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This section highlights the map units (i.e., rocks and unconsolidated deposits) that occur in Mount Rainier National Park and puts them in a geologic context in terms of the environment in which they were deposited and the timing of geologic events that created the present landscape.
The volcanoes in the north- south trending Cascade Province evolved through a history of complex plate tectonic processes and glaciation. Different tectonic models have been proposed for the development of the Cascades, but all models include subduction, addition of exotic terranes, and oblique plate movements as important components in creating today’s Cascade Range (Kiver and Harris, 1999). In general, the North American plate edge was located farther east during the Mesozoic in Nevada, western Idaho, and eastern Washington. Subduction processes added larger masses of continental materials, island- like masses called microcontinents, microplates, or exotic terranes to the western margin of North American. Addition of these microplates shifted the plate edge westward.
The first Cascade volcanoes erupted about 42 million years ago (Ma), forming the older Western Cascades. At the time, the range was oriented northwesterly but would later rotate clockwise to achieve today’s north- south orientation. Continued subduction formed rows of volcanic vents that become younger to the east. As a result of clockwise rotation, extensive fissures opened up to the east and voluminous eruptions of the Columbia River Basalt occurred about 17 Ma.
Subduction and accompanying volcanism continued for some 25 million years in the older Cascade Range. As magma rose, rocks were heated and began to expand. Large lithospheric blocks dropped downward along north- south oriented faults creating linear depressions, or grabens (fault- bounded basins). The faults opened pathways that enabled large volumes of magma to rise. The grabens filled and overflowed with overlapping shield and cinder cones to form a volcanic plateau called the Eastern, or High, Cascades.
The present generation of volcanoes is believed to be no more than 400,000- 600,000 years old. Mount St. Helens was formed only 40,000 years ago. Coincident with the growth of these recent volcanoes was the Pleistocene Ice Age, which began about 1.6 Ma. Glaciers formed on each volcanic cone when it reached sufficient height. At the peak of glaciation, a continuous icecap buried the upper Cascades from Canada to northern California, broken only in the Columbia Gorge area where elevations approached sea level (Kiver and Harris, 1999). Alpine glaciers on Mount Rainier flowed as far as 65 miles (105 km) from the mountain. Most of the glacial sculpturing seen today occurred during the Wisconsin glacial stage, which reached its maximum only 15,000- 20,000 years ago.
For Mount Rainier National Park, the terrestrial history begins in the Tertiary Period of the Cenozoic Era in the Eocene Epoch, 36.6- 52 Ma.
Tertiary Period: Eocene Epoch
On the western margin of North America, a complex tectonic framework created a complex sedimentary assemblage. In Oregon, Eocene deep- sea sedimentary fans developed west of an accreted and subsiding seamount chain in a new fore- arc basin (the Tyee forearc basin in Figure 6). The basin developed in the Coast Range, west of the older Mesozoic fore- arc basin of central Oregon (Miller et al., 1992). In the Klamath Mountains, Hornbrook Basin area, and northern part of the Great Valley fore- arc basin (California), regional uplift during the Paleocene to early Eocene caused nonmarine Eocene strata to unconformably overlie Cretaceous marine rocks. Sedimentation continued in the Great Valley fore- arc basin.
Middle and upper Eocene submarine sedimentary fans and continental- slope deposits that graded eastward into shallow-marine, deltaic, and fluvial equivalents buried newly accreted, subsiding terranes in the Pacific Northwest from about 55- 43 Ma (Christiansen and Yeats, 1992). Pull- apart basins created by oblique subduction on the northwest flank of the North Cascades were filled with thick nonmarine arkosic sandstones and conglomerates. From the North Cascades to central Idaho, extensional faulting juxtaposed upper- crustal sections against mid- crustal, metamorphic rocks in relatively continuous metamorphic core complexes. These metamorphic core complexes formed domes between which graben- like basins formed and filled with thick sediments and contemporaneous volcanics.
In latest Eocene (about 43 Ma), the coastal region rotated clockwise at a rate of one degree per million years. Lakes and alluvial basins occupied the area east of the continental divide while the Cascade Arc, an arcuate 2 6 NPS Geologic Resources Division trend of offshore volcanoes, extended from British Columbia southward into southern Oregon. South of the arc, most of California was either eroding upland or coastal plain that bordered a continental shelf and slope to the west.
Tertiary Period: Oligocene Epoch
The most explosive volcanoes were underwater where percolating water triggered steam- blasted eruptions. Andesites, rhyolites, and volcanic breccia are well exposed in the eastern part of the park where, locally, layer upon layer of volcanic debris may reach over 10,000 feet (3,000 m) thick (Kiver and Harris, 1999). These layers are exposed in highway road cuts on the east side of Backbone Ridge, and remnants of volcanic centers are well exposed in the steep cliffs below the Sarvent Glaciers, just east of Mount Rainier. Volcanic vents, now plugged with masses of solidified lava, are visible at the South Cowlitz Chimney, Double Peak, and Barrier Peak.
