7.1: The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc

Tectonic Setting of the Cascade Range

The Cascade Range Province of California is located in the northern portion of the state and composes part of a larger, regional province, which extends from Northern California through Oregon and Washington into British Columbia (Figure \(\PageIndex{1}\)). The Cascade Range (sometimes simply referred to as “the Cascades”) is known for its classic composite volcanoes (also referred to as stratovolcanoes or composite cones), including Mount Rainier (Tahoma), Mount Saint Helens (Loowit, Louwala-Clough), Mount Hood (Wy’east), and Mount Shasta (Waka-nunee-Tuki-wuki). However, the Cascade Range also includes many other volcanoes and volcanic features as well as non-volcanic, tectonically uplifted mountains. The Cascade Range is the volcanic arc mountain range produced by the subduction of the Juan de Fuca plate beneath the North American plate at the Cascadia subduction zone and also makes up part of the Ring of Fire, a series of such volcanic ranges that surround the Pacific Ocean (Figure \(\PageIndex{2}\)). The Modoc Plateau is also part of the Cascadia subduction system. This province lies to the east of the main Cascade volcanic arc and is confined to California, although similar plateau provinces can also be found east of the Cascades in Oregon, including the High Lava Plateau and Columbia Plateau.

Figure \(\PageIndex{1}\): Location of the major volcanoes of the Cascades (red triangles) in relation to the Cascadia subduction zone. The Juan de Fuca plate is forming along the Juan de Fuca ridge, which separates it from the Pacific plate, and is then subducted beneath the North American plate along the Cascadia subduction zone, indicated by the red line with teeth on it. This map was modified from the original "Volcanoes and Subduction" by Steven Earle, Physical Geology – 2nd Edition is licensed under CC BY 4.0. Access a detailed description.

Figure \(\PageIndex{2}\): Map of the global distribution of major volcanoes. Dark red triangles indicate the locations of volcanoes, dark blue shading highlights mid ocean ridges and red shading connects the volcanoes of the Ring of Fire, including the Cascade Volcanoes on the west coast of North America. "Tectonic plates and ring of fire" by Astroskiandhike is licensed under CC BY-SA 4.0.

How and Why Magma is Produced in the Cascadia Subduction Zone

North of the Mendocino Triple Junction, off the coast of California, the Juan de Fuca plate subducts beneath the North American plate. Some geologists divide the Juan de Fuca plate into two plates: the larger Juan de Fuca plate to the north and a smaller plate known as the Gorda plate to the south. For the sake of simplicity we will holistically refer to them as the Juan de Fuca plate. Before it is subducted, the Juan de Fuca plate begins its journey at the Juan de Fuca and Gorda ridges. The Gorda ridge is located 250 km (or about 150 miles) off the coast of Northern California (Figure \(\PageIndex{3}\)). Upwelling under the mid ocean ridge results in mid ocean ridge spreading, and new oceanic lithosphere is created. Near the mid ocean ridge, water from the ocean above becomes incorporated into the rock through the process of hydrothermal metamorphism.

The lithosphere that makes up the Juan de Fuca plate then travels eastward, away from the Juan de Fuca (and Gorda) ridge, until it reaches the edge of the continental slope, about 60 km (or 40 miles) off the coast of northern California, where it plunges beneath the overriding North American plate at the Cascadia Subduction Zone. From there, the subducting slab continues to sink downwards, at an angle, into the mantle beneath North America. 250 km (150 miles) or so inland from the point where it first begins to descend, the plate reaches the depth in the mantle where conditions are right for mantle melting.

At this depth, the water and other fluids that were incorporated into the crust through hydrothermal metamorphism are released through a different type of metamorphism, which occurs under high-pressure conditions deep in the mantle. This fluid is forced out of the subducting slab into the mantle above, where it acts as a flux. A flux is a chemical compound that lowers the melting temperature of a solid. In this case, the water and other fluids lower the melting temperature of the mantle. At depths of over one hundred kilometers, the mantle rock is just below its melting temperature and therefore because of the addition of water and other flux fluids, it can melt. This is called flux melting.

