The Soufrière Hills volcano (SHV) is an andesite volcano. This means that the magma forming this volcano is comprised mainly of Andesite which is made up of approximately 60% silica. Basaltic volcanoes such as the ones located in Hawaii contain lower levels of silica and as a result experience large volumes of lava flow because the lava produced has a low viscosity.  Andesite volcanoes due to their high viscosity, do not experience lava flows, but instead form domes from extruded lava. This dome usually collapses during an eruption forming part of the ensuing pyroclastic flow. The silica content in andesitic rock is partly responsible for the colour of the lava which is lighter in comparison to basaltic rock which is usually very dark(image of basalt vs andesite). Apart from affecting the lava’s colour, the silica content of magma can also help determine the shape and explosivity of a volcano. Silica has this effect by determining viscosity.

Viscosity refers to an aversion to flow. At the same temperature, honey has a higher viscosity than water and will flow less. The fact that Soufrière Hills volcano has such viscous magma means that when lava extrudes or is pushed out of the volcano, it is able to collect on top of itself with a higher angle of repose instead of flowing downward. This ability to group upwards on top of itself helps the SHV develop its height and shape. Shield volcanoes on the other hand, because of their magma’s lack of viscosity, flow downward when extruded and take their name because they take the shallow convex curve of a Roman shield. 

High viscosity makes it difficult for lava from the SHV to flow. It also makes it difficult for bubbles of gas to move quickly through magma in the magma chamber and vent. These gas bubbles increase the pressure already experienced in the magma chamber in a manner similar to shaking a closed bottle of soda. This buildup of pressure can lead to an eruption. 

In comparison to shield volcanoes, the SHV has a minor percentage of basalt in its magma (less than 10%). This basaltic magma usually resides in the lower chamber of a two chamber system.

Viscosity affects a volcano’s explosivity or potential to violently erupt.
The basaltic magma in the magma chamber of the SHV lies beneath a pool of andesitic magma because of its high viscosity. As magma evolves and viscosity increases, density decreases. Figure showing two arrows one up one down, viscosity/ density). When basaltic magma starts to rise through the andesitic magma, gas is released in the magma chamber. 

The pressure experienced in a magma chamber is an important factor when considering a volcano’s gas emissions. A specific amount of pressure is necessary to keep certain gases dissolved in the magma. The dissolved gases can only escape when the vapour pressure of the magma is greater than the confining pressure of the surrounding rocks or magma chamber. The vapour pressure is largely dependent on the volume of dissolved gases and the temperature of the magma. The minimum pressure for sulphur dioxide (So₂) to remain dissolved in magma is achieved at a depth of approximately 7km. The Soufrière Hills volcano’s andesite magma chamber is calculated to be less than 7km and this does not generate enough pressure to keep SO₂ dissolved so all SO₂ has been released or vented. 

Gas-Flux
Gas Emissions from this volcano are measured in tonnes per day (t/d). Delivery of SO₂ is not consistent over time but varies in what is called a gas flux. SHV displays an average SO₂ tonnage of 500 t/d. Constant measurement of the gas flux result in a chart that shows highs and lows or peaks and troughs. On reading a printout of such measurements, anomalies called spikes are sporadically seen where the tonnage increases dramatically from the range before it. These spikes are usually indicative of trapped gases being released from pockets in the shallow plumbing system. The curve of the long term variation usually gives a better understanding of the deep seated processes. 

Measurements of gas species at MVO is done via two separate remote sensing techniques (FTIR and DOAS) which utilize spectroscopy. Spectroscopy is originally the study of the interaction between radiation and matter as a function of wavelength or frequency. This method is used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. The two main gases of interest are sulphur dioxide (SO₂) and Hydrogen Chloride (HCL). SO₂ absorbs radiation from the Ultra Violet region of the spectrum and is easier to measure than HCl. 

FTIR
Fourier Transform Infra-Red spectroscopy. Open path or closed path system. Open path technique restricts measurements to ratios. This is the current method of measuring HCL/SO₂ at MVO. Measurements must be taken on a sunny day since solar light is the tested variable and the instruments therefore require direct sunlight.

DOAS
Differential Optical Absorption Spectroscopy. To perform DOAS, two sets of spectra are necessary;

1. The reference or background in which the light has moved through little (ideally none) of the SO₂ and 
2. One in which the light has passed through a large amount of the absorber (the gas plume).

To measure HCl, conditions need to be quite specific for example the plume needs to be in the right position and you need direct sunlight through the plume. The plume also needs to be “clean” or free from clouds for optimum readings.
Andesitic magma releases HCl and Basaltic Magma releases SO₂. Readings of HCl levels give an indication of lava extrusion while SO₂ tonnage points to volcanic activity. Because of the difficulties faced to measure HCl exclusively, an open path system is utilized to gain a ratio of HCl to SO₂ which is then used to calculate an extrusion rate.

When the instrument is pointed toward the sun, the measurement comes from direct sunlight. In a simple case the SO₂ can be quantified as being the difference in the column density between the background and the absorption spectra.

H2S reduction So₂ oxidation, hotter, oxidation, cooler reduction.

Petrology- mineral associated in rock experiments in rock

Chemistry and process, depth at which crystallisation took place- pressure

Looking at samples for minerals. Getting an idea of depth of vent and chamber.

How much info can you extract from a rock?

Generations of crystals, size of crystals, 

Basalt vs Andesite is more chemistry than process.

As magma differentiates over time. Increase in viscosity related to decrease in density so floats upwards in the magma. Basalt into Andesite or vice versa.

Physical history dictates texture and appearance.

Dome rock and pumice are the same chemical composition as evinced when you crush them up, homogenize them and run chemical tests to determine their makeup.

Cooling rate.

Basalt erupts from time to time at Soufrière hills 

chemistry influences

Evolved magma.

Basalt is mafic rock.

Minor percentage of basalt- 1-8% erupted at volcano.

Basalt is the driving force behind eruption.