Crater Lake GeologyCrater Lake Geologyand the Mt. Mazama Storyand the Mt. Mazama Story

Crater Lake: Cascade Volcanic ArcCrater Lake: Cascade Volcanic Arc Crater Lake is part of Crater Lake is part of

the Cascade Volcanic the Cascade Volcanic Arc that runs roughly Arc that runs roughly N-S from Northern N-S from Northern California up into California up into British Columbia. British Columbia.

The Cascade Volcanic The Cascade Volcanic

arc is produced by the arc is produced by the subduction of several subduction of several oceanic plates.oceanic plates.

http://en.wikipedia.org/wiki/Cascade_Volcanoes

Cascade Volcanic Arc: SubductionCascade Volcanic Arc: Subduction

The Cascade Arc is formed as the subducting oceanic plate moves The Cascade Arc is formed as the subducting oceanic plate moves deeper into the mantle, breaking down water-bearing minerals and deeper into the mantle, breaking down water-bearing minerals and relasing that water into the mantle “wedge” above the subducting relasing that water into the mantle “wedge” above the subducting plate. plate.

This water causes the mantle wedge to partially melt. The This water causes the mantle wedge to partially melt. The resulting basalt-basaltic andesite magma is less dense than the resulting basalt-basaltic andesite magma is less dense than the surrounding mantle (peridotite) and rises slowly until it either cools surrounding mantle (peridotite) and rises slowly until it either cools underground or reaches the surface as lava. underground or reaches the surface as lava.

Crater Lake: From Mt. MazamaCrater Lake: From Mt. Mazama ~7,700 years ago ~7,700 years ago

Crater Lake was Crater Lake was known as Mt. known as Mt. Mazama, a broad Mazama, a broad stratovolcano much stratovolcano much like Mt. Rainier like Mt. Rainier appears today. 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. MazamaGrowth of Mt. Mazama Mt. Mazama first began being built Mt. Mazama first began being built

400,000+ years ago as the overlapping of 400,000+ years ago as the overlapping of several stratovolcanoes and shield several stratovolcanoes and shield volcanoes.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. MazamaMt. MazamaActivity!Activity!

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

Mt. Mazama and Glacial ActivityMt. Mazama and Glacial Activity While Mt. Mazama While Mt. Mazama

was growing was growing 10,000+ years ago, 10,000+ years ago, glaciers were glaciers were actively shaping the actively shaping the volcanic landscape. volcanic landscape.

Today, evidence of Today, evidence of this glacial history is this glacial history is seen in the seen in the presence of U-presence of U-shaped valleys and shaped valleys and glacial striations.glacial striations.

http://www.shannontech.com/ParkVision/CraterLake/CraterLake7.html

http://education.usgs.gov/schoolyard/CoolGeologyActivity.html

Becoming Crater LakeBecoming Crater Lake 1.1. Eruptions of ash and pumiceEruptions of ash and pumice:: The The

cataclysmic eruption started from a vent cataclysmic eruption started from a vent on the northeast side of the volcano as a on the northeast side of the volcano as a towering column of ash, with pyroclastic towering column of ash, with pyroclastic flows spreading to the northeast. flows spreading to the northeast.

2.2. Caldera collapseCaldera collapse: : As more magma As more magma was erupted, cracks opened up around was erupted, cracks opened up around the summit, which began to collapse. the summit, which began to collapse. Fountains of pumice and ash surrounded Fountains of pumice and ash surrounded the collapsing summit, and pyroclastic the collapsing summit, and pyroclastic flows raced down all sides of the flows raced down all sides of the volcano. volcano.

3.3. Steam explosionsSteam explosions:: When the dust When the dust had settled, the new caldera was 5 miles had settled, the new caldera was 5 miles (8 km) in diameter and 1 mile (1.6 km) (8 km) in diameter and 1 mile (1.6 km) deep. Ground water interacted with hot deep. Ground water interacted with hot deposits causing explosions of steam deposits causing explosions of steam and ash. and ash.

4. 4. TodayToday:: In the first few hundred years In the first few hundred years after the eruption, renewed eruptions after the eruption, renewed eruptions built Wizard Island, Merriam Cone, and built Wizard Island, Merriam Cone, and the central platform. Water filled the new the central platform. Water filled the new caldera to form the deepest lake in the caldera to form the deepest lake in the United States. 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 TodayCrater Lake Today The last eruption The last eruption

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

Taken by Sara Auer Perry

Rock Types of Crater LakeRock 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 CharacteristicsCrater Lake Rock Characteristics

Magma with high SiOMagma with high SiO22 content contains more content contains more dissolved gas and is more viscous (less mobile) dissolved gas and is more viscous (less mobile) than those of basaltic composition. than those of basaltic composition.

High silicon-oxygen magmas will tend to be more High silicon-oxygen magmas will tend to be more explosive than those with a lower percentage. explosive than those with a lower percentage.

Crater Lake Geologic MapCrater Lake Geologic Map

Pyroclastic Ejecta Pyroclastic Ejecta (airborne material)(airborne material)

TephraTephra is the general term now used by volcanologists is the general term now used by volcanologists for airborne volcanic ejecta of any size. for airborne volcanic ejecta of any size.

Pumice: Pumice: pale clasts composed mostly of vesicular glass pale clasts composed mostly of vesicular glass which have a roughly similar composition to rhyolite. which have a roughly similar composition to rhyolite.

