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. 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
thumb|right|alt=Map of the Cascadia subduction zone and location of nearby volcanoes along coastal United States and Canada.|Area of the Cascadia subduction zone, including the Cascade Volcanic Arc (red triangles). The Garibaldi Volcanic Belt is shown here as three red triangles at the northernmost end of the arc.
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 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. 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. It represents a feature in the Squamish volcanic field.
Mount Garibaldi, one of the larger volcanoes in the southern Garibaldi Belt with a volume of , is composed of dacite lavas that were erupted in the past 300,000 years. It was constructed when volcanic material erupted onto a portion of the Cordilleran Ice Sheet during the Pleistocene period. This created the unique asymmetrical shape of the mountain. Successive landslides on Garibaldi's flanks occurred after glacial ice of the Cordilleran Ice Sheet retreated. which commonly travel only short distances from a volcanic vent due to their high viscosity. The Opal Cone lava flow represents one of the most recent volcanic features at Mount Garibaldi. A series of basaltic andesite flows were erupted from Cinder Cone about 11,000 years ago that traveled into a deep north trending U-shaped valley on the eastern flank of The Black Tusk. Subsequent volcanism produced another sequence of basaltic lava flows 4,000 years ago that flowed in the same glacial valley.
Ember Ridge, a volcanic mountain ridge between Tricouni Peak and Mount Fee, consists of at least eight lava domes composed of andesite. They were likely formed between 25,000 and 10,000 years ago when lava erupted beneath glacial ice of the Fraser Glaciation. Their current structures are comparable to their original forms due to the minimal degree of erosion. As a result, the domes display the shaps and columnar joints typical of subglacial volcanoes. The random shaps of the Ember Ridge domes are the result of erupted lava taking advantage of former ice pockets, eruptions taking place on uneven surfaces, subsidence of the domes during volcanic activity to create rubble and separation of older columnar units during more recent eruptions. The northern dome, known as Ember Ridge North, covers the summit and eastern flank of a mountain ridge. It comprises at least one lava flow that reaches a thickness of , as well as the thinnest columnar units in the Mount Cayley volcanic field. The small size of the columnar joints indicates that the erupted lava was cooled immediately and are mainly located on the dome's summit. Ember Ridge Northeast, the smallest subglacial dome of Ember Ridge, comprises one lava flow that has a thickness no more than . Ember Ridge Northwest, the most roughly circular subglacial dome, comprises at least one lava flow. Ember Ridge Southeast is the most complex of the Ember Ridge domes, consisting of a series of lava flows with a thickness of . It is also the only Ember Ridge dome that contains large amounts of rubble. Ember Ridge Southwest comprises at least one lava flow that reaches a thickness of . It is the only subglacial dome of Ember Ridge that contains hyaloclastite. Ember Ridge West comprises only one lava flow that reaches a thickness of .
thumb|right|alt=Jagged mountain with its summit hidden in clouds.|South face of [[Pyroclastic Peak, the second highest peak of the Mount Cayley massif.]]
To the northwest, Mount Cayley constitutes the largest and most persistent volcano in the central Garibaldi Belt. It is a highly eroded stratovolcano composed of dacite and rhyodacite lava that was deposited during three phases of volcanic activity. Pali Dome West consists of at least three andesite lava flows and small amounts of pyroclastic material; its vent is presently buried under glacial ice. At least three eruptions have occurred at Pali Dome East. The age of the first volcanic eruption is unknown, but it could have occurred in the past 10,000 years. The second eruption produced a lava flow that was erupted when the vent area was not buried under glacial ice. However, the flow does show evidence of interaction with glacial ice at its lower unit. This indicates that the lavas were erupted during the waning stages of the Fraser Glaciation. The third and most recent eruption produced another lava flow that was largely erupted above glacial ice, but was probably constrained on its northern margin by a small glacier. Unlike the lava flow that was erupted during the second eruption, this lava flow was not impounded by glacial ice at its lower unit. This suggests that it erupted less than 10,000 years ago when the regional Fraser Glaciation retreated.
Cauldron Dome, a subglacial volcano north of Mount Cayley, lies west of the massive glacier covering much the region. Like Pali Dome, it is composed of two geological units. Upper Cauldron Dome is a flat-topped, oval-shaped pile of at least five andesite lava flows that resembles a tuya. The five andesite flows are columnar jointed and were likely extruded through glacial ice. The latest volcanic activity might have occurred between 10,000 and 25,000 years ago when this area was still influenced by glacial ice of the Fraser Glaciation. Lower Cauldron Dome, the youngest unit comprising the entire Cauldron Dome subglacial volcano, consists of a flat-topped, steep-sided pile of andesite lava flows long and a maximum thickness of . These volcanics were extruded about 10,000 years ago during the waning stages of the Fraser Glaciation from a vent adjacent to upper Cauldron Dome that is currently buried under glacial ice.
thumb|left|alt=Rugged landscape of rubble covered with snow on a cloudy day.|Volcanic rubble in the Mount Cayley area. Its ridge-like structure provides easy travel to the north towards Mount Fee.
