DESCRIPTION:
Portland, Oregon, and Vicinity

Metropolitan Portland, Oregon, like Auckland, New Zealand, includes most of a Plio-Pleistocene volcanic field. The Boring Lava includes at least 32 and possibly 50 cinder cones and small shield volcanoes lying within a radius of 21 kilometers (13 miles) of Kelly Butte, which is 100 kilometers (62 miles) west of Mount Hood and the High Cascade axis -- (Web note: Kelly Butte is approximately 4 miles east of downtown Portland)-- . Only the Clear Lake volcanics in California lie as far west in the coterminous United States. Unlike Clear Lake, Boring lava vents have been inactive for at least 300,000 years.

The three dated samples show reversed remnant magnetism, but since tens of other determinations of Boring Lava have about equal normal and reversed magnetic polarities, the volcanoes were probably active from at least 2.7 million to less than 500,000 years ago. ...

Boring lava is characteristically a light-gray phyric olivine basalt. A specimen from Rocky Butte is predominantly labradorite, with phenocrysts of olivine, mostly altered to iddingsite. The volcanoes locally contain scoria, cinders, tuff, tuff breccia, and ash. Weathering may extend to depths of 8 meters (25 feet) or more, the upper 2-5 meters (5-15 feet) commonly being a red clayey soil.

The best and most accessible exposure is the cross-section of the cinder cone in Mount Tabor Park ... Numerous quartzite-pebble xenoliths from the underlying Mio-Pliocene Troutdale gravels which make up the bulk of Mount Tabor have been found in the cinders here. The best view of the volcanic field is from the summit in Rocky Butte Park ... where massive cliffs of flood-scoured lava form the NE face of the Butte. ...

Read MORE About It - Boring Lava Field Menu

The topography of the Portland area is varied. The area spans parts of the lower Columbia River Valley and Willamette Valley ... and extends into the Oregon Coast Range ... and into the Middle Cascade Mountains ... The floors of the lower Columbia River Valley and Willamette Valley are nearly flat to gently rolling terraced surfaces. Altitudes range from near sea level in the vicinity of Portland to nearly 400 feet on some of the higher terraces. The Willamette Valley is separated from the lower Columbia River Valley, and segments of the Willamette Valley are separated from each other by hills, composed mainly of volcanic rocks, that are more than 1,000 feet high in places. ... The Columbia River, the master stream, leaves its gorge near the east edge of the area, flows westward and is joined by the Willamette River near Portland. It then flows northward ... The Willamette River, the major tributary of the Columbia River, flows northward ...

Read MORE About It - Columbia River Menu

Overlying the Scappoose formation are the lava flows of the Columbia River basalt. The Columbia River basalt in this region is part of the vast flood of middle Tertiary basalt that extends from Idaho to the Pacific Ocean and covers wide areas in southeastern Washington and northeastern Oregon.

The Columbia River basalt is widely distributed west of the Willamette and Columbia Rivers. It underlies much of the Tualatin Mountains, and also Palatine Hill, the Rosemont area, and Petes Mountain to the south. It occurs up to an altitude of about 1,700 feet. East of the Willamette river the formation is exposed principally between Milwaukie and Gladstone, in the Willamette River gorge between Canby and Oregon City, and along the south side of the Columbia River from the east edge of the mapped area (for this report) to just west of Corbett. Columbia River basalt also crops out in a small area on the east bank of the Sandy River in the SW1/4 Sec.11, T1S.,R.4E. The basalt underlies the sediments of the intervening lowland areas and has been penetrated in many places by wells drilled for water. A map prepared by D.H. Hart and R.C. Newcomb (1956), showing altitudes on the upper surface of the Columbia River basalt beneath the Tualatin Valley, indicates that the basalt underlies the entire valley in a closed basin, whose center is Hillsboro.

The basalt is best exposed in hilly or mountainous areas where it has not been buried by later sediments. It is hard and resistant, and forms prominent ridges into which steep-walled canyons have been cut. Where erosion has exposed the underlying marine sediments of the Scappoose formation, the basalt has failed in great landslides.

The Columbia River basalt rests on an eroded surface of considerable relief, but locally in this area, at least, it had an original thickness of as much as 1,000 feet. ...

The Columbia River basalt consists of a pile of basaltic lava flows of varying thickness. Complete vertical sections of flows are rare, and at only a few places are both top and bottom exposed. The few flows that could be measured are about 50 feet thick and have pahoehoe crusts.

