Garibaldi Volcanic Belt
The Garibaldi Volcanic Belt is a northwest–southeast trending volcanic chain in the Pacific Ranges of the Coast Mountains that extends from Watts Point in the south to the Ha-Iltzuk Icefield in the north. This chain of volcanoes is located in southwestern British Columbia, Canada. It forms the northernmost segment of the Cascade Volcanic Arc, which includes Mount St. Helens and Mount Baker. Most volcanoes of the Garibaldi chain are dormant stratovolcanoes and subglacial volcanoes that have been eroded by glacial ice. Less common volcanic landforms include cinder cones, volcanic plugs, lava domes and calderas. These diverse formations were created by different styles of volcanic activity, including Peléan and Plinian eruptions.
Eruptions along the length of the chain have created at least three major volcanic zones. The first began in the Powder Mountain Icefield 4.0 million years ago. Mount Cayley began its formation during this period. Multiple eruptions from 2.2 million to 2,350 years ago created the Mount Meager massif, and eruptions 1.3 million to 9,300 years ago formed Mount Garibaldi and other volcanoes in the Garibaldi Lake area. These major volcanic zones lie in three echelon segments, referred to as the northern, central, and southern segments. Each segment contains one of the three major volcanic zones. Apart from these large volcanic zones, two large poorly studied volcanic complexes lie at the northern end of the Pacific Ranges, namely Silverthrone Caldera and Franklin Glacier Complex. They are considered to be part of the Garibaldi Volcanic Belt, but their tectonic relationships to other volcanoes in the Garibaldi chain are unclear because of minimal studies.
Geology
Background
Prior to Garibaldi Belt formation, a number of older, but related volcanic belts were constructed along the Southern Coast of British Columbia. This includes the east–west trending Alert Bay Volcanic Belt on northern Vancouver Island and the Pemberton Volcanic Belt along the coastal mainland. The Pemberton Belt began its formation when the former Farallon Plate was subducting under the British Columbia Coast 29 million years ago during the Oligocene epoch. At this time, the north-central portion of the Farallon Plate was just starting to subduct under the U.S. state of California, splitting it into northern and southern sections. Between 18 and five million years ago during the Miocene period, the northern remnant of the Farallon Plate fractured into two tectonic plates, known as the Gorda and Juan de Fuca plates. After this breakup, subduction of the Juan de Fuca Plate might have been coincident with the northern end of Vancouver Island eight million years ago during the late Miocene period. This is when the Alert Bay Belt became active. A brief interval of plate motion adjustment about 3.5 million years ago may have triggered the generation of basaltic magma along the descending plate edge. This eruptive period postdates the formation of the Garibaldi Belt and evidence for more recent volcanism in the Alert Bay Belt has not been found, indicating that volcanism in the Alert Bay Belt is likely extinct.Bedrock under the Garibaldi chain consists of granitic and dioritic rocks of the Coast Plutonic Complex, which makes up much of the Coast Mountains. This is a large batholith complex that was formed when the Farallon and Kula plates were subducting along the western margin of the North American Plate during the Jurassic and Tertiary periods. It lies on island arc remnants, oceanic plateaus and clustered continental margins that were added along the western margin of North America between the Triassic and Cretaceous periods.
Formation
The Garibaldi Belt has formed in response to ongoing subduction of the Juan de Fuca Plate under the North American Plate at the Cascadia subduction zone along the British Columbia Coast. This is a long fault zone running off the Pacific Northwest from Northern California to southwestern British Columbia. The plates move at a relative rate of over per year at a somewhat oblique angle to the subduction zone. Because of the very large fault area, the Cascadia subduction zone can produce large earthquakes of magnitude 7.0 or greater. The interface between the Juan de Fuca and North American plates remains locked for periods of roughly 500 years. During these periods, stress builds up on the interface between the plates and causes uplift of the North American margin. When the plate finally slips, the 500 years of stored energy are released in a mega-earthquake.Unlike most subduction zones worldwide, there is no deep oceanic trench present in the bathymetry of the continental margin in Cascadia. This is because the mouth of the Columbia River empties directly into the subduction zone and deposits silt at the bottom of the Pacific Ocean to bury the oceanic trench. Massive floods from prehistoric Glacial Lake Missoula during the Late Pleistocene also deposited massive amounts of sediment into the trench. However, in common with other subduction zones, the outer margin is slowly being compressed, similar to a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes, such as the magnitude 9.0 Cascadia earthquake on January 26, 1700. However, earthquakes along the Cascadia subduction zone are fewer than expected and there is evidence of a decline in volcanic activity over the past few million years. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American plates. These two tectonic plates currently converge to per year. This is only about half the rate of convergence of seven million years ago.
