5.3 Volcanic arcs and magmatism at convergent boundaries
5 min read•august 16, 2024
Volcanic arcs and magmatism at convergent boundaries are key players in plate tectonics. These fiery zones form where plates collide, creating chains of volcanoes and unique magma compositions. They're like nature's recycling centers, melting old crust and creating new land.
Understanding these processes helps us grasp Earth's inner workings. From island arcs to continental volcanoes, each setting tells a story of , melting, and explosive eruptions. It's a dynamic dance of elements and forces that shapes our planet's surface.
Volcanic Arcs at Convergent Boundaries
Formation and Characteristics of Volcanic Arcs
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- Volcanic arcs form parallel to convergent plate boundaries due to subduction processes
- Typically located 100-200 km from the trench axis
- Form a chain of volcanoes parallel to the subduction zone
- of the mantle wedge above the subducting plate generates magma
- Occurs at depths of 80-120 km
- Corresponds to the location of efficient slab dehydration
- Dehydration of the subducting oceanic lithosphere triggers partial melting
- Releases fluids into the overlying mantle
- Lowers the melting point of the mantle material
- Magma rises through the overlying plate due to its lower density
- Forms volcanic edifices at the surface
- Creates the distinctive arc-shaped alignment of volcanoes
Factors Influencing Volcanic Arc Distribution
- Subduction angle affects the location and width of the volcanic arc
- Steeper angles generally result in narrower arcs closer to the trench
- Shallower angles produce wider arcs farther from the trench
- Crustal thickness impacts magma evolution and volcano distribution
- Thicker crust may lead to more widely spaced, larger volcanoes
- Thinner crust may result in more closely spaced, smaller volcanoes
- Tectonic stress patterns influence volcano spacing and alignment
- Compressional stresses may favor clustering of volcanoes
- Extensional stresses may lead to more linear arrangements
- Magma production rates affect the size and frequency of volcanoes
- Higher rates can lead to larger, more closely spaced volcanoes
- Lower rates may result in smaller, more dispersed volcanic centers
Magmatism at Convergent Boundaries
Types of Convergent Boundary Magmatism
- Ocean-ocean convergence produces volcanism
- Characterized by andesitic to basaltic compositions
- Results in explosive eruptions (Mariana Islands, Aleutian Islands)
- volcanism occurs in ocean-continent convergence settings
- Generates more silicic magmas
- Forms larger, more complex volcanic systems (Andes Mountains, Cascades)
- Back-arc basin magmatism develops behind the volcanic front
- Produces basaltic magmas with intermediate compositions
- Occurs in extensional settings behind the arc (Lau Basin, Sea of Japan)
- Adakitic magmatism associates with melting of young, hot subducting slabs
- Creates distinctive high-silica, low-heavy rare earth element magmas
- Found in areas with subduction of young oceanic crust (Aleutian Islands)
Magmatic Processes and Events
- Flare-up events in continental arcs lead to periods of heightened activity
- Result in the formation of large igneous provinces
- Can produce significant volumes of magma over short geological timescales
- processes vary with tectonic setting
- Fractional crystallization plays a major role in arc magma evolution
- Assimilation of crustal material is more significant in continental arcs
- Magma mixing and mingling contribute to compositional diversity
- Occurs when different magma batches interact during ascent or storage
- Produces hybrid magmas with intermediate compositions
- Volatile exsolution drives explosive eruptions in arc settings
- High water content in arc magmas promotes violent eruptions
- Leads to the formation of pyroclastic deposits and ash plumes
Geochemistry of Convergent Boundary Magmas
Elemental and Isotopic Signatures
- Arc magmas show enrichment in large ion lithophile elements (LILE)
- Elements like K, Rb, Ba, and Sr are concentrated
- Reflects contribution from subduction-derived fluids
- High field strength elements (HFSE) are typically depleted
- Elements such as Nb, Ta, and Ti show relative depletion
- Indicates retention of these elements in subducted oceanic crust
- Water content in arc magmas is generally high (2-6 wt%)
- Contributes to their explosive nature
- Influences distinctive mineral assemblages (amphibole, biotite)
- Isotopic signatures reflect multiple source contributions
