🌎Plate Tectonics Unit 5 – Convergent Boundaries: Subduction Zones

Key Concepts

• Convergent boundaries occur where two tectonic plates collide and one plate is forced beneath the other into the mantle
• Subduction zones form at convergent boundaries where an oceanic plate descends beneath another oceanic or continental plate
• Plate density differences drive subduction with denser oceanic crust sinking below less dense continental or younger oceanic crust
• Subducting plates are recycled into the mantle and can contribute to the formation of magma
• Subduction results in distinctive geological features (volcanic arcs, deep ocean trenches, accretionary wedges)
• Subduction zones are sites of intense seismic activity including large magnitude earthquakes due to plate interactions
• Volcanic activity at subduction zones produces composite volcanoes and can result in explosive eruptions
• Subduction plays a crucial role in the rock cycle and contributes to the growth of continental crust over geologic time

Types of Convergent Boundaries

• Oceanic-continental convergence involves dense oceanic crust subducting beneath less dense continental crust (Nazca Plate beneath South American Plate)
• Oceanic-oceanic convergence occurs when two oceanic plates collide and the older, denser plate subducts (Pacific Plate beneath Philippine Plate)
• Continental-continental convergence happens when two continental plates collide resulting in mountain building and crustal thickening (Indian Plate and Eurasian Plate)
  • Neither plate subducts due to similar densities leading to significant compressional forces and uplift
• Variations in convergence angles and rates influence the type and intensity of geological activity at the boundary
• The age and density of the subducting plate affects the angle of subduction (slab dip) and associated volcanic activity
• Subduction polarity refers to the direction of plate descent and can vary along the length of a convergent boundary
• Double subduction zones form where two plates simultaneously subduct beneath each other in opposite directions (Molucca Sea)

Subduction Zone Formation

• Subduction initiates when one plate begins to descend beneath another often along pre-existing zones of weakness
• Age differences between colliding plates lead to density contrasts that drive subduction
  • Older oceanic crust is colder, denser, and more prone to subduction compared to younger crust
• Plate motion and far-field stresses contribute to the onset and continuation of subduction
• Subduction can nucleate at transform faults or fracture zones where there are abrupt changes in plate age and density
• Sediment accumulation and loading on the subducting plate can facilitate subduction by increasing its density
• Metamorphic reactions in the subducting slab (basalt to eclogite transition) increase density and promote further subduction
• Slab pull force generated by the weight of the descending plate is a primary driver of plate motions and subduction
• Subduction angles steepen over time as the slab sinks deeper into the mantle and becomes more negatively buoyant

Plate Interactions and Movements

• Convergence rates between plates vary globally and influence the intensity of subduction zone processes
• Faster convergence rates often correlate with increased seismic activity and more rapid recycling of oceanic crust
• Oblique convergence involves a component of lateral motion along the subduction zone resulting in transpressional or transtensional deformation
• Slab rollback occurs when the subducting plate steepens and migrates oceanward causing extension in the overriding plate (Tonga-Kermadec subduction zone)
• Back-arc basins form behind volcanic arcs due to extensional forces induced by slab rollback (Mariana Trough)
• Accretion of sediments and oceanic crust onto the overriding plate leads to the growth of accretionary wedges
• Seamount subduction can cause temporary cessation of subduction and deformation of the overriding plate
• Slab windows develop where a spreading ridge intersects a subduction zone allowing asthenospheric upwelling (Juan de Fuca Ridge and North American Plate)

Geological Features and Structures

• Deep ocean trenches form at subduction zones due to the downward flexure of the subducting plate (Mariana Trench)
  • Trenches are typically 6-11 km deep and can extend for thousands of kilometers along the convergent boundary
• Accretionary wedges develop from the scraping off and accumulation of sediments and oceanic crust at the subduction zone
  • Wedges exhibit thrust faulting and folding as a result of compressional forces
• Forearc basins are sedimentary depocenters located between the accretionary wedge and volcanic arc
• Volcanic arcs form parallel to the subduction zone where magma generated by slab dehydration and melting rises to the surface
  • Arcs can be island arcs (Mariana Islands) or continental arcs (Andes Mountains) depending on the overriding plate
• Metamorphic rocks (blueschists, eclogites) form in the subduction zone due to high-pressure, low-temperature conditions
• Ophiolites are fragments of oceanic crust and upper mantle that are emplaced onto continental margins during subduction
• Subduction erosion can remove material from the overriding plate and contribute to forearc subsidence and landward migration of the trench
• Slab dehydration releases fluids that serpentinize the mantle wedge and facilitate partial melting and magma generation

