olelog

I am one of those people who make simple drawings to help me (and hopefully also other people) understand (complex) concepts. The real world is different. It doesn’t look like our cartoons. I was once again reminded of this (sad?) fact when I read the publication “Accelerated Uplift and Magmatic Intrusion of the Yellowstone Caldera, 2004 to 2006” by Wu-Lung Chang et al. on Friday (9 November 2007) in Science.



First one of my cartoons. It shows a zone of partial melting in the mantle and magma rising up to form a magma chamber when it reaches the crust (or somewhere else, but I have drawn it here for some dubious/plausible? reasons). Through a feeder channel (or several feeder channels) the magma rises through the crust and on its way it stops to fill another magma chamber (or more magma chambers). Finally the magma reaches the surface in the form of lava during an eruption (that is of course if it ever gets this far or run out of molten material too early for that). I have drawn the magma chambers as lenticular bodies, because that is easy to draw. I might also have drawn them as droplets turned upside down, because the upwards movement is density driven. The crust, however, is far from homogenous, so the magma has to force its way through the weakest parts. Maybe along fault or fracture zones or through weaker sediment layers or wherever it is easiest. This means that a magma chamber may have practically any thinkable form, and of course that the feeder channel need not be vertical.



And now to the present magma chamber under the Yellowstone Caldera. On a 3-D drawing (fig. 3 in the paper) it looks extremely irregular. The seismatically imaged magma body looks better In 2-D, but still not exactly as in my cartoon. Here the fluid migration is not vertical but (as shown) nearly horizontal. The material migration from left to right gives rise to subsidence to the left (where material is removed in the underground) and uplift under the caldera.

It is also possible to get an idea of what magma chambers may look like by looking at old solidified magma chambers in the field. They exist in various places, but the most spectacular that I have ever seen is situated in the Patagonian Andes, in Chile (The Torres del Paine laccolith). The large grey body that I have marked as a granite intrusion was intruded about 12 million years ago. It is not all granite but most of it is granitic. Note that is lies more or less horizontal (mostly following bedding planes). The person within the red circle in the foreground shows the scale. Once upon a time (roughly speaking 10-12 million years ago) this laccolith was molten magma, and thus formed a magma chamber - you may call it a fossil magma chamber if you like.



By the way there are large lava flows and deposits of rhyolite in Yellowstone Park. Rhyolite is an extrusive igneous rock that is the volcanic equivalent of (intrusive) granite. Rhyolite has solidified quickly (cooling at the surface) and is fine-grained - granite has solidified slowly underground and is coarse-grained.

The above mentioned paper in science has attracted a lot of media attentiom, and here are a few links:

° http://www.eurekalert.org/pub_releases/2007-11/uou-yr103007.php
° http://www.foxnews.com/story/0,2933,309738,00.html
° http://www.scientificblogging.com/news_account/yellowstone_supervolcano_rising
° http://www.terradaily.com/reports/Yellowstone_Rising_999.html
° http://www.planetark.com/dailynewsstory.cfm/newsid/45273/story.htm
° http://www.physorg.com/news113754946.html
° http://www.livescience.com/environment/071108-yellowstone-volcano.html