Rainier. | 2,150-2,500 |
| Y | 5-20 | 1-5 | Coarse sand | Yellow | Mount St. Helens. | 3,250-4,000 |
| D | Absent | 0-6 | ¼-6 | Brown | Mount Rainier. | 5,800-6,600 |
| L | Absent | 0-8 | ¼-2 | Brown | Mount Rainier. | 5,800-6,600 |
| O | 1-3 | 1-3 | Flourlike to fine sand | Yellowish orange | Mount Mazama. | About 6,600 |
| R | Absent | 0-5 | ⅛-1 | Reddish brown | Mount Rainier. | 8,750-11,000? |
The next two eruptions of Mount Rainier occurred between 5,800 and 6,600 years ago. Again, pumice spread over the area east of the volcano. The older pumice, which we call layer L, covers a band only a few miles wide that extends to the southeast from the volcano ([fig. 8]). The younger pumice, layer D, covers an area at least 10 miles wide directly east of the volcano. The distribution of both deposits shows that there were strong directional winds during the eruptions. The long, narrow pattern of layer L probably was caused by strong northwesterly winds during a short-lived eruption. The pattern of layer D was caused by winds from the west.
Some time during these eruptions, hot volcanic bombs and rock fragments were thrown out of Mount Rainier’s crater and fell onto surrounding areas of snow and ice. Wholesale melting resulted, and floods descended the east flank of the volcano carrying millions of tons of ash, newly erupted rock debris, and breadcrust bombs. Breadcrust bombs seem to be solid rock, but if you would break one open, you would find that the inside is hollow or is filled with a spongy mass of black glass. Their outer surfaces are cracked like the crust of a loaf of hard-crusted bread ([fig. 9]), so we call them breadcrust bombs. They originated as blobs of soft, red-hot lava which were thrown out of the volcano’s crater. As the masses arched through the air, they quickly chilled on the outside, and a hardened skin formed around the still hot and plastic core. As their outsides cooled, gas pressure in their hot interiors caused the bombs to expand slightly and their solidified outer skin to crack. When they struck the ground, many of the bombs became flattened on one side, but they were still plastic and sticky enough to remain whole.
Bombs can be found in two deposits that form the south bank of the White River about half a mile downstream from the White River campground. The deposits are mudflows caused by the mixing of hot rock debris with the water from melted snow and ice. As the mudflows moved down the valley floor they must have resembled flowing masses of wet concrete.
Mount Rainier erupted several times between about 2,500 and 2,000 years ago. During one of the first eruptions, a mass of hot ash, rock fragments, and breadcrust bombs avalanched down the side of the volcano and buried the floor of the South Puyallup River valley. Although this hot mass flowed like a wet mudflow, the temperature of the rock debris was above 600°F. Thus, if any water had been present, it would have been in the form of steam. You can see the resulting deposit in cuts along the West Side Road on both sides of the bridge across the South Puyallup River. Innumerable bombs have rolled from the cuts into the ditches beside the road. A charcoal log found in the deposit had a radiocarbon age of about 2,500 years.
A large breadcrust bomb enclosed in a mudflow deposit that consists of a mixture of volcanic ash and rock fragments. The outcrop is on the south bank of the White River about half a mile downstream from the White River campground. (Fig. 9)
Large amounts of pumice were thrown out of the volcano at the same time as the bombs or soon after. The pumice covers most of the eastern half of the park, and fragments are scattered as far southwest as Pyramid Peak and as far northwest as Spray Park. This pumice, called layer C, is especially thick and coarse at Yakima Park and Burroughs Mountain, where it lies at the ground surface ([fig. 10]). Here the light-brown layer is 5-6 inches thick and consists of irregularly shaped pumice fragments as much as several inches across. Mingled with the pumice fragments are fist-sized chunks of light-gray rock that probably were simultaneously thrown out of the volcano by violent explosions. Some of these angular rocks were hurled as far as Shriner Peak, 11 miles east of Mount Rainier’s summit.
Pumice layer C, which consists of light-brown fragments, lies at the ground surface over much of the eastern part of the park. (Fig. 10)