10.4 Relationship Between Tectonics and Magma Composition
4 min read•july 30, 2024
Tectonic settings shape magma composition, influencing volcanic activity worldwide. From mafic basalts at mid-ocean ridges to felsic rhyolites in continental arcs, plate boundaries dictate the magma's characteristics and potential hazards.
Understanding this link helps predict eruption styles and risks. Mafic magmas often lead to effusive eruptions, while felsic magmas tend to produce explosive events. This knowledge is crucial for volcanic hazard assessment and mitigation strategies.
Tectonic Settings and Magma Composition
Influence of Tectonic Settings on Magma Generation
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- Tectonic settings (divergent, convergent, and intraplate boundaries) play a crucial role in determining the composition of magmas generated in these environments
- At divergent boundaries (mid-ocean ridges), decompression melting of the upper mantle produces mafic magmas (basaltic composition) due to the shallow melting of peridotite
- Convergent boundaries, where subduction occurs, generate a range of magma compositions depending on the depth of melting and the involvement of subducted materials
- Intraplate settings (hotspots and continental rifts) typically produce mafic magmas (basaltic composition) due to the melting of the upper mantle or mantle plumes
Role of Crustal Thickness in Magma Composition
- The thickness of the continental crust influences magma composition
- Thicker crust favors the formation of more evolved, silica-rich magmas through processes like and crustal assimilation
- In thinner oceanic crust, magmas tend to be less evolved and more mafic in composition
- The interaction between ascending magmas and the surrounding crust can further modify the magma composition through assimilation and contamination processes
Magma Types in Tectonic Environments
Mafic Magmas
- Mafic magmas (basalts) are characterized by low silica content (45-52% SiO2), high magnesium and iron content
- Typically generated in divergent, intraplate, and some convergent settings (island arcs)
- These magmas have lower viscosity, lower gas content, and higher eruption temperatures (1000-1200°C) compared to intermediate and felsic magmas
- Mafic magmas are produced by high degrees of of mantle peridotite
Intermediate Magmas
- Intermediate magmas (andesites) have moderate silica content (52-63% SiO2)
- Commonly associated with subduction zones at convergent plate boundaries
- Form through partial melting of the mantle wedge, often with contributions from the subducted slab
- Have intermediate viscosity, gas content, and eruption temperatures (800-1000°C)
- Intermediate magmas can be generated by mixing of mafic and felsic magmas or through of mafic magmas
Felsic Magmas
- Felsic magmas (rhyolites) have high silica content (>63% SiO2), low magnesium and iron content
- Typically generated in mature continental arcs or during the late stages of magmatic differentiation
- These magmas have higher viscosity, higher gas content, and lower eruption temperatures (700-850°C) compared to mafic and intermediate magmas
- Felsic magmas can be produced by low degrees of partial melting of crustal rocks or through extensive fractional crystallization of more mafic magmas
Magma Composition and Differentiation
Partial Melting
- Partial melting is the process by which magma is generated from a solid source rock
- The degree of partial melting influences the composition of the resulting magma
- Low degrees of partial melting produce magmas enriched in incompatible elements (K, Na, Ti) and more evolved compositions (felsic magmas)
- High degrees of partial melting generate magmas with compositions closer to the source rock (mafic magmas from peridotite)
- The depth and temperature of melting also affect the composition of the resulting magma
Magma Differentiation Processes
- Magma differentiation processes (fractional crystallization and assimilation) can modify the composition of magmas as they cool and evolve
- Fractional crystallization involves the removal of early-formed crystals from the melt
- This leads to the progressive enrichment of the remaining melt in silica and incompatible elements, resulting in more evolved magma compositions over time
- Assimilation occurs when magma incorporates and melts surrounding country rock
- This can potentially alter the magma composition depending on the nature of the assimilated material
- Assimilation combined with fractional crystallization (AFC) is a common process in the evolution of magmas
Magma Composition and Volcanic Hazards
Mafic Magmas and Associated Hazards
- Mafic magmas (basaltic composition) tend to have lower viscosity and higher temperature, resulting in effusive eruptions
- These eruptions are characterized by lava flows and fountains
- Hazards associated with mafic magmas include:
- Lava flows that can inundate areas and cause destruction
- Lava fountains that eject molten material into the air
- Localized tephra fall near the vent
- Examples of volcanoes with mafic magmas: Kilauea (Hawaii), Piton de la Fournaise (Réunion Island)
Intermediate Magmas and Associated Hazards
- Intermediate magmas (andesitic composition) have moderate viscosity and gas content
- They can lead to a range of eruptive styles from effusive to explosive
- Hazards associated with intermediate magmas include:
- Lava flows that can travel significant distances
- Lava domes that can grow and collapse, generating pyroclastic density currents
- Pyroclastic density currents that can travel rapidly and cause destruction
- Tephra fall that can blanket large areas and disrupt infrastructure
- Examples of volcanoes with intermediate magmas: Mount Merapi (Indonesia), Mount St. Helens (USA)
Felsic Magmas and Associated Hazards
- Felsic magmas (rhyolitic composition) have high viscosity and gas content, often resulting in explosive eruptions
- These eruptions can produce significant hazards:
- Pyroclastic density currents that can travel at high speeds and cause widespread destruction
- Tephra fall that can cover large areas, disrupt transportation and infrastructure, and pose health risks
- Lava domes that can grow and collapse, generating dangerous pyroclastic density currents
- Volcanic gases that can be toxic and contribute to atmospheric pollution
- Examples of volcanoes with felsic magmas: Yellowstone (USA), Taupo Volcanic Zone (New Zealand)
Key Terms to Review (18)
Basalt: Basalt is a dark, fine-grained volcanic rock that forms from the rapid cooling of low-viscosity lava. It is the most abundant volcanic rock on Earth and primarily constitutes the oceanic crust, playing a crucial role in the formation of various landforms and features associated with different tectonic settings.
