10.2 Volcanism at Convergent Plate Boundaries
4 min read•july 30, 2024
Convergent plate boundaries are hotspots for volcanic activity. When plates collide, one sinks beneath the other, creating prime conditions for magma formation and volcanic eruptions. This process shapes dramatic landscapes like the Andes and Aleutian Islands.
These volcanoes are often explosive, spewing ash and lava. The type of convergence—oceanic-continental or oceanic-oceanic—affects the magma composition and eruption style. Understanding these differences is key to predicting volcanic behavior and managing associated hazards.
Volcanism at Convergent Boundaries
Oceanic-Continental vs. Oceanic-Oceanic Convergence
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- Volcanism at convergent plate boundaries is driven by subduction, where one tectonic plate descends beneath another into the mantle
- Oceanic-continental convergence occurs when an oceanic plate subducts beneath a continental plate, resulting in the formation of a volcanic arc on the overriding continental plate
- Examples of oceanic-continental convergence include the Andes Mountains (South America) and the Cascade Range (North America)
- Oceanic-oceanic convergence occurs when two oceanic plates collide, and one plate subducts beneath the other, leading to the formation of an island arc
- Examples of oceanic-oceanic convergence include the Mariana Islands and the Aleutian Islands
Magma Composition and Eruption Style
- Magmas generated at oceanic-continental convergent boundaries tend to be more silica-rich and viscous compared to those at oceanic-oceanic boundaries due to the involvement of continental crust
- Volcanic eruptions at oceanic-continental boundaries are often more explosive and produce larger volumes of ash and pyroclastic material compared to oceanic-oceanic boundaries
- Island arcs formed at oceanic-oceanic boundaries are characterized by a curved chain of volcanic islands, while at oceanic-continental boundaries form a linear chain of volcanoes on the continental margin
Magma Generation in Subduction Zones
Dehydration and Partial Melting
- Magma generation in subduction zones is primarily driven by the release of fluids from the subducting oceanic plate and of the mantle wedge above the subducting slab
- As the oceanic plate subducts, it undergoes progressive metamorphism and dehydration, releasing water and other volatile components into the overlying mantle wedge
- The released fluids lower the melting point of the mantle wedge, causing partial melting and the generation of primary magmas
Magma Evolution and Composition
- The primary magmas are basaltic in composition and rise through the mantle wedge, undergoing and assimilation of crustal material
- Magma composition evolves as it ascends through the crust, becoming more silica-rich and forming andesitic to rhyolitic magmas
- The depth of magma generation and the extent of magma differentiation depend on factors such as the age and composition of the subducting plate, the convergence rate, and the thickness of the overriding plate
Volcanic Arcs and Landforms
Volcanic Arc Characteristics
- Volcanic arcs are elongated chains of volcanoes that form parallel to the subduction zone at convergent plate boundaries
- Volcanic arcs are characterized by a series of stratovolcanoes, which are tall, conical volcanoes built by alternating layers of lava flows and pyroclastic deposits
- Examples of stratovolcanoes in volcanic arcs include (Japan) and (United States)
- Volcanic arcs often have a well-defined volcanic front, which marks the surficial expression of the subducting slab at a depth of about 100-150 km
Associated Landforms and Features
- Behind the volcanic front, there may be a back-arc basin, which is an area of extension and thinning of the overriding plate, often accompanied by basaltic volcanism
- Calderas, which are large circular depressions formed by the collapse of a volcano's summit or the emptying of its magma chamber, are common features in volcanic arcs
- Volcanic arcs may also have associated geothermal systems, hot springs, and fumaroles due to the heat and fluids released by the underlying magmatic activity
Hazards of Convergent Plate Volcanism
Explosive Eruptions and Their Products
- Volcanoes at convergent plate boundaries pose significant hazards to nearby populations and infrastructure due to their explosive nature and the variety of volcanic products they generate
- can produce large volumes of ash and pyroclastic density currents (PDCs), which are fast-moving, ground-hugging flows of hot gas, ash, and rock fragments
- PDCs can travel at high speeds, causing destruction and fatalities in their path, as seen in the 1902 eruption of Mount Pelée (Martinique)
- Lahars, which are mudflows or debris flows triggered by volcanic activity, can travel long distances down river valleys, endangering communities far from the volcano
- The 1985 eruption of Nevado del Ruiz (Colombia) generated lahars that killed over 23,000 people in the town of Armero
Other Volcanic Hazards
- Lava flows, although less common at convergent boundary volcanoes, can still pose a threat to nearby settlements and infrastructure
- Volcanic gases, such as sulfur dioxide and carbon dioxide, can cause respiratory issues, acid rain, and contribute to climate change when released in large quantities
- Volcanic ash can disrupt air travel, cause damage to machinery and infrastructure, and lead to respiratory problems when inhaled
- The collapse of volcanic edifices or the formation of debris avalanches can also pose a significant hazard, as seen in the 1980 eruption of Mount St. Helens (United States)
Key Terms to Review (18)
Andesitic magma: Andesitic magma is a type of intermediate volcanic rock that is primarily composed of silica (SiO2) and contains between 52% to 66% of silica by weight. It is associated with subduction zone volcanism, where oceanic plates descend into the mantle beneath continental plates, leading to the generation of this specific type of magma. The unique composition of andesitic magma influences its viscosity and the explosive nature of the eruptions it produces.
Arc volcanism: Arc volcanism refers to the volcanic activity that occurs along convergent plate boundaries, where an oceanic plate subducts beneath a continental plate or another oceanic plate. This process leads to the formation of a volcanic arc, which is a chain of volcanoes that are typically characterized by explosive eruptions and a high level of silica in the magma. The relationship between subduction zones and arc volcanism is crucial in understanding the geology and eruption styles of these regions.
