Silverthrone Caldera
The Silverthrone Caldera, also referred to as the Silverthrone Caldera Complex, is a poorly-studied volcano in Range 2 Coast Land District of British Columbia, Canada. It lies within the Pacific Ranges of the Coast Mountains and reaches an elevation of, although some sources give the elevation as high as. The caldera is about in size and has been deeply eroded, resulting in the formation of rugged topography. Several glacial meltwater streams originating from the Silverthrone Caldera flow through valleys in the Pacific Ranges; among these streams are the Pashleth, Selman and Catto creeks and the Kingcome and Wakeman rivers. The volcano contains several named mountains, including Mounts Somolenko, Overill, Kinch, Squire, Ardern and Calli, as well as Petrovsky Peak and Silverthrone Mountain.
Volcanic rocks deposited by eruptions of the Silverthrone Caldera and associated vents include rhyolites, dacites, andesites and basaltic andesites. They are exposed in valleys, but at higher elevations, they are largely buried under glacial ice of the Ha-Iltzuk Icefield. These rocks comprise three geological units; a 750,000-year-old basal breccia unit, a 400,000-year-old unit of overlying lava flows and domes, and a less than 13,000-year-old series of lava flows and pyroclastic cones. Small magnitude, shallow earthquakes have been recorded near the volcano since 1980, but they have not been demonstrated to be magmatic in origin. The main potential hazard posed by future volcanism is to air traffic if explosive eruptions were to occur from the Silverthrone Caldera.
The Silverthrone Caldera was a source of obsidian for indigenous peoples during the pre-contact era. Geological studies have been conducted at the volcano since at least the 1960s, but its very remote location has impeded detailed fieldwork. As a result, the eruptive history of the volcano is poorly known and its affinity to the Garibaldi Volcanic Belt remains unclear. The Silverthrone Caldera was studied in the 1970s as a potential source of geothermal energy. It can be reached by helicopter or, with great difficulty, by trekking on foot through valleys.
Names and etymology
The Silverthrone Caldera has been described as the Silverthrone Caldera Complex and the Silverthrone Depression. Other terms, such as the Silverthrone volcanic complex and the Silverthrone volcanic field, refer to the caldera and associated volcanic rocks. Such terms are derived from Silverthrone Mountain, a volcanic feature associated with the caldera whose name has been reported in Canadian Alpine Journal articles as early as 1933. In a 1968 Geological Survey of Canada report, the eruptive products of the caldera were referred to as the Mount Silverthrone volcanic complex by Jack Souther, Canada's first volcanologist.The Silverthrone Caldera also shares its name with Silverthrone Glacier, an outlet glacier of the local Ha-Iltzuk Icefield which covers approximately of the southern Coast Mountains. Silverthrone is descriptive of the icy landscape; it may have been coined by Don Munday who carried out the first ascent of Silverthrone Mountain along with his wife Phyllis Munday in 1936.
Geography
Location and climate
The volcano is north of Kingcome Inlet and northwest of Knight Inlet in southwestern British Columbia, Canada. It lies in a rugged, ice-dominated portion of the Pacific Ranges which are the southernmost subdivision of the Coast Mountains. The surrounding mountains are the highest in British Columbia south of the Saint Elias Mountains; Mount Waddington northeast of the head of Knight Inlet has an elevation of and is the highest mountain entirely within British Columbia. Characterizing the landscape at higher elevations are glaciers and extensive icefields, although bedrock composed of granitoids is greatly exposed. The area is part of the Central Pacific Ranges Ecosection, one of seven ecosections comprising the Pacific Ranges Ecoregion.Moist air originating from the Pacific Ocean ascends over Queen Charlotte Sound, Queen Charlotte Strait or the Vancouver Island Ranges before reaching the Pacific Ranges. While ascending the Pacific Ranges, this air comes in contact with cold air from the British Columbia Interior and drops significant precipitation in the form of heavy rains or snow. The heavy rains are absorbed by wet mountain hemlock subalpine forests on mid-elevation slopes and wet western hemlock forests in valleys and lower elevation slopes. Alpine vegetation is restricted to a narrow band between the subalpine forests and the higher icefields. There are no settlements near the Silverthrone Caldera, although summer sport fishing recreation camps and logging operations have been in the area.
Drainage
From Kingcome Glacier in the southern part of the caldera, the Kingcome River flows south into the head of Kingcome Inlet northeast of Broughton Island. Trudel Creek, a tributary of the Kingcome River, originates from the head of Trudel Glacier and flows southwest along the inferred southeastern boundary of the Silverthrone Caldera. Charnaud Creek originates from a valley-filling lava flow adjacent to the southeastern boundary of the caldera and flows southwest into the Kingcome River. At the terminus of Pashleth Glacier in the northern part of the caldera is Pashleth Creek; it flows northwest into the Machmell River which flows west into Owikeno Lake at the head of Rivers Inlet.Selman Creek, a tributary of Pashleth Creek, flows to the northeast from Selman Lake at the northwestern end of the Silverthrone Caldera. From an unnamed glacier just south of Selman Lake at the western end of the central volcanic ridge, the Wakeman River flows south into Wakeman Sound of Kingcome Inlet. Catto Creek originates from an unnamed glacier on the central volcanic ridge and flows southwest across the inferred southwestern boundary of the caldera before it empties into the Wakeman River. Silverthrone Glacier flows southeastward from Silverthrone Mountain through a valley to Klinaklini Glacier, which lies at the head of the West Klinaklini River.
