Starting with an obvious question, what is silica? Well, it’s all around us. Silica is found on the tops of mountains, the bottom of oceans and in the deserts of the planet. It is the most common mineral phase on Earth, and forms the basis of the bulk of Earth outside the metallic core. It is made of a positively charged Si4+ ion, surrounded by four negatively charged O2- ions, which form the negatively charged SiO44- particle. Due to the size of the O2- ions, and the charge balance, silica particles are arranged as a tetrahedron. The silica tetrahedron forms the basis of most naturally occurring mineral phases outside the metallic core of the Earth, and form the crusts of the inner four planets of the solar system.
The role of silica is a subject in itself, however one aspect of silica is that it has an important role in controlling the nature of magma and volcanism. To understand this, we must first look at what effect silica has on magma.
Magma (or lava, as it is called when it reaches the surface) is molten rock, and you are probably already familiar with what it looks like . However, the orange rivers flowing down the sides of Hawaiian volcanoes are not typical of what may be found elsewhere. Hawaiian magma is ‘basaltic’, and is a prime example of what results from heating ‘mafic’ volcanic rock. Mafic rock contains relatively little silica. It is the silica content that controls the viscosity of the magma, and hence the nature of the volcanism that is seen. The low levels of silica mean that Hawaiian magma has a low viscosity, which explains why we see lava readily cascading downslope, and being churned up in the air in great fire fountains.
At the other end of the spectrum we have magma that is formed from ‘felsic’ volcanic rock, which has a much higher silicon content. The magma is described as ‘rhyolitic’, and is far more viscous due to its higher silica content. The effect of this viscosity is that flowing rivers of lava are not common. In fact, the entire nature of the eruption is different.
For all the impressive photography that they attract, eruptions like those in Hawaii, or the Mediterranean island of Stromboli, are not particularly dangerous (from a human perspective, at least). Their eruptions are almost continuous, so are predictable. Also, they are not particularly explosive. This is because the pressure from gasses being built up below the surface from tectonic processes is constantly being released, which in turn is only possible since the magma is non-viscous, and so able to be ejected from the magma chamber easily.
Eruptions like that seen at Mt St Helens, on the other hand, where the magma is rhyolitic, are generally much more dangerous. They are much less frequent, but are typically highly explosive. The reason for the difference is that the viscous magma is unable to escape from the chamber. As a result, pressure builds and and builds, with the gasses unable to escape. Eventually, the point is reached where the gases can no longer be contained, and a violent eruption ensues. Looking back through history, you can be sure that the largest eruptions (think Pinatubo, Krakatoa) will all have involved magma that is rhyolitic in nature.
In case you are struggling to remember the way it all links together, it may help to think of it in the following way. Imagine a jacuzzi. The gas bubbles out of the water without any drama. There is an amount of disturbance to the liquid, but nothing one could describe as explosive. Now imagine you fill that jacuzzi up with mud (which does typically contain silica, by the way). The effect of the more viscous liquid is that the gas can’t escape as easily. Unlike with the water, the bubbles are less frequent, but they are more explosive – more pressure is being released with each one. This is the difference between the volcanic eruptions we see with basaltic magma, and those with rhyolitic magma.