Dehydration melting: Dehydration melting refers to the process whereby water is released from minerals under high temperature and pressure, leading to the formation of magma. This occurs mainly in subduction zones, where an oceanic plate descends into the mantle, causing the surrounding rock to melt as a result of the removal of water from hydrous minerals. The resulting magma contributes to volcanic activity at convergent plate boundaries.
Effusive Eruptions: Effusive eruptions are volcanic events characterized by the gentle flow of lava onto the surface, rather than explosive activity. These eruptions typically produce basaltic lava, which is low in viscosity, allowing it to flow easily and create broad, shield volcanoes. Understanding effusive eruptions is essential for assessing volcanic hazards and their impact on surrounding environments.
Explosive eruptions: Explosive eruptions are volcanic events characterized by the violent expulsion of gas, ash, and volcanic rock into the atmosphere. These eruptions often occur when magma rises to the surface, encountering water or a buildup of gas pressure, leading to a rapid expansion and explosive release. The consequences of such eruptions can be severe, including pyroclastic flows and significant atmospheric ash fallout, which can impact climate and human activities.
Fractional Crystallization: Fractional crystallization is the process by which different minerals crystallize from a cooling magma at different temperatures, leading to the separation of various components based on their chemical composition and melting points. This process significantly influences the composition of magma as it evolves, affecting everything from its physical properties to the types of volcanic products that eventually form.
Lahar: A lahar is a destructive volcanic mudflow composed of a mixture of water, volcanic ash, and debris that flows down the slopes of a volcano. These flows can occur during or after an eruption, especially when heavy rainfall mobilizes volcanic materials, leading to rapid and often devastating movements of sediment.
Mount Fuji: Mount Fuji is an iconic stratovolcano located on Honshu Island in Japan, standing at 3,776 meters (12,389 feet), making it the tallest mountain in the country. Known for its symmetrical cone shape and cultural significance, Mount Fuji is a prime example of a stratovolcano formed by the subduction of the Philippine Sea Plate beneath the Eurasian Plate, which connects it to various geological processes and volcanic activity in Japan.
Mount St. Helens: Mount St. Helens is an active stratovolcano located in the state of Washington, known for its catastrophic eruption on May 18, 1980, which significantly altered the surrounding landscape. This volcano is a classic example of stratovolcanic activity and provides insights into volcanic behavior, pyroclastic flows, and tephra dispersal patterns associated with eruptions.
Partial Melting: Partial melting refers to the process by which only a portion of a solid material, such as rock, melts while the rest remains solid. This process is crucial in the formation of magma and influences its composition, which directly affects volcanic activity and the types of rocks formed from cooled magma.
Pyroclastic flow: A pyroclastic flow is a fast-moving current of hot gas and volcanic matter, such as ash and rock fragments, that flows down the slopes of a volcano during an explosive eruption. This deadly phenomenon is characterized by its high temperatures and speeds, making it one of the most hazardous volcanic phenomena.
Rhyolitic magma: Rhyolitic magma is a type of high-silica, low-density magma that is typically characterized by a high viscosity and a tendency to produce explosive volcanic eruptions. Its composition often includes significant amounts of quartz and feldspar, making it less fluid compared to other types of magma. This unique composition affects its physical properties, behavior in magma chambers, and the nature of volcanic activity, especially at convergent plate boundaries.
Subduction Dynamics: Subduction dynamics refers to the geological processes and interactions that occur at convergent plate boundaries, where one tectonic plate is forced beneath another. This process not only leads to the formation of deep ocean trenches but also plays a crucial role in the generation of volcanic activity, earthquakes, and the recycling of crustal material back into the mantle, contributing to various geological phenomena.
Subduction zone volcanism: Subduction zone volcanism refers to the volcanic activity that occurs at convergent plate boundaries where one tectonic plate is forced beneath another into the mantle. This process leads to the melting of the subducted plate and the surrounding mantle, generating magma that rises to form volcanoes. The unique geochemical characteristics of this magma often result in explosive eruptions and the formation of stratovolcanoes.
Tectonic plate interaction: Tectonic plate interaction refers to the processes and dynamics that occur at the boundaries where tectonic plates meet, leading to various geological phenomena. This interaction is crucial in shaping the Earth's landscape, influencing earthquakes, mountain building, and volcanic activity. Understanding these interactions is essential for studying the mechanisms behind volcanism, especially in areas where plates converge.
Trench: A trench is a deep, narrow depression in the ocean floor that forms at convergent plate boundaries where one tectonic plate is being forced beneath another. These features are often associated with intense geological activity, including the formation of volcanoes and earthquakes, as the subducting plate melts and releases magma into the mantle.
Volatiles: Volatiles are substances in magma that can easily evaporate or vaporize at surface conditions, primarily consisting of water vapor, carbon dioxide, sulfur dioxide, and other gases. These components play a crucial role in influencing magma behavior, eruption dynamics, and the characteristics of volcanic eruptions. The presence and concentration of volatiles significantly affect magma viscosity, the potential for explosive eruptions, and the overall chemistry of volcanic products.
Volcanic Arcs: Volcanic arcs are chains of volcanoes that are formed above a subducting plate at convergent plate boundaries. These arcs typically result from the melting of the subducted oceanic crust, leading to magma formation that rises to the surface and creates volcanic activity. The volcanic arcs are crucial for understanding the relationship between tectonic processes and volcanism, as they illustrate how plate interactions can lead to significant geological features.