Seismic and Volcanic Activity

• Subduction zones are the source of the world's largest and most destructive earthquakes (megathrust earthquakes)
  • Megathrust earthquakes occur along the interface between the subducting and overriding plates (2011 Tōhoku earthquake)
• Benioff zones are inclined planes of seismicity that delineate the descending slab and can extend to depths of 700 km
• Wadati-Benioff zones are characterized by intermediate-depth (70-300 km) and deep-focus (300-700 km) earthquakes within the subducting slab
• Seismic coupling refers to the degree of locking between the subducting and overriding plates and influences earthquake potential
• Slow slip events and non-volcanic tremor occur along the subduction interface and can release stress without generating large earthquakes
• Volcanic arcs develop 100-200 km above the subducting slab where fluids from slab dehydration lower the melting point of the overlying mantle wedge
• Arc volcanism produces andesitic to rhyolitic magmas with intermediate to felsic compositions
• Explosive eruptions are common in subduction zone volcanoes due to high magmatic water content and viscosity (Mount St. Helens, Pinatubo)
• Caldera-forming eruptions can occur in mature volcanic arcs where large volumes of silicic magma accumulate (Toba, Yellowstone)

Environmental and Societal Impacts

• Subduction zone earthquakes can generate destructive tsunamis that cause significant damage to coastal communities (2004 Indian Ocean tsunami)
• Volcanic eruptions can disrupt air travel, cause respiratory issues, and damage infrastructure and agriculture
  • Ash falls can contaminate water supplies and cause roof collapses due to ash accumulation
• Landslides and sector collapses of volcanic edifices can trigger debris avalanches and lahars (Nevado del Ruiz, 1985)
• Convergent boundaries are often sites of intense mineralization and ore deposit formation (porphyry copper deposits)
• Geothermal energy resources are abundant in subduction zones due to high heat flow associated with magmatism
• Subduction zones are key targets for scientific drilling projects to investigate earthquake processes and subduction dynamics (Nankai Trough Seismogenic Zone Experiment)
• Volcanic soils in arc settings are often fertile and support productive agriculture and dense populations
• Subduction zone hazards necessitate effective risk assessment, monitoring, and early warning systems to mitigate impacts on society

Case Studies and Examples

• Cascadia subduction zone: A classic example of oceanic-continental convergence with a history of large megathrust earthquakes and tsunami potential (Pacific Northwest)
• Japan Trench: Subduction of the Pacific Plate beneath the Okhotsk Plate resulting in high seismicity and tsunami risk (2011 Tōhoku earthquake and tsunami)
• Izu-Bonin-Mariana arc system: Intraoceanic subduction zone showcasing various stages of arc evolution and back-arc basin formation
• Sunda megathrust: Subduction of the Indo-Australian Plate beneath the Eurasian Plate resulting in large earthquakes and tsunamis (2004 Sumatra-Andaman earthquake)
• Lesser Antilles arc: Slow subduction of the North American Plate beneath the Caribbean Plate with a complex history of arc migration and variations in subduction angle
• Aleutian Islands: Subduction of the Pacific Plate beneath the North American Plate forming a highly active volcanic arc and deep trench (Aleutian Trench)
• Central Andes: Rapid subduction of the Nazca Plate beneath the South American Plate leading to the formation of the Altiplano-Puna plateau and large stratovolcanoes (Ojos del Salado)
• Hikurangi subduction margin: Subduction of the Pacific Plate beneath the Australian Plate in New Zealand with a mix of accretionary and erosional processes along the margin