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Crater Lake Geology
and the Mt. Mazama Story
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
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.
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/Volcanoes/Rainier/Locale/framework.html
Growth of Mt. Mazama
 Mt.
Mazama first began being built
400,000+ years ago as the overlapping of
several stratovolcanoes and shield
volcanoes.
Shield Volcano: Large volcanic
structure with gentle slopes built up
almost entirely from fluid lava flows.
Stratovolcano:Volcano composed
of alternating layers, of lava and
pyroclastic flows.
http://www.fun-costa-rica-vacations.com/volcano-vocabulary.html
Mt. Mazama
Activity!
 Mount
Mazama
lay at the
intersection of
two fault
systems, which
served as
conduits for
rising magmas
Mt. Mazama and Glacial Activity
http://www.shannontech.com/ParkVision/CraterLake/CraterLake7.html
http://education.usgs.gov/schoolyard/CoolGeologyActivity.html

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 Ushaped valleys and
glacial striations.
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/
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
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/VolRocks.php
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.
Crater Lake Geologic Map
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.
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
Left:http://www.mnh.si.edu/earth/text/dynamicearth/6_0
_0_GeoGallery/geogallery_specimen.cfm?SpecimenID
=2055&categoryID=4&categoryName=Rocks&browseT
ype=group&groupID=5&groupName=Igneous
Pyroclastic Flow



http://www.cnsm.csulb.edu/departments/geology/people/bperry/
IgneousRocksTour/VolcanoesAndLavaFlows.html
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
>500° 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.
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/curriculum/geology/images/Dike_Cross-Island_Trail_Alaska.jpg,
http://en.wikipedia.org/wiki/Sill_%28geology%29 respectively
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
Sites: Pumice Castle
http://www.siskiyous.edu/class/geol66/mazamaguide.pdf

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.
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/CraterLake/CraterLake6.html
Inset:http://volcano.oregonstate.edu/vwdocs/volc_images/north_america/crater_lake.html
Sites: Devil’s 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.gif
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