Read MORE About It - Columbia River Basalt Menu

Snow-clad Mount Hood dominates the Cascade skyline from the Portland metropolitan area to the wheat fields of Wasco and Sherman Counties. The mountain contributes valuable water, scenic, and recreational resources that help sustain the agricultural and tourist segments of the economies of surrounding cities and counties. Mount Hood is also one of the major volcanoes of the Cascade Range, having erupted repeatedly for hundreds of thousands of years, most recently during two episodes in the past 1,500 years. The last episode ended shortly before the arrival of Lewis and Clark in 1805. When Mount Hood erupts again, it will severely affect areas on its flanks and far downstream in the major river valleys that head on the volcano. Volcanic ash may fall on areas up to several hundred kilometers downwind.

Read MORE About It - Mount Hood Menu

The primary geologic formation underlying the East Buttes, Terraces and Wetlands planning area is Columbia River basalt. This formation is composed of lavas which erupted from volcanic vents east of the Cascades 17.6 million years ago and which flooded much of the Columbia River basin in one of the largest lava floods on earth.

The Columbia River basalt is locally overlain by up to 1,500 feet of sandstone and gravel deposits known as the Troutdale Formation. This formation has two distinct compositions: the lower facies consists of gravels containing quartzite, schists and granites which tie it to the ancestral Columbia River, the upper facies is primarily sandstone of basaltic origin presumably eroded from the Cascades. The deposition of these sands and gravels began ten million years ago and ceased nearly two million years ago (Price 1987).

Near the end of the Troutdale deposition until only a few hundred thousand years ago, a group of shield and cinder cone volcanoes erupted across the lower Willamette Valley. The Boring Volcanoes, as they are collectively known, are comprised mainly of high-aluminia basalts, but locally contain ash, cinders and other materials. These basalts are similar to those of Mt. Hood and other Cascade mountains and the Boring volcanism is believed to be tied to the uplift of the High Cascades. The Boring lavas were viscous and did not flow far from their source vents with explosive eruptions being rare. Three of the cinder cone volcanoes are local landmarks located within the East Buttes, Terraces and Wetlands planning area: Rocky Butte, Kelly Butte and Mount Tabor. At Rocky Butte, an intrusive body of Boring lava has been exposed by erosion and uplift. Thickness of the lava ranges from over 600 ft. at a vent to less than 50 ft. for individual flows away from the vent. Age of the lava is reported to be 1.33 million years (Swanson 1986).

During the early part of the Pleistocene period (beginning 1.6 million years ago), extensive erosion occurred in the lower Willamette Valley lowlands, scouring the lowlands and leaving the prominent volcanoes. Treasher (1942) notes that the Clackamas River once had a course east and north of Mt. Scott and nearby hills. He surmises that the Clackamas and Columbia Rivers "shifted back and forth in various channels as they cut down to their present level and must have swept past the sides of these three buttes [Mt. Tabor, Rocky and Kelly]." The rocky masses of Rocky and Kelly Buttes were resistant to the erosive forces of the rivers, but evidence of deep cuts in the sides of the buttes can be found. Unlike these two buttes, Mt. Tabor is composed mostly of sand and gravel. Treasher speculates that a combination of factors, including deflection of the rivers by Mt. Scott and Kelly Butte and the presence of erosion-resistant lava on the lower slopes, enabled Tabor to withstand the erosive forces.

The most spectacular geologic event of recent times, the series of catastrophic floods known as the Missoula Floods, is most directly responsible for the creation of the East Portland terraces. Advancing glacial ice had blocked the Clark Fork River valley in western Montana forming Lake Missoula--a lake 250 miles long and 2,000 feet deep. Repeatedly, between 16,000 and 12,000 years ago, the glacial dam failed causing some of the largest floods known on earth. The flood waters spilled across Idaho and eastern Washington, surged down the Columbia River and through the Gorge, and met head-on with the Boring volcanoes. Rocky Butte in particular stood in the immediate path of the flood waters and its facing slope was cut into a nearly vertical bluff. With the exception of the Boring volcanoes, the entire east side of Portland was submerged under up to 400 ft. of water. The East Portland terraces were formed primarily through deposition of unconsolidated sand and gravel from the flood waters and the short-lived lake in the Portland Basin.

