The Spectrum Range is a small mountain range in Cassiar Land District of northwestern British Columbia, Canada. Located on the Tahltan Highland, it borders the Skeena Mountains in the east and the Boundary Ranges of the Coast Mountains in the west. The range is bounded by the Arctic Lake Plateau in the southwest and the Kitsu Plateau in the northwest. It lies at the southern end of the Mount Edziza volcanic complex, which includes the two neighbouring plateaus and the Big Raven Plateau to the north. The range contains glaciers and is drained on all sides by streams within the Stikine River watershed. It is characterized by multi-coloured rocks, after which the range is named. Kitsu Peak, the highest point of the range, reaches an elevation of . The range is a protected area as it lies within Mount Edziza Provincial Park, one of the largest provincial parks in British Columbia.
The Spectrum Range is the eroded remains of a large lava dome whose original surface is only preserved as a few small remnants on the summits of the higher peaks. This dome, originally more than wide, formed between 3.5 and 2.5 million years ago during the second magmatic cycle of the Mount Edziza volcanic complex. Much of the dome consists of massive rhyolite and trachyte lava flows, but relatively minor basalt flows erupted later during the dome's formation. These lava flows form the nearly circular group of pyramidal peaks and long, narrow-crested ridges comprising the range; the basalt flows mainly cap the higher peaks. Volcanism in the last 2.5 million years has mostly occurred on the northwestern and southwestern sides of the range, but the precise age of the latest eruption is unknown.
Names and etymology The Spectrum Range was labelled as the Rainbow Mountains on a BC Lands map published in 1929; this was followed by the renaming of the mountain range to the Spectrum Mountains in 1945. In 1954, the form of name was changed to the Spectrum Range in accordance with Geological Survey of Canada memoir 247 published in 1948. These names for the range refer to its multi-coloured rocks; pale green, light grey and white rocks weather to bright hues of orange, yellow and red. In the 1980 book Natural Wonders of the World, the range is also referred to as the Spectrums.
Geography Location The Spectrum Range is located on the Tahltan Highland east of the Boundary Ranges of the Coast Mountains and west of the Skeena Mountains in Cassiar Land District. To the southwest, the range is bounded by the Arctic Lake Plateau, which includes adjacent volcanic features such as Wetalth Ridge, Nahta Cone and Tadekho Hill. The Kitsu Plateau bounds the range to the northwest and includes the Mess Lake Lava Field, which consists of lava flows and tephra from three pyroclastic cones. To the north and northeast, the range is bounded by Raspberry Pass and Artifact Creek valley, respectively; the latter separates Artifact Ridge from the Spectrum Range.
The Spectrum Range lies at the southern end of the Mount Edziza volcanic complex which includes the Arctic Lake and Kitsu plateaus, and Mount Edziza and the Big Raven Plateau to the north. The volcanic complex contains an intermontane plateau that is overlain by four central volcanoes along its north–south trending axis; the Spectrum Range is the southernmost and third oldest of these volcanoes. Surrounding the range is Mount Edziza Provincial Park, one of the largest provincial parks in British Columbia.
Structure
The Spectrum Range consists of long, narrow-crested ridges and a nearly circular group of pyramidal peaks. Talus and felsenmeer deposits cover large portions of the valley slopes which rise to broad, rounded crests of the interfluvial ridges. These ridges are the eroded remains of a once-continuous lava dome whose original surface is only preserved as a few small remnants on the summits of the higher peaks. The ridges and peaks decrease in elevation away from the central portion of the range; among them are the Kitsu, Yeda and Kuno peaks and the Obsidian and Yagi ridges. Kounugu Mountain at the southwestern end of the range is the only feature with Mountain in its name. BC Geographical Names describes the Exile and Outcast hills as being in the Spectrum Range. However, Canadian volcanologist Jack Souther described these features as being on the adjoining Arctic Lake Plateau.
Several cirques in the Spectrum Range are bounded by sharp-edged ridges and, apart from trees in the lower valleys, the range is devoid of vegetation. The current, approximately dome forming the range originally had a width of more than ; erosional remnants occur at the northern and southwestern edges of the range. It was also originally higher than its current elevation of as evidenced by the thick, gently dipping lava flows forming the summit of Kitsu Peak, the highest point of the range. The original volume of the Spectrum Dome is estimated to have been based on restoration calculations of the original surface.
Glaciation As a part of the Mount Edziza volcanic complex, the Spectrum Range was covered by a regional ice sheet during the Pleistocene, which receded and advanced periodically. Deglaciation from the last such cycle was essentially complete about 11,000 years ago in a steadily warming climate. This warming trend ceased about 2,600 years ago, causing glaciers to advance from the range and elsewhere along the volcanic complex as a part of the neoglaciation. The present trend towards a more moderate climate put an end to the neoglacial period in the 19th century; this has resulted in rapid glacial recession throughout the Mount Edziza volcanic complex. This rapid glacial recession is apparent from the lack of vegetation on the barren, rocky ground between the glaciers and their trim lines, which are up to apart.
