Anahim hotspot

The Anahim hotspot is a hypothesized hotspot in the Central Interior of British Columbia, Canada. It has been proposed as the candidate source for volcanism in the Anahim Volcanic Belt, a 300 km (190 mi) long chain of volcanoes and other magmatic features that have undergone erosion. This 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.
The hotspot 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. Tomographic imaging has since identified a low-velocity anomaly, indicative of an upwelling plume, that measures roughly 400 km (250 mi) deep. This measurement, however, could be an underestimate as the anomaly might originate deeper inside 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 500 km (310 mi) 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 intraplate volcanism builds peaks that rise above the surrounding landscape. Plate tectonics cause the local tectonic plate (in the case of the Anahim hotspot, the North American 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 4 km (2.5 mi) 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″ ("D double-prime") layer, approximately 200 km (120 mi) 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 1,400 km (870 mi) 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.