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| Meet the Crooked River Caldera |
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| Geologists reveal Prineville's unique geology |
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 | | PHOTOS BY SCOTT STAATS PANORAMIC VIEW COMPILED BY VANCE W. TONG/CENTRAL OREGONIAN | | The Crooked River Caldera as seen from Ochoco Viewpoint |
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By Scott Staats
Geology fascinates me. My curiosity in Prineville's geology started with the gravelly terraces that stretch from the north side of Laughlin Road near the BLM and Forest Service offices to the hospital and along Lamonta Road and the Madras Highway in places.
At first I thought these terraces resembled glacial moraines but knew that no large glaciers were in this area. River terraces maybe? Ochoco Creek couldn't have deposited that much gravel and it seemed unlikely that the Crooked River could have either.
The answer became clear after talking with a few geologists.
"The gravel terraces on the north side of town along Laughlin Road are definitely water-lain gravels and not alluvial or glacial deposits," said Carrie Gordon, geologist with the Ochoco National Forest.
To understand how these gravels got deposited, we have to take a trip far back into geologic time. According to Gordon, lava flows from Newberry Volcano flowed into the Crooked River near O'Neil, damming the river and backing up the water into the Prineville basin. River and lake gravels deposited until the lava dams broke and the lake drained.
Today, we see these remnant gravel terraces around the Prineville valley. Several gravel pits in the valley are reaping the benefits from these deposits.
"The key point I find fascinating about living here in Prineville is that we are living in the middle of volcanic country," Gordon explained. "Right now it just happens to be fairly quiet but over the last 50 to 60 million years this area has been very active from a volcanic standpoint."
I now understood how the gravels got here, but how was this basin formed? This is where it really started getting interesting.
Gordon put me in touch with Jason McClaughry, a geologist with the Oregon Department of Geology and Mineral Industries in Baker City. McClaughry and Mark Ferns have been mapping the geology of the lower Crooked River basin from Bowman Dam to Lake Billy Chinook and made an interesting find - namely the largest volcano ever discovered in Oregon.
The Crooked River Caldera is approximately 22 miles long by 12.5 miles wide and stretches from Stearns Butte south of Prineville north past Gray Butte. The caldera formed during a cataclysmic eruption about 29.5 million years ago. During the eruption, the ground surface collapsed leaving a volcanic depression that filled with ash and pumice at least a third of a mile thick.
Encircling the caldera are prominent buttes such as Powell Buttes, Gray Butte, Grizzly Mountain, Juniper Butte and Stearns Butte, all forming between 29 and 25 million years ago.
"Barnes Butte is Prineville's Wizard Island and was aged at 27.5 million years old," said McClaughry. "It post-dates the original collapse by about 2 million years and is probably part of a second collapse of a smaller caldera within the larger one."
He said the Crooked River Caldera is akin to some of the larger volcanoes on Earth and noted that Yellowstone, which is one of the largest, is about 35 miles across.
Evidence shows that the Crooked River has remained in relatively the same location within the caldera. Over the past 15 million years there have been lava flows that erupted along the Crooked River drainage from Bowman Dam to Redmond and from the vicinity of Newberry volcano, that periodically filled and clogged the Crooked River channel. Between about one and a half million years ago to 400,000 years ago, several flows entered the Crooked River near O'Neil and created a series of lava dams that backed up the river.
"That is what's responsible for the large gravel terraces that formed the nice plain at Prineville," McClaughry explained. Based on the elevation of the terraces, he said there were at least three different events that laid down these gravels.
Total depth of the gravel terraces from top to bottom is about 300 feet. The large terrace that the hospital sets on is one of the youngest and was most likely the result of a dam caused by a lava flow from Newberry Volcano that blocked the river and backed up gravels and sediment about 400,000 to 700,000 years ago.
The gravel terraces extend north of Prineville up to the base of Grizzly Mountain. The closer you get to Grizzly Mountain, the older the terraces - from 1.5 to possibly 3 million years old.
Much of fallout from the mega-volcanic eruption of the Crooked River caldera has eroded away but likely deposits near Ashwood can still be seen as well as some in the Painted Hills that are 50 to 60 feet thick. The eruption would have decimated much of central and eastern Oregon. Eruptions of this type are known to cover ten to fifteen thousand square miles or more, according to McClaughry.
During smaller volcanic eruptions, the wind plays a role in the surrounding area but in this case it wouldn't have mattered much.
"These things are like a pressure cooker with a lid on," McClaughry said, "and eventually blow out sideways along the land surface as pyroclastic flows traveling between 150 and 200 mph or more. Nothing within several miles would have lived through that."
Today's John Day Fossil Beds National Monument to the east of Prineville was directly influenced by the Crooked River Caldera. Undoubtedly, it is what wiped out some of the life there and created part of the world-famous fossil beds.
"This volcano that we discovered in Prineville and mapped out is only the second one of that age ever discovered in Oregon," McClaughry said. "This makes Prineville's geology very unique." He said that about 39.5 million years ago, a smaller caldera formed in what is now the Mill Creek Wilderness Area.
"You can't help but look at this country and realize that we're unique in so many different ways in terms of flora, fauna, soil and lithology," said Carrie Gordon.
She noted that about 60 million years ago, the area of Prineville would have been oceanfront property. Farmers in the area have good soil conditions thanks in part to the Mount Mazama eruption (Crater Lake) that dropped about a foot and half of ash and pumice in this area about 7,700 years ago.
After learning about the Crooked River Caldera, I had to drive to the Ochoco Viewpoint and take in the immensity of this mega-volcano. I looked toward Barnes Butte, Stearns Butte, Powell Buttes, Gray Butte and Grizzly Mountain and tried to imagine the violent eruption, the later lava flows that dammed the Crooked River below me and a lake filling the valley.
Standing at the viewpoint, two thoughts crossed my mind:
1) I'm glad I noticed those gravel terraces.
2) What I'd give for a time machine.
Scott Staats is a freelance outdoors writer. His column can be read every Tuesday in the Central Oregonian. He can |
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