THE TIETON ANDESITE LAVA FLOWS – OF GREAT LENGTH, BUT WHERE DID THEY COME FROM?

The Tieton andesite lava of the central Washington Cascades is long-thought to have flowed from Black Thumb in the core of the extinct Goat Rocks volcano to its terminus at the mouth of Cowiche Canyon, just northwest of the city of Yakima, Washington, a distance of about 80 km (50 miles). The Tieton andesite, best viewed along U.S. Highway 12 in Tieton Canyon, is well known for spectacular examples of columnar jointing. Don Swanson first noted the existence of two Tieton andesite flows near Rimrock Lake in his PhD thesis, but thought only one flow made it into Tieton Canyon. Our geologic mapping, supported by geochemistry and age-dating, has established the existence of two Tieton andesite flows, separated by about 250,000 years, one of which flowed down an ancestral Tieton River to near its confluence with the Naches River and the other beyond to Cowiche Canyon. The Tieton andesite flows were more likely erupted from vents at Bear Creek Mountain, about 6 km (4 miles) north of the core of the Goat Rocks, than Black Thumb, but still may be the longest known andesite lava flows in the world. Black Thumb, a dacite plug, has much different chemistry than that of Tieton andesite.

 

The older Tieton andesite lava with a silica content of about 62 wt. % and a ⁴⁰Ar/³⁹Ar date of 1.64 ± 0.07 my, flowed 74 km (46 miles) from just east of Bear Creek Mountain to Cowiche Canyon, while the shorter, younger flow with 60 wt. % SiO₂ and a ⁴⁰Ar/³⁹Ar date of 1.39 ± 0.10 my, made it to the Oak Creek Wildlife Area headquarters, a distance of about 52 km (32 miles) from its vent on the ridge north of Bear Creek Mountain. These Pleistocene intracanyon flows occupy ancestral valleys and canyons of the Tieton River and tributaries that drained the Goat Rocks volcano, and the valley of the Naches River from its confluence with the Tieton River to northwest Yakima.  In Tieton Canyon and at two localities near Rimrock Lake, the older flow is perched higher on the slope than the younger flow, suggesting a period of erosional downcutting between emplacement of the two flows.