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San Pedro, USA

Whatever Floats Your Rocks

The interesting geologic features that plate tectonics can create are amazing. What is plate tectonics, you ask? The Earth's crust is broken up into about 20 different plates (there are 7 major ones) or sections of the crust. At the boundaries of these plates, there are different kinds of movement: if two plates slide past each other, it is called a transform fault (ex: the San Andreas Fault in California); if they move away from each other, it is called a divergent plate boundary (ex: mid-ocean ridges where new ocean crust is formed); and if they move towards each other, it is called a convergent plate boundary (ex: subduction zones and mountain building). 

In the Straits of Sicily, between Sicily and Tunisia in the Mediterranean Sea, there is a convergent boundary with the African plate subducting underneath the Eurasia plate.  This means that the oceanic crust of Africa is moving underneath the Eurasian plate and making its way down into the Earth to be re-melted and eventually turned back into crust. The African ocean crust is old, cold and heavy; that makes it move quickly into the Earth at a steep angle. This plate movement causes some extension, or pulling apart, on both sides of this subduction zone. The Straits of Sicily between Sicily and Tunisia is one of those areas of extension (see image below). A rift zone has formed in this area, which means that the crust is thinning and being pulled apart which causes faults and allows magma to come up to the surface. And voilà! Volcanoes form much to the delight of geologists. 

(image taken from Civile, D., E. Lodolo, D. Accettella, R. Geletti, Z. Ben-Avraham, M. Deponte, L. Facchin, R. Ramella, R. Romeo (2010). The Pantelleria graben (Sicily Channel, Central Mediterranean): An example of intraplate ‘passive’ rift. Tectonophysics, 490, 173-183.)
We have started our exploration off the coast of Pantelleria Island to study an eruption that occurred in 1891 from the Foernster volcano, which is just a few kilometers northwest of the island. This is one of at least 30 volcanic cones off the coast of Pantelleria, most of which are only 40-200 meters below the surface of the water.  The science team began exploring this site with a transect that covered 3 volcanic vents and the ridges between them. We collected many rock samples along the way, as well as saw an abundance of different kinds of sea life including a shark. We are spending several days exploring this area of volcanoes; which, until this expedition, have had very few samples collected from it and had never before been seen by ROVs.
One of the interesting aspects of the 1891 Foernster volcanic eruption was that, during the eruption, blocks of scoria floated to the surface momentarily and re-submerged to rest on the sea floor. Scoria is a basaltic rock rich in the elements Iron (Fe), Magnesium (Mg), and Calcium (Ca), and has an abundance of vesicles or holes left from the gases that were present in the magma when it erupted. We collected a great sample of these scoria blocks as you can in the image below. Also in the image are two geologists from Italy, who have joined the science crew for this leg of the expedition as they study the scoria, which was sampled and brought to the surface moments before this image was taken. The scientists will use the images and samples collected to learn more about the 1891 eruption and hopefully this volcano and the area around it, as well.