Question: Hi! how many different types of volcano rocks are there and what are they called? or is there just one and its called a volcano rock ? :)

James Hickey answered on 15 Nov 2013:

Hey ghickman!

There are numerous different types of volcanic rock and it would take a very long time to tell you about all of them. We get different types because there are a number of different types of volcano, which have different types of eruptions, and different types of magma feeding them.

What we try to do instead is classify them into groups of similar rock depending on their texture (their appearance and physical properties) or their chemistry (what minerals make them up). I have given some examples for you below…

Texture.

Different textures usually arise from the way the rock was made, and this usually means how the rock was erupted from the volcano.

• Ignimbrites – rock formed from pyroclastic flows
• Pumice – rock formed in explosive eruptions where the gas bubbles create holes in the rock (so it has a low density and floats on water!)
• Obsidian – a type of volcanic glass that forms when sticky lava flows cool very quickly
• Scoria – similar to pumice but darker and denser
• Tuff – rock fromed from volcanic ash fall out
• Kimberlite – a very rare volcanic rock that contains lots of diamonds!

Chemistry:

Different chemistries arise from the original composition of the magma that made the volcanic rocks. This is mainly due to the amount of a mineral called silica – SiO2. More silica also makes the magma more ‘viscous’ (or sticky – for example, honey is more viscous than water). Then more viscous magmas create more explosive eruptions.

• Basalt – less than 55% SiO2 (and very common in Hawaii)
• Andesite – between 55% and 65% SiO2
• Dacite – between 65% and 75% SiO2
• Rhyolite – more than 75% SiO2