How are volcanoes differant?

rocky cjhs

Hi cjhs,

Volcanoes are different in lots of ways, but almost all of those can be blamed on one thing--the temperature of the magma that supplies the volcano. If the magma is cool, then it has a high viscosity. This means that it cannot flow easily. If you think about working in the kitchen you are familiar with this relationship. There are lots of food items that are more viscous (i.e. harder to stir) when they are cold but less viscous (i.e. easier to stir) when they are hot. Magma works the same way.

Now, if the magma is viscous then it will not flow very easily. Additionally, gas bubbles cannot rise through the magma and escape. You can imagine that if you have lots of bubbles trying to escape the potential of them building up explosive pressure is high. Sure enough, the volcanoes with the cooler magma are the explosive ones. These volcanoes are called strato volcanoes or sometimes composite volcanoes. Mt. St. Helens, Mt. Fuji, and Mt. Pinatubo are examples of strato volcanoes. What then? The explosive eruptions generate a high percentage of fine volcanic ash, and this is a common feature of strato volcanoes. When actual lava does erupt it is very viscous and usually forms lava domes or maybe short stubby lava flows. Lava domes have steep sides, and as they get inter-layered with the ash, you pretty soon end up with a cone-shaped steep-sided volcano.

However, when the magma is hot, the gas can escape more easily. This means than when an eruption does happen, either the gas has already escaped through pre-existing cracks in the volcano or it just bubbles through the lava during the eruption. A lava fountain at a Hawaiian volcano looks very spectacular but it isn't really an explosion. The lava flows that are produced also have low viscosities and they flow away from the vent very easily. They also are pretty difficult to pile up into a steep mountain so the volcanoes formed from hot magma have gradual slopes and are caled shield volcanoes.

There are numerous photos of both strato volcanoes and shield volcanoes in VolcanoWorld. Take a look at some and you'll be able to get a good idea of some of these things I'm talking about.

Sincerely,

Scott Rowland 

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