America's Volcanic Past
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
MORE America's Volcanic Past - Washington State
Visit A Volcano - Mount Rainier National Park
Volcanic Highlights and Features:
| [NOTE: This list is just a sample of various Mount Rainier features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Mount Rainier.] |
Mount Rainier is an active
volcano that first erupted about half a million years ago.
Because of Rainier's great height (14,410 feet
above sea level) and northerly location,
glaciers have cut deeply into its lavas, making it appear
deceptively older than it actually is.
Mount Rainier is known to have erupted as recently as in the
1840s, and large eruptions took place
as recently as about 1,000 and 2,300 years ago. Mount Rainier
and other similar volcanoes in the
Cascade Range, such as Mount Adams and Mount Baker, erupt
much less frequently than the more
familiar Hawaiian volcanoes, but their eruptions are vastly more
destructive. Hot lava and rock debris from Rainier's eruptions have melted snow and glacier ice and
triggered debris flows (mudflows) -
with a consistency of churning wet concrete - that have swept
down all of the river valleys that head on the volcano.
Debris flows have also formed by collapse of
unstable parts of the volcano
without accompanying eruptions. Some debris flows have traveled as far
as the present margin of Puget Sound,
and much of the lowland to the east of Tacoma and the south of
Seattle is formed of pre-historic debris from Mount Rainier.
Excerpt from: Sisson, 1995, History and Hazards of Mount Rainier, Washington: USGS Open-File Report 95-642 |
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| Mount Rainier National Park |
Columbia Crest:1
The summit lava
cone is most clearly recognized from the northeast (Sunrise), where it floors the large Emmons and Winthrop Glaciers that slope
smoothly up to Rainier's summit. Broad lobes on the glaciers' surfaces show the locations of the youngest lavas from Rainier's
summit, now buried under hundreds of feet of ice. The summit itself is formed of two small overlapping craters, each about a
quarter mile in diameter; the younger of these forms a nearly perfect circle of radially-outward-sloping lavas. The shallow floors
of these craters are filled with snow and ice, but the raised rims are snow-free year-round because of high winds and because
much of the ground is still hot. Steam or warm mist, at or just below boiling temperature, rises from the crater rims in many areas
and has melted an intricate system of caves into the base of the crater-filling ice. On calm days, a faint odor of sulfur can also be
smelled. The hot ground, steam, and sulfur smell, as well as the little-eroded shape of the summit craters attest to Rainier's recent
activity.
Columbia Crest:5
Today there are three distinct summits, or high points,
at the top of Mount Rainier. The lower
two, Liberty Cap and Point Success,
are remnants of the sides of an old, higher cone. The
third and highest summit, Columbia Crest (14,410 feet)
lies in the rim of a small recent lava
cone. This cone is indented by two craters,
the larger of which is about 1/4 mile in diameter.
Both craters are nearly filled with snow and ice,
into which a system of tunnels and caves are
melted by volcanic heat and steam.
Columbia Crest and "Layer C":4
"Layer C", which is approximately 60% of the post-glacial tephras by volume, is also the most widespread,
covering much of the eastern half of Mount Rainier National Park with 2-30 centimeters of lapilli, blocks, and bombs. It is also the coarsest of the
Rainier tephras; 25-30 centimeter bombs can be found 8 kilometers to the east of the summit. Vertical and horizontal changes in scoria,
lithic fragments and pumice, and lateral variations of "Layer C" indicate it was deposited by more than one event, or in a single, extended eruption
during a change in wind direction. Isopachs and isopleths for this layer indicate an origin at the summit of Mount Rainier; however, it does not occur
on snow-free parts of Columbia Crest cone. Apparently this 250-meter-high summit cone is younger than the 2,200-year age of "Layer C".
Observation Rock and Echo Rock:2
Observation Rock and Echo Rock are dissected satellitic volcanoes,
which erupted olivine andesite
upon Mount Rainier's northwestern flank late in its history.
Liberty Cap:5
Today there are three distinct summits, or high points,
at the top of Mount Rainier. The lower
two, Liberty Cap and Point Success,
are remnants of the sides of an old, higher cone. The
third and highest summit, Columbia Crest (14,410 feet)
lies in the rim of a small recent lava
cone. This cone is indented by two craters,
the larger of which is about 1/4 mile in diameter.
Both craters are nearly filled with snow and ice,
into which a system of tunnels and caves are
melted by volcanic heat and steam.
Osceola Mudflow:3
The largest lahar originating at Mount Rainier in the last 10,000 years is known as the Osceola Mudflow. This cohesive
lahar, which occurred about 5600 years ago, was at least 10 times larger than any other known lahar from Mount Rainier. It was the
product of a large debris avalanche composed mostly of hydrothermally-altered material, and may have been triggered as magma
forced its way into the volcano. Osceola deposits cover an area of about 550 square kilometers (212 square miles) in the Puget
Sound lowland, extending at least as far as the Seattle suburb of Kent, and to Commencement Bay, now the site of the Port of
Tacoma. The communities of Orting, Buckley, Sumner, Puyallup, Enumclaw, and Auburn are also wholly or partly located on top of
deposits of the Osceola Mudflow and, in some cases, of more recent debris flows as well.
Point Success:5
Today there are three distinct summits, or high points,
at the top of Mount Rainier. The lower
two, Liberty Cap and Point Success,
are remnants of the sides of an old, higher cone. The
third and highest summit, Columbia Crest (14,410 feet)
lies in the rim of a small recent lava
cone. This cone is indented by two craters,
the larger of which is about 1/4 mile in diameter.
Both craters are nearly filled with snow and ice,
into which a system of tunnels and caves are
melted by volcanic heat and steam.
The Palisades:2
At the northwestern corner of White River Park a great
cliff of columnar jointed black rock, known as The Palisades, rises
abruptly from the headwaters of Lost Creek.
The rock in the cliff is a rhyodacite welded tuff, as much as 800 feet thick,
which rests upon and grades into an underlying plug of rhyodacite.
The welded tuff extends south from The Palisades, forming
an irregular-topped plateau about a mile long and half a mile wide,
rimmed by striking cliffs, 50 to 500 feet high, carved
from joint columns more than 100 feet long.
Ptarmigan Ridge:2
Thick intracanyon flows, the earliest Mount Rainier lavas exposed, provide striking evidence of the ruggedness of that
surface. Some flows are banked against ancient canyon walls as precipitous as any of those one can see in the park today.
Others filled canyons to depths of 2,000 feet, but the original walls of these canyons have long since been removed by
erosion. One immense flow, making up the bulk of Ptarmigan Ridge, is at least 1,200 feet thick. Such thick flows could
not have spread upon a surface of low relief; they were confined in canyons.
Willis Wall, Sunset Amphitheater, and the headwall above South Tahoma
Glacier:2
Evidence of vigorous glacial erosion has been recognized. Impressive too, are the great cirques, whose steeply slanting
headwalls truncate scores of (lava) flows.
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