Scoria: Scoria: darker clasts composed mostly of vesicular darker clasts composed mostly of vesicular glass which have a roughly similar composition to basalt.glass which have a roughly similar composition to basalt.

VesicleVesicle: A small cavity in a glassy igneous rock that is : A small cavity in a glassy igneous rock that is formed when bubbles of gas or steam expand during the formed when bubbles of gas or steam expand during the cooling and solidification of the rock itself. cooling and solidification of the rock itself.

Tuff (welded and non)Tuff (welded and non)

Consolidated volcanic ash Consolidated volcanic ash (particles of glass) & pumice (particles of glass) & pumice ejected from vents during a ejected from vents during a volcanic eruption.volcanic eruption.

Welded tuff is a pyroclastic Welded tuff is a pyroclastic rock, of any origin, that was rock, of any origin, that was sufficiently hot at the time of sufficiently hot at the time of deposition for the particle of deposition for the particle of volcanic ash to become volcanic ash to become fused together (note the fused together (note the deformation of the pumice). 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&browseType=group&groupID=5&groupName=Igneous

Pyroclastic FlowPyroclastic Flow A ground-hugging avalanche of A ground-hugging avalanche of

hot ash, pumice, rock hot ash, pumice, rock fragments, & volcanic gas that fragments, & volcanic gas that rushes down the side of a rushes down the side of a volcano up to 100 km/hr. volcano up to 100 km/hr.

The temperature within a The temperature within a pyroclastic flow may be >500° pyroclastic flow may be >500° C, sufficient to burn & C, sufficient to burn & carbonize wood. carbonize wood.

Once deposited, the ash, Once deposited, the ash, pumice, and rock fragments pumice, and rock fragments may deform (flatten) and weld may deform (flatten) and weld together because of the together because of the intense heat and theintense heat and the weight of weight of the overlying material. the overlying material.

http://www.cnsm.csulb.edu/departments/geology/people/bperry/IgneousRocksTour/VolcanoesAndLavaFlows.html

Dike vs. SillDike vs. Sill

DikesDikes are tabular or sheet-like bodies of magma are tabular or sheet-like bodies of magma that cut through and across the layering of that cut through and across the layering of adjacent rocks. They form when magma rises adjacent rocks. They form when magma rises into an existing fracture, or creates a new crack into an existing fracture, or creates a new crack by forcing its way through existing rock, and by forcing its way through existing rock, and then solidifies. then solidifies.

A A sillsill is an intrusive body of magma that pushes is an intrusive body of magma that pushes its way its way betweenbetween layers of sediments. layers of sediments.

Pics fromPics from http://www.answersincreation.org/curriculum/geology/images/Dike_Cross-Island_Trail_Alaska.jpg, http://www.answersincreation.org/curriculum/geology/images/Dike_Cross-Island_Trail_Alaska.jpg, httphttp://en.wikipedia.org/wiki/Sill_%28geology%29 ://en.wikipedia.org/wiki/Sill_%28geology%29 respectivelyrespectively

Sites: Phantom ShipSites: 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 CastleSites: 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

Sites: Wineglass TuffSites: Wineglass Tuff During the eruption of Mt. During the eruption of Mt.

Mazama the giant column Mazama the giant column of airborne ash & gasses of airborne ash & gasses collapsed and generated collapsed and generated pyroclastic flows. These pyroclastic flows. These flows made the Wineglass flows made the Wineglass Welded Tuff, seen right. Welded Tuff, seen right.

The Wineglass is The Wineglass is composed of ash (glass composed of ash (glass particles) which have been particles) which have been fused or welded together fused or welded together by the hot temperatures of by the hot temperatures of that and subsequent that and subsequent pyroclastic flows. 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 BackboneSites: Devil’s Backbone

The Devils Backbone is a The Devils Backbone is a vertical wall of dark andesite vertical wall of dark andesite lining the cliff face and lining the cliff face and measuring about 1,000 feet measuring about 1,000 feet long by 50 feet across near long by 50 feet across near the top.the top.

A dike formed by molten A dike formed by molten lava that created and filled lava that created and filled cracks, as it forced its way cracks, as it forced its way up through the rock and then up through the rock and then solidified. It has been left solidified. It has been left standing by the erosion of standing by the erosion of the surrounding material. the surrounding material.

http://www.dartmouth.edu/~volcano/images/DCLp08.gif

SummarySummary Crater Lake is a member of the Cascade Volcanic Crater Lake is a member of the Cascade Volcanic

Arc, formed due to subduction of oceanic plates Arc, formed due to subduction of oceanic plates that is still occurring today.that is still occurring today.

Mt. Mazama was a stratovolcano that erupted Mt. Mazama was a stratovolcano that erupted ~7700 years ago to produce a caldera that is now ~7700 years ago to produce a caldera that is now known as Crater Lake. known as Crater Lake.

Before its cataclysmic eruption, Mt. Mazama was Before its cataclysmic eruption, Mt. Mazama was heavily glaciated. heavily glaciated.

Rocks at Crater Lake range from basalt to rhyolite.Rocks at Crater Lake range from basalt to rhyolite.

Key features that we will see are dikes, Key features that we will see are dikes, pyroclastic flow deposits, glacial striations, etc.pyroclastic flow deposits, glacial striations, etc.

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