After the volcanic activity subsided, the Ohanapecosh Formation was compressed into broad folds. Zeolites (a group of hydrous aluminosilicate minerals common as replacement minerals in volcanic rocks) and other minerals replaced most of the original minerals, and these new minerals firmly cemented the fragmental debris into hard rocks. They also imparted the dark gray and green colors typical of Ohanapecosh deposits. Uplift of the entire area followed. In the warm, wet climate, streams and rivers carved deep valleys into this hilly terrain. Valleys as deep as 1,500 feet (457 m) were eroded into the underlying Ohanapecosh Formation.
Stevens Ridge Formation: During the Oligocene and Miocene (Figure 2), thin layers of pumice and ash were deposited over the hilly terrain. These layers are the initial deposits of the Stevens Ridge Formation (Figure 7). The first thin ash falls had little affect on the many trees growing on the hills, but that was about to change. The most catastrophic event ever to befall the area blasted a series of searing hot ash flows over the area and smothered the former landscape. The ash flows consisted of mixtures of volcanic dust, bits of pumice, and other hot volcanic fragments buoyed up and greatly mobilized by hot volcanic gases that expanded and lubricated the flows so that flow velocities may have reached 60 or 80 miles per hour (97- 129 km/hr). Flowing into the valleys carved in the underlying Ohanapecosh Formation, the first ash flows choked streams, disturbed the soil, and killed trees that stood in their path. Subsequent ash flows eventually filled the valleys with deposits hundreds of feet thick and completely covered the pre- existing hills. When the hot volcanic ash was deposited, the heat fused the small bits of volcanic glass and pumice together into a rock known as welded tuff.
Only remnants of the Stevens Ridge ash flows survive in the park. Highway road cuts near the top of Backbone Ridge contain pieces of wood, bits of soil, and other debris chaotically mixed into the lower part of the basal Stevens Ridge ash flow. This ash flow can be traced northwestward to the lower slopes of Stevens Ridge where it is visible along the Stevens Canyon highway. Other ash flows are well exposed in the cliffs on the southern slope of Stevens Ridge.
Tertiary Period: Miocene Epoch
Although the volcanoes have been dissected and mostly removed by erosion, feeder dikes to the volcanoes still remain. The dikes can be traced for about a mile along the surface and vary from 6 inches (15 cm) to 20 feet (6 m) thick. Swarms of dikes that cut the Stevens Ridge and Ohanapecosh Formations may be found on Backbone Ridge and on Stevens Peak, Mount Wow, and in the headwaters of the North Fork of the Puyallup River.
Lava also was forcefully injected between underground layers of Ohanapecosh and Stevens Ridge strata, which, when cooled, formed tabular bodies known as sills. These sills can be found in the low country along the Muddy Fork of the Cowlitz River and near Longmire. The Box Canyon of the Cowlitz slices into one of these thick Fifes Peak Formation sills.
Following the eruptions of Fifes Peak lava, the area was once again compressed and folded. The folds that formed after deposition of the Ohanapecosh Formation became tighter. Faults broke the strata of the Ohanapecosh, Stevens Ridge, and Fifes Peak Formations as the rocks shifted to relieve the compressive stresses. For example, near vertical movement caused 910 meters (3,000 feet) of displacement between the base of the Stevens Ridge Formation and the Stevens Ridge Formation strata in Stevens Canyon. Continued plate collisions on the western margin caused this folding and faulting and set the stage for the second major episode in the geologic history of MORA – the emplacement of the Tatoosh pluton.
Granitic Intrusive Rocks of Miocene and Pliocene Age
Tatoosh Pluton and Associated Intrusives: About 9 Ma (late Miocene), the rate of collision between lithospheric plates slowed and the angle of descent of the subducting plate steepened, like it is today (Figure 9). In the Miocene and Pliocene, a great upward surge of molten rock stopped short of the surface and intruded the rocks of the Ohanapecosh, Stevens Ridge, and Fifes Peak Formations, solidifying as intrusive igneous bodies, called plutons (Figures 7, 10) (Fiske et al., 1963, 1988). Dikes and sills riddle the bedded formations. Some of these intrusive bodies are large enough to be mapped; some are not. Some of the magma erupted onto the surface although all but the welded tuff at The Palisades has been eroded from the park.
Igneous activity such as eruption of the lava flow at Bee Flat occurred sporadically throughout the rest of the Pliocene time. But uplift and erosion were the dominant processes at the time, and these processes developed the unconformity that separates the Pliocene from the Pleistocene (Map Unit Properties Table). Over time, the thin roof of older rocks was partly eroded away to expose the top of a large and complex granitic pluton. Some of the best exposures are found in the rugged cirques and peaks of the Tatoosh Range, from which the pluton is named (Appendix A). The granodiorite of the Tatoosh pluton is also exposed in the Carbon and White River valleys and part of the upper Nisqually River valley. Mapping reveals that the Tatoosh pluton completely underlies Mount Rainier and forms a platform upon which the volcano grew (Figure 10) (Fiske et al., 1988).
