The modern glaciers that grace the slopes of Mount Rainier are much smaller descendants of the vast ice masses that repeatedly overwhelmed the Cascade Range during Pleistocene time.
Extensive glaciation in this region both preceded and followed the development of Rainier's modern cone, 500,000 years ago. At times of glacial maximum, ice volumes filled valleys to the brim and extended for more than 100 km (60 mi) down channel into the lowlands surrounding Mount Rainier. Since the Little Ice Age (A.D. 1250 to mid-1800'a), the glaciers have been receding and ice volumes decreasing. Today, 25 glaciers cover 92 km2 (35 mi2) of the volcano, and they are the headwaters for five major rivers, including some that provide water for hydroelectric power and irrigation.
Major Glacial Periods
Hayden Creek (~170 to 130 ka)
Icecaps covered the high elevations of Mount Rainier during this period of alpine glaciation. Long valley filling glaciers flowed down the Cowlitz River channel to a distance of about 105 km (63 mi) and down the Nisqually River valley 48 km (30 mi) to the west end of Alder Lake. Within present day Mount Rainier National Park, Hayden Creek till was found on a ridgetop that is 600 m (nearly 2000 ft) above the adjacent valley floor – these upland valley glaciers were extremely thick.
Evans Creek (22 to 15 ka)
Glaciers were substantially smaller in the Cascade Range during Evans Creek time than they were during the preceding Hayden Creek glaciation. Although icefields or an icecap covered the slopes of Mount Rainier and parts of nearby mountains, glaciers away from the volcano were wholly confined within valleys. From origination point on Mount Rainier, these valley glaciers extended for 64 km (38 mi) down the Cowlitz, 30 km (19 mi) down the Nisqually, 38 km (24 mi) down the White, and about 26 km (16 mi( down obth the Puyallup and Mowich river valleys.
From the late Pleistocene (12 ka) to the end of the Little Ice Age (mid-1800's) several glacial advances, much smaller than Hayden and Evans Creek periods, occurred at Mount Rainier. The McNeeley Drift includes two sets of moraines dated at approximately 11,300 and 9,500 years old. The Little Ice Age is the most recent episode of several periods of renewed glacial advances that have occurred during the last 10,000 years.
Influence of glaciers on Mount Rainier topography
Glaciers mantled Mount Rainier for most or all of its 500,000-year lifespan. Many lava flows that erupted during ice ages were unable to melt entirely through the thick, valley-filling ice, and instead chilled and hardened beside it. With continued eruptions, lava flows would skirt the margin of the valley- filling glacier, advancing only where ice was thin or absent. As the glaciers retreated, and the valleys emptied of ice, lava flows were left perched high on the sides and crests of ridges, much like docks stranded along the former shores of a shrinking lake or reservoir. This process accounts for why lava flows cap many of the ridges radiating from Mount Rainier, instead of filling the deep valleys. Today's glaciers are too small to exert much control on the movement of lava. Instead, eruptions of lava and of pyroclastic flows will melt snow and ice, causing destructive lahars.
The Ricksecker Point lava flow, which erupted 40,000 years ago, was important for developing understanding that glaciers directed the courses of lava flows at Mount Rainier. Canyons 250 m (800 feet) deep flank Ricksecker Point on the north and south, but 40,000 years is too brief to excavate to such depths by erosion. Impoundment of the lava flow against thick glaciers that filled the valleys to the north and south, followed by emptying of the canyons during ice retreat, provides an explanation for the great depth of the flanking canyons despite the relative youth of the lava flow. One of the biggest pieces of evidence for understanding that Mount Rainier's lava flows followed ridges adjacent to valley filling glaciers are large horizontally oriented columnar joints along the margins of lava flows. The orientation of these columns suggests that lava cooled laterally rather than from the upper and lower margins of a lava flow.