Mount Taranaki and Volcanic Ash

The volcano of Mount Taranaki itself is what is called a composite cone. This means that the mountain is a structure built up by multiple, successive eruptions which have produced lava and ash that gradually built up into a cone shape over time. The magma that the rocks of Taranaki are composed of are what is known as andesitic igneous rock.

The type of igneous (volcanic) rock that forms when lava cools is dependent on the characteristics of the magma that the lava had been before being released from beneath the Earth's surface. At one end of the spectrum are the mafic magmas such as basalt. These magmas are hot, runny and flow easily. They allow gas to be released quite easily, and so are not associated with explosive eruptions as a result. Felsic magmas such as rhyolite are found at the other end of the spectrum. These magmas are comparatively cooler and are thicker and blockier than mafic magmas. As a result, these magmas do not flow as easily and tend to trap gases such as water vapour and carbon dioxide (Nelson, 2015). This means that felsic magmas tend to be closely associated with violent, explosive eruptions.
 
The Andesitic magma that was erupted from Taranaki lies between basalt and rhyolite on the mafic-felsic magma continuum, although it is closer to the basaltic end than the rhyolitic end. When someone thinks about a volcanic eruption, they are quite likely to think of huge lava fountains and glowing rivers of molten rock. Although this occur in some volcanic eruptions, these are actually considered quieter eruptions. A large proportion of volcanic events are somewhat more explosive. In these more explosive events, three different types of solid material can be released. These are:

• Ash                        Fine material, less than 2 mm in diameter
• Lapilli                    Mid-range material. About 2 to 64 mm diameter
• Blocks                   Angular rocks more than 64 mm across

(Dale, et al., 2005)

Volcanic ash will be the focus here. It isn't really ash in the sense of the stuff you have to clean out of your fireplace every once in a while. Rather, volcanic ash is somewhat similar to sand. It is composed of a hard, glassy, silicon-based material. It is very angular and this makes it very abrasive, like sandpaper. The material is also covered by a coating of soluble salt ions. These ions are reactive. They are charged particles, and can cause corrosion damage to metallic structures.

The direction ash travels is dependent on the wind at the time of the eruption. The distance ash will travel will depend on how heavy the individual particles are, the strength of the wind at the time of the eruption and the altitude which the ash particles reach. Heavier material such as blocks, lapilli and coarser grained ash will fall to earth again quite close to the volcanic vent, while fine grained ash will be carried further by the wind.

This diagram shows how coarse particles ejected from a volcano will fall to earth closer to the vent, while fine particles are capable of travelling much further distances (Wilson et al. 2012).

This is a picture of a grain of volcanic ash by scanning electron microscope. The scale shows 100 micrometres or 100 millionths of a metre. This particular ash particle is approximately 650 micrometres long (about 0.65 mm). The jagged surface is due to gas bubbles called vesicles exploding as the magma is erupted (Wilson et al. 2012).

Volcanic Ash has the capability to affect and/or interact with many different "spheres" of the Earth. Some of these include:

• Geosphere:       The geological systems deep within the earth.
• Atmosphere:    The various layers of gases enveloping the planet.
• Biosphere:       The living environment: plants, animals, bacteria, etc 
• Hydrosphere:   Water systems - Rivers, Lakes, Streams, Underground Aquifers
• Anthrosphere   The human environment; infrastructure, residential, commercial and industrial

The intensity of a volcanic eruption is rated on the VEI scale or "Volcanic Explosivity Index." This is a scale from 0 to 8, where 0 is an extremely mild eruption and 8 is cataclysmically devastating. The Oraunui eruption of the volcano at Lake Taupo approximately 27,000 years ago is an example of a VEI 8 eruption. Large eruptions upward of 5 are rare, so Taranaki would be quite unlikely to have an event like this. An eruption of VEI 4 would likely be the largest that the volcano would produce.