Boring Lava Field

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Boring Lava Field
Bobs mountain.jpg
Bob's Mountain in Washington State
Location Oregon and Washington, U.S.
Coordinates 45°18′N 122°30′W / 45.3°N 122.5°W / 45.3; -122.5Coordinates: 45°18′N 122°30′W / 45.3°N 122.5°W / 45.3; -122.5
Highest point
 – elevation
 – coordinates
Larch Mountain[1]
4,061 feet (1,238 m)
45°18′N 122°30′W / 45.3°N 122.5°W / 45.3; -122.5
Geology Volcanic field[2]
Age Pleistocene[2]
Last eruption > 300,000 years ago[2]
Locations of Boring vents in the Portland, Oregon area

The Boring Lava Field is an extinct Plio-Pleistocene volcanic field zone with at least 32 cinder cones and small shield volcanoes lying within a radius of 13 miles (21 km) of Kelly Butte, which is approximately 4 miles (6 km) east of downtown Portland, Oregon, in the United States. The name is derived from the town of Boring, Oregon, which lies just to the southeast of the most dense cluster of lava vents. The zone became active at least 2.7 million years ago, and has been extinct for about 300,000 years.[2]

The Portland metropolitan area, including suburbs, is one of the few places in the continental United States to have extinct volcanoes within a city's limits; Bend, Oregon is another; for example Pilot Butte is a 500 foot high cinder cone located within the Bend city limits.

Geography[edit]

The Boring lava deposits received their name based on their proximity to the town of Boring,[3][4] and were given this name by Ray Treasher in 1942.[5] They lie to the west of the town.[5]

Multnomah Creek drains from Larch Mountain, one of the volcanic cones in Boring Lava Field.[6] Local streams near the community of Boring receive seepage from the local aquifer. This unit, part of the greater Troutdale sandstone aquifer, is also made of sandstone and conglomerate and bears water well.[7] It also supplies water to domestic wells in the Mount Norway area.[8] Boring Lava is known to have formed intrusions into local sedimentary rock, and thus it may guide flow of groundwater locally.[8]

Geology[edit]

There are 90 volcanic centers within a 20 miles (32 km) radius of Troutdale and more than 32 vents within a 13 miles (21 km) radius of Kelly Butte. Mostly small cinder cone vents, these volcanoes also include some larger lava domes from shield volcanoes at Mount Sylvania, Highland Butte, and Larch Mountain. The Boring Lava Field marks the densest volcanic vicinity in this group, with 20 volcanic vents encompassing an area of 36 square miles (93 km2).[5]

Considered an outlier of the Cascade Range, Boring Lava Field lies to the west of the major Cascade crest. It shows a similar composition to the High Cascades that run through Oregon and southern Washington state, with Pliocene to Quaternary basalt lava flows and breccias.[9] It was active during the late Tertiary into the early Quaternary.[6] Like the surrounding High Cascades, Boring Lava Field erupted lava made of olivine basalt and basaltic andesite. The olivine basalt deposits have fine to medium textures, and the basaltic andesite lava flow deposits have relatively little pyroclastic rock in them.[10] Dark gray to light gray in color, Boring Lava produces columnar and platy joints, which can be seen in Oregon east of Portland and in Clark County in Washington state.[8] It is often used to refer to the local deposits erupted by vents in the field near Portland, which reach thicknesses of more than 400 feet (120 m).[9] Boring Lava has a more mafic (rich in magnesium in iron) composition than the nearby volcano Mount Hood, but they have similar ages.[6] It overlaps with volcaniclastic conglomerate from other Cascade eruptions in Multnomah County and the northern part of Clackamas County.[11][8] The Boring lava also contains tuff, cinder, and scoria. It is characterized by plagioclase laths that show a pilotaxitic texture with spaces between them that show a diktytaxitic texture.[12]

While the volcanic rock of Boring Lava was being emplaced over rock from the Troutdale formation,[13] there was deformation that uplifted and dropped fault blocks to the southeast of Portland.[14] Along the Washougal River, a large landslide occurred as a result of failure due to the Boring lava pushing down on rock from the Troutdale formation.[15] Eruptive vents on the western edge of the field formed along a fault line that trended to the northeast, located north of present-day Carver.[14]

Boring Lava was erupted by vents in the volcanic field,[4][16] and has been exposed at elevated topographic levels in intact volcanic cones and dissected lava plains. There is likely more lava deposited under Quaternary sedimentary mantle throughout the region.[4] Activity was confined to a relatively concentrated area.[17]

Subfeatures[edit]

Trimble (1963) argued that the Boring Lava Field was produced by eruptive activity at 30 volcanic centers. [18] These include shield and cinder cone volcanoes.[18]

Oregon vents[edit]

The following vents are in Oregon:

Washington vents[edit]

The following vents are in Washington:

