USGS Publications Warehouse

Abstract

Mount Rainier National Park includes 378 square miles of rugged terrain on the west slope of the Cascade Mountains in central Washington. Its mast imposing topographic and geologic feature is glacier-clad Mount Rainier. This volcano, composed chiefly of flows of pyroxene andesite, was built upon alt earlier mountainous surface, carved from altered volcanic and sedimentary rocks invaded by plutonic and hypabyssal igneous rocks of great complexity. The oldest rocks in the park area are those that make up the Olmnapecosh Formation of late Eocene age. This formation is more than 10,000 feet thick, and consists almost entirely of volcanic debris. It includes some lensoid accumulations of lava and coarse mudflows, heaped around volcanic centers., but these are surrounded by vastly greater volumes of volcanic clastic rocks, in which beds of unstratified coarse tuff-breccia, about 30 feet in average thickness, alternate with thin-bedded breccias, sandstones, and siltstones composed entirely of volcanic debris. The coarser tuff-breccias were probably deposited from subaqueous volcanic mudflows generated when eruption clouds were discharged directly into water, or when subaerial ash flows and mudflows entered bodies of water. The less mobile mudflows and viscous lavas built islands surrounded by this sea of thinner bedded water-laid clastics. In compostion the lava flows and coarse lava fragments of the Ohanapecosh Formation are mostly andesite, but they include less abundant dacite, basalt, and rhyolite. The Ohanapecosh Formation was folded, regionally altered to minerals characteristic of the zeolite facies of metamorphism, uplifted, and deeply eroded before the overlying Stevens Ridge Formation of Oligocene or early Miocene age was deposited upon it. The Stevens Ridge rocks, which are about 3,000 feet in maximum total thickness, consist mainly of massive ash flows. These are now devitrified and altered, but they originally consisted of rhyodacite pumice lapilli and glass shards, which compacted and welded into thick massive units during emplacement and cooling. Subordinate water-laid clastic rocks occur t(ward the top of the formation, and thin-bedded pyroclastic layers occur between some of the ash flows. Exposures on Backbone Ridge and on Carbon River below the mouth of Cataract Creek show that in places the thick basal Stevens Ridge ash flows swept with great violence over an old erosion surface developed on rocks of the Ohanapecosh Formation. Masses of mud, tree trunks, and other surface debris were swirled upward into the base of the lowermost ash fiery, and lobes and tongues of hot ash were forced downward into. the saprolitic mud. The Stevens Ridge Formation is concordantly overlain by the Fifes Peak Formation of probable early Miocene age, which consists of lava flows, subordinate mudflows, and minor quantities of tuffaceous clastic rocks. The lavas are predominantly olivine basalt and basaltic andesite, but they include a little rhyolite. They are slightly to moderately altered: the ferromagnesian phenocrysts are generally replaced by saponite, chiprite, or carbonate ; the glass is devitrified ; and the rocks are locally permeated by veinlets of zeolite. Swarms of diabase sills and dikes are probably intrusive equivalents of the Fifes Peak lavas. The upper part of the Fifes Peak Formation has been mostly eroded from Mount Rainier National Park, but farther north, in the Cedar Lake quadrangle, it attains a thickness of more than 5,000 feet. The Fifes Peak and earlier formations were gently folded, faulted, uplifted, and eroded before the. late Miocene Tatoosh pluton worked its way upward to shallow depths and eventually broke through to the surface. The rise of the pluton was accompanied by .the injection of a complicated melange of satellitic stocks, sills, and dikes. A favored horizon for intrusion of sills was along or near the unconformity that separates the Ohanapecosh and Stevens Ridge Formations. Near the major plutonic centers sill is piled on sill along this nonconformity to form a complex that resembles a huge cedar-tree lacolith. At several places the magma broke through to the surface, initiating a series of explosive volcanic eruptions. Only small remnants of the resulting pyroclastic rocks have survived erosion : one .of these remnants forms The Palisades, in the northeastern part of the park, and consists of a mass of welded tuff at least 800 feet thick, which grades downward into a plug of rhyodacite. Other plugs and stocks in the area northeast of the park contributed pumiceous debris to the Ellensburg Formation of early Pliocene age in the adjacent Snoqualmie and Mount Aix quadrangles. Extensive erosion unroofe5 the Tatoosh pluton and carved canyons as deep as 4,000 feet into. the plutonic rocks and sill complexes. This occurred before the Mount Rainier' volcano began to take form, probably in early Pleistocene time. The first Mount Rainier eruptions were voluminous lava flows of pyroxene andesite. These flows spilled westward down the canyons of the ancestral Mowich and Puyallup Rivers, astride which the volcano grew. They also obliterated, a northeastward-flowing river, remnants of whose canyon now lie buried beneath the lava fill of Grand Park .and Old Desolate. With the smoothing of the pre-Rainier topography by these early intracanyon flows, the volcano began to grow into its present form. Despite rapid contemporaneous erosion by streams and glaciers, the overlapping streams of lava, subordinate mudflows, and thin pyroclastic deposits eventually built a huge cone that was about 1,000 feet higher than the present mountain. Its summit area was then either engulfed by collapse or else deeply hollowed out by erosion. Still later a small cone, which culminates in Columbia Crest, grew along the southeastern edge of the old summit rim. The smoothly mounded sides of this younger cone, almost unscarred by erosion, contrast strikingly with the older intensely eroded cone that forms the main bulk of the mountain. During the building of the Columbia Crest cone thin falls of pumice and ash spread outward for several miles from the base of the volcano.. The youngest extensive pumice sheet, about 500 to 600 years old, probably records the last significant eruption from Mount Rainier. Rapid erosion, which had so effectively reduced and sharpened the older cone, is still continuing. The huge glaciers, rasping down the mountain's sides, are particularly potent agents of erosion; but avalanches, rockfalls, and a variety of other downslope movements also are rapidly stripping debris from the steeper slopes. Mudflows and destructive slurry floods of melt water, sand, and rock occasionally surge through the lower canyons, eroding great gashes into the unconsolidated glacial and stream deposits that choke the upper ends of the valleys and spreading huge fans of detritus farther down. The Kautz flood of October 2, 1947, is typical: it moved 15 million cubic yards of debris several miles downstream and redeposited it in a huge fan that temporarily blocked the Nisqually River.