Tephra – the tiny sub millimeter to large boulder sized rock fragments and volcanic glass ejected from a volcano during an eruption – is something we became well acquainted with during the field work inside Okmok caldera. During or shortly after rainfall, the fine grain tephra inside the caldera became melting mud in the badlands and the active alluvial fans. At the water line near a caldera lake or on a locally saturated drainage channel, it also created a dangerous quicksand for unsuspecting field geophysicists (more on that in a later post). So you might be thinking all would be well on a rare dry sunny day in the caldera, but nope, that’s when the very fine-grained tephra ash becomes wind-borne and blows into every nook and cranny of our field equipment, laptops as well as your nose, eyes, mouth, ears, hair, beard, etc. So by the time we were leaving the island, I was thinking how nice it will be to be away from the tephra for awhile.
Just when we thought all the tephra was behind us, it reared its angular grains on our unsuspecting seafloor MT receivers. Nearly all of the receivers deployed close to the island in shallow water have been coming up filled with tephra, especially the receivers near Umnak Pass and those on the south side of Umnak. On past projects we’ve had instruments come up filled with sand and sediments but that stuff easily rinses off. However, unlike sand grains that tend to be well rounded during lengthy transport down rivers, volcanic tephra is incredibly angular and wedges itself into the nooks and crannies of the instruments. In particular we keep finding it wedged in between our cylindrical electrodes and the long yellow arm tubes housing them. This has made pulling out the electrodes nearly impossible, but after a bit of trial and error we found that we can thread the electrode wire down a long hollow broom tube and ram that inside the yellow arm and push the electrode back out. Every cruise we learn something new and useful.
Here’s a pile of black tephra that washed out of one of the receiver frames (the photo doesn’t do justice to the heinousness of the tephra):
Here’s some photos showing the MT receiver recoveries from yesterday morning and afternoon: