Magma Formation, Transport, and Volcanic Processes

Posted on Mar 9, 2025 in Geology

Magma Formation and Volcanic Processes

Mid-Ocean Ridges

Thinning of lithosphere due to slab pull forces from the denser and older subducting slab. Adiabatic decompression drives melting of the mantle. Production of primitive basalt (~50% SiO2) called Mid-Ocean Ridge Basalt, or MORB. Volcanism is generally effusive (not explosive), producing voluminous basaltic lava flows and pillow lavas.

Continental Rifts

Melting due to thinning of the lithosphere, decompression-driven. More alkaline basalt than MORB (K2O + Na2O), lower degree of partial melting and at greater depth. Other types: kimberlites (> 250 km depth), trachytes and phonolites (crust), carbonatites (500-600 °C). Most dangerous eruptions!

Ocean-Ocean Subduction: Island Arc

Ocean-Continent Subduction: Continental Arc

Dehydration of the subducting slab following heating releases fluids to the mantle wedge, promoting partial melting. Melt ascends because of its low density, but only a small amount makes it to the surface.

  • Island arcs: Island Arc Basalts, higher SiO2 content (48-53 wt %) than MORBs.
  • Continental arcs: Assimilation of crustal material and fractional crystallization generates a wide range of magmas, from basalts (~50 wt %) to rhyolites (~75 wt %), although intermediate magmas are the most common.

Intraplate Volcanism

Related to mantle plume.

  • Oceanic hotspots
  • Continental hotspots
  • Early phase of continental rifts
  • Large Igneous Provinces: eruptions involving huge volumes of tholeiitic basalt.

Mantle plume (hot upwelling mantle) originating from core-mantle boundary or upper-lower mantle boundary eventually partially melting following adiabatic decompression.

  • Ocean Island Basalts (OIB): mostly tholeiite, some alkali basalt.
  • Continental hotspots: assimilation of crustal material producing magmas with wide range of SiO2 (50 to 75 wt %).

Origin and Transport of Magma

  • Decompression melting: mid-ocean ridges, subduction zones backarcs, continental rifts, ocean islands.
  • Fluid melting: subduction zones.

Buoyancy is a force induced by the density difference between two bodies.

Continental Arcs

  1. Basalt rises towards continental crust, stops at neutral buoyancy and accumulates.
  2. Fractional crystallization decreases magma density by crystallizing dense minerals, to a point that the magma can rise again.

=> Andesite (and more evolved magmas) at continental arcs: Density filtration + Crustal assimilation + Magma mixing

Silicic Magmas

Processes in magma reservoirs producing silicic magmas:

  • Assimilation of crustal material by melting of surrounding rocks.
  • Fractional crystallization: storage of mafic magma underneath volcanoes. Crystallization following the Bowen series with slow cooling of the magma. Formation of a mush = silicate melt + crystals + volatiles.
  • Depth of magmatic reservoirs: usually 3-15 km.

Fractional crystallization and crustal assimilation: Increase in H2O and SiO2 contents, promoting buoyant rise = potentially explosive system.

Magma Density

Magma density depends on:

  • P ↗ ρ ↗
  • T ↗ ρ ↘ Limited effect, even less for SiO2 and Al2O3 -rich melts
  • Composition: Fe, Mg, Ti, Ca ↗ ρ ↗; Li, Na, K ↗ ρ ↘
  • Volatile content: volatile ↗ ρ ↘

Changes in density affect magma transport and properties such as viscosity.

Magma = silicate melt at depth
Lava = silicate melt erupted at the surface

Viscosity

Resistance to flow. Can vary by more than a factor of 1014, depending on composition.

Magma viscosity depends on:

  • Temperature: T ↗ μ ↘
  • Composition: SiO2 ↗ μ ↗
  • Dissolved H2O content: H2O ↗ μ ↘
  • Crystal ↗ μ
  • Spherical bubbles: μ ↗
  • Deformed bubbles: μ ↘

Crystallization and degassing both affect the magma composition and the volatile content.

Role of Volatiles

Volatiles exsolve from a silicate melt after the later had reached oversaturation. Volatiles may separate from the magma through outgassing, or stay and build up pressure leading to fragmentation.