Lying at the northern portion of the Mount Cayley volcanic field is a subglacial volcano named Slag Hill. At least two geologic units compose the edifice. Slag Hill proper consists of andesite lava flows and small amounts of pyroclastic rock. Lying on the western portion of Slag Hill is a lava flow that likely erupted less than 10,000 years ago due to the lack of features indicating volcano-ice interactions.
Ring Mountain, a flow-dominated tuya lying at the northern portion of the Mount Cayley volcanic field, consists of a pile of at least five andesite lava flows lying on a mountain ridge. Its steep-sided flanks reach heights of and are composed of volcanic rubble. This makes it impossible to measure its exact base elevation or how many lava flows constitute the edifice. With a summit elevation of , Ring Mountain had its last volcanic activity between 25,000 and 10,000 years ago when the Fraser Glaciation was close to its maximum. Northwest of Ring Mountain lies a minor andesite lava flow. Its chemistry is somewhat unlike other andesite flows comprising Ring Mountain, but it probably erupted from a volcanic vent adjacent to or at Ring Mountain. The part of it that lies higher in elevation contains some features that indicate lava-ice interactions, while the lower-elevation portion of it does not. Therefore, this minor lava flow was likely extruded after Ring Mountain formed but when glacial ice covered a broader area than it does currently, and that the lava flow extends beyond the region in which glacial ice existed at that time.
Northern segment
thumb|right|alt=Glacially covered mountain with vegetation on its lower flanks.|Northern flank of the Mount Meager massif. The volcanic vent that produced its latest eruption 2,350 years ago is the bowl-shaped depression in the middle of this image.
The Mount Meager massif is the most voluminous composite volcano in the Garibaldi chain and British Columbia, as well as the most recent to erupt. It has a volume of and consists of an eroded stratovolcano, ranging in composition from andesite to rhyodacite. Several dissected lava domes and volcanic plugs are present on its glaciated summit, as well as a clearly defined volcanic crater with a lava dome placed within it. This is the largest recorded Holocene explosive eruption in Canada, originating from a volcanic vent on the northeastern flank of Plinth Peak. Subsequent pyroclastic flows were sent down the flanks of Plinth Peak for and were later succeeded by the eruption of a lava flow that demolished many times. This created thick agglutinated rubble that successfully blocked the adjacent Lillooet River to form a lake. Subsequently, the breccia dam collapsed to produce a catastrophic flood that deposited house-sized boulders more than downstream. After the flood took place, a small dacite lava flow was erupted that later solidified to form a series of well-preserved columnar joints. This is the last phase of the 2350 BP eruption, and subsequent stream erosion has cut through this lava flow to form a waterfall. Seismic data suggest that these volcanoes still contain active magma chambers, indicating that some Garibaldi Belt volcanoes are likely active, with significant potential hazards. The seismic activity corresponds with some of Canada's recently formed volcanoes and with persistent volcanoes that have had major explosive activity throughout their history, such as Mount Garibaldi and the Mount Cayley and Mount Meager massifs.
History
Human occupation
People have used resources in and around the Garibaldi Volcanic Belt for centuries. Obsidian was collected by the Squamish Nation for making knives, chisels, adzes and other sharp tools in pre-contact times. This material appears in sites dated 10,000 years old up to protohistoric time periods. The source for this material is found in upper parts of the mountainous terrain that surround Mount Garibaldi. At Opal Cone, lava of the Ring Creek flow was normally heated to cook food because its pumice-like texture is able to maintain heat. It also did not break after it was used for a long period of time.
A large pumice outcrop adjacent to the Mount Meager massif has been mined several times in the past, and extends more than in length and in width with a thickness of about . The deposit was first hired by J. MacIsaac, who died in the late 1970s. In the mid-1970s the second hirer, W.H. Willes, investigated and mined the pumice. It was crushed, removed then stored close to the village of Pemberton. Later, the bridge that was used to access the pumice deposit was washed out. Mining operations resumed in 1988 when the deposit was staked by L.B. Bustin. In 1990, the pumice outcrop was bought by D.R. Carefoot from the owners B. Chore and M. Beaupre. In a program from 1991 to 1992, workers evaluated the deposit for its properties as a construction material, absorber of oil and stonewash. About of pumice was mined in 1998 by the Great Pacific Pumice Incorporation.
The hot springs associated with Meager and Cayley have made these two volcanoes targets for geothermal explorations. At Mount Cayley, temperatures of to more than have been measured in shallow boreholes on its southwestern flank.
