The type of lava depends on the composition of the molten rock (magma) which rises within the crust to erupt through the volcano. This in turn controls:

• how easily the lava flows;
• how far the lava flows;
• how violent the eruptions are.

Lavas, and the magma from which they originate, are classified by their chemical composition.

The chemical principally used in this classification is silica (SiO₂), which is combined with metallic elements - aluminium, potassium, sodium, calcium, magnesium, iron etc - to form silicate minerals.
If the amount of silica is particularly high, a proportion will crystallise separately as the mineral quartz (SiO₂).

In this review of the shapes of volcanoes, a simple two-fold classification of lavas is adopted:

• Lavas with a silica percentage of around 45% flow easily and, in exceptional conditions, very quickly (up to 60km/hour). These lavas, erupted at a higher temperature (1000 - 1200^oC) and with a relatively low viscosity, may flow for many kilometres from the vent before solidifying.
• Lavas with a high silica percentage of around 65% flow slowly - only a few meters in an hour - because the silica makes the lava sticky or viscous. The lava, at a temperature of around 900^oC, will travel only a few kilometres before solidifying.

The presence of volatiles dissolved in the lava also affects its behaviour. Volatiles help the lava to flow more easily, but after they escape, the lava will quickly solidify.

Geopix Study Topic 'Types of Lava'

• gives full images of the lavas shown below:
• gives full images of additional types of lava:
• describes different types of lava in more detail; for example: silicic/acid and mafic/basic, pillow lava, pahoehoe/ropy and aa, plateau lavas, vesicular lava;
• explains how each type of lava is formed;
• suggests how to answer exam questions on this topic.

The Volcanoes: 'Types of Lava' Study Topic is part of the Geopix Volcanoes Study Guide CD.
To order this CD go to the Geopix Online Store

The Study Topic contains the following pages: