Silverthrone Caldera | Read 10 Min

Silverthrone Caldera
Silverthrone Caldera Complex
Approximate extent of the Silverthrone Caldera
Highest point
Peak	Silverthrone Mountain
Elevation	2,860 m (9,380 ft)
Coordinates	51°31′03″N 126°06′47″W / 51.51750°N 126.11306°W / 51.51750; -126.11306
Dimensions
Silverthrone Caldera Location in British Columbia

The Silverthrone Caldera, also referred to as the Silverthrone Caldera Complex, is a 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 2,860 metres (9,380 feet), although some sources give the elevation as high as 3,160 m (10,370 ft). The caldera is about 25 by 20 kilometres (16 by 12 miles) in size and has been deeply eroded, resulting in the formation of rugged topography. Several glacial meltwater streams originating from the volcano flow through valleys in the Pacific Ranges. These include the Pashleth, Selman and Catto creeks and the Kingcome and Wakeman rivers. The caldera 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 3,600 km² (1,400 mi²) 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 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 caldera is not well known and its affinity to the Garibaldi Volcanic Belt remains unclear. The volcano was studied in the 1970s as a potential source of geothermal energy. It can be reached by helicopter or by trekking on foot through nearby 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 3,600 km² (1,400 mi²) of the southern Coast Mountains. Silverthrone is descriptive of the icy landscape; it may have been coined by Don Munday who, with his wife Phyllis Munday, carried out the first ascent of Silverthrone Mountain in 1936.

Geography

Location and climate

The volcano is 55 km (34 mi) north of Kingcome Inlet and 60 km (37 mi) 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 4,016 m (13,176 ft) 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 2,860 m (9,380 ft), 2,865 m (9,400 ft) and 3,160 m (10,370 ft). Mount Somolenko, 2,660 m (8,730 ft) 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, 2,380 m (7,810 ft) in elevation, lies between the Trudel and Kingcome glaciers. Its name is an extension of the World War II theme; James Kinch was a signalman of the Royal Canadian Corps of Signals who died on September 7, 1941. Petrovsky Peak, west of Mount Kinch, reaches an elevation of 2,300 m (7,500 ft).

Mount Ardern, southwest of Silverthrone Mountain at the head of the Kingcome River, consists of loose volcanic rock. It has an elevation of 2,360 m (7,740 ft) and is named after James Ardern, a Canadian Army soldier killed in action on October 3, 1944, during World War II. Between the Kingcome and Silverthrone glaciers is Mount Squire, another peak composed of loose volcanic rock with an elevation of 2,390 m (7,840 ft). 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. South of Silverthrone Mountain is Mount Calli, which reaches an elevation of 2,420 m (7,940 ft) 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 2,730 m (8,960 ft); Silverthrone Mountain is the highest.

Geology

Tectonic setting

The relationship of the Silverthrone Caldera to other volcanoes in southwestern British Columbia remains unclear. This is because there have been very few geological studies conducted at the caldera. It has been considered to be part of the Garibaldi Volcanic Belt, but it lies 190 km (120 mi) 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.

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. 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. 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 25 km (16 mi) thick. Further studies of the magmatic products of the caldera are required to provide further insights on mantle and slab processes.

The tectonic settings of Silverthrone and its closest prominent neighbour, the Franklin Glacier Complex about 55 km (34 mi) 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 separated from the Juan de Fuca Plate along the Nootka Fault about 4 million years ago. The Silverthrone and Franklin Glacier complexes lie inboard of the Explorer Plate; subduction of this tectonic plate beneath the North American Plate occurs at a rate of about 2 centimetres (0.79 inches) per year. However, both tectonic plates are currently locked to some degree in the Cascadia subduction zone.

Structure

The Silverthrone Caldera is a roughly 25 km (16 mi) long and 20 km (12 mi) wide subsidence structure in the central Coast Crystalline Complex. Such structures form when magma chambers are partially emptied during volcanic eruptions, resulting in the land surface subsiding and the area above the magma chamber collapsing. Subsidence of the area above the magma chamber results in the formation of steep-sided ring faults; these are cylindrical fractures around the edges of calderas. Calderas as large as Silverthrone form as a result of massive Plinian eruptions, which send ash columns high into the stratosphere and create large-scale pyroclastic flows. These caldera-forming eruptions are orders of magnitude larger than the 1980 eruption of Mount St. Helens; they range from 6 to 8 on the Volcanic Explosivity Index. Their extreme explosivity is caused by silica-rich magma which cools on the land surface in the form of volcanic rocks such as dacite and rhyolite. Silverthrone is one of the largest centres of Quaternary acidic volcanism in British Columbia as determined from reconnaissance mapping.