Mixing Magmas, El Hierro

 

Steaming basanite/rhyolite clasts from the eruption at El Hierro in late November 2011. Image courtesy of INVOLCAN and @teideano.

The break is done! First update looks at the continuing activity at El Hierro in Canary Islands continues. The “jacuzzis” persist to the south of the island (see above) with some indication that they might be closer to shore than before – however, they still don’t appear to be in shallow enough water or vigorous enough activity to be Surtseyan in nature. Plenty of the the steaming, black floating pumices are still being spotted on the water’s surface (see below). You can see some video of the “jacuzzis” with a helicopter hovering over them (video) and it clearly shows the ocean steaming. All the activity so far has been focussed to the south of El Hierro and the Instituto Español de Oceanografía (IEO) now says that there are “no signs” of an impending eruption to the north of the island.
IGN posted some information (spanish) on the petrology of the clasts being erupted from the submarine vents and it looks like the floating clasts don’t float for long: “The high vesicularity of both liquids is what gave a relatively low density, allowing them to reach the surface of the water and survive in it for few hours until the gas vacuoles were filled with water, finally sinking to the seabed.” (Google translation). This is typical for pumice clasts in most situations – once water saturates the rock, it loses the ability to float. IGN has now collected some impressively-sized pieces, too, measuring up to 2 meters.

 The "jacuzzis" from the submarine vents south of El Hierro as seen on November 24, 2011. Image courtesy of INVOLCAN.

Compositionally, the clasts are a mix, as we’ve known for a while, but it seems that even the end members of the mix might be a mix. The dark material is low silica basanite (43-45 wt% silica) that is normal for El Hierro. The lighter material is more peculiar – not only is it different from the basanite, but it might be a mixture in itself. It contains both trachyte (similar to dacite but more sodium, potassium and calcium – 64-65 wt% silica) and rhyolite (74 wt% silica). [SPECULATION] That compositional different might suggest that the two true end members are the basanite and the rhyolite while the trachyte might be a mix of the two. Considering the clasts preserve the light and dark of the basanite and rhyolite, then the mixing that occurs during (or right before) the eruption is incomplete. Some samples preserve more of the mixing, thus contain the trachyte. The magmas are stored in very different conditions as well – basanite at 1200°C, likely in the newly intruding dike, and the rhyolite/trachyte at 850-900°C, likely as a crystal mush that the dike intersects. [/SPECULATION] There isn’t much of the lighter magmas either – IGN estimates there is roughly 10:1 volumetrically basanite to rhyolite/trachyte – similar to what is found in Iceland, where small volumes of rhyolites are found at many Icelandic volcanoes. The lighter rhyolite/trachyte might not be that old either – research at La Palma in the Canary Islands suggests that the time to differentiate a phonolite (similar to rhyolite) from a basanite is less than 1750 years