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.

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

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.

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  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.

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.”

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 NameSiO 2 Rhyolite70% or more DaciteApproximately 65% AndesiteApproximately 60% Basaltic-AndesiteApproximately 55% Basalt50% or less

Crater Lake Rock Characteristics  Magma with high SiO 2 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: Left: _0_GeoGallery/geogallery_specimen.cfm?SpecimenID =2055&categoryID=4&categoryName=Rocks&browseT ype=group&groupID=5&groupName=Igneous

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 >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. IgneousRocksTour/VolcanoesAndLavaFlows.html

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 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.

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.

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: Inset:

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.

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.