• Question: What is the difference between the different types of volcano?

    Asked by megansaurus to Davie, Gemma, James P, James V, Nuala on 28 Jun 2012.
    • Photo: Gemma Purser

      Gemma Purser answered on 28 Jun 2012:


      Im sure James V will be able to explain lots more on this one but Its to do with how runny or not the lava is inside them, a term known as viscosity. Some volcanoes have really sticky lava and so they make taller steep volcanoes. Others have really runny lava and so there volcanes have gentle sided slopes and aren’t so high.
      Different types of lava occur because they are formed by liquifying different types of rocks under ground. These different types of rock contain different elements (and in different amounts) such as silica, aluminium, potassium and in some stange cases things like carbonate.
      Most volcanoes erupt lava made of basalts that contain alot of silica and they are very hot at about 1000 degrees cesius but there are some strange volcanoes in africa that erupt lava made of carbonatites and they are cooler at 500-600 degrees celsius.

    • Photo: James Verdon

      James Verdon answered on 28 Jun 2012:


      Hi megansaurus,
      It’s all to do with how sticky the magma is, which is controlled by how much silicate is in the melt. Also, the amount of volatiles they have is also important. Volatiles are things like CO2 and water in the magma, which can form gasses, making the lava ‘fizzy’.

      If a magma has a low amount of silicate, it will be runny. It’s easy for the runny magma to reach the surface and get out. Volcanoes with runny magma won’t have big explosive eruptions, the mamga will just run out, like in the video below, creating a relatively flat volcano, called a shield volcano. Hawaii is a good example of this.

      If the magma has much higher silicate concentration, as well as higher amounts of volatiles, then you get a very different eruption. The magma will be really sticky, so it won’t be able to get out easily. Instead, it will build-up underneath the volcano. The gasses from the volatiles will also be trying to get out, but won’t be able to. Gradually, the pressure will build up until eventually, something gives and releases the pressure. When the pressure is released, all the gasses try to escape at once. This creates an explosive eruption. A good analogy is shaken up coke bottle. The coke is full of gas trapped in a liquid under pressure. If you take the top off too quickly, then whoosh!!! you get an explosive eruption. It’s exactly the same for these types of volcanoes. If the coke had gone flat, you wouldn’t get an explosion, and so it is with volcanoes – the more volatiles in the magma, the more explosion you can get.

      Because the magma is much stickier, these sort of volcanoes tend to build up the types of cones that you’re probably familiar with – big steep mountains with a vent at the top.

    • Photo: James Pope

      James Pope answered on 28 Jun 2012:


      Hi megansaurus,

      A really interesting answer from James V, what I can add is that when we find the remains of volcanoes from the geological past, we can often find their lava or pyroclastic flows (hot flows of ash) in the rocks of regions and from that we can work out what type of volcano they were millions of years ago, based on what we know now about magma and modern active volcanoes!

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