The Osceola Mudflow, also known as the Osceola Lahar, was a

debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented Rock (geology), rock flow down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. ...

and

lahar A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock, pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a valley, river valley. Lahars are o ...

in the U.S. state of

Washington Washington most commonly refers to: * George Washington (1732–1799), the first president of the United States * Washington (state), a state in the Pacific Northwest of the United States * Washington, D.C., the capital of the United States ** A ...

that descended from the summit and northeast slope of

Mount Rainier Mount Rainier ( ), also known as Tahoma, is a large active stratovolcano in the Cascade Range of the Pacific Northwest in the United States. The mountain is located in Mount Rainier National Park about south-southeast of Seattle. With an off ...

, a volcano in the

Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...

during a period of eruptions about 5,600 years ago. It traveled down the west and main forks of the White River, passed the location of present-day Enumclaw then reached

Puget Sound Puget Sound ( ; ) is a complex estuary, estuarine system of interconnected Marine habitat, marine waterways and basins located on the northwest coast of the U.S. state of Washington (state), Washington. As a part of the Salish Sea, the sound ...

in several areas, including near the present day sites of

Tacoma Tacoma ( ) is the county seat of Pierce County, Washington, United States. A port city, it is situated along Washington's Puget Sound, southwest of Seattle, southwest of Bellevue, northeast of the state capital, Olympia, northwest of Mount ...

and Auburn. The Osceola flow began either as an avalanche or series of avalanches near the summit of Mount Rainier but had transformed to a lahar within of where it was initiated as it incorporated significant amounts of water from within the volcano's hydrothermal system. The sector collapse formed a wide horseshoe-shaped crater, open to the northeast, almost the same size as the crater produced by the

1980 eruption of Mount St. Helens In March 1980, a series of volcanic explosions and pyroclastic flows began at Mount St. Helens in Skamania County, Washington, United States. A series of Phreatic eruption, phreatic blasts occurred from the summit and escalated until a major ...

. Most of the Osceola crater has been filled in by subsequent lava eruptions, most recently about 2,200 years ago. With a volume of and an areal extent of about , the Osceola Mudflow buried a large portion of the Puget Sound lowland with hydrothermally altered volcanic material that is estimated to have been traveling at up to downstream from the source region. Many communities in

King King is a royal title given to a male monarch. A king is an Absolute monarchy, absolute monarch if he holds unrestricted Government, governmental power or exercises full sovereignty over a nation. Conversely, he is a Constitutional monarchy, ...

and Pierce

counties A county () is a geographic region of a country used for administrative or other purposesL. Brookes (ed.) '' Chambers Dictionary''. Edinburgh: Chambers Harrap Publishers Ltd, 2005. in some nations. The term is derived from the Old French denoti ...

, notably

Kent Kent is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Essex across the Thames Estuary to the north, the Strait of Dover to the south-east, East Sussex to the south-west, Surrey to the west, and Gr ...

, Enumclaw, Orting, Buckley,

Sumner Sumner may refer to: Places Antarctica * Mount Sumner, a mountain in the Rare Range, Antarctica * Sumner Glacier, southern Graham Land, Antarctica Australia * Sumner, Queensland, a suburb of Brisbane New Zealand * Sumner, New Zealand, a seasi ...

, Puyallup and Auburn, are wholly or partly located on top of Osceola Mudflow deposits which reach a depth of up to . The flow may have buried embayments of Puget Sound. The Osceola Mudflow was Mount Rainier's signature event during the

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

. It was named after the unincorporated community of

Osceola Osceola (1804 – January 30, 1838, Vsse Yvholv in Muscogee language, Creek, also spelled Asi-yahola), named Billy Powell at birth, was an influential leader of the Seminole people in Florida. His mother was Muscogee, and his great-grandfa ...

Description

The age of the mudflow has been determined by the

radiocarbon dating Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for Chronological dating, determining the age of an object containing organic material by using the properties of carbon-14, radiocarbon, a radioactive Isotop ...

of the wood found within the debris. Eight samples range in age from 4425 ± 310 to 5040 ± 150 yr B.P. The average of the eight ages is 4832 ± 43 yr B.P.. Corrected for changes in atmospheric

Carbon 14 Carbon-14, C-14, C or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic matter is the basis of the radiocarbon dating method pioneered by Willard Libby and colle ...

(14C), is between 5603 and 5491 yr B.P. From these samples the Mudflow is thus dated to about 5600 yr B.P. . Since 1898, geologists recognized that Mount Rainier was historically higher. In 1963, D.R. Crandell inferred that the “missing summit” had collapsed down the northeast side becoming the Osceola Mudflow. A semicircular amphitheater would have opened to the northeast. The depression then filled over time with ice (

Emmons Glacier Emmons Glacier is on the northeast flank of Mount Rainier, in Washington. At , it has the largest surface area of any glacier in the contiguous United States. The glacier was named after the geologist Samuel Franklin Emmons after his involvement ...

) and lava flows from a central vent. Russell Cliff, Liberty Cap, Point Success, and Disappointment Cleaver, surround this feature. Extrapolating the dipping lava beds upward, it has been estimated the height of Mount Rainier was about . This is similar to the depression in Mount St. Helens from the eruption of May 18, 1980. Using information from the Mount St. Helens crater the volume of Mount Rainier collapsed has been calculated to be An analysis of the clay and altered minerals like

smectite A smectite (; ; ) is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secon ...

kaolinite Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina () ...

halloysite Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are oxygen (55.78%), silicon (21.76%), aluminium (20.90%), and hydrogen (1.56%). It is a member of the kaolinite group. Halloysite typic ...

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

cristobalite Cristobalite ( ) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, Si O2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members o ...

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

, and

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

in Osceola deposits with the same minerals in outcrops on Mount Rainier show that the mudflow began as a huge avalanche from this area. The volume of the mudflow is estimated at from an examination of outcrops and well logs plus an estimation of the volume parcels from across the identified flow. The mudflow centers about from Mount Rainier. The Osceola volume of plus the Paradise lahar of is more than the of the “missing summit.” The material would have expanded during the

avalanche An avalanche is a rapid flow of snow down a Grade (slope), slope, such as a hill or mountain. Avalanches can be triggered spontaneously, by factors such as increased precipitation or snowpack weakening, or by external means such as humans, othe ...

, and it would have added materials from along the route, accounting for the difference. The Osceola debris underlies the Puyallup River valley and the Puyallup and Duwamish embayments of

. Osceola Mud has a depth of about (two-thirds below sea level) north of Auburn. Near Orting the flow is deep. Down valley, near

on the Auburn and Puyallup deltas, of the prehistoric Puget Sound, the mudflow is deep. Research shows that of Osceola debris spread underwater and covers 157 in prehistoric Puget Sound. The Osceola debris increased sedimentation after the Osceola Mudflow filled the Duwamish and Puyallup arms of Puget Sound.

References

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External links

Mount Rainier's Osceola Mudflow
Nick Zentner (2017) YouTube 1:03:52 Lahars Mount Rainier Geology of Washington (state) Landslides in the United States Prehistoric volcanic events Holocene Sector collapses