Anahim hotspot
The Anahim hotspot is a volcanic hotspot located in the West-Central Interior of British Columbia, Canada. One of the few hotspots in North America, the Anahim plume is responsible for the creation of the Anahim Volcanic Belt. This is a long chain of volcanoes and other magmatic features that have undergone erosion. The chain extends from the community of Bella Bella in the west to near the small city of Quesnel in the east. While most volcanoes are created by geological activity at tectonic plate boundaries, the Anahim hotspot is located hundreds of kilometres away from the nearest plate boundary.
This hotspot's existence was first proposed in the 1970s by three scientists who used John Tuzo Wilson's classic hotspot theory. This theory proposes that a single, fixed mantle plume builds volcanoes that then, cut off from their source by the movement of the North American Plate, become increasingly inactive and eventually erode over millions of years. A more recent theory, published in 2001 by the Geological Society of America, suggests that the Anahim hotspot might be supplied by a mantle plume from the upper mantle rather than a deep-seated plume proposed by Wilson. The plume has since been tomographically imaged, showing it to be roughly deep. This measurement, however, could be an underestimate as the plume might originate deeper within Earth.
Volcanism as early as 14.5 million years ago has been linked to the Anahim hotspot, with the latest eruption having taken place in the last 8,000 years. This volcanic activity has produced rocks that show a bimodal distribution in composition. While these rocks were being deposited, the hotspot coincided with periods of crustal extension and uplift. Activity in modern times has been limited to earthquakes and volcanic gas emissions.
Theories
Tectonic plates generally focus deformation and volcanism at plate boundaries. However, the Anahim hotspot is about from the nearest plate boundary. While studying the Anahim Volcanic Belt in 1979, Canadian geologists Mary Bevier, Richard Armstrong and Jack Souther used the hotspot theory to explain this zone of volcanism so far from regular conditions. The theory was first invented by Canadian geophysicist John Tuzo Wilson in 1963 to explain the formation of the Hawaiian Islands.Wilson's stationary hotspot theory
In 1963, Wilson proposed that small, long lasting, exceptionally hot areas of magma exist under Earth's surface; these heat centres create thermally active mantle plumes, which in turn sustain long-lasting volcanic activity. This "intra-plate" volcanism builds peaks that rise above the surrounding landscape. Plate tectonics cause the local tectonic plate to slowly slide over the hotspot, carrying its volcanoes with it without affecting the plume. Over hundreds of thousands of years, the magma supply for the volcano is slowly cut off, eventually going extinct. No longer active enough to overpower erosion, the volcano slowly erodes away. As the cycle continues, a new volcanic centre manifests and a volcanic peak arises anew. The process continues until the mantle plume itself collapses.This cycle of growth and dormancy strings together volcanoes over millions of years, leaving a trail of volcanic mountains and intrusions extending from coastal British Columbia across the Coast Mountains into the Interior Plateau. According to Wilson's theory, the Anahim volcanoes should be progressively older and increasingly eroded the further they are from the hotspot and this is easily observable; the oldest rock on coastal British Columbia, that of the Gale Passage dike swarm, is about 14.5 million years old and deeply eroded, while the rock at Nazko Cone, the hotspot's present centre, is a comparatively young 0.34 million years of age or less. Radiocarbon dating of peat directly above and below a tephra layer extending from Nazko Cone suggest that the latest eruption occurred approximately 7,200 years ago.
Geophysicists believe that hotspots originate at one or two major boundaries deep in the Earth, either a shallow interface in the lower mantle between an upper convecting layer and a lower non-convecting layer, or a deeper D″ layer, approximately thick and immediately above the core-mantle boundary. A mantle plume would initiate at the interface when the warmer lower layer heats a portion of the cooler upper layer. This heated, buoyant and less-viscous portion of the upper layer would become less dense due to thermal expansion and rise towards the surface as a Rayleigh-Taylor instability. When the mantle plume reaches the base of the lithosphere, the plume heats it and produces melt. This magma then makes its way to the surface, where it is erupted as lava.
Arguments for the validity of the hotspot theory generally centre on the steady age progression of Anahim volcanoes and nearby features: a similar eastward-younging spatiotemporal trend exists for the Yellowstone hotspot track to the southeast. The presence of two hotspot tracks on the same continent and their general agreement between each other provides a unique tool in assessing and testing the motion of North America.
