Yamsay Mountain - Geography and Geology

Geography and Geology

Like other volcanoes in the Cascades Range, Yamsay was produced by subduction of the oceanic Juan de Fuca Plate under the North American Plate. Annual convergence between the two continues at a rate of 4 centimetres (1.6 in) per year. The Cascades make up the majority of the North American West Coast volcanoes; they have been volcanically active for about 36 million years. The rock making up the Cascades is slightly older, dated between 55 and 42 million years old. In an attempt to organize the volcanoes by age, scientists typically divide them into two provinces: the High Cascades – the younger volcanoes – and the Western Cascades, comprising the older volcanoes.

Yamsay, along with the Simcoe volcanic complex, Medicine Lake Highlands, and Newberry Volcano, make up a wayward range of shield volcanoes between 50 kilometres (31 mi) and 60 kilometres (37 mi) east of the main Cascades Range. Volcanism at these centers is unlike other eruptions in the Cascades, and does not stem from fissures as would Basin and Range volcanism. The volcanoes appear to be isolated from the major volcanic ranges in the Northwestern United States, although scientists group them as part of the Cascades.

Recent volcanism in the Cascades, lasting from 7 to 2 mya, produced largely basaltic and basaltic andesite-based material (up to 50 percent). On the eastern side of the High Cascades, however, volcanoes erupted dacite, andesite, and rhyolite. These materials made up several types of eruptions: lava flows, domes, and pyroclastic flows.In the most recent volcanic activity, andesite, basalt, and rhyolite have been erupted from High Cascades volcanoes, particularly Three Sisters-Broken Top, Mount Hood, Mount Jefferson, and Crater Lake.