Yukon Volcanism

The Yukon has a long history of volcanic activity. The oldest volcanic rocks are ~ 1.6 billion years old, found in the Wernecke breccias north of Dawson City and in the Peel Plateau  ^( 1 ) . The youngest volcanic rocks, which erupted from Yukon's only "active" volcano, are less than 10,000 years old and can be found near Fort Selkirk, west of Carmacks in south-central Yukon  ^( 1 ) . Volcanic rocks found across the Yukon vary greatly in age and have fascinating formation histories. For example, some deposits originated as flood basalts on an ocean floor, while others are the result of crustal thinning due to continental rifting.

This virtual field experience focuses on volcanic rocks that are part of the Northern Cordilleran Volcanic Province (formerly the Stikine Volcanic Belt), which extends from southern British Columbia, through the Yukon, and into eastern Alaska. This province hosts more than 100 volcanic centres that erupted from 20 million years ago until as recently as 200 years ago  ^( 1 ) ! The volcanic centres represent a wide range of eruption styles, from small single-eruption flows to expansive multi-flow plateaus.

This volcanic belt was created from tectonic activity related to the formation of the Cordilleran mountain ranges. In Canada, the subduction of oceanic crust and the accretion of allochthonous (exotic) terranes ( 1 ) approximately 185 million years ago spurred the formation of the Cordillera. During this period of tectonism, the crust experienced transpression, a combination of compression and lateral movement, resulting in complexly deformed rocks and mountain uplift. After ~ 45 million years of transpression, the region transitioned to a transtensional regime, a combination of extension and lateral movement, which led to thinning of the crust. Lateral movement along faults created local zones of crustal weakness. Magma generated in the Earth's mantle rose buoyantly through these zones of weakness and erupted on surface, resulting in the volcanic features we see today. This transtensional tectonic phase with associated volcanism is responsible for creating the Northern Cordilleran Volcanic Province  ^( 1 ) .

This Virtual Geology field experience will focus on eruptive centers in the Yukon that are part of the Northern Cordilleran volcanic province. The virtual field sites highlighted include:

• Alligator Lake volcanic complex - which hosts world-class examples of volcanic features such as columnar basalt rosettes, volcanic ejecta, samples from the Earth's mantle, and more!
• Fort Selkirk volcanic field - which is home to at least seven volcanic centers along the Yukon River, including evidence of a sub-glacial eruption and Yukon's youngest volcano.

Select a field site below to begin your exploration of volcanism in the Yukon. If you want to learn more about igneous rocks and volcanology, continue to scroll past the interactive map for additional information.

An igneous rock (from Latin ignis, meaning "fire") is a rock that is produced from the cooling and solidification of hot, molten rock. Igneous rocks are very common, making up 95% of the Earth's crust.

Igneous rocks vary compositionally and depending on the minerals present in the rock, they are broadly classified into one of the following categories:

• Felsic: light coloured, composed dominantly of feldspar-silicate minerals
• Intermediate: a sub-equal combination of both mafic and felsic minerals
• Mafic: dark coloured, composed dominantly of magnesian-ferric minerals

There are two main genetic categories of igneous rocks that are used to describe where the magma cooled and crystallized. These two groups are intrusive and extrusive.

Intrusive rocks form when magma cools and solidifies under the Earth's surface. In this environment, the surrounding rocks act as insulation, resulting in slow conductive cooling of the magma body. This allows the crystallizing minerals to have a long time to grow, resulting in textures characterized by coarse mineral grains. The proper term for this texture is phaneritic, meaning one can see individual mineral grains with the unaided eye. Intrusive igneous rock bodies are called plutons.

Extrusive rocks form when magma reaches the Earth's surface at an eruptive centre; once on surface it is referred to as lava. Lava cools and solidifies rapidly on the Earth's surface. The rapid cooling shortens mineral crystal growth time, resulting in textures characterized by fine minerals grains. The proper term for this texture is aphanitic, meaning one cannot see individual mineral grains with the unaided eye. Extrusive rocks, commonly called volcanic rocks, are associated with many different styles of eruption and eruptive locations on Earth's surface.

Volcanology is the study of all aspects of volcanism, and there are many sub-disciplines. For example, physical volcanologists study the eruption process and related deposits; geochemists study the composition of the lava, gases, and volcanics; geophysicists study seismic activity which is important in volcanic monitoring and early warning systems; and remote sensing specialists use electronic equipment and modelling software to monitor and predict the timing and severity of a potential eruption. All of these sub-disciplines are important in order to fully understand the mechanisms, intricacies, and risks associated with volcanism.

• Volcanic edifice: the landform that is built by layers of lava and pyroclastic material
• Magma chamber: the magma source in the Earth's crust that ascended upwards from the mantle
• Conduit: the channel or passage that allows magma from the magma chamber to travel up to the vent
• Vent: the opening in the Earth's crust from which lava, pyroclastic material, and gases are erupted
• Crater: a bowl-shaped depression that surrounds the vent at the top of the edifice
• Flank: the side of the volcano

Volcanoes are classified into three main categories, each with typical characteristics, such as viscosity (resistance to flow) of the magma, amount of dissolved gas, magma composition, tectonic setting, and eruption style.

The three main types of volcanoes are shield volcanoes, stratovolcanoes, and cinder cones. Depending on the style of eruption, other landforms such as lava domes, fissure vents, and calderas can arise from erupting magma.

