The Bella Bella and Gale Passage dike swarms are two parallel

dike swarm A dike swarm (American spelling) or dyke swarm (British spelling) is a large geological structure consisting of a major group of parallel, linear, or radially oriented magmatic dikes intruded within continental crust or central volcanoes ...

s on the Central Coast of

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...

, Canada. They range in age from 14.5 to 12.5 million years old. They are both chemically

bimodal In statistics, a multimodal distribution is a probability distribution with more than one mode (statistics), mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, ...

, consisting of rocks such as

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...

and

comendite Comendite is a hard, peralkaline igneous rock, a type of light blue grey rhyolite. Phenocrysts are sodic sanidine with minor albite and bipyramidal quartz. Iddings, Joseph Paxson, 1913, ''Igneous rocks: composition, texture and classification'', v ...

. They form the westernmost extent of the

Anahim Volcanic Belt The Anahim Volcanic Belt (AVB) is a west–east trending chain of volcanoes and related magmatic features in British Columbia, Canada. It extends from Athlone Island on the Central Coast, running eastward through the strongly uplifted and deeply ...

Athlone Island Athlone (; ) is a town on the border of County Roscommon and County Westmeath, Ireland. It is located on the River Shannon near the southern shore of Lough Ree. It is the second most populous town in the Midlands Region with a population of 21 ...

Dufferin Island Dufferin Island is an island on the Central Coast of British Columbia, Canada, on the south side of Seaforth Channel just northwest of Bella Bella. It was named in 1876 by Captain Chatfield and the officers of HMS Amethyst after Frederick Hamil ...

and

Denny Island Denny Island ( cy, Ynys Denny; ) is a small uninhabited rocky island of , with scrub vegetation, in the Severn Estuary. Its rocky southern foreshore marks the boundary between England and Wales. Above high water mark, the island is reckoned adm ...

. The Bella Bella and Gale Passage dike swarms are

petrographically Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The classi ...

similar to the

shield A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry or projectiles such as arrows, by means of a ...

complexes in the central Anahim Volcanic Belt. As a result, the swarms are thought to represent the roots of a

peralkaline Peralkaline rocks include those igneous rocks which have a deficiency of aluminium such that sodium and potassium are in excess of that needed for feldspar. The presence of aegerine (sodium pyroxene) and riebeckite (sodium amphibole) are indicative ...

magma system in which they are the magma conduits connecting the underlying

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upw ...

to the volcanic centre at the surface, which has been extensively eroded to remnants of eruptive

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

References

Dike swarms Anahim Volcanic Belt Central Coast of British Columbia Miocene volcanism {{Canada-geology-stub