- Subducted sediments and altered oceanic crust influence composition
- Mantle wedge provides the primary magma source
- Crustal contamination affects magmas in continental settings
Magma Series and Compositional Trends
- Calc-alkaline magma series characterizes magmatism
- Defined by iron depletion during differentiation
- Contrasts with tholeiitic series typical of mid-ocean ridges
- Magma compositions range from basaltic to rhyolitic
- Andesitic compositions are particularly characteristic of mature arcs
- Basaltic compositions more common in island arcs and back-arc basins
- Trace element ratios serve as indicators of slab contributions
- Ba/La and Ce/Pb ratios used to assess fluid and sediment input
- Sr/Y and La/Yb ratios indicate garnet fractionation or slab melting
- Rare earth element (REE) patterns show distinctive features
- Light REE enrichment relative to heavy REE
- Eu anomalies indicate plagioclase fractionation or accumulation
Subduction Processes and Volcanic Arcs
Subduction Zone Dynamics
- Subduction angle influences volcanic arc location and width
- Steeper angles result in narrower arcs closer to the trench
- Shallower angles produce wider arcs farther from the trench
- Slab thermal structure affects dehydration reactions
- Controls the flux of fluids into the mantle wedge
- Influences subsequent magma generation processes
- Convergence rate impacts the subduction zone thermal regime
- Faster rates generally lead to colder slabs and less magma production
- Slower rates may allow for more efficient slab heating and dehydration
- Age and composition of the subducting plate affect magma chemistry
- Older, colder slabs may dehydrate at greater depths
- Younger, hotter slabs may partially melt, producing adakitic magmas
Mantle Wedge Processes and Crustal Influences
- Mantle wedge dynamics play a crucial role in subduction zones
- Corner flow circulates material in the wedge
- Small-scale convection enhances heat and mass transfer
- Crustal thickness of the overriding plate influences magma evolution
- Thicker crust promotes longer residence times and more differentiation
- Thinner crust allows for more rapid magma ascent and less modification
- Tectonic erosion or accretion modifies subduction zone geometry
- Erosion can steepen the subduction angle over time
- Accretion may lead to a shallowing of the subduction angle
- Stress regime in the overriding plate affects magma ascent paths
- Extensional settings facilitate easier magma transport to the surface
- Compressional settings may lead to more complex magma plumbing systems
Key Terms to Review (16)
Andesite: Andesite is an intermediate volcanic rock that primarily consists of plagioclase feldspar and is typically associated with subduction zones. This rock type forms when magma rises from the mantle through the Earth's crust, often at convergent plate boundaries where an oceanic plate is subducting beneath a continental plate, leading to the creation of volcanic arcs. Its composition reflects a mix of silica content, resulting in its characteristic gray to dark color.
Benioff Zone: The Benioff Zone refers to the inclined plane of seismic activity that occurs at convergent boundaries, particularly where one tectonic plate subducts beneath another. This zone is characterized by the presence of earthquakes at varying depths and is closely associated with volcanic arcs and magmatism that result from the melting of subducted materials. The dynamics of the Benioff Zone provide insights into the complex processes occurring at convergent boundaries and their role in shaping the Earth's geology.
Continental arc: A continental arc is a curved chain of volcanoes that form along the edge of a continental plate, typically at a convergent boundary where an oceanic plate is subducting beneath it. This geological feature arises due to the melting of the subducted oceanic plate, which generates magma that rises to create volcanic activity on the continental crust above. The formation of continental arcs is closely linked to the processes of magmatism and the tectonic interactions at convergent boundaries.
Convergent Boundary: A convergent boundary is a tectonic plate boundary where two plates move toward each other, often resulting in one plate being forced beneath the other in a process known as subduction. This interaction leads to significant geological features and phenomena, including earthquakes, volcanic activity, and mountain building, reflecting the dynamic nature of Earth's lithosphere.