Shallow hotspot theory
Another hypothesis is that the Anahim hotspot is supplied by a miniplume. These mantle plumes have their roots in the upper mantle but they may later originate from the lower mantle. Arguments for an Anahim miniplume are centred on the existence of two small dike swarms at the western end of the Anahim Volcanic Belt. This assumption is in turn based on the notion that giant dike swarms mark the arrival of deep-seated mantle plumes.History of study
In 1977, Jack Souther produced a synthesis of volcanism in the Canadian Cordillera and delineated several Neogene-to-Quaternary volcanic belts throughout British Columbia. One of these was the linear Anahim Volcanic Belt, which included the Wells Gray-Clearwater volcanic field at its eastern end. However, its origin had not yet been understood. In 1979, two volcano tectonic models had been proposed by Jack Souther, Mary Bevier and Richard Armstrong. This included a hotspot and a propagating crack controlled by stress fields related to large-scale plate tectonics of western North America.Garry C. Rogers of the Geological Survey of Canada speculated in his 1981 NRC Research Press article McNaughton Lake seismicity—more evidence for an Anahim hotspot? that earthquake swarms at McNaughton Lake may be related to the Anahim hotspot. Rogers noted that if the seismicity is related to a hotspot the surface expression must be lagging behind the passage of the hotspot. An alternative theory proposed by Rogers is that if the Anahim hotspot is located under the Wells Gray-Clearwater area, the stress field surrounding the hotspot must precede it by approximately.
In 1987, Canadian volcanologist Catherine Hickson revealed that the Wells Gray-Clearwater volcanic field is not part of the Anahim Volcanic Belt, but rather a separate centre that most likely represents an area of lithospheric decompression melting caused by rifting along pre-existing crustal fractures. The Wells Gray-Clearwater volcanic field has since not been considered part of the Anahim Volcanic Belt and the Anahim hotspot is now believed to be in the area of Nazko Cone.
The existence of an Anahim hotspot was supported in a detailed Bulletin of Volcanology report by Kuehn et al.. This included new geochemical and geochronometric data for the Baldface Mountain and Satah Mountain volcanic fields, as well as for Nazko Cone. The data obtained indicated that volcanism in the two volcanic fields were contemporaneous with the Itcha Range shield volcano and that both fields agree with the vector of the North American Plate motion over a hotspot in the British Columbia Interior. It was also noted that the trace and rare-earth element patterns of mafic lavas in the Anahim Volcanic Belt are similar to ocean island basalts, providing more evidence for a hotspot.
Characteristics
Position
The Anahim hotspot has been imaged through seismic tomography and is estimated to be wide. Recent high-resolution local tomography indicates a possible lower-mantle plume and a pond of plume material is evidenced by a large low-velocity zone in the upper mantle. These low seismic velocity zones often indicate hotter and more buoyant mantle material. The low-velocity zone is flanked on both sides by high-velocity anomalies of variable amplitude. In the north, high-velocities may reflect the remains of batholithic roots that formed as a result of continuous subduction along the northern continental margin 150 to 50 million years ago. High velocities in the south represent the subducting Juan de Fuca slab. Centered near Nazko Cone, the low-velocity zone extends to a depth of approximately. However, it may extend deeper southward beneath the Juan de Fuca Plate through the transition zone into the lower mantle. This has led to the conclusion that the Anahim hotspot is supplied by a mantle plume over slab edge flow.Movement
Individual volcanoes drift southwest from the hotspot at a rate of about per year with each successive volcanic centre spending about two million years actively attached to the plume. The oldest Anahim volcano, situated on the Central Coast of British Columbia, formed 14.5 million years ago. If any prior record in the form of seamounts existed off the British Columbia Coast, this record would presumably have been subducted under North America with the Farallon/Juan de Fuca plates and lost. Thus it is unknown if the hotspot existed in the Pacific Ocean prior to being located on the North American continent from ongoing plate motion. However, past geologic field mapping and geochemical studies suggest massive plutons could be present in the offshore continental shelf. These suspected bodies are aligned with the northeast-trending Anahim Volcanic Belt, whose age progression suggests these suspected offshore plutons could be of Miocene age. An earlier displaced portion of the hotspot track might exist on Haida Gwaii as part of the Masset Formation. However, further analyses of Masset volcanic rocks are still required to determine if they are compositionally and istopically similar to alkalic lavas found on the mainland.Magma
The composition of the volcanoes' magma has changed significantly with time as they grow over the hotspot and migrate away. Volcanic activity 14.5 to 3.0 million years ago was predominately felsic, producing large volumes of rhyolite and trachyte lava. This can be explained by the presence of thick granitic structures under these volcanoes, which have been tectonically compressed from being near the North American Plate margin. A unique characteristic of the felsic lava flows is that although they were high in silica content, the flows were overly fluid in nature. This is because the peralkaline content of these felsic lavas decreased the viscosity of the flows a minimum of 10–30 times over that of calc-alkaline felsic flows. Evidence for explosive volcanism exists in the form of pumice flows, bedded tuffs, intensely shattered basement rocks and the high content of coarse basement clasts in rhyolite breccias.Magma production of the Anahim hotspot has shifted from more felsic to more mafic compositions in the last 3.0 million years. For instance, much of the magma created between 3.0 and 0.33 million years ago was igneous phonolite, trachyte, trachyandesite, basalt and basanite; the volcanoes built during this period are almost entirely made of these rock types. Other igneous rocks such as phonotephrite are present in smaller quantities; these occur in the Satah Mountain volcanic field. Volcanic eruptions in the last 0.33 million years have been mainly basanitic and have occurred at the youngest eruptive centre, Nazko Cone. Basanites produced by these eruptions are significantly more undersaturated than basalts from older Anahim volcanoes in the west and may indicate an eastward shift toward a deeper or less depleted mantle source. It is also possible that as the North American Plate moves over the Anahim hotspot, it underlies thinner continental crust. The overall chemistry and mineralogy of the Anahim magmas are analogous to regions of incipient continental rifting above a mantle plume.
