How do silica and viscosity related to lava

Mt St Helens. These magmas erupt as basalts or intrude as gabbro, and are far less viscous. Eruptions are generally effusive. Temperature: Magma temperatures reflect the melting points of their mineral components.

Not surprisingly, magmas formed by partial melting of mantle rocks are much hotter — well over o C for some Hawaiian basalts — than is the case for crustally derived melts.

Rhyolites may reach the surface at temperatures of less than o C, and so have much higher viscosity. 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. 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. Our Oxbridge-graduate consultants are available between 9. Silica And Volcanism. Types of 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. Types of Eruption 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.

An Analogy In case you are struggling to remember the way it all links together, it may help to think of it in the following way.

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Now what? The outer oxygen atoms in each tetrahedron can share electrons with the outer oxygen atoms of other tetrahedra. The sharing of electrons in this manner results in the development of covalent bonds between tetrahedra. In this way Si-O tetrahedra can link together to form a variety shapes: double tetrahedra shown here, C , chains of tetrahedra, double chains of tetrahedra, and complicated networks of tetrahedra.

As the magma cools, more and more bonds are created, which eventually leads to the development of crystals within the liquid medium. Thus, the Si-O tetrahedra form the building blocks to the common silicate minerals found in all igneous rocks.

However, while still in the liquid state, the bonding of tetrahedra results in the polymerization of the liquid, which increases the "internal friction" of the magma, so that it more readily resists flow. Magmas that have a high silica content will therefore exhibit greater degrees of polymerization, and have higher viscosities, than those with low-silica contents. The amount of dissolved gases in the magma can also affect it's viscosity, but in a more ambiguous way than temperature and silica content.

When gases begin to escape exsolve from the magma, the effect of gas bubbles on the bulk viscosity is variable. Although the growing gas bubbles will exhibit low viscosity, the viscosity of the residual liquid will increase as gas escapes.

The overall bulk viscosity of the bubble-liquid mixture depends on both the size and distribution of the bubbles. Although gas bubbles do have an effect on the viscosity, the more important role of these exsolving volatiles is that they provide the driving force for the eruption.

This is discussed in more detail below. As dissolved gases are released from the magma, bubbles will begin to form. Bubbles frozen in a porous or frothy volcanic rock are called vesicles , and the process of bubble formation is called vesiculation or gas exsolution. The dissolved gases can escape only when the vapor pressure of the magma is greater than the confining pressure of the surrounding rocks.

The vapor pressure is largely dependent on the amount of dissolved gases and the temperature of the magma. Gas escape through vertical vesicle cylinders Vesicle-rich flow top. Explosive eruptions are initiated by vesiculation, which in turn, can be promoted in two ways: 1 by decompression , which lowers the confining pressure, and 2 by crystallization, which increases the vapor pressure. In the first case, magma rise can lead to decompression and the formation of bubbles, much like the decompression of soda and the formation of CO 2 bubbles when the cap is removed.

This is sometimes referred to as the first boiling. Alternatively, as magma cools and anhydrous minerals begin to crystallize out of the magma, the residual liquid will become increasingly enriched in gas. In this case, the increased vapor pressure in the residual liquid can also lead to gas exsolution.

This is sometimes referred to as second or retrograde boiling. Both mechanisms can trigger an explosive volcanic eruption. The amount of dissolved gas in the magma provides the driving force for explosive eruptions.