Mount Edziza Plateau

The Mount Edziza volcanic complex is a large and potentially active north-south trending complex volcano in Stikine Country, northwestern British Columbia, Canada, located southeast of the small community of Telegraph Creek. It occupies the southeastern portion of the Tahltan Highland, an upland area of plateau and lower mountain ranges, lying east of the Boundary Ranges and south of the Inklin River, which is the east fork of the Taku River. As a volcanic complex, it consists of many types of volcanoes, including shield volcanoes, calderas, lava domes, stratovolcanoes, and cinder cones.

Most of the Mount Edziza volcanic complex is encompassed within a large provincial park called Mount Edziza Provincial Park. Named after Mount Edziza, this park was established in 1972 to preserve the volcanic and cultural treasures unique to the northern British Columbia area. The Mount Edziza volcanic complex is remote, and, without roads, accessible only along trails. The easiest access is from Highway 37 and a spur road from Dease Lake to Telegraph Creek. From Kinaskan Lake, on Highway 37, a poorly maintained trail extends west for into the heart of the complex. From Telegraph Creek another trail extends east for to the north slope of Mount Edziza.

Geology

Origins

The Mount Edziza volcanic complex began forming about 7.5 million years ago and has grown steadily since then. Like other volcanoes in northwestern British Columbia, the Mount Edziza volcanic complex has its origins in continental rifting—a long divergent plate boundary where the lithosphere is being pulled apart. Here, the continental crust of the North American Plate is being stretched at a rate of about per year. This incipient rifting has formed as a result of the Pacific Plate sliding northward along the Queen Charlotte Fault, on its way to the Aleutian Trench, which extends along the southern coastline of Alaska and the adjacent waters of northeastern Siberia off the coast of Kamchatka Peninsula. As the continental crust stretches, the near-surface rocks fracture along steeply dipping cracks parallel to the rift known as faults. Hot basaltic magma rises along these fractures to create passive lava eruptions, known as effusive eruptions.

The rift zone has existed for at least 20 million years, and has created a line of volcanoes called the Northern Cordilleran Volcanic Province, also called the Stikine Volcanic Belt, stretching from the Alaska-Yukon border to near Prince Rupert, British Columbia. Several presently dormant volcanoes in the province are potentially active, three of them having erupted in the last few hundred years, two witnessed by First Nations and placer miners during the 18th and 19th centuries. The Tseax Cone, which last erupted in the 18th century, is the southernmost historically active volcano in the province, while Prindle Volcano in easternmost-central Alaska, which erupted during the Pleistocene period, is generally considered the northernmost.

Structure

The Mount Edziza volcanic complex is Canada's second largest volcano of young volcanic activity, with an area of , exceeded only by Level Mountain north of Edziza, which has an area of . Four central volcanoes, known as Armadillo Peak, Spectrum Range, Ice Peak, and Mount Edziza, lie along the northerly trending axis of an oval, composite shield volcano. The composite shield volcano consists of overlapping shields, two of which are clearly noticeable on maps. The composite shield volcano forms a broad lava plateau, long and wide, mainly made of basaltic lava flows; it is dotted with cinder cones and surrounded by steep ridges called escarpments, which expose layers of black columnar basaltic lava flows with distal rock fragments and pyroclastic deposits. More light-coloured magmas of mainly trachyte and comendite with very little aluminum are mainly confined to the four central volcanoes and associated lava dome. The lava plateau is flanked by Klastline River to the north, Mess Creek and larger Stikine River to the west and the Iskut River to the east. Elevations of the lava plateau are with volcanic mountains rising above sea level. Three sections of the lava plateau have official names; these are the Arctic Lake, Big Raven, and Kitsu plateaus. The history of the Mount Edziza volcanic complex includes at least two periods of regional glaciation, when deep ice sheets covered the land, and several lesser advances of mountain glaciers.

Stratovolcano composition

The steep-sided, symmetrical stratovolcanoes in the region were built by repeated eruptions of thick, slow-moving lava that have commonly flowed only a few kilometres from the vent. Explosive eruptions are often associated with these volcanoes, depositing alternating layers of volcanic ash, cinders, blocks, and globes of molten rock called volcanic bombs or lava bombs, which are added to its slopes to create the stratovolcano. Edziza's stratovolcanoes contain a fine-grained silica-rich volcanic rock called trachyte; they have not erupted for thousands of years, allowing erosion to destroy the original cone, creating craggy ridges and rock outcrops of more resistant materials.

Caldera composition

Circular calderas in the Mount Edziza volcanic complex were formed as a result of emptying the magma chamber beneath a volcano. If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice above it. A roughly circular fracture— a "ring fault"— develops around the edge of the chamber. These ring fractures serve as feeders for fault intrusions that are also known as ring dikes. Secondary volcanic vents form above the ring fracture. As the magma chamber empties, the centre of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions. These caldera collapses are relatively small compared to most other caldera collapses. The largest caldera at the Mount Edziza volcanic complex is about in diameter while most calderas are at least in diameter. Volcanic eruptions accompanying these collapses produced trachyte and a white, sodic rhyolite called comendite.

