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Geology tour hits highlights of volcanic Eastern Sierra |
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Friday, 28 August 2009 |
Sports and Outdoors
By Catherine Billey
Mammoth Times Staff Writer
 Mammoth Times Photos/Catherine Billey Dr. Terry Wright, Professor Emeritus in Geology at Sonoma State University, explains to an attentive group organized by the Eastern Sierra Land Trust that the Panum Crater just south of Mono Lake is a perfect example of a rhyolitic plug-dome volcano.
A geology tour led by Dr. Terry Wright, Professor Emeritus in Geology at Sonoma State University, took 28 people from the northern edge of the Owens Valley to the Panum Crater near Mono Lake for a four-site tour on Aug. 22. It finished with a luncheon of local foods prepared by land trust staff in an aspen grove near DeChambeau Creek.
People on the tour came from Washington State, Southern California, France and Mammoth.
“We thought the tour was a great success,” said outreach coordinator Serena Dennis.
“Climbing up to see the crater was awesome because you don’t see stuff like that every day. And gnarled rocks like toothpaste,” said Carli Olson, 16.
Wright ensured that courtesy would prevail during the tour by playing a tune on a recorder he produced from his pocket, which he said he would repeat, if anyone held private conversations during his presentations.
No one did. And why would they? The geology of the Eastern Sierra is fascinating. Among other things, Wright brought a particularly notable aspect of local geology to life: the enormous Long Valley Caldera.
Long Valley Caldera
The Long Valley Caldera is a volcanic depression about 20 miles in east-west dimension and 10 miles going north-south, running roughly around through Crowley Lake, Glass Mountain and Mammoth Mountain.
“It was formed about 750,000 years ago during the eruption of the Bishop tuff,” Wright said.
To the south, the volcanic deposit blanketed an older glacial formation, the Sherwin Till, with hundreds of feet of pumice and ash that could be seen during the tour, and created the Volcanic Tablelands.
The fact that there have been eruptions in the caldera as recently as 600 years ago means that magma is bubbling close to the surface – within three kilometers, Wright said.
The Long Valley eruption that likely took place in one day sent two feet of ash as far away as Nebraska.
“This was a monster,” Wright said.
Holding up a piece of Bishop ash deposit, he said the tuff is what helps geologists identify other ages of mountain building in the Sierra Nevada.
“Dating eruptions gives an idea of the age of other rocks.”
The gray granite of the Sierra Nevada escarpment near Mount Tom and Pine Creek can be seen dramatically mixed with molten lava rock from deep within the earth in another geologic age.
Further shaking things up, fault lines in the White Mountains are pushing south while the Sierra Nevada Range is moving north along the shear zone.
The Crowley Lake Overlook provides an excellent perspective on the caldera.
“After the blast, the whole area subsided because there was nothing to support the overlying rock. It sank, but there was still magma there.”
Wright also explained how several “little” volcanos - called resurgent domes - were formed when magma rose up after the eruiption, including Mammoth Mountain.
Farther up Highway 395 at the next two stops – Vista Point near the June Lake Loop, where the Aeolion Buttes are observed, and the Panum Crater, which provides a perfect example of a rhyolitic plug-dome volcano – the group enjoyed the short hike into the crater. It provided a different geological example than what had been seen at the caldera sites. There, “magma came up and hit the groundwater, forming a paste that oozed out and flopped over,” Wright said, actually demonstrating with toothpaste. “We’re looking at the last phase of the ooze.”
Looking toward Bloody Canyon in the Eastern Sierra, he pointed out coulees - the parts of a volcano that blasted out and formed avalanches in the ancient landscape.
“Looking at the Sierra Nevada, you can also see the moraines where glaciers came out of the mountains creating U-shaped valleys.”
But glaciation is a whole other aspect of geology, one that might be addressed in some future tour (spectacular examples exist at Convict Lake).
Local foods on private lands
Jean Pierre Besnard of Paris, France, was amazed that an arid environment like the Eastern Sierra could yield such delicious food. And what a yield it was.
Land trust staff set up three tables with flowers and wine within the aspen grove. Nearby, a fourth table hosted by Dennis and Executive Director Karen Ferrell-Ingram, was heaped with locally grown foods.
While storm clouds gathered above, the group enjoyed repast from Eastern Sierra Alpers Trout from the Inland Aquaculture Group; goat cheese, green beans, potatoes, eggs and lettuce provided from Simis Ranch; peaches from Apple Hill Farms in Wilkerson, south of Bishop; bread baked at Great Basin Bakery in Bishop; onions and peppers from the Seizmic Gardens (they sell at the Bishop Farmer’s market); and lettuce, heirloom and cherry tomatoes and cucumber from land trust staff gardens.
No one seemed to mind when a few rain drops began to fall.
Dennis said that because the tour had been so popular, the land trust will likely organize another one next summer. |
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Last Updated ( Friday, 04 September 2009 )
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