Types of Lava

Extreme Earth Events - 12ESS

Finn LeSueur


Akoranga 8 Mahi Tuatahi

  1. Open the Viscosity of Lava research document on Google Classroom
  2. Use it to create 2x bullet points for each section of your handout

Lava Flows & Eruptions

  • How explosive a volcanic eruption is, and how far the lava will travel depends on the viscosity
  • Viscosity: A measure of how liquid/or not the material is
    • High viscosity –> thick, molassas-like substance
    • Low viscosity –> like water

Silica Content

  • Silica is a silicon-oxygen compound found in rock.
  • It has the greatest effect on lava viscosity.
  • High silica content –> high viscosity.
  • The particles are more tightly bound, this means it moves slower and is more explosive.

Gas Content

  • Typically water, carbon dioxide, sulfur dioxide
  • Gas can be compressed into a liquid (like soda), under the Earth
  • As the magma rises and the pressure is released, the gas will try escape the magma (highly explosive)
  • Magma with high gas content is not very viscous, but becomes more viscous as the gas escapes

Water Content

  • Water is introduced through subduction of oceanic plates into the mantle
  • Higher water content –> lower viscosity
  • Think of this as diluting honey with water


  • Hot –> higher viscosity
  • Magma composed of melted crust has a lower temperature
  • Magma composed of melted mantle rocks has a higher temperature

Types of Lava

  • Different minerals have different levels of silica in them
  • Recall: silica has the greatest effect on viscosity (therefore shape & explosiveness) of a volcano


  • Lower silica content
  • Typically less than 52%
  • Often found at divergent plate boundaries/hot spots
  • Less viscous lava/less explosive eruptions
  • Fissure/dome volcanoes


  • Dark grey rock
  • 52-66% silica
  • Often found at subduction zones without rising to the surface
  • Medium viscosity/medium explositive
  • Stratovolcanoes


  • Typically higher than 66% silica content
  • Grey/pink in colour
  • Found at convergent boundaries where subducted crust then rises to the surface
  • Strato/composite volcanoes

Task / Ngohe

  1. Collect a Volcano Composition and Silica Content worksheet from the front
  2. Cut and glue it into your book
  3. Put paper cuttings (uncrumpled) in the recycling bin
  4. Use your knowledge from these notes and your previous worksheet to help you answer the questions!