Goat Rocks

Goat Rocks is an extinct stratovolcano in the Cascade Range, located between Mount Rainier and Mount Adams in southern Washington, in the United States. Part of the Cascade Volcanoes, it was formed by the subduction of the Juan de Fuca Plate under the western edge of the North American Plate. The volcano was active from 3.2 million years ago until eruptions ceased between 1 and 0.5 million years ago. Throughout its complex eruptive history, volcanism shifted from silicic explosive eruptions to voluminous, mafic activity.

The volcano and the surrounding Goat Rocks Wilderness, established in 1964, are named after the numerous mountain goats which live in the area. The variable climate of the wilderness area supports a host of wildlife, including marmots, pikas, deer, and elk. Various peaks within the wilderness can be climbed or scrambled, and other activities such as hiking are permitted. The area is rife with minerals and resources, including coal, cobalt, copper, and base metals.

Geography and geology

Goats Rocks volcano is located in southern Washington, west of Yakima,Wood (1992), p. 161. at latitude 46.50° N and longitude 121.45° W.Wood (1992), p. 160. This region of the Cascades was originally occupied by Native Americans, who hunted and fished in its vicinity and used its trails as trade routes.Barstad, p. 2. Goat Rocks lies in a zone of intermittent volcanism which has produced many small volcanic vents, also including the Mount Adams volcanic field and Indian Heaven. Situated in the eastern portion of the Cascade Range, Goat Rocks lies at the northwest corner of the Klickitat River basin. As a member of the Cascade Volcanoes, Goat Rocks was produced by the subduction of the oceanic Juan de Fuca plate under the western edge of the continental North American Plate. This fault, known as the Cascadia subduction zone, lacks the deep oceanic trench usually found at convergent plate boundaries, which can probably be explained by its slow rate of subduction. According to the United States Geological Survey, the Nazca and North American plates converge at a rate of to each year, just half of their convergence rate as recently as 7 million years ago.

The Cascade Volcanoes in Washington are restricted to four belts; Goat Rocks forms one segment of the north–south trending Mount Adams belt that also incorporates Tumac Mountain, Adams, and the King Mountain Fissure Zone. Of the four Cascade arcs, this belt contains the second most volcanoes, and its volcanoes are predominantly made of calc-alkalic to tholeiitic to basaltic andesite lava. Goats Rocks and its immediate vicinity are underlain by pre-Tertiary greywacke and argillite, and these deposits are cut by several prominent northward-trending faults.Church and Close (1984), p. 1063. Overlaying these deposits are Cenozoic volcanic rocks, including the thick, Eocene-aged Ohanapecosh Formation, comprised by chemically altered basalt and andesite lava flows and bedded andesite and dacite volcaniclastic rock. In turn, the Stevens Ridge Formation overlies the Ohanapecosh layer, featuring quartz-bearing silicic tuff, rhyolite, and other volcaniclastic rocks. Under both layers is the Russell Ranch Formation, to in thickness, which contains deposits of lapilli and tuff hinting at a caldera, although geologists have failed to identify clear evidence for the formation of a caldera in this zone.Church et al., p. 4.

Together with Mount Adams, Mount Rainier, and Mount St. Helens, the Goat Rocks volcano is part of a triangle of volcanoes, an arrangement not found elsewhere in the Cascades. This may be related to a mid- crust zone with abnormally high electrical conductivity, the Southern Washington Cascades Conductor (SWCC), which is not well understood by geologists. Scientists from the United States Geodynamics Committee hypothesized in 1994 that this anomaly is associated with the thrust of a large deposit of sedimentary rock against a continental margin. However, Hill et al. (2009) dismiss this view as it proves inconsistent with maps made from magnetotelluric readings of the area. Instead, they propose that the anomaly is linked to the region's volcanism, which has caused partial melting of the crust.Hill et al. (2009), pp. 785–788.

The oldest Cascade stratovolcano formed after the Columbia River Basalt Group, Goat Rocks is roughly equal in volume to Mount St. Helens and smaller in volume than Mount Baker, at approximately . Rough estimates place the composition of the volcano as 40% tuff and breccia.Smith, p. 10. The remains of its lava flows can be found in the Tieton, Klickitat, and Cispus river valleys, their elevation reversed by erosion to make them resemble ridges. These flows have been thoroughly eroded, but testify to the volcano's previous size.Smith, p. 9.

Glaciation

Although it may have once been a towering stratovolcano, Goat Rocks has undergone extensive erosion since its extinction. Once eruptions ceased, the ongoing forces of glacial erosion, as well as hydrothermal alteration of the volcano's interior, stripped away layers of volcanic ash deposits and lava flows, uncovering the rocky lava spines where magma had cooled and hardened within the conduits of the volcano.Harris, p. 254. Nearly all the remaining volcanic debris has been partially covered by a younger layer of rock.

The Goat Rocks area is notable for its extensive glaciers, despite its modest elevation and southerly location relative to the rest of the Washington Cascades. Four major glaciers mantle the north and northeast slopes of the peaks: Conrad Glacier,Beckey, p. 72. McCall Glacier, Meade Glacier, and Packwood Glacier, along with numerous smaller permanent snowfields.

Major peaks in the Goat Rocks vicinity

The elevation of the Goat Rocks volcano and its immediate vicinity varies from to at its highest point, Gilbert Peak. Because of extensive glaciation and erosion, the peaks surrounding Goat Rocks are at most moderate in height.

Eruptive history

Goat Rocks is a stratovolcano with a somewhat complicated eruptive history. It first became active approximately 3.2 million years ago during the Pliocene

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