Mount Rainier erupts explosively to produce small to moderate volumes of tephra, but volumetrically the volcano is primarily effusive. Explosive eruptions have deposited a dozen layers of frothy tephra (pumice and scoria). Mild explosive activity during effusive eruptions of gas-poor magma and ash from pyroclastic flows have also generated at least 25 layers of glassy ash that lie between the pumice layers.
Future tephra and ash rich eruptions will distribute the products downwind, most often toward the east, away from Puget Sound's large population centers. Airborne plumes of volcanic ash can greatly endanger aircraft in flight and seriously disrupt aviation operations. Although seldom life threatening, volcanic ash fallout on the ground can be a nuisance to residents, affect utility and transportation systems, and entail substantial clean-up costs.
Because Mount Rainier is covered in such large quantities of ice and snow, eruptions that include quantities of tephra fall significant enough to produce pyroclastic flows may melt the snow, which could cause lahars to form.