- Associated Press - Sunday, July 19, 2015

CORVALLIS, Ore. (AP) - On a recent sunny afternoon, a small knot of retired geologists and soil scientists stood beside a road cut in northwest Corvallis having an argument.

The exposed hillside above them, just across Walnut Boulevard from the Timberhill Shopping Center, was mostly light-brown dirt and sandstone, with chunks of dark-colored rock mixed in.

Bob Lillie, a former Oregon State University geology professor, pulled several rolled-up diagrams out of a map tube and began pointing out various rock formations while the others leaned in to look.

Phil Sollins, whose specialty at OSU was forest ecosystems and soils, quickly lost patience. “Let’s look at the rocks,” he suggested. Then he charged up the hill and started whacking away at an outcrop with the business end of a claw hammer.

Ralph Nafziger, a geochemist who worked at the U.S. Bureau of Mines in Albany, and Courtney Cloyd, an ex-Forest Service geologist, were hard on his heels, and a slightly disgruntled Lillie followed after rolling up his maps. Soon everyone was peering at Sollins’ newly collected sample, wrangling over crystalline structure and trying to decide if they were looking at basalt or gabbro.

Geology can be like that. The earth reveals itself in small glimpses, often in ways that are ambiguous, if not downright confusing. That can be especially true in places like the Corvallis Fault, a half-mile-wide swath that runs along the boundary between the Oregon Coast Range and the Willamette Valley.

“It’s a chaotic zone,” Lillie said. “It’s jumbled.”

And that’s precisely what makes it so interesting to geologists: This unassuming little rift in the earth’s crust marks the spot where titanic forces have come together to shape this part of the world, and will continue to shape it in the future. Though it’s not an active fault, it’s a rich repository of evidence that helps to tell the tale of how the Pacific Northwest came to be.

Earlier this month, the four retired scientists toured the fault zone, looking for clues to the region’s geological history. They were following a route laid out nearly four decades ago in “Field Guide to the Geology of Corvallis and Vicinity, Oregon,” a scientific paper written by R.D. Lawrence and three colleagues from the OSU geology department. The monograph was published in the April 1977 edition of The Ore Bin, the newsletter of the Oregon Department of Geology and Mineral Industries, and is still considered the standard work on the subject.

Each stop on the tour opened a fascinating window on the past.

ICE AGE FLOODING

The trek began at Avery Park, where the meandering Marys River pools into a popular swimming hole.

Low summer flows have exposed a section of riverbed composed of baseball-sized cobbles, rounded and smoothed by centuries of flowing water and cemented together in a matrix of sandstone. Lillie speculated these rocks might be part of a formation known as the Linn gravels, eroded fragments of the Cascades that were washed down into what is now the Willamette Valley between 28,000 and 36,000 years ago.

Just above this layer, the riverbank is made of fine-grained silt. It’s an even more recent deposit, laid down in one of the most dramatic episodes in the region’s history: the Missoula Floods.

Toward the end of the last ice age, a lobe of the continental ice sheet creeping down from Canada would periodically block the Clark Fork of the Columbia River, impounding enormous volumes of water in a virtual inland sea geologists call Glacial Lake Missoula. But ice dams are inherently unstable structures, and this one would burst every century or so, sending an estimated 50 cubic miles of water roaring toward the Pacific.

Forty or more such floods occurred over several thousand years and shaped the Northwest’s topography in dramatic ways, such as scouring out the rugged Channeled Scablands region of eastern Washington. When the wall of water hit a sharp bend in the Columbia at Portland’s West Hills, the flow would back up into the Willamette Valley as far south as Eugene, submerging all but the highest hills for days at a time.

“The ice dam would break in Montana and bring in 300 feet of water up the Willamette Valley,” Lillie said. “Imagine that: 300 feet of water in the Willamette Valley.”

When the floodwaters receded, they left behind a gift: a deep layer of silt that is largely responsible for the valley’s far-famed fertility.

WHERE PLATES COLLIDE

In a strange reversal of the usual order of things, driving uphill from the river on Kings Boulevard brings the group to far older geological formations. At the road cut just north of Walnut, everybody piles out of Sollins’ van for a peek inside the Corvallis Fault.

Here can be seen several of the major ingredients of the region’s geology, all jumbled together: the Siletz River Volcanics, a type of basalt formed in ocean-floor eruptions some 55 million years ago; Tyee Sandstone, a layer of compressed marine sediments that dates back about 41 million years (there’s also a slightly younger layer of marine sandstone in the region known as the Spencer Formation); and intrusions of gabbro, a volcanic rock that started out as underground magma and rose toward the surface between 30 million and 35 million years ago.

How did all these different kinds of rocks get here? And why are they mixed together in unexpected ways?

The answer, as Lillie explains in “Beauty from the Beast,” his just-published book on Northwest geology, is plate tectonics.

Here’s the short version: The earth’s crust is not a solid, spherical shell but rather an assemblage of plates that fit together like mammoth puzzle pieces, floating atop a semisolid layer called the mantle. Driven by heat rising up from deep within the earth, different plates move in different directions; here in the Pacific Northwest, a slice of oceanic crust known as the Juan de Fuca Plate is running head-on into the North American Plate.

Oceanic crust tends to be heavier than continental crust, so for the most part the Juan de Fuca Plate is diving beneath the North American Plate in a process called subduction. Over the last 200 million years, this process has been responsible for creating most of what we know as Oregon, Washington and British Columbia, adding millions of acres of land to a continental margin that used to end somewhere in the vicinity of Idaho.

