Volcanoes in the neighborhood


Last updated 1/23/2024 at 9:24am

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If you like local history, sooner or later you may take a deep dive. You might end up wondering about the sandy earth at your feet or the age of the jagged peaks in the evening skyline. Maybe you watched too many dinosaur movies over the holidays and started imagining what Sisters was like millions of years ago.

The origin stories of the mountains and landscapes of Sisters fill geology books. They involve plates in the earth and under the sea, subduction zones, millions of years of volcanic eruptions, and huge sheets of glacial ice.

For a small place, Sisters is rich in volcanoes, counting at least seven strato/composite or shield volcanoes in our skyline along with dozens of large and small cinder cones. We live in the shadow of ancient extinct volcanoes (Mt. Washington, Three Fingered Jack, Mt. Jefferson) and a few volcanos that are still a little alive (South Sister).

It's part of a string of fire mountains along a crack in the earth's core where plates collide and heat and molten rock escape. Called the Cascade Volcanic Arc, a recent U.S. Geologic Survey report counts at least 466 volcanoes, 25 miles north and south of the Three Sisters (https://www.usgs.gov/volcanoes/three-sisters/science/geology-and-history-summary-three-sisters).

Most say the age of dinosaurs was about 230 to 66 million years ago. Back then, Oregon was covered by shallow seas. After it dried out, deep volcanic deposits covered everything, including any dinosaurs. There are fossils in the Metolius Basin on the north end of Green Ridge buried under layers of lava and ash. But instead of an entombed T-Rex, they are the remains of tiny plants called phyto diatoms. These microscopic algae lived in water, and died long ago, leaving their white silica cell walls forming diatomaceous earth.

The story continues 45 million years ago when the "old Cascades" were erupting. This ancestral mountain range had small low volcanoes that built up their surroundings with huge amounts of lava and mudflows. Between 18-36 million years ago the old Cascade volcanoes became more notable, sending quantities of ash in the sky, which winds carried far to the east.

Geologists believe that the landscape around Sisters today is less than 9 million years old and that most is younger than 500,000 years old. This accounts for the dry sandy soils in your garden. They are geologically speaking, young soils, of volcanic ash and glacial debris with little organic material.

The Metolius Watershed Analysis (USFS 1996) report by Geologist Larry Chitwood notes our oldest rocks are the remnants of the Castle Rocks volcano on the north end of Green Ridge. Believed to be 8-9 million years old, the jagged castle-like spires drew the attention and an appropriate name by explorer Henry Abbott in 1855. The Metolius River flows around the northern base of this ancient volcano for 8 miles. In its day, it would have looked like a smaller version of Black Butte, which did not yet exist.

Five to seven million years ago, we would have seen a different mountain range to the west. Hundreds of overlapping shield volcanoes erupted to build the present-day Cascade crest and almost buried the Castle Rocks volcano. Eruptions from the old Cascades spread an expansive apron of lava, compacted volcanic dust, and ash all the way to Madras and Bend.

A must-have book for local history lovers, "Oregon Sisters Country" by Ray Hatton, includes a chapter where Chitwood drops you into the geologic action. "Then the earthquakes began. If you arrived on just the right day four and a half million years ago you would feel the first of hundreds of major earthquakes...". The Metolius basin would sink 2000 feet along a north-south fault line becoming part of the Cascade Graben, a trenched valley with Green Ridge on its east side, soaring 2000 feet above the river.

Our skyline still has the eerie skeletons of the next batch of volcanoes, extinct Three Fingered Jack and Mt. Washington. Active between 500,000 to about 250,000 years ago, their peaks were carved by glacial ice to jagged spires, uncovering lava hardened in the ancient vents. Ground up mountainsides of glacial debris filled the Metolius basin.

Reported dates of Mt. Jefferson's activity vary but Chitwood says it was a site of major volcanic activity built of at least six volcanoes on top of each other. The U.S. Geological Survey reports it erupted repeatedly beginning about 300,000 years ago, the largest explosion sent ash to southwest Idaho. Many eruptions occurred while it was buried under the deep glacial ice which carved its sharp top and uncovered layers of past mountains.

Of the Three Sisters, North Sister is said to be the oldest, last erupting about 55,000 years ago. Middle, then South Sister started building about the same time. South Sister last erupted 2000 years ago, but scientists measured a bulge and clusters of small earthquakes there in the year 2000.

Black Butte is an anomaly. It is about a million years old but still has the silhouette of a classic stratovolcano. Forming inside the rain shadow of the Cascades it was untouched by the erosive power of the glaciers. Chitwood described "molten rock building its castles in the Cascades and ice tearing them down." He concluded the high peaks "suffered tremendously at the hands of the glaciers." The lakes in the receding tracks of melting glaciers on the Three Sisters and Mt. Jefferson are found in the highest concentrations of neoglacial moraine dammed lakes in the U.S., but the glaciers are a story in themselves. The last volcanic activity in the area appears to have been from spatter cones extending towards Blue Lake crater about 1350 years ago. To the north, Mt. St. Helens erupted dramatically in 1980.

Our volcanoes shade us from the west side rain and inspire those sunset photos. We live but for a short moment of time in this old and young landscape while the mountains slumber quietly for now.


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