Columbia River Basalts

Columbia River Basalt Group

In general, the region of the lower Snake River and the Columbia River’s course through the Columbia Plateau and Columbia River Gorge experienced volcanic activity starting some 55 million years ago, although the expedition would primarily encounter rock units of Miocene-age.^[6]Terry L. Tolan, Marvin H. Beeson, and Beverly F. Vogt, “Exploring the Neogene History of the Columbia River: Discussion and Geologic Field Trip Guide to the Columbia River Gorge—Part 1 … Continue reading Following their descent of the Clearwater River, the Lewis and Clark expedition passed through what is commonly referred to as the Columbia River Basalt Group (CRBG), one of the most extensive accumulations of basaltic lava on earth, deposited by eruptions that occurred between 17.5 and 6 million years ago. Basalt, of which more than 90 percent of all volcanic rocks are composed, is hard and dense with a dark, crystalline appearance. Unweathered surfaces of basalt are black, grayish-black, and dark to medium gray, but the rock weathers readily to a spectrum of lighter grays and rusty-colored appearances (such as reddish-gray, reddish-brown, yellowish-brown, and yellowish-orange) because of oxidation of its iron-rich mineral content.

For our purposes, it is unnecessary to describe the exhaustively complex sequence of over three hundred individual basalt flows of the CRBG, but it will be useful to know the general sequence of the major eruptive episodes in reference to the various basalt formations the Lewis and Clark expedition encountered. Initial eruptions between 17.5–17 million years ago formed the geographically limited Imnaha Basalt, but it was the more than one hundred individual lava flows between 17 and 15.5 million years ago that resulted in the Grande Ronde Basalt, the most voluminous and extensive formation composing 85 percent of the CRBG’s total volume.^[7]Donald A. Swanson, Kenneth A. Cameron, Russell C. Evarts, Patrick T. Pringle, and Joseph A. Vance, “Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and … Continue reading Later lava flows between 15.5 and 14.5 million years ago formed the Wanapum Basalt, which was followed by a long sequence of eruptive events lasting between 14 and 6 million years ago that deposited lavas collectively known as the Saddle Mountain Basalt.^[8]Ibid. The composition of basalt and how it differs from other extrusive volcanic rock requires a brief digression into the functional mineralogy of magma. If the silica (the same silicon-oxygen … Continue reading Despite their low representations in volume, the expedition would encounter Wanapum Basalt and the Saddle Mountain Basalt rocks quite frequently during their journey because of their greater surface exposure.

The areal extent of these lava flows is simply astonishing, covering the upper third of the state of Oregon, the lower half of Washington State and much of the western border of Idaho, some 163,700 ² in extent.^[9]Terry L. Tolan, Stephen P. Reidel, Marvin H. Beeson, James Lee Anderson, Karl R. Fecht, and Donald A. Swanson, “Revisions to the Estimates of the Areal Extent and Volume of the Columbia River … Continue reading That is 63,205 square miles, an area the size of Pennsylvania, New Jersey, Delaware, and Connecticut with enough lava left over to cover Washington, D.C. five times over. Geologists often refer to the CRBG lavas as “flood basalts,” which concisely summarizes their behavior when they broke out onto the earth’s surface. Such a flowable tendency explains why some CRBG lavas, discharging from fissures or vents located in northeastern Oregon, eastern Washington, and western Idaho,^[10]Tolan, et al., “Neogene History,” 46:8, 88. could flow westward at speeds of 3 to 9 miles per hour (or faster) for hundreds of miles and, in some instances, breach both the ancestral Cascade and Coast mountain ranges, and reach the Pacific Ocean.^[11]Marvin H. Beeson, Rauno Perttu, and Janice Perttu, “The Origin of the Miocene Basalts of Coastal Oregon and Washington: An Alternative Hypothesis,” Oregon Geology, 41:10 (October 1979): … Continue reading

Absence of Basalt Nomenclature in the Journals

Our “following the floods” story begins as the Corps of Discovery proceeded down the Snake River (beginning at present-day Lewiston, Idaho, and Clarkston, Washington, where it meets the Clearwater River) near the eastern edge of the basalt flows. This is where the expedition journal keeper’s observations of this dark-colored rock commence:

[11 October 1805:] The hills on the river are…rocky; the rocks of a dark colour. —Gass

[12 October 1805:] The hills or assents from the water is faced with a dark ruged Stone. —Clark

Despite the brevity of these and subsequent observations, the journal keepers captured the prevailing geologic feature, cliffs of dark-colored basalt, along this part of the Snake and Columbia Rivers route. An enduring mystery, however, and a question with no satisfactory answer, is why they never described the rocks they were encountering over the next several hundred miles as “basalt.” It is true that the mineralogical nature of basalt was unknown at this time, and that modern expertise in mineralogy and diagnostic indicators such as paleomagnetism are required to distinguish one basalt flow from another. Yet, there was an unmistakable description of basalt and its common columnar shape in Richard Kirwan’s Elements of Mineralogy,^[12]Richard Kirwan, Elements of Mineralogy, 2nd Edition, Vol. 1 Earths and Stones (London: Printed by J. Nichols for P. Elmsly, in the Strand, 1794), and Richard Kirwan, Elements of Mineralogy, 2nd … Continue reading believed to be the only geology and mineralogy reference book carried by Lewis and Clark. Kirwan’s descriptions of basalt were easily recognizable and distinctive, whether he was describing the color (“greyish black, sometimes bluish, or brownish black; when withered [weathered], the surface is greyish, or reddish brown), shape (“generally of a columnar form, streight or curved, perpendicular or inclined”), geometry (“pentangular, hexangular, octangular”), or occurrence (“found in considerable masses”).

There is strong evidence the captains referred to Elements of Mineralogy at various times throughout the expedition,^[13]For instances where the captains used terminology that is highly reminiscent of Kirwan’s technical phrasing, see pages 190-195 in John W. Jengo, “‘Mineral Productions of Every … Continue reading but it remains unexplained why they did not attempt any rock identification of this dominant lithology. This absence of basalt nomenclature in the journals is particularly puzzling given that virtually all the consolidated rocks the expedition encountered here were volcanic basalts, including miles upon miles of the characteristic hexagonal, columnar-jointed columns evident all along this portion of the route.

Lewis and Clark’s geologic observations from the confluence of the Clearwater and Snake Rivers to the mouth of the Sandy River may be less obvious at first glance than their remarks along the Missouri River, yet some of their most valuable and trenchant notations were made along this route. That many of these observations were made at times of great anticipation and unremitting apprehension is remarkable enough. To have their descriptions of natural phenomena confirmed by subsequent accounts of missionaries and expanded upon by modern scholarly research justifies recognizing the journalists of the corps as first-rate geological observers befitting their command of the aptly-named Corps of Volunteers for North Western Discovery.