Small Silicic Volcanic Flows
Melba A. Martin, Ed. D., JPL/NASA Solar System Ambassador, Chinle High School Science Consultant & IEP Facilitator, Arizona
Rick Alm, B. S., ISTA President Elect, Science Chairman, Bonneville High School, Idaho
Charlotte Jennings, M. Ed., Nightingale-Bamford School, New York
Helen Hill, B. A., Rainwater Observatory and Planetarium, French Camp Academy, Mississippi
Small volcanic formations usually consist of a portion of a larger volcanic structure. They occur over a relatively brief period of time by geologic standards (monogenetic). They vary in the type of “parent” structure and the examples sited in this report also vary in their appearance and mineral composition.
Small silicic volcanic formations fall into two categories based upon eruption type: effusive and explosive. Effusive means a relatively quiet outpouring of lava flows with little or no explosive activity. Explosive is defined as any eruption in which the release of dissolved gases tears the magma into fragments and ejects them into the air.
As observers of the western and high Cascades, it was concluded that the Big Obsidian Flow, Glass Buttes, and Llao Flow were effusive eruptions. The Paulina Ash Flow and the Tumalo Outcrop showed the products of explosive eruptions. The Nimrod Granite Outcrop was a unique small silicic formation, the only silicic intrusion (not eruption) in our area.
Effusive Small Silicic Formations
Glass Buttes Complex
The Glass Buttes (pictured above) are domed volcanos that are small rounded hillocks formed by viscous lava types containing little water. These buttes are east of Brothers, Oregon, and erupted ~ 4.9 million years ago. The buttes are composed of obsidian and rhyolite, two igneous rocks with the same chemical compositions but different structures. Obsidian is a black to brown shiny volcanic glass. Rhyolite is a mixture of silicate minerals that is a light colored fine-grained igneous rock, with 68% or higher silica. Rhyolite is nearly all small mineral crystals of quartz and feldspars.
Big Obsidian Flow
A Formation on the Newberry Shield Volcano
Pictured above is a small section of the Big Obsidian Flow. The Big Obsidian Flow is the youngest event in the eruptive history of the Newberry Shield Volcano. This formation is approximately 1,300 years old. The flow formed from a slow-moving lava that flowed approximately 100–150 cm per day. The top cooled more quickly, forming pumice and obsidian that broke into fragments as the flow moved. Inside, the flow cooled slowly enough that some of the lava crystallized into rhyolite. The whole flow is roughly 1.8 kilometers long and 20 meters thick. There is also evidence of light to dark gray pumice that is highly porous with very low density. The pumice in this area fell 500 years prior to the Big Obsidian Flow and is beneath and around the obsidian layer. Big Obsidian Flow is interesting because it is a small silicic flow erupted from the Newberry Shield Volcano — a basaltic feature.
Llao Rock Obsidian Flow, Crater Lake
At Llao Rock (pictured above on the rim of Crater Lake to the right of Wizard Island) the observers noted good examples of high silica and low water lava flows. The Llao Rock eruptive center is one of the final events in the eruption of Mount Mazama and the ultimate formation of Crater Lake. The earliest eruptions of Mazama had been basaltic in nature, but the later events like Llao showed an increase in silica content.
Llao Rock is composed of rhyodacite, a light colored igneous rock of 62–68% silica content. The Llao Rock Eruptive Center spreads laterally for 1.8 km and is 375 m thick. This pumice fall included a jumbled mixture of clastic material of varying size with rounded surfaces.
A “finger” of the Llao Rock obsidian flow was studied at a road-cut. At this cross section the observers documented a core of obsidian that graded out to a mixture of rough-edged rhyolite and dacite. This is similar to the Big Obsidian Flow of Newberry Volcano.
Explosive Small Silicic Formations
Explosive silicic formations include the eruptions of high silica and water content lavas. Observed examples include the Paulina Ash Flow and the Tumalo Outcrop.
The Paulina Ash Flow was observed in the 20 m high rock wall at Paulina Creek Falls. The rocks are organized in layers. The upper third of the outcrop is massive. The unit below contains multiple thin layers reported to contain ash, lapilli, and volcanic bombs. The interface between the two layers is reported to be gradational. Some of the rock wall faces are red or white — the colors appear to represent coatings on the rock surfaces. The rocks around the falls are gray fine-grained andesites with lithoclasts and quartz and plagioclase crystals. There are regions in the boulders that contain evidence of flow (“swirls”).
The bottom unit is interpreted to contain layers of different types of tephra, coming from Newberry Volcano. The younger, upper Paulina Creek Falls unit is interpreted to be a massive andesitic ash flow that also erupted from Newberry Volcano. The lower unit may have been formed during an explosive event that blasted ash and fragments of volcanic rock into the air. The ash and lapilli, and bombs eventually settled through the air. Later a hot flow of ash came down the volcano side, and laid down the upper ash flow deposit. The rock faces are stained white on the surface from calcium deposits. The red is from staining from iron minerals.
The Tumalo Outcrop records several events. It contains several horizontal layers of volcanic rock offset by a series of normal faults. The lower unit is orange and contains poorly sorted pumice fragments and other volcanic rock fragments in ash. The unit does not show stratification and the fragments are not rounded. This unit is capped by a unit of well rounded volcanic rocks arranged in layers with layers of finer grained material. The overlying white and buff-colored layers contain a mixture of welded dacite-rhyolite with fragments of pumice and other volcanic rocks. This unit is layered and it appears that the size of materials increases from the bottom to the top.
The upper white and buff-colored tuffs at the Tumalo Outcrop.
The lower unit is interpreted as a welded tuff laid down by a pyroclastic flow during a volcanic eruption. A fast flowing stream later laid down the rounded rocks. This was followed by two ash falls that created the upper buff-colored and white units.
The Nimrod Outcrop is an oddity that needs to be mentioned in this report - it is one of the few granitic formations in this area. It is composed of a fine-grained light colored granite and includes small quartz and plagioclase feldspar crystals. The iron staining on this formation may have come from the surrounding basalt. The crystal size in this granite formation indicates that the subsurface cooling occurred at a “fast” pace for slow cooling. The outcrop is actually a small granitic feature that some geologists describe as a volcanic neck. It was essentially a pipe from the magma chamber to the surface from which lava erupted — probably explosively as the rocks are rich in silica. At some point the Nimrod lava stopped flowing and slowly cooled.
The Nimrod Outcrop showing the light colored, fine grained granite.
Connections to Volcanism on Other Planets
These small highly silicic formations that frequently occur on Earth have not been documented to occur elsewhere in the solar system. Two examples of possible silicic volcanism off the Earth are the Pancake Volcanos on Venus and the Gruithuisen Domes on the Moon. These features may be dome volcanos. They are larger in scale than the small silicic features we observed in the field, but appear to have been formed from a low viscosity lava, perhaps one high in silica or partially crystallized.
Pancake domes on Venus. These domes are approximately 20 km across.
Unusual Volcanoes on Venus
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.