Columbia River Basalt Group

The Columbia River Basalt Group eruption began approximately 17 million years ago at the eastern edge of present-day Oregon (the Steens Mountain area, where the Steens Basalt formation defines the southernmost CRBG outflow). The eruption centre migrated northwestward over the next several million years, with the largest eruptions occurring approximately 16 to 15 million years ago at the Chief Joseph Dike Swarm in eastern Washington and western Idaho. The Chief Joseph Dike Swarm consists of approximately 21,000 individual feeder dikes (vertical sheets of basalt that fed surface eruptions), most concentrated in a north-south band approximately 100 km wide and 300 km long along the Washington-Idaho border. Individual basalt flows erupted in eruptive bursts measured in days to weeks: the largest flows (the Roza Member of the Wanapum Basalt and the Sentinel Bluffs Member of the Grande Ronde Basalt) each erupted approximately 1,300 to 1,500 cubic kilometres of lava in roughly a decade, with individual lava sheets running from the eruptive centre to the Pacific Ocean in days. The eruption rate (cubic kilometres per year averaged over the active eruption period) was the highest sustained eruption rate ever documented on Earth from a continental flood basalt province.

• Eruption begins ~17 mya at Steens Mountain (southern Oregon); migrates northwestward; largest eruptions ~16-15 mya at Chief Joseph Dike Swarm (eastern WA/western ID border)
• Chief Joseph Dike Swarm: ~21,000 individual feeder dikes in a north-south band ~100 km wide and 300 km long; most concentrated dike swarm in any continental flood basalt province
• Largest individual flows (Roza, Sentinel Bluffs): ~1,300-1,500 cubic km of lava in ~a decade per flow; individual lava sheets ran from eruptive centre to Pacific Ocean in days
• Sustained eruption rate (cubic km per year over active period) is the highest ever documented for a continental flood basalt province

The Columbia River Basalt Group is divided into five formations representing distinct phases of the eruption sequence. The Steens Basalt (~17 to 16.6 mya) is the oldest and southernmost: a relatively small-volume formation that crops out in southeastern Oregon (Steens Mountain area) and represents the initial eruption phase. The Imnaha Basalt (~17.5 to 16 mya, slightly overlapping Steens) crops out in northeastern Oregon and southeastern Washington and represents the second phase. The Grande Ronde Basalt (~16 to 15.6 mya) is by far the most voluminous formation, accounting for approximately 70 percent of total CRBG volume; Grande Ronde flows reached the present-day Pacific coast and the Willamette Valley, and the modern bedrock of most of Washington wine country is Grande Ronde Basalt. The Wanapum Basalt (~15.6 to 15 mya) is the next most voluminous formation and crops out across much of central Washington; the Roza Member of Wanapum is a prominent individual flow visible at many wine-country roadcuts. The Saddle Mountains Basalt (~14.5 to 6 mya) is the youngest and least voluminous formation; it consists of multiple smaller flow members spread over a long eruptive interval and forms the youngest exposed basalt at many vineyard sites including the Wahluke Slope.

• Steens Basalt (~17-16.6 mya): oldest, southeastern Oregon, relatively small volume, initial eruption phase
• Imnaha Basalt (~17.5-16 mya): northeastern Oregon, southeastern Washington, second phase
• Grande Ronde Basalt (~16-15.6 mya): ~70 percent of total CRBG volume; reached Pacific coast and Willamette Valley; modern bedrock of most WA wine country; parent material for Jory and Laurelwood soils
• Wanapum Basalt (~15.6-15 mya): second most voluminous, central WA; Roza Member prominent at many wine-country roadcuts. Saddle Mountains Basalt (~14.5-6 mya): youngest, multiple smaller flows including Wahluke Slope

