Physics of Volcanoes


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Volcanoes are one of natures most interesting and dangerous phenomenons. The way volcanoes operate can be understood, on a basic level, by just some simple physics and chemistry, this paper will investigate and explain some of the basic physics that govern the behavior of volcanoes.

Magma is composed of three main components

* viscous silicate melt
* crystals
* volatiles (gases)

The way that these three components interact is one of the main focuses of petrologists and volcanologists.
Also, the way that these components interact is govered by some of the basic laws of physics.

Viscosity
In layman's terms viscosity is a fluids resistance to flow and is important in many volcanic processes. Viscosity is defined as the internal resistance to flow by a substance when a shear stress is applied. Many factors affect a magma's viscosity. Temperature is one of them, as a with all fluids as a the temperature increases the viscosity also increases. This example can be seen when examining a rhyolitic melt, when the temperature decreases from 1300°C to 600°C the viscosity increases by more than eight orders of magnitude. The silica content and the water content also affect a magmas viscosity. The more silica is in a magma the more viscous the magma will be due to the strong silica-oxygen bonds. When water is added to a magma it has the ability to break the silica-oxygen bonds and therfore, the viscosity decreases.

Rheology
Rheology is the study of how materials flow and it is very important when examining volcanogenic processes. Two main types of flows can be defined: laminar and turbulent. Laminar flow occurs when the motion of the particles of fluid is very orderly with all particles moving in straight lines parallel to the walls. Turbulent flow occurs when the streamlines or flow patterns of a fluid are disorganized and there is an exchange of fluid between these areas. The Reynolds number (Re) can be calculated to determine whether a fluid is turbulent or laminar.

When the Reynolds number is less than 10 .... it is considered laminar, when it is greater than 100 it is considered turbulent. The areas in between are defined as transitional and can go either way.

Rheology and viscosity are imporatnat to volcanologists because it will determine if Magma Chambers will convect and overturn. When the Rayleigh's and Reynolds numbers are high enough it is possible for these chambers to over turn which can lead to volcanic eruptions.

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The way that lavas flow is dependent on many factors including: viscosity, rheologic properties, temperature and composition. Many studies have been completed to determine how lavas will act when sitting on a sloping surface. It has been determined by Hulme in 1974 that a lava must create a critical thickness, t, before it will flow.

An explosive eruption involves three different processes: fragmentation of the magma; blasting of the fragmented mass through the vent to the surface, and the ascent of the eruption column. An explosive eruption will only take place if the pressure within the magma exceeds the strength of the surrounding rock. These explosive eruptions are driven by the thermal energy stored in the magma. The thermal energy is transferred into kinetic energy of the eruption column through the expansion of gases into growing vesicles (air bubbles).

Magma's viscosity, dissolved volatiles and mass eruption rate are all variables that will effect the eruption. Lionel Wilson recognized that there were three different types of explosive eruptions and estimated 'modified Bernouilli equation' to derive ejecta velocities for eruptions. A second equation that Wilson derived is the 'Gun Barrel Equation' which is used to investigate propelling objects at high velocities through narrow vents.

There are three major types of explosive eruptions:

It is important to note that eruption styles can change. Changes in eruption styles are a function of changes in magma composition and crystallinity, the amount and nature of exsolved volatiles, vent and conduit geometry and access to external water.

Plinian
In a plinian eruption a mixture of gas and particles are emitted from the vent at velocities from 100 – 400m/s, these type of eruptions can last from minutes to hours. Plinian eruptions are usually initiated by the fracturing of rocks overlying the magma, which causes the magma to rise. As the pressure decreases as the magma rises, over saturation of the magma occurs causing over saturation of the magma and results in the release of dissolved volatile components. The acceleration and volume increase of these exsolved fluids result in fragmentation. It is at this point in the magma’s ascent that there is a very high value of mixture acceleration which affects the magma rise velocity. In figure 1 the different zones of an eruption column are seen along with the variation in density, pressure and velocity of the erupting mixture (Cioni et al, 2000).

Strombolian

Strombolian eruptions are one of the least violent types of eruptions. Due to the low viscosity of the magma, typically basalt or basaltic andesite, the gases in the magma are able to escape resulting in reduced intensity and magnitude. Gases are usually released in rhythmic bursts which disrupt the top of the magma column so only that portion of the column is erupted. In figure 2 (Vergniolle and Mangan, 2000) the eruption column of a strombolian volcano is seen. The eruptive process occurs in 2 phases and is based upon the gas-volume ratio of the magma.

Vulcanian

Vulcanian eruptions are defined as small to moderate eruptions that eject material to heights up to <20km and are characterized by discrete violent eruptions. These eruptions occur by ballistic ejection of blocks and bombs, by atmospheric shock waves and the emission of tephra and juvenile components. Since the distance of the material ejected is so large it is obvious that the pressure required to accelerate them must also be large. Velocities have been calculated to be up to 200m/s, but observations at volcanoes, like Arenal (below) have been up to 400m/s. Today’s estimates of pressures needed to produce these velocities are expected to be~10 M Pa.

Refrences Cited

Cioni, R. et al, 2000, Plinian and Subplinian Eruptions. Encyclopedia of Volcanoes, 477-494

Francis, Peter. Volcanoes, A Planetary Perspective. Oxford Press New York, 1993.

Gaskell, T.F., Physics of the Earth. Thanes and Hudson, 1970.

Morrissey, M., Mastin, L., 2000, Vulcanian Eruptions. Encyclopedia of Volcanoes, 463-475.

Verginiolle, S., Mangan, M., 2000, Hawaiian and Strombolian Eruptions. Encyclopedia of Volcanoes, 447-461.



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