Mt. Rainier is a stratovolcano composed of lava flows interlayered with pyroclastic flows and mudflows. Mt. Rainier (14,410 feet) is the highest mountain in the Pacific Northwest and supports 26 officially named glaciers, the largest glacier system in the 48 contiguous states. Crevasses form when the glacier flows over convex-upward irregularities in the underlying bedrock. Note extensive rockfall and morainal debris on lower slopes of glacier.