Spatial and Temporal Overview of Volcanos in Central Oregon
Chris Bronstad, Key Peninsula Middle School, Washington
A sufficiently water-hydrated group of scientists and educators took a three day field trip through volcanic areas of central Oregon in order to observe and interpret volcanic features. The field trip started in Eugene, Oregon and traveled as far east as Riley while completing a circuitous route through the region.
Clarissa Cole, Evanston H.S., Wyoming
Mike DeNoon, Blue Valley West H.S., Kansas
Al Dorn, Univ. of Nebraska Omaha, Nebraska
Jim Hill, Rainwater Observatory, Mississippi
Carol B. Ivers, Foran H.S., Connecticut
The field party examined volcanic activity that resulted from two distinct geologic processes — subduction processes and Basin and Range processes. These processes are reflected in the features and deposits of Crater Lake and Newberry Volcano, respectively.
Crater Lake National Park
Crater Lake is an 8 km diameter caldera formed by the collapse of Mount Mazama, a large composite volcano of the Cascades Volcanic Range. Composite or stratovolcanos are the product of multiple eruptions of silica- and gas-rich magma. They form steep-sided volcanic cones as layers of andesitic to rhyolitic lava build up, interspersed with pyroclastic deposits.
Volcanism in the Mount Mazama region began about 420,000 years ago. The rocks record a change in the silica content of the magma through time. Older exposures, unseen during our fieldtrip, include basaltic lava flows. The lower, older units observed in the caldera walls are andesitic to dacitic. The upper units (e.g., Llao Rock, Wineglass Welded Tuff) are rhyodacitic to rhyolitic in composition. The final eruption that caused collapse of Mount Mazama occurred approximately 7,700 years ago and involved highly explosive volcanism that deposited a 100 m thick blanket of silica-rich ash and pumice.
Crater Lake rim showing numerous layers of andesitic to rhyolitic flows.
South of Bend is Newberry Volcano, one of the largest young shield volcanoes in the Cascades. It covers 1300 km2 and rises to an elevation of about 2,434 m at the top of Paulina Peak.
Newberry Volcano began forming approximately 600,000 years ago. Its lower units are comprised of basaltic and basaltic-andesitic flows. More recent volcanism, observed in the caldera, includes rhyolitic to andesitic pyroclastic deposits and dacitic to rhyolitic lava flows and domes. Big Obsidian Flow marks the most recent activity. The flanks of the volcano are dotted with basaltic cinder cones and lava flows.
Newberry shield and caldera complex. Note the abundance of dark basaltic rock.
Crater Lake marks the location of the ancient stratovolcano Mount Mazama, part of the Cascades Volcanic Range. The Cascades volcanos result from the subduction of the Juan de Fuca oceanic lithospheric plate under the North American continental lithospheric plate.
Model of plate tectonic processes associated with the Cascades Volcanic Range.
Plate Tectonics and the Cascade Range
Newberry Volcano lies at the edge of the region influenced by extension in the Basin and Range province of the United States. It is unclear whether extension is being driven by a shallowly subducted hot plate (perhaps involving a spreading center) or tied to hot-spot activity; many geologic models have been proposed. The Newberry region probably also is influenced by the plate tectonic processes of the Cascades region.
Different volcanic rocks and features associated with the two areas of study reveal a complex zone involving both andesitic volcanism and basaltic volcanism. The presence of basaltic flows in the lower portions of Mount Mazama, and the abundance of basaltic flows contributing to Newberry Volcano, are counter to textbook models for volcanic materials resulting from subduction of oceanic lithosphere beneath a continental plate. Likewise, the rhyolitic to andesitic volcanism observed at Newberry Volcano is counter to models of the Basin and Range. In these models, the descending plate, sediment, water, and upper mantle partially melt to form magma that feeds andesitic stratovolcanos. As one instructor observed “I was struck by how “un-textbook” the Southern Cascades volcanism is …”
Basalt forming the Newberry Shield Volcano could result from mantle-derived magma that works it way through the thinned, extended crust of the Basin and Range. Other features observed during the field trip, such as the cinder cones and the flood basalts of the Horse Ridge Summit, are typical of areas with extended crust. Alternatively, perhaps the magma sourcing the Newberry volcano has differentiated over time, becoming increasingly silica rich, and producing the rhyolitic units that formed in the last few thousand years.
An explanation of the abundance of basalt flows in the older portions of Mount Mazama is more difficult. Perhaps the crust in the Cascades Volcanic Region is stretched and thinned by the descent of the relatively young and hot (buoyant) oceanic plate and basalt magma derived from the mantle makes its way to the surface. As the plate continues to descend, more typical andesitic and rhyolitic magma forms and contributes to the volcanos. More research is needed to understand if other stratovolcanos of the High Cascades are built on basaltic shield volcano foundations. Several appear to have this structure (e.g., the Three Sisters, Mount Bachelor, Mount Thielsen, and Mount McLoughlin).
Perhaps there is an evolution throughout this region of Southern Cascade volcanos beginning with basaltic flows and evolving to more silicic flows. Newberry Volcano may be too young to have evolved far beyond the basalt stage.
The volcanism on other planets appears to be primarily basaltic. We have no direct evidence of physiographic features or silicic magmas that might indicate a process analogous to plate tectonics.
There are some volcanic features — Gruithuisen Domes on the Moon and Pancake Volcanos on Venus - that may have a relatively silica-rich composition. These features have steep sides, suggesting they formed from a more viscous magma than the surrounding basaltic volcanic features. Their presence suggests that a basaltic magma may have partially differentiated in the interior of these planetary bodies.
Gruithuisen Domes on the Moon. The dome to the left is 20 km wide and 1200 m high. The dome on the right is
13 km wide and 1550 m high. Their steep sides suggest they were formed by lava that had a relatively high viscosity.
Pancake domes on Venus. The domes are approximately 20 kilometers across.
Unusual Volcanoes on Venus