Mountainous parts of the Yakama Nation lands in south-central Washington are mostly covered by basaltic lava flows and cinder cones that make up the Simcoe Mountains volcanic field. The accompanying geologic map of the central part of the volcanic field has been produced by the U.S. Geological Survey (USGS) on behalf of the Water Resources Program of the Yakama Nation. The volcanic terrain stretches continuously from Mount Adams eastward as far as Satus Pass and Mill Creek Guard Station. Most of the many hills and buttes are volcanic cones where cinders and spatter piled up around erupting vents while lava flows spread downslope. All of these small volcanoes are now extinct, and, even during their active lifetimes, most of them erupted for no more than a few years. On the Yakama Nation lands, the only large long-lived volcano capable of erupting again in the future is Mount Adams, on the western boundary.

The geologic map presented here extends, east-west, from Satus Creek to the Klickitat River and, north-south, from Signal Peak to Indian Rock. In various colors, the map shows the areas covered by about 223 different eruptive units, mostly lava flows and cinder cones, while stars mark vents where many of them erupted. Shown in plain gray, the basement beneath the Simcoe Mountains volcanic field is the Columbia River Basalt Group, regional “flood basalts” of enormous volume and extent that erupted far to the east and long before the Simcoe volcanics.

Although the number of past eruptions is large, few were great explosions that fed towering eruption plumes or spread ash over huge areas downwind. Most were localized basaltic lava fountains (like some in Hawaii) where showers of molten fragments reached heights of a few hundred feet. Most of them also poured out tongues of lava that were channelled along stream valleys for a few miles downstream or, occasionally, as far as 10 miles. Because the basalt so common here is one of the most fluid kinds of lava, it tends to flow farther and faster than most other types of lava before it cools and solidifies.

Lava compositions other than various types of basalt are uncommon here. Andesite is abundant on and around Mount Adams but is very rare east of the Klickitat River. The only important nonbasaltic composition in the map area is rhyolite, which crops out in several patches around the central highland of the volcanic field, mainly in the upper canyons of Satus and Kusshi Creeks and Wilson Charley canyon. Because the rhyolites were some of the earliest lavas erupted here, they are widely concealed by later basalts and therefore crop out only in local windows eroded by canyons that cut through the overlying basalts.