Fort Rock Basin

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Fort Rock Basin
Fort Rock and basin.jpg
Fort Rock and its basin
Map showing the location of Fort Rock Basin
Map showing the location of Fort Rock Basin
Location Lake County, Oregon, USA
Coordinates 43°18′N 121°00′W / 43.3°N 121°W / 43.3; -121Coordinates: 43°18′N 121°00′W / 43.3°N 121°W / 43.3; -121[1]
Elevation 4,331–5,630 ft (1,320–1,716 m)[1]
Geology maar field
Age About 50,000 to 100,000 years old[2]
Last eruption About 50,000 years ago[2]

The Fort Rock-Christmas Lake Valley Basin is the basin of a former inland sea that existed in that region from Pliocene through late Pleistocene time.[3] The Fort Rock basin maar field includes over 30 hydrovolcanic landforms spread over an area of 1,500 square miles (4,000 km2).[1][2]

Moffitt Butte[edit]

Moffitt Butte is a dissected basaltic tuff ring, 4,600 feet (1,400 m) in diameter and 400 feet (122 m). Moffitt Butte is not associated with a lake basin, as is the case for Fort Rock and Hole-in-the-Ground, but rising magma probably encountered permeable aquifers beneath the cone. A line of tuff rings between Moffitt Butte and the Fort Rock-Christmas Lake Valley Basin are located roughly along what would have been an early drainage route between the Fort Rock Basin and the La Pine Basin. The crater floor of Moffitt Butte is about 260 feet (80 m) above the surrounding plain. A smaller vent and small tuff ring, 1,700 feet (520 m) in diameter, are located on Moffitt Butte's southwestern flank. The crater of the smaller vent is filled with lava that issued from a dike on its northwest edge.[4]

Table Rock[edit]

Table Rock is an erosional remnant of a tuff cone, which at present is a symmetrical cone about 5,020 feet (1,530 m) in diameter at the base, tapering to a diameter of about 1,180 feet (360 m) at a height of 1,180 feet (360 m) above the surrounding plain. The cone is capped with flat-lying basalt which once filled the crater, but erosion has modified the original cone, exposing the once ponded basalt lava lake. Dikes extend north and south of the crater's lava lake. On the lower flanks of the cone, the rocks are mostly palagonite lapilli-tuff. Near the summit, the uppermost palagonites are overlain by massive cinders and bombs from fire-fountaining that preceded the filling of the crater with lava.[5]

Notable Vents[edit]

Name Elevation Location Last
eruption
Big Hole [2][6]     possibly 20,000 years ago
Black Hills [2]      
Boat-Wright Ranch [2]      
Flat Top [2]      
Flatiron [2]      
Fort Rock [7]   43°22′N 121°04′W / 43.37°N 121.06°W / 43.37; -121.06 50,000 to 100,000 years ago
Hole-in-the-Ground [8]   43°25′N 121°12′W / 43.41°N 121.20°W / 43.41; -121.20 50,000 to 100,000 years ago
Horning Bend [2]      
Moffitt Butte [4]   43°31′N 121°26′W / 43.51°N 121.44°W / 43.51; -121.44 50,000 to 100,000 years ago
Reed Rock [2]      
Ridge 28 [2]      
St. Patrick Mountain [2]      
Sand Rock [9]   43°22′N 120°19′W / 43.37°N 120.32°W / 43.37; -120.32 50,000 to 100,000 years ago
Seven-Mile Ridge [2]      
South Green Mountain [2]      
Table Mountain [2]      
Table Rock [5]   43°10′N 120°53′W / 43.17°N 120.88°W / 43.17; -120.88 50,000 to 100,000 years ago
Wastina Butte [2]      
Wildcat Butte [2]      

See also[edit]

References[edit]

  1. ^ a b c Wood, Charles A.; Jűrgen Kienle (1993). Volcanoes of North America. Cambridge University Press. pp. 203–204. ISBN 0-521-43811-X. 
  2. ^ a b c d e f g h i j k l m n o p q "Oregon Volcanoes - Fort Rock Basin Maar Field". Deschutes & Ochoco National Forests - Crooked River National Grassland. United States Forest Service. 2004-01-09. Retrieved 2008-05-17. 
  3. ^ Heiken, G. H.; R. V. Fisher and N. V. Peterson (2006-03-28). "A Field Trip to the Maar Volcanoes of the Fort Rock - Christmas Lake Valley Basin, Oregon". Geological Survey Circular 838: Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California. United States Geological Survey. Retrieved 2008-05-17. 
  4. ^ a b "Oregon Volcanoes - Moffitt Butte Volcano". Deschutes & Ochoco National Forests - Crooked River National Grassland. United States Forest Service. 2004-01-09. Retrieved 2008-05-17. 
  5. ^ a b "Oregon Volcanoes - Table Rock Volcano". Deschutes & Ochoco National Forests - Crooked River National Grassland. United States Forest Service. 2003-11-26. Retrieved 2008-05-17. 
  6. ^ Lorenz, Volker (1970). "Some Aspects of the Eruption Mechanism of the Big Hole Maar, Central Oregon". Geological Society of America Bulletin (Geological Society of America) 81 (6): 1823–1830. doi:10.1130/0016-7606(1970)81[1823:SAOTEM]2.0.CO;2. ISSN 0016-7606. 
  7. ^ "Oregon Volcanoes - Fort Rock Volcano". Deschutes & Ochoco National Forests - Crooked River National Grassland. United States Forest Service. 2003-12-24. Retrieved 2008-05-17. 
  8. ^ "Oregon Volcanoes - Hole-in-the-Ground Volcano". Deschutes & Ochoco National Forests - Crooked River National Grassland. United States Forest Service. 2003-12-24. Retrieved 2008-05-17. 
  9. ^ "Oregon Volcanoes - Sand Rock Volcano". Deschutes & Ochoco National Forests - Crooked River National Grassland. United States Forest Service. 2004-01-09. Retrieved 2008-05-17.