Extensive flows of the Columbia River Basalt (CRB) Group in Washington, Oregon, and Idaho are dominantly inflated compound pahoehoe sheet lavas. Early studies recognized that CRB lavas are compound pahoehoe flows, with textures suggesting low flow velocities, but it was thought that the great thickness and extent of the major flows required very rapid emplacement as turbulent floods of lava over a period of days or weeks. However, small volume (< 1 km³) compound pahoehoe flows on Kilauea, Hawai'i, demonstrate that such flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger (10–20 km3) pahoehoe flows in Iceland. The vertical distribution of vesicles and other morphologic features within CRB lava flows indicate that they grew similarly by inflation. Small pahoehoe lobes at the base and top of many CRB pahoehoe lava flows indicate emplacement in a gradual, piecemeal manner rather than as a single flood. We propose that each thick CRB sheet flow was active for months to years and that each group of flows produced by a single eruption (a flow field) was emplaced slowly over many years.
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