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DESCRIPTION:
The Boring Lava Field, Portland, Oregon



Boring Lava Field

From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.170-172, Contribution by John E. Allen
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.

Northwest of the town of Boring, 20 eruptive centers are concentrated within around 100 square kilometers (39 square miles). Vents in the east part of this cluster average less than 2.6 kilometers (1.6 miles) in diameter and 333 meters (1,090 feet) in height above their bases. Lava from Highland Butte and Larch Mountain shield volcanoes from gently sloping plains covering many tens of square kilometers. Well logs indicate that in most places except near vents, Boring lava is between 30 and 60 meters (100-200 feet) thick.

Partial summit craters remain only at Bobs Hill, 33 kilometers (20.5 miles) northeast of Portland, and at a low cone enclosing a lake (Battleground Lake) north of Battleground, Washington, 33 kilometers (20.5 miles) north of Portland. Most other volcanoes still have a low cone shape and are mantled with loess above 122 meters (400 feet) elevation. Below this they were scoured by the cataclysmic Bretz floods from Glacial Lake Missoula around 13,000 to 15,000 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 northeast face of the Butte.

From: Allen, 1975, Volcanoes of the Portland Area, Oregon: State of Oregon, Department of Geology and Mineral Industries, The ORE-BIN, v.37, no.9, September 1975
Map, Vents of the Boring Lava Field, click to enlarge [Map,InlineGIF]
Location Map and Table: Location and elevation of 95 vents, including multiple vents, in the Portland area
-- Modified from: Allen, 1975, Ore-Bin no.37, no.9

Within a 13-mile radius of Kelly Butte there are over 32 volcanic vents; within a 20-mile radius centered at Troutdale there are 90 volcanic centers. Most of these were originally small cinder cones like Pilot Butte and Lava Butte near Bend, Oregon, but some of them, such as Mount Sylvania in southwest Portland, Highland Butte 10 miles southeast of Oregon City, and Larch Mountain south of the Columbia River Gorge, were low, broad lava domes of the type called "shield volcanoes".

The densest concentration of volcanic vents lies west of the town of Boring, where 20 centers occur within an area of about 36 square miles. Because of this grouping near Boring, Ray Treasher (1942) first gave the name "Boring lava" to the lava, cinders, and ash which emanated from volcanic centers in the Portland area within a time span of from perhaps 10 million to less than 1 million years ago (Trimble, 1963). Some like Bob's Mountain in Washington, may be very young indeed.

"The Boring lava is composed mainly of basaltic flow rocks, but locally contains tuff-breccia, ash, tuff, cinders and scoriaceous phases" (Trimble, 1963). The Boring Lava, originating in the Portland area, is quite different from Yakima Basalt (Columbia River Basalt), which originated outside the area. The Boring, as compared to the Yakima, is gray rather than dark gray to black, and the jointing is generally massive or blocky rather than columnar or brickbat. Still more characteristic of the Boring Lava, as seen in thin section, is the meshwork of minute plagioclase laths (polotaxitic texture) commonly with open spaces between the laths (diktytaxitic texture). The Boring Lava contains olivine, rare in Yakima Basalt, and there is a very distinct geochemical difference between the two types of lavas (Beeson, personal commun., 1975).

From: Swanson, et.al., 1989, IGC Field Trip T106: Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: American Geophysical Union Field Trip Guidebook T106.
Allen (1975) located more than 30 Boring vents within 21 kilometers of here and more than 90 vents (about 50 of which he classified as "certain") within 32 kilometers of Troutdale (5 kilometers northeast of Gresham). The Boring in this area is between about 1.3 million years old (Rocky Butte, 3 kilometers northeast of here) and 2.1 million years old (in bluffs near Oregon City, 18 kilometers south of here), judging from unpublished K-Ar ages (Sherrod, oral commun., 1988) and a K-Ar age of 1.56 +/- 0.2 million years on a flow at Bear Prairie 29 kilometers east-northeast of here (Tolan and Beeson, 1984).

The setting of the basalt field is puzzling and not understood. The vents lie well west of the crest of the Cascades, and those such as Kelly Butte, Mount Tabor, and the vents in the Portland Hills are in and even west of the Portland basin.

From: Trimble, 1963, Geology of Portland, Oregon and Adjacent Areas: USGS Bulletin 1119, 119 p.
The post-Troutdale Pliocene was a time of renewed small-scale discontinuous volcanic activity. Boring volcanoes dotted the land surface in the vicinity of Portland. The lavas were viscous and in most cases did not flow far from their source vent, but a few vents were explosive. In some areas, notably along the foothills of the Cascade Mountains east of the Sandy River and in the areas southeast of Oregon City, several vents furnished enough lava to form a lava plain. The lavas southeast of Oregon City displaced the ancestral Willamette River to the west, where it cut its present gorge. ... This fourth episode of Tertiary volcanism continued into the early or middle part of the Quaternary.

