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The Grand Ronde Basalt of the Columbia River Basalt Group.
Thick stacks of laterally extensive lava flows typify this flood basalt province. Photo by Thor Thordarson.
Area covered by Columbia River flood basalts shown in gray. Dashed lines are dike swarms. The outer limits of the Chief Joseph dike swarm are marked by CJ (vents for the flows in the Imhaha, Grande Ronde, and Wanapum Formations and Saddle Mountains Basalt). The Grande Ronde (GR) and Cornucopia (C) dike swarms are within the Chief Joseph dike swarm. The Monument Dike Swarm (M) was the vent for the Picture Gorge Basalt. The Paso Basin is near the confluence of the Columbia and Snake Rivers. Map based on Hooper (1997).
Almost everything about this volcanic province is impressive. The Columbia River Flood Basalt Province forms a plateau of 164,000 square kilometers between the Cascade Range and the Rocky Mountains. In all, more than 300 individual large (average volume 580 cubic km!) lava flows cover parts of the states of Idaho, Washington, and Oregon. At some locations, the lava is more than 3,500 m thick. The total volume of the volcanic province is 175,000 cubic km. Eruptions filled the Pasco Basin in the east and then sent flows westward into the Columbia River Gorge. About 85% of the province is made of the Grande Ronde Basalt with a volume of 149,000 cubic km (enough lava to bury all of the continental United States under 12 m of lava!) that erupted over a period of less than one million years. Flows eventually reached the Pacific Ocean, about 300 to 600 km from their fissure vents. The Pomona flow traveled from west-central Idaho to the Pacific (600 km), making it the longest known lava flow on Earth (the major- and trace-element compositions of the flow do not change over its entire length).
Feeder dikes form the vents for the flood basalts and they trend to the north-northwest to south-southeast across eastern Oregon and western Idaho (Swanson and others, 1975). Hundreds of vents have been recognized and mapped. Small vents, such as spatter cones, are associated with the feeder dikes. The vents systems are 50 to more than 200 km long and a few kilometers wide. Some vents are hidden under younger flows. Photo of dike in the Chief Joseph dike swarm cutting across Grande Ronde Basalt. Photograph courtesy of Stephen Reidel.
Most of the flows in the Columbia River Flood Basalt Province are tholeiitic basalt. Representative samples are given below. Data from Wright and others (in press) presented in Swanson and others (1989).
1 2 3 SiO2 53.84 50.94 52.00 Al2O3 14.37 14.27 15.04 FeO* 11.37 13.50 10.45 MgO 5.25 4.57 7.19 CaO 8.97 8.56 10.39 Na2O 2.92 2.85 2.23 K2O 1.10 1.25 0.65 TiO2 1.75 3.12 1.62 P2O5 0.23 0.68 0.24 MnO 0.19 0.25 0.18 FeO* = total FeO. 1. High MgO Grande Ronde basalt. 2. Roza Member of the Wanapum Basalt. 3. Pomona Member of Saddle Mountains Basalt.
Volcanism began about 17.5 million years ago and ceased about 6 million years ago.
Most of the volume of the Columbia River Flood Basalt Province (85%) was erupted in only 1.5 million years from 17 to 15.5 million years ago. Volume of each formation, in cubic kilometers, is given in parentheses. Black dots separate formations. Data from Tolan and others (in press) presented in Swanson and others (1989).
Comparison of the Roza Member (~ 14.5 million years ago, volume=1300 km3, emplacement=5-15 years, eruption rate=2600-8100 m2/s) of the Columbia River Flood Basalt Province to lava flows from 1. Kupaianaha (1986-1992, ~0.5km3, 5.6 years, 2-5m2/s), 2. Mauna Loa (1859, 0.27m3, 10 months, 4 m3/s), and 3. Laki (1783-1784, 14.7 km3, 8 months, 1150-4250 m3/s). From Self and others (1997).
The tectonic origin of the flood basalts is not simple. Hooper (1997) identified three major factors:
1. the Yellowstone hot spot;
Many flood basalt provinces are associated with known hot spots and the Yellowstone hot spot may have influenced magma generation for the Columbia River flood basalt but the vents were 300-400 km north of the hot spot track and the chemistry of the basalts suggest a source in the lithospheric mantle not the asthenosphere as expected for hot spot magmas.
The area and volume of the Columbia River Flood Basalt Province are impressive but the volume is one-tenth the volume of other large igneous provinces such as Deccan, Parana, Karoo, and the Siberian Traps.