Random thoughts on Volcanology

I am currently studying for a Volcanology exam I have in the morning. I’m going to muse about random things we’ve covered in an attempt to know them better.

Magma resevoirs

Magma reservoirs are continuous regions of magma , and serve as the magmatic source for volcanism. Resevoirs are made up of two key components: the magma chamber, and the magma mush. Typically magma reservoirs are described or depicted simply as a magma chamber, but within what people commonly view as the chamber, there is a zone called the mush.

The magma chamber is the zone where eruptable magma is stored, eruptable meaning a non-rigid supply of magma. Generally, if the composition of the magma contains less than 50% crystals, it is considered part of the magma chamber.

The magma mush zone exists where the magma is rigid with greater than 50% crystal content. In the image below, the magma mush is represented by where the crystals are settling.

Volume and size of magma reservoirs can be estimated by measuring the volume of eruptions in the form of lava and pyroclastic deposits. Both lava and pyroclastics were once magma within the chamber. Lava measurements are 1:1 when inferring volume of magma, converting a measurement of pyroclastic however, is more complicated.

Taking measurements of pyroclastic deposits and turning them into value of magma volume requires using the dense rock equation; (volume of pyroclastic deposit) x [(density of pyroclastics)/(density of magma)].

Indirect measurements of magma reservoirs can be made by studying seismic waves in a few ways. One way is  through the observation seismic wave attenuation. Attenuation meaning the weakening of a force, this basically means observing when seismic waves slow down as they pass through the earth. Waves will become attenuated when they pass through a magma reservoir because the magma is a liquid. Seismic waves travel faster through country rock than molten rock.

This also means that magma reservoirs can be detected by locating anomalies in seismic reflections, again because of the properties of solid vs. liquid.

The more interesting way to infer size of magma reservoirs is by analysis of earthquake foci (aka the hypocenter or the spot within the earth, at depth, below the epicenter). Earthquakes only occur where brittle deformation can take place, meaning no liquids. As volcanoes typically occur in seismically active places and can causes earthquakes themselves, you can use that information to map a magma reservoir.

The beginning of the class was focused a lot of magma compositions and tectonic settings while this part of the class has focused on eruption types. So the above may seem a little out of place but it fits.

Here’s some eruption classification junk:

Volcanic eruptions can be broadly split into effusive and explosive eruptions. Effusive meaning non-explosive while explosive eruptions are rather self-explanatory. Within explosive eruptions there are Phreatomagmatic eruptions which are explosive eruptions caused by the violent interaction between magma and some external water source.

To get more descriptive there is also the Lacroix classification system that classifies eruptions by comparing them against iconic eruption types.

Hawaiian Eruptions and predominantly effusive, with lava fountaining common and primarily basaltic lavas/magma.

Strombolian Eruptions are moderate, discrete eruptions with lavas/magmas ranging from basaltic to andesitic in composition.

Violent Strombolian Eruptions are similar to Strombolian except that they have moderate sustained explosions and have a sustained ash plume.

Surtseye Eruptions are basaltic, phreatomagmatic eruptions with a large “rooster tail” ash plume.

Vulcanian Eruptions are eruptions with moderate-strong, sustained explosions with intermediate composition magmas that are sometimes phreatomagmatic.

Plinian Eruptions are eruptions with very continuous blasts and a very high eruption plume. When interacting with water, they are the largest of the phreatomagmatic eruptions.

Outside of those classification schemes, there are also those of George Walker, which measures pyroclastic fall desposits, and the more common Volcanic Explosivity Index which is a semi-quantitative scale of eruption magnitudes using a variety of measurable quantities associated with an eruption.

I am tired now.

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