About 5,600 years ago an enormous landslide (3.8 km³⁾ removed the volcano's summit and altered core to form a deep horse-shoe shaped crater (see landslide story). Subsequent eruptions filled the collapse crater and built a new summit cone. The new geophysical data show that intense hydrothermal alteration has penetrated no more than 20-50 m beneath the surface of the summit cone. The voluminous areas of altered rock are limited to an old dike system on the west side of the volcano and to portions of the now-buried former walls of the collapse crater.

These results suggest that future collapses of hydrothermally altered rocks will be most probable on the upper west side of the volcano in and around the Sunset Amphitheater (photo above), about 650 m below the summit. This means that landslides from Sunset Amphitheater 2,600 and 500 years ago left substantial altered material at great height, setting the state for future collapses and lahars from the volcano's west side. Collapse of fresh, unaltered rock from other sides of the volcano, however, is also possible. During future eruptions, all sides of the volcano are at risk from lahars and pyroclastic flows.

The experimental use of high-resolution helicopter-borne magnetic and electromagnetic surveys over Mount Rainier illustrates a new capability for "sensing" the locations and sizes of altered zones within a volcano. Large volumes of altered rock located high within a steep volcano are potential sources of landslides. Such landslides typically transform into lahars that travel many tens of kilometers downstream from a volcano.

Sensors suspended from a helicopter (like those at left) were used to analyze rock beneath the surface of Mount Rainier for their magnetic and electrical-resistivity properties. The sensors were flown above the volcano along east- and west-trending lines spaced 250 m apart; the lowest sensor was flown about 45 m above the ground; the upper sensor was flown about 75 m above the ground.

The transmitter and receiver coils in the electromagnetic (EM) sensor measure the electromagnetic response of the ground at different frequencies to obtain information from different depths. The deepest that the lowest frequency EM data can penetrate the surface at Mount Rainier is about 150 m beneath the ice. The survey collected four frequencies (837 Hz, 4,341 Hz, 4,737 Hz, and 33 kHz) of electromagnetic measurements from the lower sensor and total-field magnetic data from the upper sensor.

Altered rock is nonmagnetic and has low electrical resistivity

The techniques used to identify altered volcanic rocks within a volcano are effective because altered rocks have low magnetization and electrical resistivities. In contrast, fresh volcanic rocks have relatively high magnetic signatures and high resistivities.

High-resolution aeromagnetic data acquired over Mount Rainier primarily reflect the topographic expression of the normally-magnetized, highly-magnetic lavas that comprise the volcano. Ridges generally produce magnetic highs, while valleys produce lows. Because intense hydrothermal alteration reduces the high magnetization of fresh volcanic rocks, the aeromagnetic data shows a magnetic low over an area that is altered. The location and approximate volume of altered rock is determined from the data.

The electromagnetic (EM) measurements from Mount Rainier show that the resistivities of unaltered young volcanic rocks are generally above 3000 ohm-m while those of old volcanic rocks range from 1500-3000 ohm-m. The resistivity of ice is 1M ohm-m based on analogs with other temperate glaciers and modeling. Measured resistivities of a few saturated altered rock samples from Mount Rainier and from magnetotelluric data range from about 10 to 500 ohm-m. These results show that the resistivities expected from the altered rocks are at least an order of magnitude lower than those of the unaltered rock.

Map showing location of Mount Rainier and pathways of lahars of the past 6,000 years (in orange). Black box shows the location of the maps below (a and b). 

The Osceola collapse event 5,600 years ago sent altered debris northeast and south into the Puget Sound. The Round Pass and Electron collapse events 2,700 and 500 years ago, respectively, sent altered debris west to Orting and Puyallup; the Puyallup River flows toward the northwest and empties into Puget Sound near Tacoma.