Explosive Eruption Dynamics 

Petrology of Volcanic Rocks

In order to understand volcanic eruptions, the physical and chemical nature of the erupted rocks must be examined. Various characteristics give important insights as to how the rocks are formed, and thus what processes occur in volcanoes. Petrological study can also illuminate pre- and syn-eruptive conditions, better constraining such factors a viscosity, starting pressure/depth, and ascent rates.  

Igneous petrology offers numerous approaches for studying the formation of volcanic rocks.

Petrology has several approaches to studying the origin of volcanic rocks. Two very broad classifications are "descriptive," and experimental. What I am terming "descriptive" petrology, is the classification and examination of rocks from the field. This work can take the form of detailed geochemical analysis, textural studies, and studies of various mineral assemblages.

Experimental petrology expands upon observational petrology, and quantifies the pressure, temperature, and chemical conditions necessary for certain rocks to form. Essentially, experimental petrologist examine a rock from the field, determine its texture and mineral assamblage, and then recreate this rock in the lab using high P-T furnaces. The results of these investigations reveal at what pressures, temperatures, oxygen fugacities, etc. the rock formed at. These data can also reveal ascent or cooling rates.

 

Discussion of different analysitcal techniques -- probe, FTIR, XRF, textural work

The effects of water on volcanic rocks

One very important result of petrological studies has been an acknowledgement of the very important role that water plays in volcanic rock formation. To put in simply, without water dissolved in silicate (or other melt) there would be no volcanoes. . . And the world would be a very boring place.

Water lowers the melting point of mineral assemblages dramatically. Calcite, a carbonate mineral melts at ~1300C when dry, but with the addtion of a few weight percent water, calcite melts at ~670C -- this is the eruptive temperature observed at carbonatite volcanoes.

In silicate systems, melting, or liquidus, temperatures are dropped several hundred degrees, thereby allowing magmas to form, and volcanoes to erupt.

Water is also the driving component of explosive volcanism. At high pressure, water is soluble in silicate melts, however, a tatmosphereic pressure, water is not soluble. This is the same phenomena as opening a beer -- with the cap on, the bottle is at high pressure, and CO2 is dissolved, but when the pressure decreases (the beer is opened) the CO2 exsolves and forms bubbles -- the beer vesiculates.

In a rhyolite melt at ~4 km depth, 4 wt% water can be dissolved in the melt (Wallace and Anderson, 2000). However, when the pressure is dropped to 1 atmosphere, virtually no water is soluble in the melt and it must all exsolve. This exsolution of volatile (i.e. water) creates bubbles, which undergo a very rapid volume expansion, on the order of 1000x; it is this expansion that fragments the magma. For a more detailed discussion, see "Magma Rising to the Surface."

Analytical Techniques

there are numerous methods of analyzing volcanic rocks. These range from whole rock analysis tools, such as X-Ray Flourescene (XRF), to microanalytical techniques, such as the Electon Microprobe (probe, or EPMA), the Ion Microprobe, and Fourier Transform Infrared Spectroscopy (FTIR) for analysis of water and CO2. For textural work, the Probe and the petrographic miscropscope are very useful as well as the Scanning Electron Microscope.

Rock textures

Physical textures of rocks can reveal a great deal about how the rock formed. The images below show pumices from the KS1 eruption of Ksudach Volcano, Kamchatka, Russia. The different textures are a product of different ascent rates and velocities.

Back Scattered Electron (BSE) image from the EPMA of gray pumice. Note that the vesicles are round.

BSE image of white pumice. Note that bubble are all stretched. This texture indicates shear in the volcanic conduit.

BSE image of gray pumice -- the microlite concentrations indicate a slow ascent rate.