Unlike Mount Edziza, whose ice cap was approximately in 1992, the Spectrum Range is covered with relatively small separate glaciers; they occupy cirques on most peaks greater than in elevation. The largest of these glaciers is Nagha Glacier, which initiates just northwest of Yeda Peak and terminates at the head of the valley between Yagi Ridge and the Kitsu Plateau. Yeda Glacier, an informally named glacier at the head of Ball Creek, existed south of Yeda Peak in 1988.
Drainage Extending outward from the Spectrum Range is a crudely radial drainage system characterized by deeply incised valleys. This system lies within the Stikine River watershed, which includes the entire Mount Edziza volcanic complex. Kitsu Creek originating from the northern side of Kitsu Peak is unique in that it flows northwest across the Kitsu Plateau rather than through a valley. It has one named tributary, Nagha Creek, which also flows northwest from the Spectrum Range. Tadekho Creek originates from between Kuno and Yeda peaks and flows to the northwest. Kitsu and Tadekho creeks both flow into Mess Creek, which is a northwest-flowing tributary of the Stikine River.
The Little Iskut River originates from Little Ball Lake just south of Kounugu Mountain and flows to the northeast where it collects Stewbomb Creek flowing east from the range. Stewbomb Creek has one named tributary, Artifact Creek, which originates adjacent to Kitsu Peak and flows through a valley between the Artifact and Obsidian ridges. Ball and More creeks both flow south from the southern end of the range near Yeda Peak, the former of which contains an east-flowing tributary called Chachani Creek. The Little Iskut River and Ball and More creeks are tributaries of the Iskut River, which flows south and then west into the Stikine River.
Geology Background The Spectrum Range is part of the Northern Cordilleran Volcanic Province, a broad area of volcanoes extending from northwestern British Columbia northwards through Yukon into easternmost Alaska. The dominant rocks that make up these volcanoes are alkali basalts and hawaiites, but nephelinites, basanites and peralkaline phonolites, trachytes and comendites are locally abundant. These rocks were deposited by volcanic eruptions from 20 million years ago to as recently as a few hundred years ago. Volcanism in the Northern Cordilleran Volcanic Province is thought to be due to rifting of the North American Cordillera, driven by changes in relative plate motion between the North American and Pacific plates.
Composition
The Spectrum Range consists mainly of trachyte, comendite and pantelleritic trachyte and rhyolite of the Spectrum Formation, the fifth oldest stratigraphic unit of the Mount Edziza volcanic complex. More than 90% of these volcanic rocks were erupted as lava; individual flows reach thicknesses of up to . In contrast, less than 10% of them were erupted as pumice and pyroclastic flows. Rhyolites are the most silicic rocks; they contain up to 77% silica. The trachytes and rhyolites are overlain locally by eroded remnants of the Kitsu Member. This stratigraphic unit consists of alkali basalt flows that preserve the unmodified upper surface of the original Spectrum Dome. Obsidian Ridge contains high-quality obsidian, which was used by early Tahltan people for making stone tools.
Most of the felsic lavas forming the Spectrum Range are peralkaline. Because of their unusual major element geochemistry, peralkaline lavas can be 2–3 times less viscous than their metaluminous equivalents. There is a consensus in the scientific community that peralkaline magmas evolve from melts generated in Earth's mantle, but there is less agreement on the nature of the mantle source. The source that produces alkali basalts parental to silicic peralkaline rocks has been variously attributed to metasomatically enriched mantle, asthenospheric mantle enriched by "primitive" material from a deep mantle plume, or mantle enriched by recycled mid-ocean ridge basalt. Highly alkaline but not strictly peralkaline rhyolites are also present; they are chemically and mineralogically similar to other felsic lavas in the Spectrum Range.
Caldera In the middle of the Spectrum Range at the base of the volcanic pile is a buried depression that may be a caldera or an irregular collapse structure. It occurs within a roughly circular area about in diameter and was likely formed by collapse of a shallow magma chamber during the eruption of the Spectrum Formation lavas. At least of vertical caldera collapse may have resulted if the magma chamber was similar in diameter to this circular area. However, the evidence is fragmentary because of poor exposure of the depression and of the bounding vertical faults. The southwestern side of a northwesterly-trending, nearly vertical fault adjacent to Stewbomb Creek has dropped at least and is cut by parallel rhyolite dikes.
Basement Underlying nearly all of the Spectrum Range is the Kounugu Member of the Nido Formation, one of many stratigraphic units forming the Mount Edziza volcanic complex. Basaltic lava flows of this Pliocene-age geological member are exposed around the perimeter of the range and are limited only to the area south of the broad east–west valley of Raspberry Pass. They issued from at least four separate eruptive centres that have since been either deeply eroded or completely destroyed by erosion. These eruptive centres have been named Vanished Peak, Lost Peak, Swarm Peak and Exile Hill.
Also underlying the Spectrum Range are flat-lying basalt flows of the Raspberry Formation, the oldest stratigraphic unit of the Mount Edziza volcanic complex. This geological formation is of Late Miocene age and originated as a composite shield volcano that erupted lava from at least three locations near Raspberry Pass. The Nido and Raspberry formations are underlain by the Stikinia terrane, a Paleozoic and Mesozoic suite of volcanic and sedimentary rocks that accreted to the continental margin of North America during the Jurassic.
Eruptive history