As the oceanic crust burrowed deeper under the continent, heat and pressure squeezed out the moisture content, sending superheated water upward to melt portions of the continental crust, which erupted at the surface and created the volcanic Cascade Range. The most recent example occurred just 35 years ago, when Mount St. Helens exploded in a tremendous blast that killed 57 people and lopped more than 1,000 feet off the top of the peak.

And in some cases, the original geologic sequence is reversed. The hills northwest of the Corvallis Fault, outriders of the Coast Range, are a good example of this mixed-up process.

“The hills are being thrust over, the hard material is being shoved over the sediment,” Lillie said.

DETECTIVE WORK

Teasing out exactly what happened and when in the dim past of the planet is painstaking work. Geologists must sift through the available evidence at each site they examine, trying to determine if they’re looking at an undisturbed sequence of depositions or the chaotic results of some long-ago cataclysm or complex series of events.

From Kings and Walnut, the four scientists swing over to Highland Drive and head north, following the fault line and stopping at road cuts to look for clues. Sollins rummages in the back of the van and produces a thick-bladed hoe, which he uses to scrape away vegetation and expose likely-looking rocks. He also puts his hammer to good use, cracking open samples to expose their mysterious interiors.

Hanging from a cord around his neck, Cloyd has a 10-power magnifying loupe that he uses to get a better look at crystal structures within the rocks. He points out a piece of vesicular basalt studded with Rice Krispie-sized pockets of whitish material. The white stuff, he explains, is probably feldspar deposited long after the basalt formed. The vesicles — gas bubbles left behind by the volcanic eruption that created the basalt — were filled in by a silica-rich mineral solution that seeped into the rock after it had cooled.

“It’s really detective work, what geologists do — interpreting what you see,” Cloyd said. (Not that they always agree, he added later: “For every five geologists, you get at least seven opinions.”)

From there the group traces the fault zone along narrow, winding lanes up Vineyard Mountain, the high point of the ridge above Lewisburg. More road cuts, some of them hundreds of feet above the valley floor, expose bands of sandstone or outcroppings of basalt. Both tell part of the story of ancient mountain-building in the Pacific Northwest.

“As the Coast Range was uplifted, the Tyee was eroded away,” Sollins said. “As it got shoved up, as soon as it developed a slope, it started eroding.”

Part of the tale’s complexity lies in the fact that both processes were going on at the same time.

“We call that the dynamic duo: simultaneous uplift and erosion,” Lillie said.

And, really, the story’s even more complicated than that. If the Coast Range was all uplifted Tyee sandstone, it might have been worn down to a nub by now. But that layer of hard basalt, scraped off the oceanic crust and transported overland rather than being subducted, gives the tale a different ending.

“That’s one of the reasons this mountain is here,” Cloyd said at a stop to examine a prominent band of volcanic rock, “because this is more erosionally resistant by far.”

Even harder is gabbro. Unlike the basalt that makes up much of the Coast Range, which cooled quickly after erupting on the ocean floor, gabbro forms deep underground. As a result, it cools much more slowly, providing time for the formation of interlocking crystals that make it much more durable.

“It’s like granite,” Lillie said, “but it has more iron, so it’s generally darker-looking than granite.”

The gabbro found in this area rose up through the crust as molten magma, intruding through the older volcanic and sedimentary layers. Today the highly resistant rock can sometimes be seen exposed at the surface, often near the tops of mountains and hills.

“That’s why you see quarries there,” Lillie said. “They’re going after the gabbro. It’s building stone.”

The final stop on Thursday’s tour came at one such quarry, an abandoned working behind the home of Roz and Scott Keeney that reportedly provided the stone for the foundations of the Benton County Courthouse. The semicircular cliff of dark gray stone makes a dramatic backdrop to their property just off Witham Hill Drive.

“We call this the amphitheater,” Scott Keeney said.

STUDY IN CONTRASTS

Sitting on the Keeneys’ deck under a canopy of spreading oak trees, Lillie reflects on the day’s tour. He starts with the good news.

Despite its ominous-sounding name, he stresses, the Corvallis Fault probably poses no risk to mid-valley residents. Seismographs monitoring the area have detected no significant motion along the zone, which appears to be inactive.

The bad news, however, lies just offshore, where the subducting Juan de Fuca Plate is wedged firmly against the North American Plate. Rather than sliding smoothly past one another, the two titanic land masses appear to have been stuck for the last three centuries or so, while the tectonic pressure continues to mount.

That point of contact between the two plates is known as the Cascadia Subduction Zone, and geologists generally agree that it will be the site of a truly massive earthquake when the plates eventually lurch into motion again — with devastating consequences for the Pacific Northwest.

The last one, which had an estimated magnitude of 8.7 to 9.2, occurred on January 26, 1700. Scientists were able to pinpoint the date through written records in Japan, which was struck by a tsunami generated by the Cascadia quake.

Geological evidence points to similarly destructive subduction zone earthquakes in the region every 200 to 600 years, which means the next one could happen anytime in the next three centuries. Despite advances in construction techniques and disaster preparedness, “the Big One” is certain to hit the region hard.

But Lillie also points out that the powerful geological forces that threaten the Pacific Northwest are the same ones that shaped the beautiful landscape we call our home, this green and fertile valley cradled between majestic mountain ranges.

“We learn to take the beauty with the beast,” he said with a shrug.

“We have landslides and earthquakes and tsunamis and volcanic eruptions, right? That’s the beast part. But we also have the mountains and the valley and the coastline. We wouldn’t have all that beauty without the beast.”

___

Information from: Gazette-Times, https://www.gtconnect.com

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