Crater Lake Geology

1
Crater Lake Geology

• and the Mt. Mazama Story

2
Crater Lake Cascade Volcanic Arc

• Crater Lake is part of the Cascade Volcanic Arc
  that runs roughly N-S from Northern California up
  into British Columbia.
• The Cascade Volcanic arc is produced by the
  subduction of several oceanic plates.

http//en.wikipedia.org/wiki/Cascade_Volcanoes
3
Cascade Volcanic Arc Subduction

• The Cascade Arc is formed as the subducting
  oceanic plate moves deeper into the mantle,
  breaking down water-bearing minerals and relasing
  that water into the mantle wedge above the
  subducting plate.
• This water causes the mantle wedge to partially
  melt. The resulting basalt-basaltic andesite
  magma is less dense than the surrounding mantle
  (peridotite) and rises slowly until it either
  cools underground or reaches the surface as lava.
  

4
Crater Lake From Mt. Mazama

• 7,700 years ago Crater Lake was known as Mt.
  Mazama, a broad stratovolcano much like Mt.
  Rainier appears today.

Above The cataclysmic eruption of Mount Mazama
7,700 years ago, as depicted in this painting by
Paul Rockwood (image courtesy of Crater Lake
Natural History Association). Left Mt. Rainier,
WA courtesy of USGS http//vulcan.wr.usgs.gov/Volc
anoes/Rainier/Locale/framework.html
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Growth of Mt. Mazama

• Mt. Mazama first began being built 400,000 years
  ago as the overlapping of several stratovolcanoes
  and shield volcanoes.

StratovolcanoVolcano composed of alternating
layers, of lava and pyroclastic flows.
Shield Volcano Large volcanic structure with
gentle slopes built up almost entirely from fluid
lava flows.
http//www.fun-costa-rica-vacations.com/volcano-vo
cabulary.html
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Mt. MazamaActivity!

• Mount Mazama lay at the intersection of two fault
  systems, which served as conduits for rising
  magmas

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Mt. Mazama and Glacial Activity

• While Mt. Mazama was growing 10,000 years ago,
  glaciers were actively shaping the volcanic
  landscape.
• Today, evidence of this glacial history is seen
  in the presence of U-shaped valleys and glacial
  striations.

http//www.shannontech.com/ParkVision/CraterLake/C
raterLake7.html
http//education.usgs.gov/schoolyard/CoolGeologyAc
tivity.html
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Becoming Crater Lake

• 1. Eruptions of ash and pumice The cataclysmic
  eruption started from a vent on the northeast
  side of the volcano as a towering column of ash,
  with pyroclastic flows spreading to the
  northeast.
• 2. Caldera collapse As more magma was erupted,
  cracks opened up around the summit, which began
  to collapse. Fountains of pumice and ash
  surrounded the collapsing summit, and pyroclastic
  flows raced down all sides of the volcano.
• 3. Steam explosions When the dust had settled,
  the new caldera was 5 miles (8 km) in diameter
  and 1 mile (1.6 km) deep. Ground water interacted
  with hot deposits causing explosions of steam and
  ash.
• 4. Today In the first few hundred years after
  the eruption, renewed eruptions built Wizard
  Island, Merriam Cone, and the central platform.
  Water filled the new caldera to form the deepest
  lake in the United States.

Figures modified from diagrams on back of 1988
USGS map Crater Lake National Park and Vicinity,
Oregon. http//pubs.usgs.gov/fs/2002/fs092-02/
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Crater Lake Today

• The last eruption at Crater lake was a small
  dacite dome which formed under lake level
  adjacent to the Wizard Island Platform, 4800
  years ago.

Taken by Sara Auer Perry
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Rock Types of Crater Lake
Name SiO2

Rhyolite 70 or more
Dacite Approximately 65
Andesite Approximately 60
Basaltic-Andesite Approximately 55
Basalt 50 or less

http//volcanoes.usgs.gov/images/pglossary/VolRock
s.php
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Crater Lake Rock Characteristics

• Magma with high SiO2 content contains more
  dissolved gas and is more viscous (less mobile)
  than those of basaltic composition.
• High silicon-oxygen magmas will tend to be more
  explosive than those with a lower percentage.

