page_title: Tuya
section_title: Formation
hierarchical_section_title: Tuya / Formation
caption_attribution_description: This is the majestic Santiam Pass, elevated at. 4817 ft above sea level in central Oregon Cascades. It is located about 18 mi (29 km) northwest of Sisters, between volcanic peaks of Three Fingered Jack to the north and Mount Washington to the south. U.S. Highway 20 connects eastern Oregon with the valley of the Santiam River on the west via Santiam Pass. One of the 19 or 20 lakes by the name of Lost Lake is located beside the highway just west of Santiam Pass. While the pass was known by native peoples, the first recorded crossing of the pass was in April 1859 by an expedition searching for a cattle trail over the Cascade Range from the Willamette Valley to Central Oregon. This expedition was led by Andrew Wiley, who reportedly climbed a tall tree on a mountain near Lost Prairie to help determine the route. Lost Prairie is located along Hackleman Creek between Fish Lake and Tombstone Prairie. Wiley later helped establish the Santiam Wagon Road. The prominent glaciated peak on the photo is the Three Fingered Jack, a Pleistocene volcano elevated 7,841 foot (2,390 meter), deeply glaciated and consists mainly of basaltic andesite lava. Mount Jefferson can also be seen from its left. The typical tuya volcano Hogg Rock, which is clearly visible from the photo below the Three Fingered Jack where US 20 Highway wraps in a nearly 180-degree curve below cliffs on the south and west sides, is only about 2 miles (3 km) north of Hayrick Butte, a somewhat larger tuya of similar age and composition. A tuya is a type of subglacial volcano, formed when lava erupts underneath an overlying glacier or ice sheet and then melts through to the surface and pools, producing the flat plateau on top with near-vertical walls along the ice-contact margin as the lava cools and hardens. Unlike most tuyas which have steep cliffs on all sides, Hogg Rock has a break in its cliffs on the east side, with a moderate slope which leads down to a small road and gravel pit. Santiam Pass was also the intended route over the Cascades for the Oregon Pacific Railroad, an early transcontinental railroad project led by Col. T. Egenton Hogg for whom Hogg Rock is named. Although the working line never extended east of Santiam Pass, the pass is today the eastern terminus of the Oregon Pacific Railroad Linear Historic District, which is listed on the US National Register of Historic Places. Taken from Hoodoo Ski Bowl summit.
context_page_description: A tuya is a type of distinctive, flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet. They are rare worldwide, being confined to regions which were covered by glaciers and had active volcanism during the same period.
  As lava that erupts under a glacier cools very quickly and cannot travel far, it piles up into a steep-sided hill. If the eruption continues long enough, it either melts all the ice or emerges through the top of the ice and then creates normal-looking lava flows that make a flat cap on top of the hill. Discovering and dating the lava flows in a tuya has proven useful in reconstructing past glacial ice extents and thicknesses.
context_section_description: Tuyas are a type of subglacial volcano that consists of nearly horizontal beds of lava capping outward-dipping beds of fragmental volcanic rocks, and they often rise in isolation above a surrounding plateau. Tuyas are found in Iceland, British Columbia, the Santiam Pass region in Oregon, the Tyva Republic in eastern Russia, the Antarctic Peninsula and beneath the West Antarctic Ice Sheet. Tuyas in Iceland are sometimes called table mountains because of their flat tops.
  S. Holland, a geographer for the British Columbia government, described tuyas in the following way:
  "They have a most interesting origin ... [they were] formed by volcanic eruptions which had been thawed through the Pleistocene ice-sheet by underlying volcanic heat. The lavas capping the mountains were extruded after the volcanoes were built above lake-level, and the outward-dipping beds were formed by the chilling of the lava when it reached the water's edge."
  Because they erupt under ice and water, tuyas have phreatomagmatic eruptions creating layers of breccia and hyaloclastite above pillow lavas. If the volcano breaches the surface of the glacier it will be topped by a subaerially erupted lava plateau.
caption: Hogg Rock (foreground), Oregon
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