The westernmost CRBG flows reached the Willamette Valley approximately 15 to 10 million years ago, flowing down the ancestral Columbia River channel and inundating much of what is today the Willamette Valley floor. These flows are the parent material for two of the Willamette Valley's most consequential wine soils. The Jory soil series (named for Jory Hill in Marion County, Oregon, where the soil was first formally described in the 1940s) is a red iron-rich clay produced by tropical-style weathering of basalt over millions of years; Jory soils sit directly on basalt bedrock at the surface of the Dundee Hills and elsewhere in the central Willamette Valley. The deep red color comes from oxidized iron (hematite) released as the basalt's ferromagnesian minerals weather to clay. Jory soils produce Pinot Noir with richer red fruit, silkier mid-palate texture, and slightly fuller-bodied register than the marine-sedimentary Willakenzie soils of Yamhill-Carlton. The Laurelwood soil series (named for the Laurelwood schoolhouse in Washington County, Oregon) is a windblown loess deposit that sits over Columbia River Basalt bedrock in the Tualatin Mountains; the Laurelwood is silt-textured at the surface but rests on basalt at depth, producing Pinot Noir with elegant fruit, fine-grained tannin, and a floral aromatic register that distinguishes it from both Jory and Willakenzie sites.

• Westernmost CRBG flows reached the Willamette Valley ~15-10 mya; flowed down ancestral Columbia River channel; inundated much of present Willamette Valley floor
• Jory soil series: red iron-rich clay from tropical-style weathering of basalt over millions of years; iron oxide (hematite) produces deep red color; sits directly on basalt bedrock at Dundee Hills surface
• Jory Pinot Noir register: richer red fruit, silkier mid-palate texture, slightly fuller-bodied; distinguishes Dundee Hills from Yamhill-Carlton's marine-sedimentary Willakenzie register
• Laurelwood soil series: windblown loess over Columbia River Basalt bedrock; silt-textured at surface, basalt at depth; produces Pinot Noir with elegant fruit, fine tannin, floral register

Washington wine country sits directly on the Columbia River Basalt Group bedrock. The Walla Walla Valley sits on exposed Grande Ronde Basalt, with Touchet Bed silts (Missoula Flood slack-water deposits) layered above the basalt at most vineyard sites. Red Mountain in the eastern Yakima Valley sits on a basalt anticline (a fold in the CRBG that produced the mountain itself), with basalt-derived soils mixed with loess and alluvial deposits; the basalt mineral signature contributes to Red Mountain Cabernet's structural intensity and dense extracted tannin. Wahluke Slope sits on Saddle Mountains Basalt (the youngest CRBG formation) overlain by loess and Missoula Flood gravels. The Rocks District of Milton-Freewater AVA on the Oregon side of Walla Walla deserves special mention: the cobblestone surface that defines the Rocks consists of basalt cobbles eroded from upstream CRBG outcrops along the Walla Walla River and deposited as alluvial fan material. The fist-sized to softball-sized basalt cobbles cover much of the vineyard surface and produce the distinctive ferrous iron-stained mineral register that defines Rocks District Syrah from Cayuse Vineyards, Reynvaan Family Vineyards, and the dozen-plus other producers working the AVA's distinctive terroir.

• Walla Walla Valley: exposed Grande Ronde Basalt; Touchet Bed silts (Missoula Flood slack-water) layered above; most vineyard sites have basalt at depth, silts at surface
• Red Mountain (eastern Yakima Valley): basalt anticline produced the mountain; basalt-derived soils mixed with loess and alluvial deposits; basalt mineral signature contributes structural intensity to WA's Cabernet flagship
• Wahluke Slope: Saddle Mountains Basalt (youngest CRBG) overlain by loess and Missoula Flood gravels; the broad south-facing slope produces the warmest sustained ripening in eastern Washington
• Rocks District of Milton-Freewater: alluvial basalt cobblestones eroded from upstream CRBG outcrops along Walla Walla River; fist-to-softball-sized cobbles cover vineyard surface; produces ferrous iron-stained mineral register in Cayuse + Reynvaan Syrah