Dacite: Dacite is a volcanic rock that is intermediate in composition, primarily composed of silica, and typically found in explosive volcanic eruptions. It plays a significant role in understanding the processes of volcanism at convergent plate boundaries, where tectonic plates collide and generate magma. Dacite often forms in volcanic arcs above subduction zones, indicating the complex interplay between plate tectonics and magmatism.
Earthquakes: Earthquakes are sudden releases of energy in the Earth's crust, resulting from tectonic movements that create seismic waves. These movements can occur at different types of plate boundaries, affecting geological formations and human structures alike, and they are often linked to various geological processes such as subduction, rifting, and faulting.
Explosive volcanism: Explosive volcanism refers to the violent eruption of magma from a volcano, which occurs when gas pressure builds up in the magma and is released suddenly, resulting in the ejection of volcanic materials such as ash, pumice, and volcanic rock. This type of volcanism is closely associated with convergent tectonic boundaries where oceanic plates subduct under continental plates, leading to the formation of explosive volcanic arcs.
Island arc: An island arc is a curved chain of volcanic islands that forms along a convergent tectonic plate boundary, where one oceanic plate subducts beneath another. This geological formation results from the melting of the subducting plate, which generates magma that rises to the surface, creating a series of islands. Island arcs are often characterized by their volcanic activity and associated tectonic features, reflecting the dynamic processes at play at these boundaries.
Magma differentiation: Magma differentiation is the process by which different types of igneous rocks form from a common magma source due to variations in temperature, pressure, and chemical composition as the magma cools and crystallizes. This process plays a crucial role in the formation of volcanic arcs at convergent boundaries, where subduction leads to melting and the generation of diverse magmatic compositions.
Oceanic-continental convergence: Oceanic-continental convergence is a tectonic process where an oceanic plate collides with a continental plate, leading to significant geological features and events. This type of boundary is crucial in the formation of mountain ranges and volcanic activity, showcasing the dynamic nature of Earth's lithosphere as these two different types of plates interact.
Partial melting: Partial melting is the process in which only a portion of a solid material melts, leading to the generation of liquid magma while leaving the remaining solid material unchanged. This process is significant in the context of tectonic activity, especially at convergent boundaries where subduction occurs, as it contributes to the formation of volcanic arcs and the generation of magma that can fuel volcanic eruptions.
Plate interaction: Plate interaction refers to the various ways tectonic plates interact with each other at their boundaries, resulting in geological phenomena such as earthquakes, volcanic activity, and mountain building. The nature of these interactions—whether convergent, divergent, or transform—shapes the Earth's surface and plays a crucial role in the movement of plates, influencing both the formation of features like volcanic arcs and the mechanisms driving plate motion.
Stratovolcano: A stratovolcano, also known as a composite volcano, is a steep, conical volcano characterized by its layered structure formed from alternating eruptions of lava and tephra. These volcanoes are typically associated with explosive eruptions due to the viscosity of their magma, which often contains high levels of silica. Stratovolcanoes are commonly found at convergent plate boundaries, where oceanic plates subduct beneath continental plates, leading to the formation of volcanic arcs.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle as the plates converge. This process is crucial in shaping Earth’s features, influencing everything from the formation of oceanic trenches to the creation of mountain ranges and volcanic activity.
Tectonic uplift: Tectonic uplift refers to the process by which Earth's crust is raised due to tectonic forces, often associated with the movement of tectonic plates. This phenomenon can lead to the formation of mountain ranges and elevated terrains, significantly influencing geological features and landscapes. As tectonic uplift occurs, it also interacts with erosion and sedimentation processes, playing a critical role in shaping the Earth's surface over time.
Tsunamis: Tsunamis are large ocean waves generated by significant disturbances in or near a body of water, most commonly caused by underwater earthquakes, volcanic eruptions, or landslides. These waves can travel across entire ocean basins and cause devastating effects when they reach coastlines, making them an important consideration in the context of geological activities and plate interactions.