Volcanoes
Over the last 14.5 million years, the Anahim hotspot has created at least 40 volcanoes. They can be organized into three groupings: the eastern section, which comprises several small cinder cones and is the location of all modern volcanic activity; the central section, which consists of predominantly shield volcanoes; and the western section, all of which have since been reduced to remnants of eruptive breccia and high-level plutons and dike swarms. These form one of the six Neogene-Quaternary volcanic provinces in British Columbia.Volcanic characteristics
The Anahim volcanoes are grouped into three types: volcanic cones, shield volcanoes and lava domes. The shields are characterized by their large size and their symmetrical shape. They are the most prominent of the three volcano types, with the Rainbow Range being the highest at around above sea level. Their outer slopes merge with older flat-lying basalt flows of the Chilcotin Group, which covers a large percentage of the Interior Plateau. The more abundant lava domes and volcanic cones are much smaller in size. These comprise two extensive volcanic fields in the vicinity of the Itcha Range.Although many Anahim volcanoes are surrounded by Chilcotin Group basalt flows, the exact nature of their relationship is unknown. It is unlikely that the Anahim volcanoes ever were a source area for the Chilcotin basalts as they have distinct transitional geochemistries. The Chilcotin Group is interpreted to be related to back-arc extension behind the Cascadia subduction zone.
Evolution and construction
Each volcano type produced by the Anahim hotspot has its own unique life cycle of growth and erosion. Volcanic cones have their origins from tephra accumulating around vents during Strombolian eruptions. They are composed of trachyte, trachyandesite, basalt, phonolite, basanite and to a lesser extent phonotephrite. In contrast, lava domes are formed mainly by viscous trachytic magma that erupts effusively onto the surface and then piles up thick around vents. Most of these features are formed only by a single volcanic eruption and are therefore monogenetic in nature. However, in some cases several eruptions occur at a single locus to create larger, polygenetic centres. Once activity has ended, erosion eventually reduces them into volcanic remnants such as lava plugs.Shield volcanoes undergo at least two stages of volcanic activity. The initial shield stage is the most productive volcanically and features repeated eruptions of large volumes of predominately fluid peralkaline felsic magmas that become progressively more evolved. During this stage, a small summit caldera may form, as is the case for the Ilgachuz Range. After the shield stage has been completed, the post-shield stage succeeds. This stage of activity is characterized by small volumes of more mafic lavas expressed as small cinder cones and capping flows. Dissection of the shield by stream erosion is also apparent, resulting in the creation of deeply incised radial valleys.
Prolonged erosion eventually removes most if not all traces of the volcanoes to expose their underlying solidified magma systems. Such systems can be below the surface with rocks ranging from hypabyssal to plutonic. Exposure of the King Island Pluton and the Bella Bella and Gale Passage dike swarms are prime examples of this phase of erosion.
Tectonic history
Extensional tectonics
Rifting and crustal extension in Queen Charlotte Sound up to about 17 million years ago has been linked to the Early Miocene passage of the Anahim hotspot. Yorath and Chase proposed that subcrustal melting above the Anahim plume resulted in weakening of the regional crust, setting the stage for rift development. Later, widespread volcanism produced subearial basalt and rhyolite flows in the region of the rift and along transcurrent faults that extend towards the northwest. Haida Gwaii was displaced approximately to the north along a series of faults extending through Sandspit and Louscoone Islet. This period of rifting and crustal extension contributed to the formation of the Queen Charlotte Basin.While the rift was in development, a conservative plate boundary would have extended northwards from the landward end of the rift. Such a plate boundary might have been similar to the Gulf of California – San Andreas fault system in the U.S. state of California. This type of configuration need have existed for only a few million years to have generated the of opening in the rift. Alternatively, the Haida Gwaii block may have been only partially coupled to the offshore plate during a longer period of oblique convergence. Bathyal sediments, perhaps as young as 15 million years, were deposited within the rift zone during and after the rifting took place as the Anahim hotspot passed by.