Figure \(\PageIndex{3}\): Block diagram showing production of new crust at the Gorda Ridge, subduction of the oceanic lithosphere along the Cascadia trench, and associated flux melting and back-arc upwelling. "Cascades and Modoc Plateau Block Diagram" by Emily Wright is is licensed under CC BY 4.0. Access a detailed description.

There is a common misconception that many people develop about why volcanoes occur near subduction zones. Be sure that you don’t have this misconception by completing the activity, “melting and subduction zones”.

Query \(\PageIndex{1}\)

Video \(\PageIndex{1}\): Flux Melting Demonstration with Ice and Salt

Flux melting in the context of subduction zones seem like a strange concept at first, but there are other cases where flux melting is more familiar. One example occurs in cold climates; local highway maintenance departments sometimes salt bridges and roads to prevent ice from forming. In this case, the salt acts as the flux, not the water. The video, "Flux Melting Demonstration with Ice and Salt" demonstrates an example of flux melting that can be used as an analogy to better understand the process at subduction zones. The video includes a narration, but can also stand on its own with the sound turned off.

Magma Generation Beneath the Modoc Plateau

Magma generation beneath the Modoc Plateau has a different cause. The chemistry of the volcanic rock suggests that there was no flux involved in the original melting of the magma. Here, melting is a result of back-arc upwelling (Figure \(\PageIndex{3}\)) and other similar processes. The motion of the downgoing Juan de Fuca slab results in the upward flow of mantle material into the space above the subducting slab (known as the mantle wedge). This convection brings relatively hot mantle material upward and also decreases the pressure. Because solid rock is denser than liquid magma, high-pressure conditions keep mantle rock solid even though it is very hot. A decrease in these high-pressure conditions can therefore result in melting. Even within the Cascade Range province, rock chemistry suggests that some magma originates from this mantle flow around the subducting slab, while other rocks are sourced from flux melting.

Whatever the cause of melting, once magma is produced in the mantle, volcanoes are likely to occur. Since liquid magma is less dense than the surrounding mantle, the magma produced by this melting rises upwards through the mantle, much like cooking oil added to water. Some of this magma may cool again before reaching the surface forming intrusive igneous rock, but some will reach the surface, erupt as lava, and form volcanoes.

Magma Evolution During the Rise to the Surface

The Cascade Range and the Modoc Plateau are host to many different types of volcanoes and a variety of volcanic rocks. The variety found in the Cascade Range is a result of the processes that occur as magma undergoes its journey from the mantle to the surface.

Because the mantle is composed of many different minerals, it does not melt uniformly. As minerals with lower melting points turn into liquid magma, those with higher melting points remain as solid crystals. Each mineral has a unique melting and crystallization temperature. Mafic minerals (those high in iron and magnesium) have higher melting temperatures, so they are usually the last minerals to melt and the first to crystallize as magma cools. Since most rocks are made of many different minerals, when they start to melt, some minerals begin melting sooner than others. This is known as partial melting and it is one part of a process called magmatic differentiation, which describes the physical and chemical changes that take place as magma slowly rises and cools into solid rock.

The most important example of partial melting occurs as magma is generated from mantle rock. The chemistry of mantle rock (peridotite) is ultramafic, low in silica, and high in iron and magnesium. When peridotite begins to melt, the silica-rich portions melt first due to their lower melting point (see 2.4: Igneous Rocks). As this continues, an increasingly silica-rich magma forms, and the ultramafic mantle rock produces a mafic magma.

Liquid magma is less dense than the surrounding solid rock, so it rises through the mantle and crust. As magma begins to cool and crystallize, further magmatic differentiation changes the chemistry of the resultant rock into a more felsic composition. This happens two ways through both assimilation and fractionation. During assimilation, pieces of country rock with a different, often more felsic, composition are added to the magma. These solid pieces may melt, which further changes the composition of the magma.