Quaternary Period: Pleistocene Epoch
At the time of the great Pleistocene Ice Age, the Tatoosh Mountains were undergoing glaciation in a very cold climate. A series of relatively fluid andesitic lava flows early in the Pleistocene flowed onto the rugged terrain carved into the mountain (Figure 7; Appendix A). Intracanyon lava flows filled ancient canyons to depths of 2.000 feet (610 m) and traveled up to 15 miles (24 km) from the young volcano (Fiske et al., 1988). Evidence of mudflow and glacial deposits interlayered with the lava flows indicates that glaciation was ongoing throughout this time.
These intra- canyon flows now form ridges or flat benches far above today’s canyons. Originally, the flows filled the canyons, but the lava was more resistant to erosion than the surrounding strata. The vigorous erosion by rivers and glaciers preferentially cut the weaker rocks, excavating far below the floors of the old filled canyons.
Good exposures of old intra- canyon flows can be seen at Burroughs Mountain and the flat surface of Yakima Park where a stream of lava flowed about 7 miles (11 km) down the canyon of the ancestral White River. The end of this flow is exposed in the highway road cuts below Sunrise Ridge. The present- day canyon cut by the White River lies to the south side of the intra- canyon flow. The thick lava flow that underlies Rampart Ridge, just west of Longmire, is another example of a ridge that was once a canyon. Originally, the flow poured into the upper canyon of the Nisqually River. The ridges of old intracanyon flows stand in stark contrast to the younger flows that now lie at the bottom of a few present- day canyons.
The projecting fingers of intra- canyon lava lie in the shadows of the main cone of Mount Rainier. The cone is built from hundreds of thin lava flows, breccia deposits, and debris and mudflows. The cone began to be built from short streams of lava interspersed with ejections of breccia and ash. About 75,000 years ago, the volcano reached its greatest height of perhaps 15,500 to 16,000 feet (4,700 to 4,900 m) (Harris et al., 1995). Dikes radiated from the center of the volcano, like spokes in a wheel, and a plug of solidified magma filled the central vent.
Quaternary Period: Holocene (Recent) Epoch
About 5,000 years ago, the weak clay zones, steep slopes, high elevation, and abundant water and ice combined with a volcanic eruption to produce the Osceola Mudflow, one of Earth’s largest known mudflows. The upper 3,000 feet (910 m) of Mount Rainier collapsed as distinct blocks of rocks quickly disintegrated into a giant debris avalanche, and mixing with abundant water and clay particles, transformed into a wall of mud cascading down the White River drainage. Thinning away from the mountain, the Osceola Mudflow deposited 100 feet (3o m) of mud near the present site of Enumclaw, 35 miles (56 km) downstream (Figure 4). Traveling at about 40 miles per hour (64 km/hr), the mudflow buried the site of an Indian village and eventually poured into Puget 2 8 NPS Geologic Resources Division Sound, 75 miles (120 km) from the mountain (Kiver and Harris, 1999).
Mount Rainier was built from material erupting from a central vent. A plug of solidified lava now fills this vent and is exposed in the precipitous east wall of Sunset Amphitheater (Appendix A). Two small plugs on the northwestern flank of Mount Rainier denote satellite volcanoes and three thick radial dikes extending outward from the summit area mark locations where lava erupted and flowed onto the lower slopes of the volcano. Two prominent dikes can be seen at Puyallup Cleaver and St. Elmo Pass where they stand as resistant ribs above the rocks they intrude.
Although Mount Rainier is the second highest peak in the conterminous United States, the mountain’s peak was once considerably higher than it’s 14,410 feet (4,390 m) today. The truncated lava flows in the upper part of the mountain slant upward to a former summit that was at least 100 feet (30 m) higher than the present summit (Fiske et al., 1988). Three hypotheses have been proposed as reasons for the summit’s removal. Some geologists believe the pinnacle of Mount Rainier was removed by a violent explosive eruption, but fragments from this eruption have not been found. Others believe that avalanches and glacial erosion rapidly ate away the upper part of the mountain. Still others see the former summit collapsing into the central vent when the column of lava temporarily subsided.
The largest rockfalls on Mount Rainier in historic time occurred in December 1963, when a series of rockfalls, hundreds of feet across, fell from the steep north face of Little Tahoma Peak onto Emmons Glacier (Crandell, 1969A, B; Kiver and Harris, 1999). When the rock hit the glacier, it shattered into dust and countless fragments and then avalanched down the steep ice surface at a tremendous speed. At the end of the glacier, the rock debris traveled toward the valley floor over a layer of compressed air that reduced the friction and permitted the avalanche to move almost 2 miles (3 km) beyond the end of the glacier. This avalanche passed over a small wooden gage house about 5 feet (1.5 m) high without damaging it and then slammed into the north base of Goat Island Mountain. The force of the avalanche scraped away trees and bushes in the path of the flow. A later avalanche stopped just short of the gage house, but unfortunately, when the trapped wind was expelled from beneath the rock debris, the gage house was destroyed. At least seven rockfalls and avalanches from Little Tahoma Peak occurred in a matter of minutes or hours and some came to rest only a short distance up- valley from the White River Campground.
Today, fumaroles and occasional steam explosions on the flanks of Mount Rainier indicate continued heat flow, and seismic tremors suggest that future volcanic eruptions are possible.
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