Scientists have estimated that there have been at least 13 significant earthquakes along the Cascadia subduction zone in the past 6,000 years. The most recent, the 1700 Cascadia earthquake, was recorded in the oral traditions of the First Nations people on Vancouver Island. It caused considerable tremors and a massive tsunami that traveled across the Pacific Ocean. The significant shaking associated with this earthquake demolished houses of the Cowichan Tribes on Vancouver Island and caused several landslides. Shaking due to this earthquake made it too difficult for the Cowichan people to stand, and the tremors were so lengthy that they were sickened. The tsunami created by the earthquake ultimately devastated a winter village at Pachena Bay, killing all the people that lived there. The 1700 Cascadia earthquake caused near-shore subsidence, submerging marshes and forests on the coast that were later buried under more recent debris.
Many thousand years of dormancy are expected between large explosive eruptions of volcanoes in the Garibaldi Belt. A possible explanation for the lower rates of volcanism in the Garibaldi chain is that the associated terrain is being compressed in contrast to the more southern portions of the Cascade Arc. In continental rift zones, magma is able to push its way up through the Earth's crust rapidly along faults, providing less chance for differentiation. This is likely the case south of Mount Hood to the California border and east-southeast of the massive Newberry shield volcano adjacent to the Cascade Range in central Oregon because the Brothers Fault Zone lies in this region. This rift zone might explain the massive amounts of basaltic lava in this part of the central Cascade Arc. A low convergence rate in a compressional setting with massive stationary bodies of magma under the surface could explain the low volume and differentiated magmas throughout the Garibaldi Volcanic Belt. In 1958, Canadian volcanologist Bill Mathews proposed there could be a connection between regional glaciation of the North American continent during glacial periods and higher rates of volcanic activity during regional glacial unload of the continent. However, this is hard to predict due to the infrequent geological record in this region. But there is specific data, including the temporal grouping of eruptions synglacially or just postglacial within the Garibaldi Belt, that suggests this could be probable.
Glaciovolcanism
Dominating the Garibaldi chain are volcanoes and other volcanic formations that formed during periods of intense glaciation. This includes flow-dominated tuyas, subglacial lava domes and ice-marginal lava flows. Flow-dominated tuyas differ from the typical basaltic tuyas throughout British Columbia in that they are composed of piles of flat-lying lava flows and lack hyaloclastite and pillow lava. They are interpreted to have formed as a result of magma intruding into and melting a vertical hole through adjacent glacial ice that eventually breached the surface of the glacier. As this magma ascends, it ponds and spreads into horizontal layers. Lava domes that were formed mainly during subglacial activity comprise steep flanks made of intense columnar joints and volcanic glass. Ice-marginal lava flows form when lava erupts from a subaerial vent and ponds against glacial ice. The Barrier, a lava dam impounding Garibaldi Lake in the southern segment, is the best represented ice-marginal lava flow in the Garibaldi Belt.Flow-dominated tuyas and the absence of subglacial fragmental deposits are two uncommon glaciovolcanic features in the Garibaldi chain. This is due to their different lava compositions and decline of direct lava-water contact during volcanic activity. The lava composition of these volcanic edifies changes their structure because eruption temperatures are lower than those associated with basaltic activity and lava containing silica increases thickness and glass differentiation temperatures. As a result, subglacial volcanoes that erupt silicic content melt less qualities of ice and are not as likely to contain water close to the volcanic vent. This forms volcanoes with structures that display their relationship with the regional glaciation. The surrounding landscape also changes the flow of meltwater, favouring lava to pond within valleys dominated by glacial ice. And if the edifice is eroded, it could change the prominence of fragmental glaciovolcanic deposits as well.