Early impressions
The belt of volcanoes has been the subject of myths and legends by First Nations. To the Squamish Nation, Mount Garibaldi is called Nch'kay. In their language it means "Dirty Place". This name of the mountain refers to the volcanic rubble in the area. This mountain, like others located in the area, is considered sacred as it plays an important part of their history. In their oral history, they passed down a story of the flood covering the land. During this time, only two mountains peaked over the water, and Garibaldi was one of them. It was here that the remaining survivors of the flood latched their canoes to the peak and waited for the waters to subside. The Black Tusk on the northwestern end of Garibaldi Lake and Mount Cayley northwest of Mount Garibaldi are called ta<u>k</u>'ta<u>k</u>mu'yin tl'a in7in'axa7en in the Squamish language, which means "Landing Place of the Thunderbird".
Protection and monitoring
thumb|left|alt=Flat-topped, steep-sided mountain rising above the surrounding mountainous landscape.|[[The Table (British Columbia)|The Table, a flow-dominated tuya rising above the southwestern side of Garibaldi Lake.]]
A number of volcanic features in the Garibaldi Belt are protected by provincial parks. Garibaldi Provincial Park at the southern end of the chain was established in 1927 to protect the abundant geological history, glaciated mountains and other natural resources in the region. It was named after the stratovolcano Mount Garibaldi, which in turn was named after the Italian military and political leader Giuseppe Garibaldi in 1860. To the northwest, Brandywine Falls Provincial Park protects Brandywine Falls, a high waterfall composed of at least four basaltic lava flows with columnar joints. Its name origin is unclear, but it may have originated from two surveyors named Jack Nelson and Bob Mollison. As a result, volcano monitoring is less important than dealing with other natural processes, including tsunamis, earthquakes and landslides. Recent seismic imaging from Geological Survey of Canada employees supported lithoprobe studies in the region of Mount Cayley in which scientists found a large reflector interpreted to be a pool of molten rock roughly below the surface. The existence of hot springs at the Mount Meager massif and Mount Cayley indicates that magmatic heat is still present beneath or near these volcanoes. This long history of volcanic activity along a still active plate boundary indicates that volcanic eruptions in the Garibaldi Belt have not ended and risks for future eruptions remain. It is estimated that over 200 eruptions have occurred throughout the entire Cascade Volcanic Arc in the past 12,000 years, many of them in the United States. Many eruptions in the western United States have sent large amounts of tephra in southern British Columbia. However, all major cities in southwestern British Columbia with populations more than 100,000 are located west of the Garibaldi Volcanic Belt and prevailing winds travel eastwards. Therefore, these communities are less likely to have large amounts of tephra. In the Lower Mainland, a thick layer of volcanic ash can deposit once every 10,000 years and once every 1,000 years. More minor amounts of volcanic ash can be expected more commonly. During Mount St. Helens' eruption in 1980, of tephra was deposited from southeastern British Columbia to Manitoba. An eruption column released during Peléan activity would discharge large amounts of tephra that would endanger aircraft. Tephra may also melt the large sheets of glacial ice east of Garibaldi and cause floods. This could later endanger water supplies from Pitt Lake and fisheries on the Pitt River. An explosive eruption and the associated tephra may also create temporary or longer-term water supply difficulties for Vancouver and most of southern British Columbia. The water reservoir for the Greater Vancouver drainage area is south of Mount Garibaldi.
Landslides and lahars
Several landslides and lahars have occurred throughout the Garibaldi Belt. At the Mount Meager massif, considerable landslides have occurred from Pylon Peak and Devastator Peak in the past 10,000 years that have reached more than downstream in the Lillooet River valley. At least two significant landslides from the southern flank of Pylon Peak 8,700 and 4,400 years ago dumped volcanic debris into the adjacent valley of Meager Creek. More recently, a large landslide from Devastation Glacier buried and killed a group of four geologists on July 22, 1975. The estimated volume of this landslide is . A considerable landslide as large as Meager's largest throughout the Holocene would likely produce a lahar that would devastate most of the growth in the Lillooet River valley. If such an event would occur without it being identified by authorities who would send out a public warning, it would kill hundreds or even thousands of residents. Because of this, computer programs would be able to identify the approaching information and activate an automatic notice when a large lahar is identified. A similar system for identifying such lahars exists at Mount Rainier in the U.S. state of Washington. Although there are no known eruptions from the massif in the past 10,000 years, it is associated with a group of hot springs. Evans (1990) has indicated that a number of landslides and debris flows at Mount Cayley in the past 10,000 years might have been caused by volcanic activity.
Lava flows
The threat from lava flows in the Garibaldi Belt is minor unless an eruption takes place in winter or under or adjacent to areas of glacial ice, such as ice fields. When lava flows over large areas of snow, it creates meltwater. This can produce lahars that could flow further than the associated lavas. If water were to enter a volcanic vent that is erupting basaltic lava, it may create a massive explosive eruption. These explosions are generally more extreme than those during normal basaltic eruptions. Therefore, the existence of water, snow, or glacial ice at a volcanic vent would increase the risk of an eruption having a large impact on the surrounding region. Subglacial eruptions have also caused catastrophic glacial outburst floods.