Lahars & Flows

Mt. Rainier is well-known for being one of the most glacier-covered mountains in North America, and during the last Ice Age, these glaciers were even thicker and denser than they are now. These glaciers were dense enough that, when an eruption occurred and lava flowed out, they would redirect and divert the lava flows. The lava would skirt around the thick edges of ice and slightly melt the outside of the glacier, preferring to flow where the ice was thinner or more brittle. This formed distinctive ridge lines, and these volcanic valleys are visible today due to the reduction of those ancient, massive glaciers. Many of these ridges are made of hexagonal columns of a volcanic rock called andesite. The hexagonal shape is not a crystal structure, but is a result of how lava flows cooled: when the hot lava came into contact with the Ice Age glaciers, the ice was durable enough to cool the lava, which shrunk as it cooled and cracked in a hexagonal pattern.

Over the last 10,000 years, Mt. Rainier has undergone at least 60 massive lahars and other landslides that dramatically changed its shape and height. Eruptions produce both pyroclastic flows and lava flows, and this in combination with chemically changing rocks creates a perfect environment for landslides and mudflows. These debris flows have reached as far as 60 miles away, all the way to modern-day Tacoma and Puget Sound, and displace and change the course of surrounding rivers and lakes. They also lay down new layers of mud, rock, and sediment, burying previous landscapes.