Convergent Boundary: A convergent boundary is a tectonic plate boundary where two plates move towards each other, resulting in one plate being forced beneath the other in a process known as subduction. This interaction can lead to significant geological features such as mountain ranges, deep ocean trenches, and volcanic activity, illustrating the dynamic nature of Earth's crust and the processes that shape its internal structure and influence magma composition.
Divergent boundary: A divergent boundary is a tectonic plate boundary where two plates move away from each other, leading to the formation of new crust as magma rises to the surface. This process is primarily associated with mid-ocean ridges and rift valleys, where the movement of plates creates spaces for molten rock to erupt and solidify, contributing to the dynamic nature of Earth's surface and its internal processes.
Effusive eruption: An effusive eruption is a volcanic event characterized by the gentle flow of low-viscosity lava, which results in the formation of broad, shield-shaped volcanoes. These eruptions are generally less explosive than other types, allowing lava to spread out over large areas, creating distinct landforms and contributing to the landscape's evolution.
Explosive eruption: An explosive eruption is a volcanic eruption characterized by the violent expulsion of magma, gas, and volcanic ash into the atmosphere. This type of eruption is typically associated with high-viscosity magma that traps gas, leading to intense pressure buildup and a sudden release, resulting in an explosive release of materials.
Feldspar: Feldspar is a group of rock-forming minerals that make up about 60% of the Earth's crust, primarily composed of aluminum silicate combined with varying amounts of potassium, sodium, and calcium. These minerals play a significant role in the composition of igneous rocks and are crucial for understanding the processes that generate magma and the relationships between tectonics and magma chemistry.
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.
Hotspot: A hotspot is a volcanic region that has experienced active volcanism for a significant period of time, typically resulting from a plume of hot material rising from deep within the Earth's mantle. These regions are often characterized by a chain of volcanoes formed as tectonic plates move over the stationary hotspot, illustrating key processes in Earth's internal structure and plate tectonics.
Magma chamber: A magma chamber is a large underground pool of molten rock located beneath the Earth's surface, where magma accumulates and resides before it can erupt as lava. This chamber plays a crucial role in volcanic activity and is instrumental in determining the composition, behavior, and style of eruptions.
Magma differentiation: Magma differentiation is the process by which a single magma source evolves into different types of magma, resulting in variations in composition and physical properties. This process is influenced by factors such as temperature, pressure, and the presence of crystals that may settle out or react with the liquid magma, leading to diverse volcanic rock types.
Mantle convection: Mantle convection is the process by which heat from the Earth's interior causes the mantle to move in a continuous cycle, driving tectonic plate movements and influencing volcanic activity. This process plays a crucial role in the dynamics of the Earth's lithosphere and is central to understanding how magma forms and rises, as well as how tectonic plates interact and reshape the Earth's surface over geological time.
Olivine: Olivine is a common mineral found in the Earth's mantle and is primarily composed of magnesium and iron silicate. It plays a crucial role in understanding magma composition, generation processes, and the relationship between tectonics and magma. As a major component of ultramafic rocks, olivine can influence the properties of magma and provide insights into the conditions under which it forms.
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.
Plate subduction: Plate subduction is the geological process where one tectonic plate moves under another and sinks into the Earth's mantle. This process typically occurs at convergent boundaries, where oceanic plates are forced beneath continental plates or other oceanic plates, leading to the formation of deep ocean trenches and volcanic arcs. Subduction is a key mechanism that drives plate tectonics and influences magma composition in volcanic regions.
Rhyolite: Rhyolite is a light-colored volcanic rock that is high in silica content, typically greater than 70%. This composition makes it one of the most viscous types of magma, which can lead to explosive volcanic eruptions. Rhyolite is closely linked to the processes of magma generation and the tectonic settings that influence its formation, often resulting from the melting of continental crust or in subduction zone environments.
Subduction Zone: A subduction zone is a tectonic boundary where one tectonic plate is forced under another, leading to intense geological activity, including the formation of volcanoes and earthquakes. These zones are critical in shaping volcanic landforms, influencing eruption styles, and playing a significant role in magma composition and global volcanic events.
Tectonic uplift: Tectonic uplift is the geological process that occurs when the Earth's crust is raised due to tectonic forces, often associated with the movement of tectonic plates. This uplift can lead to the formation of mountain ranges and elevated terrains, playing a critical role in shaping the landscape. The dynamics of tectonic uplift are closely tied to other geological processes, including magma composition and volcanic activity, as well as the overall structural evolution of the Earth's crust.
Volcanic Arc: A volcanic arc is a chain of volcanoes formed above a subduction zone, where one tectonic plate is forced under another. This geological feature typically creates a curved shape due to the dynamics of plate tectonics, and it reflects the interaction between the descending oceanic plate and the overlying continental or oceanic plate. Volcanic arcs are significant in understanding both the internal structure of the Earth and the composition of magma produced during subduction processes.