Geology of Mount Rainier – Kirk Schleiffarth

The rugged and heavily glaciated Cascade Range stretches from northern California through Oregon, Washington and into southern British Columbia.  The Cascades are the result of millions of years of accretion, uplift, and arc volcanism.  Lassen Peak and Mt. St. Helens were the only volcanoes to erupt during the 20th century with major eruptions in 1915 and 1980, respectively.  Both eruptions produced immense ash columns, devastating pyroclastic density currents and lahars (volcanic mudflows).  Fortunately both eruptions took place far from any major population centers.  There are other volcanoes in the range however, that pose a major threat to the ever-growing population of the Pacific Northwest; namely, Mt. Rainier.
Three hikers make their way around Mt. Rainier by way of the 93-mile Wonderland Trail

Mt. Rainier is not only the highest mountain in the Cascade Range and the most prominent and most glaciated peak in the contiguous United States, but is considered one of the most dangerous volcanoes in the world.  This isn’t because Mt. Rainier will necessarily produce a larger eruption than other volcanoes, but because of its proximity to a densely populated area.  Previous eruptions left lahar deposits in the Puget Sound lowlands near Tacoma and spread tephra (ash and pumice) over much of Washington State.   

I have spent a lot of time on and around Mt. Rainier.  My first time at Mt. Rainier was in the fall of 2011 on a climbing trip with a couple of overconfident buddies; we spent an entire day wandering around on a heavily crevassed glacier in whiteout conditions at 12,000 ft.  In March 2012, I tried again via a more technical route in even worse conditions; we turned around due to 80 mph winds and low visibility. Finally, in May 2012, we made it happen and successfully climbed Mt. Rainier via the Ingraham Glacier route in perfect conditions.  I went back in August to attempt a 48-hour madman hike around Mt. Rainier via the Wonderland Trail (93 miles).  We made it 80 miles before we collapsed.  The following year in May 2013, I successfully climbed the volcano again, but this time I brought my skis and enjoyed the ride down.  In September 2013, my friend convinced me to try to the circumnavigation again; we gave up after a long 45-mile, 15,000 ft. elevation gain day.  Needless to say, I have a very personal relationship with Mt. Rainier and it became very clear that the volcano is active and ever evolving.
Little Tahoma Peak (left of Mount Rainier) is an eroded remnant of an ancient lava flow on Mt. Rainier

Climber’s make their way up Mt. Rainier with a backdrop of the Tatoosh Mountains and Mt. Adams
The volcano has a long and complex history of growth and erosion.  Thunderous rockslides and avalanches, glaciers slowly carving the mountain and bulldozing rock downhill, mud- and silt-saturated glacial streams, and hot, sulfur-rich fumaroles spewing from the summit crater were a few of the geologic processes I witnessed while on the mountain.  Putting the geologic history of any volcano in the correct sequence can be complicated.  The clues to its past however, are obvious if you know where to look.  Many of the rocks at Mt. Rainier are visibly hydrothermally altered, or chemically weakened by hot groundwater.  As a result, landslides have played a major role in the shape and geomorphology of the volcano and the deposits downstream.  In fact, much of the northern and western flanks of the mountain slid away in catastrophic landslides, which transitioned to dense mudflows that traveled up to 70 miles away from the volcano in previous centuries.  Many of the suburbs of Tacoma and Seattle sit atop these old mudflow deposits.  Landslides and mudflows are amongst the most dangerous hazards associated with Mt. Rainier because they can occur without an eruption and offer no warning to scientists that constantly monitor the volcano for signs of unrest.  Many people in the Seattle-Tacoma area are in harms way and will have no warning. 
Volcanic hazards associated with Mount Rainier (from the USGS)
Mt. Rainier from the White River drainage and the future location of a destructive lahar

In general however, most hazards associated with volcanoes are related to eruptions and usually offer good warning signs.  Prior to an eruption, magma must rise from depth into the volcano.  The magma, which is under significant pressure, breaks the rock and creates small earthquakes that are recorded by seismometers.  Additionally, high-precision GPS and tiltmeters located on the volcano can track any deformation that occurs as the magma causes the volcano to swell.  The rising magma will degas large amounts of SO2, CO2, and H2O.  Any significant change in gas concentrations, which are regularly monitored, could indicate magma is on the move.  If a major eruption were to occur, it would start showing signs of unrest weeks, if not months, in advance. 

Gibraltar Rock is an arete composed of altered andesitic lavas and pyroclastic surge deposits
The quiet giant at Reflection Lakes

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