As many as five distinct terraces are now evident in east Portland. Perhaps the best example of the first terrace (at 150 ft. mean sea level) is the Overlook Bluff, discussed later in this report. Other terrace levels can be observed along NE Glisan Street and other east-west streets in the area. Evidence of erosion during and after the time of the Missoula Floods can be seen in several deep swaths cut into the depositional surfaces and bedrock. One such swath passes from Rocky Butte and Mt. Tabor to the southwest toward Lake Oswego. The most easily recognized example of this erosion is Sullivan's Gulch, a resource site covered later in this report.

Boring Lava Field Information - Menu

The majority of the Johnson Creek drainage basin is characterized by the geologic classification of alluvium. Alluvial deposits include all of the material in the channels of present-day streams, their flood plains, and abandoned channels. Alluvium consists of very poorly consolidated gravel and sand in the stream channels, gravel and sand lenses usually overlain by silt and minor clay on the flood plain, and organic material usually in abandoned channels beneath several feet of silt or clay. Alluvial soils are deposited and subject to erosion and re-deposition by water.

The thickness of the alluvium is variable. The sand and gravel is generally thin and rests on bed rock in small stream channels where gradients are high. The smaller flood plain deposits of silt and gravel tend to be narrow, thinning out at the canyon sides, whereas the larger flood plains may contain recent alluvium up to 30 feet thick or more. The primary geologic formation underlying Powell Butte is Columbia River basalt. This formation is composed of lava which erupted from volcanic vents east of the Cascades 17.6 million years ago and which flooded much of the Columbia River basin in one of the largest lava floods on earth.

The Columbia River basalt is locally overlain by up to 1,500 feet of sandstone and gravel deposits known as the Troutdale Formation. This formation has two distinct compositions: the lower facies consists of gravels containing quartzite, schists and granites which tie it to the ancestral Columbia River, the upper facies is primarily sandstone of basaltic origin presumably eroded from the Cascades. The deposition of these sands and gravels began ten million years ago and ceased nearly two million years ago.

Near the end of the Troutdale deposition until only a few hundred thousand years ago, a group of shield and cinder cone volcanoes, including Powell Butte, erupted across the lower Willamette Valley. The Boring Volcanoes, as they are collectively known, are comprised mainly of high-alumina basalts, but locally contain ash, cinders and other materials. These basalts are similar to those of Mt. Hood and other Cascade mountains and the Boring volcanism is believed to be tied to the uplift of the High Cascades. The Boring lavas were viscous and did not flow far from their source vents, with explosive eruptions being rare.

Portland's West Hills are the southern part of the Tualatin Mountains. Geological evidence suggests that these mountains were formed by a variety of forces acting over millions of years. The oldest known rock in the West Hills is known as the Scappoose formation, a thick bed of sandstone and shale believed to be deposited in an ancient ocean twenty-two million years ago. These sedimentary rocks were subsequently covered by a thick layer of lava during a period of regional vulcanism. This lava is an igneous rock called Columbia River Basalt. Geologists think this basalt is sixteen million years old. Columbia River Basalt comprises most of the bulk of the West Hills. The period of regional vulcanism was followed by a long period of weathering in which basalt was reduced to clay. The red crust now found on exposed basalt was deposited during a period when the West Hills enjoyed a tropical climate. Tectonic forces reshaped and reoriented the originally flat basalt into the Tualatin Mountains at the same time the Coast and the Cascade Ranges were forming. This period of squeezing and folding was followed by a second period of deposition of waterborne sediment. The resulting rock is known as the Troutdale Formation. This rock contains quartz, and is found on the lower slopes of the West Hills. A second period of more local vulcanism is quite recent in geological history. These smaller volcanoes produced a type of igneous rock called Boring lava. Most of the taller isolated hills in Portland are Boring volcanoes. Boring lava is also a type of basalt and can be distinguished from Columbia River Basalt by its gray color. Boring lava can be found along the ridge tops and west slopes of the West Hills. The most recent geological formation is a layer of wind blown silt deposited during the last ice age. This formation is a yellow-brown clay called Portland Hills Silt. This silt covers the upper portion of the West Hills. The deepest known deposit of this clay is a layer fifty-five feet thick found at the crest of the West Hills in Forest Park.

Return to:
[Portland Area Volcanoes and Volcanics Menu] ...