Mountains
Silverthrone Mountain is the highest point of the Silverthrone Caldera, whose summit has variously been given elevations of, and. Mount Somolenko, in elevation, lies within the caldera south of Silverthrone Mountain between the Klinaklini and Kingcome glaciers. It is named after Nicholas Somolenko, a leading aircraftman of the Royal Canadian Air Force who was killed in World War II when his aircraft was shot down on June 7, 1944. Just southwest of Silverthrone Mountain also between the Klinaklini and Kingcome glaciers is Mount Overill, so-named after William Overill who was a Canadian Army soldier killed in action on October 6, 1943, during World War II. Mount Kinch, in elevation, lies between the Trudel and Kingcome glaciers. Its name is an extension of the World War II theme; James Kinch was a Canadian Army soldier who died on September 7, 1941.Petrovsky Peak, west of Mount Kinch, reaches an elevation of. Between the Kingcome and Silverthrone glaciers is Mount Squire, a high peak made of loose volcanic rock. Its name is also an extension of the World War II theme; James Squire was a Canadian Army soldier killed in action on July 18, 1943. Mount Ardern, southwest of Silverthrone Mountain at the head of the Kingcome River, also consists of loose volcanic rock. It has an elevation of and is named after James Ardern, a Canadian Army soldier who was killed in action on October 3, 1944, during World War II. South of Silverthrone Mountain is Mount Calli, which reaches an elevation of north of the head of the Kingcome River. It is named after Canadian Army soldier Kenneth Calli who was killed in action on August 15, 1944, during World War II.
These mountains are part of the Silverthrone Group, an extensive group of mountains bounded in the east by the Klinaklini and North Klinaklini rivers, in the north by Owikeno Lake, Rivers Inlet and the Sheemahant River, in the west by the Pacific Ocean and in the south by Knight Inlet. Many of the mountains in this group consist of volcanic rocks, but it also includes non-volcanic mountains such as Mount Fitzgerald, whose summit consists of granite. The highest mountains in the Silverthrone Group are Silverthrone Mountain, Rampart Peak, Crumble Peak and an unnamed peak with an elevation of ; Silverthrone Mountain is the highest.
Geology
Tectonic setting
The relationship of the Silverthrone Caldera to other volcanoes in southwestern British Columbia remains unclear due to there having been very few geological studies conducted at the caldera. It has been considered to be part of the Garibaldi Volcanic Belt, but it lies west-northwest of the main trend of this volcanic zone, making its connection to the Garibaldi Volcanic Belt questionable. The volcano has also been included as part of the much older Pemberton Volcanic Belt, which overlaps with the trend of the Garibaldi Volcanic Belt near Meager Creek to the southeast. Both volcanic belts were formed by subduction zone volcanism along the continental margin of western North America in the last 29 million years and are part of the Cascade Volcanic Arc. The types of volcanic rocks found at the Silverthrone Caldera are comparable to those in continental arcs; they belong to the calc-alkaline magma series. Likewise, the lifespan of the caldera is comparable to most of the large evolved eruptive centres in the Cascade Volcanic Arc, which have lifespans ranging from 100,000 to 1,000,000 years.Silverthrone is sometimes excluded from the Garibaldi Volcanic Belt and the Cascade Volcanic Arc due to it being sketchily known and ambiguous in its affinity. When included, Silverthrone is the northernmost major eruptive centre of both the Garibaldi Volcanic Belt and the Cascade Volcanic Arc. However, the relationship of the minor Milbanke Sound Cones further to the northwest with volcanoes of the Garibaldi Volcanic Belt also remains unclear. This is because little is known about these volcanic cones; they may reflect a northern extension of the Garibaldi Volcanic Belt or they may have formed as a result of a different geological process. Further studies of the magmatic products of the Silverthrone Caldera are required to provide additional insights on mantle and slab processes. The Smithsonian Institution's Global Volcanism Program lists the tectonic setting of the Silverthrone Caldera as a subduction zone and the underlying continental crust more than thick.
The tectonic settings of Silverthrone and its closest prominent neighbour, the Franklin Glacier Complex about to the east-southeast, appear to differ from other volcanoes in the Garibaldi Volcanic Belt. The main portion of this volcanic belt, which extends from the Salal Glacier volcanic complex in the north to the Watts Point volcanic centre in the south, is the result of subduction of the Juan de Fuca Plate beneath the North American Plate. Immediately north of the Juan de Fuca Plate is the Explorer Plate, which are separated by the Nootka Fault. The Silverthrone and Franklin Glacier complexes lie inboard of the Explorer Plate which is subducting under the North American Plate at a rate of about per year. However, both tectonic plates are currently locked to some degree in the Cascadia subduction zone.