The three main stages of a volcano are active, dormant, or extinct. There are many inconsistencies and contradictory definitions when categorizing volcanoes. It is important to remember that there is not a single definition of these terms that is agreed upon amongst volcanologists, but here is a general summary:

• Active: A volcano that erupts on a regular basis or that has erupted during the current geologic epoch, the Holocene (i.e., in the last 10 000 years). This classification does not imply that the volcano is currently erupting, but rather that the magma source is young and still active.
• Dormant or "potentially active": A volcano that is not currently erupting and has not erupted in the last 10 000 years, but has the potential to erupt again in the future.
• Extinct: A volcano that has no written record of an eruption, there is no sign of activity, and/or it is cut off from the magma supply.

It is possible for a volcano that is classified as extinct to have an unexpected (and often catastrophic) eruption. A few examples of this scenario includes the volcanoes of Yellowstone, Toba, and Vesuvius. All three of these now-active volcanoes were thought to be extinct before their infamous eruptions.

Basalt is a mafic, aphanitic, extrusive igneous rock that is composed primarily of the minerals plagioclase, pyroxene, and olivine. This rock forms when basaltic magma erupts onto the surface as lava, cooling and crystallizing these minerals rapidly. Basalt is the most common volcanic rock on Earth, covering most of the ocean floor!

Some magmas have a large volume of gas dissolved in the molten rock. When the magma erupts as lava on the Earth’s surface, the dissolved gas tries to exsolve (escape) out of the lava. The outer surface of the lava cools and crystallizes rapidly, forming a crust which prevents the gas from exsolving and forces it to stay trapped in the lava. The lava solidifies, creating a network of rock around the trapped gas bubbles called vesicles. Two common types of rocks that have vesicles are scoria and vesicular basalt.

rock volume > vesicle volume = vesicular basalt

vesicle volume > rock volume = scoria

There are two main types of eruptions:

• Effusive, when lava flows are the predominant expulsion. Lava flows are outpourings of molten rock onto Earth's surface. Depending on the composition of the lava, these flows have the ability to travel great distances from their vent.
• Explosive, when ejected pyroclastic material is the predominant expulsion. Fragments ejected during explosive eruptions are termed pyroclasts (from Greek meaning “fire-broken”). A pyroclast can be ejected volcanic material, or may be solid pieces of pre-existing rock that were broken off during the eruption. The term pyroclastic material is used to describe all types of pyroclasts, which are subdivided by size.

If the eruptive style of the volcano is more effusive, it typically produces lava flows. A lava flow is a stream of molten lava that is erupted from the volcanic vent. The speed at which the lava flow advances depends on a few factors, including the rate at which the lava erupts from the vent, the topography of the landscape over which the lava is flowing, and the viscosity of the lava.

Have you ever noticed a hexagonal shape in nature - perhaps in a dried-out mud puddle, or maybe the shape of a honeycomb? That's because hexagons are the most efficient shape when trying to minimize the perimeter for a given area. In other words, it's the most surface area inside the shape for the smallest total length on the edges. This is the easiest shape for natural phenomena to release stress, distribute surface tension, or arrange molecules. Hexagons are the most mechanically stable shape, as forces acting on all sides are equal, dispersing energy or stress equally around all 6 sides of the shape.

How does this relate to rocks? A common feature related to lava flows is columnar jointing. This phenomenon can be seen in lava flow examples all over the world, including Giant's Causeway in Ireland, the young basaltic flows of Iceland, Devil's Tower Monument in Wyoming, USA, or even right here in Whitehorse, Yukon!

How do these naturally geometric columns form? When a large-volume lava flow erupts onto the Earth's surface, the top surface that is exposed to the air and the bottom surface that is in contact with the ground both start to cool. This decrease in temperature causes the lava to contract and shrink, which increases the surface tension on these outer surfaces of the lava flow. To allow the lava flow to become physically stable, big cracks called tension joints begin to form, which release the surface tension. What angle do these tension joints intersect at? You guessed it - 120°! As the lava flow continues to cool, the temperature gradient starts to migrate deeper into the flow, which enables these hexagonal cracks to propagate from the edges towards the center of the lava flow.

The resulting rocks are straight, regular, parallel columns of basalt. This type of basalt column occurrence is called colonnade. Another texture of basalt columns is called entablature. This texture is recognized by chaotic orientations of columns that vary in size and orientation. This texture forms when meteoric water (rain) infiltrates down the tension joints between the upper colonnade columns and disrupts the hot lava in the core of the flow. The addition of cold water to this system changes the temperature cooling gradient from being parallel to the ground, to being chaotic and unpredictable. Since the formation of columns is perpendicular to the cooling gradient, the resulting columns are also chaotic and unpredictable in orientation.

A great, easily accessible example of basalt columns is in Miles Canyon, just 10 km south of Whitehorse, YT. At this location, the Yukon River has been eroding away the basalt columns for thousands of years. This erosion has been cutting the canyon deeper and deeper, exposing beautiful columnar basalt cliffs!

Just as volcanic edifices can change over time, so can the rocks that erupted! A common example of this is amygdaloidal texture. After a vesicular rock cools and crystallizes completely, the vesicles create a network of open pore space that water and other fluids can flow through. Over time, water that is rich in dissolved mineral constituents percolates through this pore network and slowly precipitates minerals into the cavities of the vesicles. These secondary minerals are often hydrothermal minerals, including quartz, calcite, or zeolite minerals. These mineral-filled vesicles are called amygdules, and a rock that has experienced this process can be described as amygdaloidal.

The amygdules may be mistaken for phenocrysts, which are primary igneous minerals that crystallized in the magma. A good method of decifering a phenocryst from an amygdule is to look at the shape of the mineral. Phenocrysts are typically angular and may be elongated, while amygdules are rounded, since they take the shape of the pre-existing vesicle/air bubble.