  • Battle Ground Lake - Elevation 509 feet (155 m)[28] (miniature crater lake)
  • Bob's Mountain - Elevation 2,110 feet (643 m)[19] (contains intact summit crater)
  • Bob's Mountain (N) - Elevation 1,775 feet (541 m)[19]
  • Bob's Mountain (S) - Elevation 1,690 feet (515 m)[19]
  • Brunner Hill - Elevation 680 feet (207 m)[19] (2 vents)
  • Green Mountain - Elevation 804 feet (245 m)[19] (logged off during World War I)
  • Mount Norway - Elevation 1,111 feet (339 m)[19]
  • Mount Pleasant - Elevation 1,010 feet (308 m)[19]
  • Mount Zion - Elevation 1,465 feet (447 m)[19]
  • Nichol's Hill - Elevation 1,113 feet (339 m)[19]
  • Pohl's Hill - Elevation 1,395 feet (425 m)[19]
  • Prune Hill (W) - Elevation 555 feet (169 m)[19]
  • Prune Hill (E) - Elevation 610 feet (186 m)[19] (overlooks Camas, Washington)
  • Tum-Tum Mountain - Elevation 1,400 feet (427 m)[19] (located in Chelatchie, Washington)
The buttes of the Boring Lava Field are visible toward the center of this panorama of Portland, Oregon

Eruptive history[edit]

The products of the Boring Lava Field were erupted discontinuously over an erosion surface.[29] Activity took place during the late Tertiary and early Quaternary, in what is now the Portland area as well as the surrounding area, with a particularly concentrated pocket of activity to the east.[4] Nearly all of these eruptions were confined to single vents or small vent complexes, with the exception of a lava plain southeast of present-day Oregon City.[16] Boring Lava generally consists of flowing lava; only one eruptive deposit contains tuff, ash, and tuff breccia, and one vent to the northeast of the Carver area displayed evidence of explosive eruptions that later became effusive.[30]

References[edit]

  1. ^ a b "Larch Reset". NGS data sheet. U.S. National Geodetic Survey. Retrieved 2008-11-18. 
  2. ^ a b c d Wood, Charles A.; Jűrgen Kienle (1990). Volcanoes of North America. Cambridge University Press. pp. 170–172. 
  3. ^ Treacher 1942, p. 10.
  4. ^ a b c d Trimble 1963, p. 36.
  5. ^ a b c Allen 1975, p. 145.
  6. ^ a b c Dougall 2007, p. 14.
  7. ^ Swanson et al. 1993, p. 25.
  8. ^ a b c d Swanson et al. 1993, p. 28.
  9. ^ a b Swanson et al. 1993, p. 13.
  10. ^ Swanson et al. 1993, p. 12.
  11. ^ Swanson et al. 1993, p. 14.
  12. ^ Allen 1975, p. 149.
  13. ^ Trimble 1963, p. 34.
  14. ^ a b Swanson et al. 1993, p. 17.
  15. ^ Trimble 1963, p. 31.
  16. ^ a b Trimble 1963, p. 37.
  17. ^ Trimble 1963, pp. 36–37.
  18. ^ a b Allen 1975, p. 147.
  19. ^ a b c d e f g h i j k l m n o p q r s Allen, John E. (September 1975). "Volcanoes of the Portland Area, Oregon" (PDF). The Ore Bin. Portland, Oregon: State of Oregon Department of Geology and Mineral Industries. 37 (9): 150. Retrieved 2009-06-16. 
  20. ^ a b c Allen, John E. (September 1975). "Volcanoes of the Portland Area, Oregon" (PDF). The Ore Bin. Portland, Oregon: State of Oregon Department of Geology and Mineral Industries. 37 (9): 151. Retrieved 2009-06-16. 
  21. ^ "Powell Butte". NGS data sheet. U.S. National Geodetic Survey. Retrieved 2008-04-02. 
  22. ^ "Rocky Butte Reset". NGS data sheet. U.S. National Geodetic Survey. Retrieved 2008-04-02. 
  23. ^ "Mount Scott". Geographic Names Information System. United States Geological Survey. 
  24. ^ "The Boring Lava Field: Portland, Oregon". United States Geological Survey. Retrieved May 21, 2010. 
  25. ^ "Sylvania Reset". NGS data sheet. U.S. National Geodetic Survey. Retrieved 2008-11-17. 
  26. ^ "Feature Detail Report ID 1136814: Mount Tabor Summit". 
  27. ^ "Mount Talbert". Geographic Names Information System. United States Geological Survey. 
  28. ^ "Battle Ground Lake". Geographic Names Information System. United States Geological Survey. Retrieved 2012-05-03. 
  29. ^ Trimble 1963, p. 1.
  30. ^ Trimble 1963, p. 38.

Sources[edit]

External links[edit]