Lava dome composition

Edziza's rounded, steep-sided lava domes were built by eruptions of very thick light-coloured magma, including trachyte. Such magmas are typically too thick to move far from the vent it extrudes from, causing it to solidify quickly and build on previous volcanic extrusions, creating a characteristic dome-like shape. The thickness of the magma is attributed to high levels of silica, a naturally occurring silicon dioxide found in various crystalline and amorphous forms. Edziza's domes reach heights of several hundred metres, and grew slowly and steadily for months to years. The sides of these structures are composed of unstable rock debris. Due to the possibility of the building of gas pressure, the dome can experience more explosive eruptions over time. When part of a lava dome collapses while it still contains molten rock and gases, it can produce a pyroclastic flow, a super-heated mix of gas, ash, and pumice.

Characteristics of lava dome eruptions include shallow, long-period and hybrid seismic activity, which is attributed to excess fluid pressures in the contributing vent chamber. Other characteristics of lava domes include their spherical dome shape, cycles of dome growth over long periods, and sudden onsets of violent explosive activity. The average rate of dome growth may be used as a rough indicator of magma supply, but it shows no systematic relationship to the timing or characteristic of lava dome explosions.

Cinder cone composition

The steep conical cinder cones of Edziza were formed by lava fountain eruptions, emitting particles and blobs of congealed lava from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinder around the vent to form a circular or oval cone. Edziza's cinder cones have bowl-shaped craters at their summits and rise more than a hundred metres above their surroundings. Cinder cones are widespread in British Columbia as well as throughout other volcanic terrains of the world.

Eve Cone, a black cinder cone of the Mount Edziza volcanic complex, is one of the most famous symmetrical and best preserved cinder cones in Canada, reaching a height of and a topographic prominence of .

Shield volcano composition

Edziza's shield volcanoes are built almost entirely of fluid lava flows. They formed as a result of lava flowing out in all directions from central summit vents and from groups of vents, building a broad, gently sloping cone of flat, domical shape. They are built up slowly by the accretion of thousands of lava flows of highly fluid basaltic lava, which spread widely over great distances, and then cool as thin, gently dipping sheets. In some shield volcano eruptions, basaltic lava has poured out quietly from fissure vents instead of central vents, flooding the surrounding countryside with lava flow upon lava flow, forming Edziza's broad lava plateau.

Lava plateaus similar to Edziza's can be found elsewhere in North America, including the Snake River Plain in Idaho, and the Columbia River Basalt Group in southeastern Washington, and eastern Oregon, United States; they can also be found in Iceland.

Subglacial mound composition

Subglacial mounds (SUGM) of the Mount Edziza volcanic complex are an unusual type of subglacial volcano formed when subglacial eruptions began melting overlying glacial ice at a time when this region was covered by glacial ice during the Pleistocene and early Holocene periods. These subglacial eruptions were not hot enough to melt a vertical pipe right through the overlying glacial ice, instead forming mounds of hydrated volcanic rock made up of volcanic fragments called hyaloclastite and lava that solidified into pillow-shaped masses called pillow lava deep beneath the glacial ice field. Once the glaciers had retreated, the subglacial volcanoes would be revealed, with a unique shape as a result of their confinement within glacial ice.

Eruptive history

The lava domes, calderas, stratovolcanoes, subglacial mounds and cinder cones forming the volcanic complex were constructed in five phases, each of which began with the effusion of dark olivine basalt which formed the flat-lying shield volcanoes and concluded with the eruption of light-coloured magma. This cyclical behavior is attributed to the episodic rise of basic, mantle-derived alkali basalt both to the surface and partly into crustal reservoirs where the light-coloured magmas with very little aluminum were created by prolonged crystal fractionation. The silica-rich trachyte and comendite lavas are similar to those associated with the most violent eruptions on Earth.

Armadillo Peak eruptive period

The first phase of activity resulted in the creation of Armadillo Peak seven million years ago, today represented by an eroded remnant of a small caldera flanked by steep-sided light-coloured secondary lava domes, including Cartoona Peak, Tadeda Peak, IGC Centre, and Sezill Volcano, and a thick pile of interlayered light-coloured lava flows, pyroclastic flows, air-fall pumice, and epiclastic deposits. It is the most central of the four central volcanoes and its summit is capped by of fine-grained silica-rich trachyte lava flows which were ponded inside the caldera to produce a lava lake six million years ago during its final stage of activity.

Spectrum Range eruptive period

The second phase of activity began three million years ago, emplacing rhyolitic magma thick and long during a single event of activity. A broad circular lava dome was eventually created called the Spectrum Range. This is the southernmost of the four central volcanoes and is over wide and up to thick on the southwestern flank of Armadillo Peak and north of the Arctic Lake Plateau. Named for its extensive colourful alteration, it overlies a basal shield volcano and contains deeply carved circular valleys displaying portions of massive silica-rich comendite and trachyte lava flows which comprise the lava dome. The deeply carved circular valleys also display the bounding faults of a buried, cogenetic caldera approximately across. More than of rhyolite and trachyte was erupted during the Spectrum Range dome eruptive period, with its activity ending 2,500,000 years ago.

