Magma

Magma is the molten or semi-molten natural material from which all Igneous rocks are formed. Found beneath the surface of the Earth.

Magma Sources and Types

• Recall: lava is magma (i.e., molten rock) that flows on the earth’s surface

• Temperature should be high enough and pressure low enough to melt rock

  • Most magma originates in upper mantle (50-250 km deep in Earth)

• Most volcanoes found in one of these 3 settings:

  1. Divergent boundary – oceanic and continental
  2. Convergent boundary – above subduction zones
  3. Hot spots (i.e., not associated with plate boundary)

• Magma composition and physical properties dictate how the volcano erupts

• Composition determined by the rock material that is melted and extent of melting

• Raw materials and melting processes controlled by the tectonic setting

Origin of Magma

• Rocks in asthenosphere are close to melting point but needs • A reduction in pressure, like in a spreading ridge OR

• Water to lower melting temp, like in a subducting plate that warms up and the water within rocks/minerals are released

• Resulting magma is less dense and will begin to rise up through solid rock

• Or can sit in a magma chamber where cooling and fractional crystallization will occur (i.e., higher temperature minerals start to crystallize and fall out)

Magma compositions vary in silica (SiO2), iron, magnesium, and gases

• Mafic magma – low in SiO2 (45-50 %) but high in iron and magnesium. Low viscosity, flow easily. Gas escapes easily.
• Magma in asthenosphere is ‘ultramafic’
• Intermediate magma – intermediate range of SiO2 (50-65 %), iron, and magnesium. Intermediate viscosity.
• Felsic magma – high in SiO2 (up to 75 %) with some aluminum, but low in iron and magnesium. High viscosity, flow slowly. Gases get trapped, cause explosions.

Tectonic setting partially drives magma decomposition and rocks formed at surface

• Spreading ridges ⇒ basalts
  • Mafic because not restricted on way to surface
• Continental rift ⇒ basalts or rhyolites (felsic) or andesite (intermediate)
  • Magma interacted with more material on its path to the surface
• Ocean hot spots ⇒ basalts
• Continental hot spots ⇒ andesite-rhyolites

• Magmatic activity in subduction zone is complex
• Mostly basaltic to andesitic (i.e., more mafic)
  • But can mix with sediments while subducting plate or interact with overlying crust while traveling back to surface ⇒ more felsic