Fractionation or fractional crystallization is another process that increases the silica content, making a magma more felsic. As a magma diapir rises through the crust, the temperature drops and some minerals will crystallize and settle to the bottom of the magma chamber, leaving the remaining melt depleted of ions in those minerals. For example, olivine is a mafic mineral with a high melting point, making it one of the first minerals to crystallize in cooling magma (Figure \(\PageIndex{4}\)a). The solid olivine crystals are denser than the surrounding liquid magma, so they will settle to the bottom of the magma chamber (Figure \(\PageIndex{4}\)b). Because olivine contains more iron and magnesium and less silica than other common igneous minerals, the settling olivine removes a significant amount of iron and magnesium from the magma but does not remove much silica. This means the remaining melt becomes more silica-rich and felsic (Figure \(\PageIndex{4}\)c). As the mafic magma further cools, and other mafic minerals crystallize, which remove even more low-silica components from the magma, it becomes even more felsic. This process of separating magmas into differing compositions can occur multiple times as the magma moves upward from one magma chamber to the next.

Figure \(\PageIndex{4}\): An example of crystal settling and the formation of a zoned magma chamber. Olivine minerals form (a), then settle to the bottom (b), leaving more felsic magma behind at the top of the magma chamber (c). "Crystal Settling" by Steven Earle is licensed under CC BY 4.0. Access a detailed description.

When mafic magma rises through the thick continental crust of the North American plate, the journey is much longer than when magma rises through thin oceanic crust. This gives the magma more time to react with the surrounding country rock and to undergo fractionation. Much of the continental lithosphere is felsic (i.e. granitic), therefore the process of assimilation can now also occur. Thus the mafic magma tends to assimilate felsic rock, becoming more silica-rich as it migrates through the lithosphere and changing into intermediate or felsic magma by the time it reaches the surface.

Video \(\PageIndex{2}\): Partial Melting of Igneous Rocks

The video “Partial Melting of Igneous Rocks” describes the process of partial melting and describes how different types of magma form in association with different types of plate settings. Pay attention to the descriptions of magma associated with subduction zones and use this information to predict which types of rocks are common in the California Cascades.

Each magma diapir travels its own unique journey originating in the mantle (Figure \(\PageIndex{5}\)). Not all diapirs make it to the surface (see 9.2: Basement Geology of the Sierra Nevada: The Core of an Ancient Volcanic Arc and Chapter 15: Peninsular Ranges), but volcanoes are formed by those that do. Some of these diapirs change significantly and erupt as felsic lava, while some change less and erupt as mafic or intermediate lava. The result is that the volcanic rocks of the Cascade Range include a variety of compositions and contribute to a variety of volcano shapes and sizes.

Figure \(\PageIndex{5}\): Rising magma diapirs in mantle and crust. Fractional crystallization and assimilation occur as diapirs rise through the crust. The thickness of the Earth and atmospheric layers is not to scale. "Magmatism and Volcanism" by An Introduction to Geology is licensed under CC BY 4.0. Access a detailed description.

Query \(\PageIndex{2}\)

Acknowledgments

Parts of the text on this page were taken with some editing from existing Open Educational Resources by Earl (2019) and Johnson et al. (2017). Links to the original text can be found in the reference section on this page.

References

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3. Humphreys, E. D., & Grunder, A. L. (2022). Tectonic controls on the origin and segmentation of the Cascade Arc, USA. Bulletin of Volcanology, 84(12), 102. https://doi.org/10.1007/s00445-022-01611-2
4. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2017). Chapter 4: Igneous Processes and Volcanoes. In An Introduction to Geology. https://slcc.pressbooks.pub/introgeology/chapter/4-igneous-processes-and-volcanoes/
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