Ice Peak eruptive period

Ice Peak, high, which overlaps the northern flank of Armadillo Peak, began to form during Edziza's third phase of activity, starting 1,600,000 years ago when the regional Cordilleran Ice Sheet began retreating. It is a stratovolcano that was constructed when large areas of the Edziza lava plateau were free from glacial ice and now enclosed by glacial deposits. However, additional parts of the Mount Edziza volcanic complex were likely still covered by glacial ice. The volcanic activity from Ice Peak during this period produced both basic and intermediate to light-coloured lava flows and pyroclastic rocks which mixed with meltwater to produce debris flows. As Ice Peak began to form, basic lava spread to the flanks of the cone where it formed meltwater lakes and combines with and forms part of the adjacent shield volcano. As lava continued to flow into these meltwater lakes, pillow lava and solidified rubble was created. Many of the lava flows with compositions of trachyte and basalt were however erupted just below the surface of the soil. Continuous volcanic activity eventually made Ice Peak reach an elevation of when three viscous, intermediate, and light-coloured lava flows built up around secondary lava domes parallel with its western side during its final stage of activity 1,500,000 years ago and develops nearly all of the steep, higher flanks of the volcano. These viscous light-coloured lava flows are displayed at two cliffs with broad faces, known as Ornostay Bluff and Koosick Bluff and contain basic rock made of solidified rubble overlain by large fractured lava with massive, poorly established pillars.

Two cinder cones on Ice Peak's southern flank called Camp Hill and Cache Hill and possibly first erupted when glacial ice was still existing on the Edziza lava plateau. As lava flowed into the glacial ice above a vent, pools of meltwater were created. Continuous lava eruptions that flowed into the meltwater pools were cooled and fractured. This fragmental material was interrupted by explosions of steam, water, ash, rock, and volcanic bombs called phreatic eruptions. Camp Hill was eventually developed and over time it grew above water level inside the meltwater lake. Later eruptions produced a pyroclastic cone on top of the original fragmental cone. Cache Hill erupted when nearly all the glacial ice had retreated. The first lava flows from Cache Hill flowed through and dammed a river valley, which eventually ponded to produce a small lake. Subsequent lava flows traveled into the lake to produce pillow lava and solidified rubble. During the long period of Ice Peak activity, high-altitude glaciers developed and melted cutting valleys into the volcano. The current high summit of Ice Peak is a remnant of the western rim of a small summit caldera, which has been nearly destroyed by erosion from high-altitude glaciation. Near the end of Ice Peak activity 1,500,000 years ago, this high-altitude glacial ice combined with the regional ice forming part of the Cordilleran Ice Sheet. It is likely that only the tallest mountains might have been visible over the Cordilleran Ice Sheet which was at least thick. A small volume of intermediate lava was erupted from Ice Peak compared to the other central volcanoes.

Mount Edziza eruptive period

The fourth phase of activity began one million years ago when the Cordilleran Ice Sheet retreated from the upper flanks of the adjacent lava plateau, creating Mount Edziza proper which is the most northerly of the four central volcanoes. It is a steep-sided stratovolcano and the largest and highest of the peaks that form the volcanic complex with an elevation of , overlapping the northern flank of Ice Peak. The stratovolcano is made up of a fine-grained volcanic rock called trachyte and is associated with several lava domes which were formed by trachyte lava flows and explosive eruptions. Its smooth northern and western flanks, only slightly channeled by erosion, curve up to a circular summit ridge which surrounds a central, ice-filled caldera in diameter. Many glaciers cover Mount Edziza proper, including the Tencho Glacier on its southern flank. Active cirques on the eastern flank have breached the caldera rim, exposing the remnants of numerous lava lakes which ponded in the caldera 900,000 years ago and rest on hydrothermally altered breccia of the main conduit. Piles of pillow lava and hyaloclastite, formed by subglacial eruptions, are found on the flanks of Mount Edziza and nearby Ice Peak, as well as on the surface of the surrounding shield volcano. Pillow Ridge on Edziza's northwest flank was formed when basaltic lava erupted beneath the regional Cordilleran Ice Sheet when it was close to its greatest thickness.

Central volcano flank eruptive period

The fifth and final phase of eruptive activity occurred from secondary volcanic vents along the flanks of the four central volcanoes starting 10,000 years ago. This phase of activity began at a time when remnants of glacial ice were still present and continued after the glacial period. The initial flank eruptions, quenched by glacial meltwater, formed hyaloclastite tuff rings, whereas later activity created 30 small cinder cones, primarily of basaltic composition, including Mess Lake Cone, Kana Cone, Cinder Cliff, Icefall Cone