Volcanoes II: Magma Formation, Composition, and Properties

Magma Formation

  • magma forms in 3 ways:

    1. increase in temperature (without significant change in pressure)
    2. decrease in pressure (without significant change in temperature)
    3. introduction of water volatiles

Magma Transport

  • magma rises by:
    • cracking the rock above it
    • following existing fractures and faults
    • melting additional rock in its path
  • magma rises buoyantly because it has a lower density than the rock above it

Different Types of Volcanoes

  • volcanoes show a wide variety in their activity mostly due to the composition of the magma
    • viscosity and gas content

Viscosity

  • viscosity is affected by:

    1. temperature

      1. the hotter the magma is, the lower its viscosity will be
      2. cooler magma will have more crystals (solidified minerals), contributing to increased viscosity

    2. magma composition

      1. silicate minerals (minerals containing SiO2) are “sticky”
      2. magma with a higher silica (SiO2) content has a higher viscosity

Magma Composition

  • different types of magma are defined by their silica (SiO2) content, which gives them different properties
    • felsic
    • intermediate
    • mafic
    • mafic magma has the highest temperature, lowest SiO2 content, and low viscosity (runniest)

Gas Content

  • the other thing that creates different types of volcanoes is the gas content of the magma
    • gas content of magma is typically 1-10%
    • if magma has a higher viscosity (stickier), any gas present gets trapped
    • trapped gases expand near the surface as the magma rises and is erupted from the volcano
    • more gas in a vicious magma = more explosive eruption (more hazardous)

Why Do Volcanoes Differ?

  • high gas content and vicious magma = explosive volcanism
    • explosive volcano with lots of pyroclastic material
    • several associated hazards
  • low gas content and low viscosity magma = effusive volcanism
    • “tame” volcano with lots of lava flows
    • fewer hazards

Where does Magma Form?

  1. temperature:

    1. happens at subduction zones

      1. sub-ducted slab descends into Earth and is heated up, but doesn’t melt

    2. increasing pressure increases melting temperature

      1. sub-ducted plate doesn’t melt, it’s just incorporated into plastic asthenosphere

    3. increased temperature is a minor process in formation of magma at hot spots

  2. decompression melting

    1. happens at divergent boundaries

      1. pulling apart plates reveals material below and lowers pressures

    2. happens at hot spots

      1. magma is created as mantle plume reaches lower pressures closer to surface

  3. adding water/volatiles

    1. happens at subduction zones

      1. water in sub-ducting plate is liberated, then causes mantle above that plate to melt