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Crater Lake Geologic Map
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Pyroclastic Ejecta (airborne material)

• Tephra is the general term now used by
  volcanologists for airborne volcanic ejecta of
  any size.
• Pumice pale clasts composed mostly of vesicular
  glass which have a roughly similar composition to
  rhyolite.
• Scoria darker clasts composed mostly of
  vesicular glass which have a roughly similar
  composition to basalt.
• Vesicle A small cavity in a glassy igneous rock
  that is formed when bubbles of gas or steam
  expand during the cooling and solidification of
  the rock itself.

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Tuff (welded and non)

• Consolidated volcanic ash (particles of glass)
  pumice ejected from vents during a volcanic
  eruption.
• Welded tuff is a pyroclastic rock, of any origin,
  that was sufficiently hot at the time of
  deposition for the particle of volcanic ash to
  become fused together (note the deformation of
  the pumice).

Above http//www.earth.ox.ac.uk/oesis/rocks/ign7
.html Lefthttp//www.mnh.si.edu/earth/text/dynami
cearth/6_0_0_GeoGallery/geogallery_specimen.cfm?Sp
ecimenID2055categoryID4categoryNameRocksbrow
seTypegroupgroupID5groupNameIgneous
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Pyroclastic Flow

• A ground-hugging avalanche of hot ash, pumice,
  rock fragments, volcanic gas that rushes down
  the side of a volcano up to 100 km/hr.
• The temperature within a pyroclastic flow may be
  gt500 C, sufficient to burn carbonize wood.
• Once deposited, the ash, pumice, and rock
  fragments may deform (flatten) and weld together
  because of the intense heat and the weight of the
  overlying material.

http//www.cnsm.csulb.edu/departments/geology/peop
le/bperry/IgneousRocksTour/VolcanoesAndLavaFlows.h
tml
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Dike vs. Sill

• Dikes are tabular or sheet-like bodies of magma
  that cut through and across the layering of
  adjacent rocks. They form when magma rises into
  an existing fracture, or creates a new crack by
  forcing its way through existing rock, and then
  solidifies.
• A sill is an intrusive body of magma that pushes
  its way between layers of sediments.
• Pics from http//www.answersincreation.org/curricu
  lum/geology/images/Dike_Cross-Island_Trail_Alaska.
  jpg, http//en.wikipedia.org/wiki/Sill_28geology
  29 respectively

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Sites Phantom Ship

• Phantom Ship consists partly of a dike from the
  Phantom Cone that has been exposed by erosion and
  projects above the lake surface on the southern
  side of the caldera.
• http//www.siskiyous.edu/class/geol66/mazamaguide.
  pdf

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Sites Pumice Castle

• Pumice Castle (ribbed structure on the right,
  just above the trees) is a formation exposed on
  the southeastern wall of the caldera.
• It is composed of welded and non-welded ash-flow
  tuff layers that were erupted 50,000 to 60,000
  years ago during the growth of Mount Mazama.

• http//www.siskiyous.edu/class/geol66/mazamaguide.
  pdf

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Sites Wineglass Tuff

• During the eruption of Mt. Mazama the giant
  column of airborne ash gasses collapsed and
  generated pyroclastic flows. These flows made the
  Wineglass Welded Tuff, seen right.
• The Wineglass is composed of ash (glass
  particles) which have been fused or welded
  together by the hot temperatures of that and
  subsequent pyroclastic flows.

Top http//www.shannontech.com/ParkVision/CraterL
ake/CraterLake6.html Insethttp//volcano.oregonst
ate.edu/vwdocs/volc_images/north_america/crater_la
ke.html
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Sites Devils Backbone

• The Devils Backbone is a vertical wall of dark
  andesite lining the cliff face and measuring
  about 1,000 feet long by 50 feet across near the
  top.
• A dike formed by molten lava that created and
  filled cracks, as it forced its way up through
  the rock and then solidified. It has been left
  standing by the erosion of the surrounding
  material.

http//www.dartmouth.edu/volcano/images/DCLp08.gi
f
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Summary

• Crater Lake is a member of the Cascade Volcanic
  Arc, formed due to subduction of oceanic plates
  that is still occurring today.
• Mt. Mazama was a stratovolcano that erupted 7700
  years ago to produce a caldera that is now known
  as Crater Lake.
• Before its cataclysmic eruption, Mt. Mazama was
  heavily glaciated.
• Rocks at Crater Lake range from basalt to
  rhyolite.
• Key features that we will see are dikes,
  pyroclastic flow deposits, glacial striations,
  etc.

http//www.jvphotography.net/panoramas/panoramas2.
htm