The Geology of Mt' Rainier

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The Geology of Mt. Rainier

• Sarah Holcomb
• Geology 200
• Lecture Final

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History

• Captain George Vancouver sailed into Puget Sound
  in 1792. He "discovered" a huge mountain that
  was thus named in honor of his friend, the Rear
  Admiral Peter Rainier. This mountain was (and
  still is) a living volcano that was already known
  and valued by the Nisqually, Cowlitz, Yakama,
  Puyallup, and Muckleshoot native peoples. As the
  inhabitants of the river valleys surrounding the
  mountain, they hunted and gathered in the land
  under the shadow of the great mountain. For them,
  the mountain was a powerful entity and presence
  separate from the resources it provided. Those
  who followed the Captain also came to appreciate
  both physical and spiritual presence of Rainiers
  natural resources. Mount Rainier is a place where
  the physical things realm of rocks, glaciers,
  water, plants, and animals present a higher
  meaning challenge, strength, renewal, beauty,
  and enjoyment.

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Statistics

• Northern Cascade Mountains, Washington
• Stratovolcano
• 14,410 Feet
• Latitude 46.87 N
• Longitude 121.758 W
• Mainly andesitic rock types
• Most recent eruption 2,200 years ago

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Pre-Rainier

• There are no solid geological remains of a
  proto-Rainier due to an 11 million year hiatus in
  volcanic activity. However, petrographical
  recordings indicate that during its initial
  growth it was one of the most explosive mountains
  in the Cascade Mountain Range.

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Pre-Rainier

• Earliest signs of a proto-Rainier lie to the west
  of the present mountain
• Lily Formation 2.9 million to 840,000 years ago
• Thick sequence of volcanic debris thought to be
  the earliest Mt Rainier deposit

Andesite under a microscope and raw
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Formation

• Underlined by middle-Tertiary volcanic rocks
• Gently warped along a NW-trending system of folds
• Intruded by the Tatoosh Pluton
• Made up of Granodiorite and Quartz Monzonite
• Intrusion occurred 17.5 to 14.1 Million years ago

Quartz Monzonite and Granodiorite
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Formation Original Dome

• Early lava flows formed a small shield on the
  craggy Tertiary basement rock
• Features dominantly lava flows and breccias from
  vents created by plutonism

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Formation Main Cone

• Built up on the smaller original dome
• On rugged land with 700 m of relief eroded
• Erosion mainly in the Tatoosh Pluton and the
  Stevens Ridge Formation
• Early lava flows from Mt Rainier were channeled
  along deep canyons that radiated out from the
  mountain

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Formation Modern Cone

• The flows stacked up to form a mound near the
  main vent (laying the foundation for our modern
  cone )
• Most of the main cone was built up by hundreds of
  layers thin lava flows interbedded with breccia
  and minor tephra from the eruptions

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Modern Cone Cont.

• Radial dikes have been linked to being secondary
  sources for the cone forming flows
• Two satellite formations on the Northwest flank
  erupted in the late Pleistocene
• Echo Rock and Observation Rock
• About 270 cubic kilometers of lava has erupted
  from Mt Rainier in the past 1 million years

Echo Rock and Observation Rock
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Modern Cone Cont.(again)

• Thick pumice layers NE, E, and SE of Mt Rainier
  are interpreted to have erupted between 70,000
  and 30,000 years ago
• The unnamed layers are an Order of Magnitude
  larger than any Holocene tephra layers
• Order of Magnitude class of magnitude of any
  amount where each class contains values of a
  fixed ratio to the class preceding it (most
  commonly in ratios of 10)

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Glaciation

• The majority of Mt Rainier was completely formed
  by the beginning of the last ice age roughly
  25,000 years ago
• From about 25,000 to about 15,000 years ago,
  glaciers extended from the mountain to the
  perimeter of the present Puget Sound Basin.
• Because of the mountains steep slopes, the
  glaciers were able to carve out deep grooves from
  the cone and surrounding areas

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Lahars

• Lahar a flow of pyroclastic material mixed with
  water.
• Due to the eruptions causing volcanic growth and
  Mt Rainiers steep sides, lahars of great
  magnitude frequently flowed down the mountain,
  some even reaching the present Puget Sound

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Modern Glaciation

• Mt Rainier currently holds 26 major glaciers on
  its slopes
• Glaciers cover 36 square miles of the mountains
  surface
• The glacier also have a volume of roughly one
  cubic mile

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Present Day Mt Rainier

• Holocene explosive eruptions at Mt Rainier are
  dominated by lahars
• Over 60 have been identified
• Some have been notably eruption induced (ex.
  Paradise Flow)

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Modern Activity

• The height of Mt Rainier has been eroded by a
  series of eruptions, collapses, and lahars
• Hydrothermal Alteration affects the stability of
  the rock layers
• The single most catastrophic event in recent
  history was 5,600 years ago the Osceola Mudflow
  (known as on of the largest in the world)
• Mt Rainier lost 1,000 feet of elevation
• Travelled down both of the forks of the White
  River, as far as the Puget Sound

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Modern Activity Cont.

• The most recent eruption of Mt Rainier was 2,200
  years ago
• Covered the eastern half of present day Mt
  Rainier National Park with up to 1 foot of
  lapilli, bombs, and blocks
• Geologists estimate that Mt Rainier is about
  2,000 feet shorter today than is was at the peak
  of its growth
• Due to the rate of erosion exceeding the rate of
  growth

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Modern Hazards

• Due to Mt Rainiers steep slopes and the amount
  of people that live in the river valleys around
  its base, it is known as one of the most
  dangerous modern volcanoes in the world
• Avalanches
• Lahars
• Floods
• Landslides

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Modern Hazards Cont.

• Mt Rainier is one of the 16 volcanoes worldwide
  known as a Decade Volcano, which are worthy of
  particular study in light of their history of
  large, destructive eruptions and proximity to
  highly populated areas
• Studies are conducted by the International
  Association of Volcanology and Chemistry of the
  Earth's Interior (IAVCEI)
• Explosive eruptions
• Lava flows
• Tephra fallout
• Pyroclastic flows

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Bibliography

• www.emporia.edu/earthsci/student/zellers1/rainier.
  htm
• www.nps.gov/archive/mora/ncrd/glaciers.htm
• vulcan.wr,usgs.gov/Volcanoes/Rainier/description_r
  ainier.html
• en.wikipedia.org/wiki/Ice_Age
• en.wikipedia.org/wiki/Decade_Volcano