Deadly Mount Sinabung eruption, May 21, 2016

Sinabung as viewed from the east.
Photo by Tom Casadevall, U.S.G.S., 1987

Mount Sinabung in the North Sumatra province of Indonesia, erupted on Saturday killing seven people in a village.  The village, Gembar, is one of four villages inside a 2.5 mile danger zone from which 5,000 residents were evacuated at the time. Villagers still enter the zone intermittently to tend to property. A video showing parts of the eruption and its ashy aftermath is available from The Guardian here.
     Sinabung is a highly active stratovolcano that rises to an elevation of 2460 meters. It is part of a subduction zone setting where the continental crust is >25 kilometers thick. The major products are andesite, basaltic andesite and dacite. There are four active craters at the summit. All four craters discharge sulfurous gases and form sulfur deposits which are mined by the local people.
    There are unconfirmed reports of an eruption in 1881, and solfataric activity was noted high on the volcano in 1912. It appeared to be dormant from this date until an explosive eruptions (VEI 2) in the summer of 2010. The eruption of August 27th was phreatic, with the initial emission of a grayish white plume followed by black plumes that reached 2000 m above the crater. The rocks and ash erupted came from altered rock in the crater and its deeper hydrothermal system.
    The events of 2010 were followed by a lava dome-forming eruption accompanied by explosive eruptions in September, 2013. Lava extrusion continued through 2014 at a rate of about 3.5 cubic meters per second, reaching about 0.1 cubic kilometer in volume. Sixteen people were killed in February 2014, and 30,000 local residents had to be evacuated. Magma mixing before the eruption is indicated by the presence of magic blobs and "plagioclase microlites more calcic than the phenocryst rims," and absence of a reaction rim on hornblende phenocrysts (Nakada et al., AGU abstract, 2014).
     As of the date of this post, the area remains under high alert and efforts are being made to evacuate any people in the danger zone.

Empress of China on display at Tuscon Gem and Mineral Show; rhodochrosite

The $1.6 million dollar Rhodochrosite mineral from China
on display at the Tuscon Gem and Mineral Show
photographer David J. Eicher, link is here which is the blog
of the State Geologist of Arizona

Rhodochrosite is a manganese carbonate mineral, MnCO3, and so it belongs to the family of minerals like calcite, CaCO3. It is a very soft mineral, with a hardness of 3.5-4 on a scale of 10, but it is about three and a half times as dense as ice. Calcium, magnesium and zinc frequently substitute for the manganese, changing the color to lighter shades of red or pink.  because it is soft, it's difficult to cut and is rarely used in jewelry.  It was first discovered in Romania, where it was found in silver mines.   It is the state mineral of Colorado because very large crystals are not common, and some of the best occur in the Sweet Home Mine in Colorado.  According to Wiki, the Incas believed that rhodochrosite is the blood of their former rulers, turned to stone, and thus it is sometimes called "Inca Rose."

How does rhodochrosite relate to geological fluid dynamics? Searching for an answer to this on the WWW led me to an interesting WWW site on "giant crystals", which has some additional links.  Rhodochrosite forms in regions of the crust where warm to hot water circulate to form hydrothermal systems. It often occurs in association with copper, lead, and silver sulfides.  In some places, like stalactites in the Capillitas Mine in Argentina, it forms like stalactites, from water dripping through Mn-rich rocks (here's an earlier post on ice stalactites.)   As far as I can tell, there is very little published about the Chinese rhodochrosites, and even less on this spectacular sample.  Generally rhodochrosite is leached by acid to extract the manganese.

In hydrothermal systems, water is rarely pure, but is a salty brine.  The water can come from a body of crystallizing magma at depth, or from metamorphic reactions deeper in the crust, or it can originate at the surface of the earth from meteorological processes and then trickle down into the crust where it is heated.  There it can dissolve minerals in the rocks and become loaded with  interesting cations, such as manganese.  Upon circulating back toward the surface, the water can become saturated with these ions because it is cooling, or because it can boils, or because it intersects waters of different chemistry.  These changes cause minerals to precipitate.  Here's a good Britannica encyclopedia article about mineral deposits.