Click for Trimble Boring Lava Boring Lava - Excerpt from Trimble, 1963

Boring Lava Field - Points of Interest

Map, Vents of the Boring Lava Field, click to enlarge [Map,InlineGIF]
Location Map and Table: Location and elevation of 95 vents, including multiple vents, in the Portland area
-- Modified from: Allen, 1975, Ore-Bin no.37, no.9



Battle Ground Lake, Washington


Map, Battle Ground Lake, click to enlarge [Map,130K,InlineGIF]
Topo Map, Battle Ground Lake, 1:25,000
-- Basemap courtesy TopoZone, 2003

Aerial Photo, Battle Ground Lake, click to enlarge [Image,120K,JPG]
USGS Aerial Photo, Battle Ground Lake




Bear Prairie, Washington




Bob's Mountain (Bob's Hill), Washington




Brunner Hill, Washington




Chamberlain Hill, Oregon




Cook's Butte, Oregon




Devil's Rest, Oregon




Elk Point, Oregon




Green Mountain, Washington

Map, Green Mountain, click to enlarge [Map,141K,InlineGIF]
Green Mountain, 1:25,000
-- Basemap courtesy TopoZone, 2003

Image, Green Mountain from the roof of CVO, click to enlarge [Image,85K,JPG]
Green Mountain as seen from the roof of CVO. View is from the south.
-- USGS Photo by Gene Iwatsubo, June 13, 2003




Highland Butte, Oregon




Hunsinger Peak, Oregon




Kelly Butte, Oregon

Map, Kelly Butte, click to enlarge [Map,144K,InlineGIF]
Kelly Butte, 1:25,000
-- Basemap courtesy TopoZone, 2003




Larch Mountain, Oregon




Lenhart Butte, Oregon




Lookout Point, Oregon




Mount Norway, Washington




Mount Pleasant, Washington




Mount Scott, Oregon

Map, Powell Butte and Vicinity, click to enlarge [Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003




Mount Sylvania, Oregon

Map, Mount Sylvania, click to enlarge [Map,172K,InlineGIF]
Mount Sylvania, 1:25,000
-- Basemap courtesy TopoZone, 2003




Mount Tabor, Oregon

Map, Mount Tabor, click to enlarge [Map,156K,InlineGIF]
Mount Tabor, 1:25,000
-- Basemap courtesy TopoZone, 2003

Aerial Photo, Mount Tabor, click to enlarge [Image,163K,JPG]
USGS Aerial Photo - Mount Tabor, Portland, Oregon, 2000




Mount Talbert, Oregon

Map, Powell Butte and Vicinity, click to enlarge [Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003




Mount Zion, Washington




Nesmith Point, Oregon




Nichol's Hill, Washington




Pepper Mountain, Oregon




Pohl's Hill, Washington




Powell Butte, Oregon

Map, Powell Butte and Vicinity, click to enlarge [Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003

Map, Powell Butte, click to enlarge [Map,151K,InlineGIF]
Powell Butte, 1:25,000
-- Basemap courtesy TopoZone, 2003

Map and Aerial Photo, Powell Butte, click to enlarge [Map and Aerial Photo, 156K,InlineGIF]
Powell Butte, 1:100,000
-- Images courtesy USGS and TerraServer, 2003

Aerial Photo, Powell Butte, click to enlarge [Image,140K,JPG]
USGS Aerial Photo - Powell Butte, Portland, Oregon, 2001




Prune Hill, Washington




Rocky Butte, Oregon

Map, Rocky Butte, click to enlarge [Map,160K,InlineGIF]
Rocky Butte, 1:25,000
-- Basemap courtesy TopoZone, 2003




Ross Mountain, Oregon




Scout Camp, Oregon




Swede Hill, Oregon




TV Hill, Oregon




Walker Peak, Oregon




Willamette National Cemetery, Oregon

Map, Powell Butte and Vicinity, click to enlarge [Map,144K,InlineGIF]
Powell Butte and Vicinity, 1:100,000
-- Powell Butte, Mount Scott, Mount Talbert, Willamette National Cemetery
-- Basemap courtesy TopoZone, 2003


Volcanic History

From: East Buttes, Terraces and Wetlands Conservation Plan, Bureau of Planning, City of Portland, July 1993, from: City of Portland, Oregon, Website, February 2002
The primary geologic formation underlying the East Buttes ... 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 Mount 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 ... 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 feet at a vent to less than 50 feet 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 Mount 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 [Mount 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, Mount Tabor is composed mostly of sand and gravel. Treasher speculates that a combination of factors, including deflection of the rivers by Mount 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 feet 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 Mount 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.

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05/01/07, Lyn Topinka