Today is the turn of Mt. Etna. Having recently visited Etna this post may be a little longer than the others so I apologize! A recent Daily Volcano Quote by the Volcanism Blog, available here summarises Etna extremely well! I have previously posted on etna in the following posts on lava stalactites, xenoliths, Rifugio Sapienza and a view of the composite cone. You can view all of these by clicking on the category for Etna along my sidebar.

Etna is a stratovolcano which has been built onto an older shield volcano. The entire volcanic complex is ~ 500,000 years old. The reason for its location is complex and there are many theories but it is generally belived that it's location is down to the extension of the crust because of the reversal of the Ionian slab.

The first edifice started forming ~180,000 years ago and is built up of a large number of pyroclastics and lavas. The last 14,000 years of edifice formation is referred to as the Recent Mongibello, which is also the name of the current summit crater (Mongibello).

Etna is characterised by smaller explosive eruptions such as Hawaiian and Strombolian activity accompanied by lava flows which occur both from the summit craters and eccentrically (on the flanks of the volcano). More rarely Etna has seen basaltic plinian eruptions in 122 B.C. and 44 B.C. Baslatic plinian eruptions are rare themselves.The lavas which are generated at Etna tend to be either ʻaʻā  or pahoehoe in nature. Pahoehoe is the lava which is commonly seen on Hawaii, it is runny and less viscous. ʻAʻā is more viscous and is generally characterised by a more blocky morphology. Etna experiences regular outbursts of activity the most recent occuring within recent months.

Tomorrow is the turn of Galeras, Colombia. The following are a couple of interesting articles on Etna. For more general information see the global volcanism program. Thanks for reading!

Gillot, P.Y et al. 1994. The evolution of Mount Etna in the light of potassium-argon dating. Acta Vulcanol. 5, pp. 81-87

Guest, J.E et al. 1984. The valle del bove, Mount Etna: Its origin and relation to the stratigraphy and structure of the volcano. Journal of Volcanology and Geothermal Research 21 (1-2), pp. 1-23

Monaco, C et al. 2005. Tectonic control on the eruptive dynamics at Mt. Etna Volcano (Sicily) during the 2001 and 2002-2003 eruptions. Journal of Volcanology and Geothermal Research 144, pp. 211-233

Lava stalactites form when lava level in a tube drops and lava which has stuck to the ceiling begins to droop down. As it does so it cools, therefore forming stalactites!
Below is an example from the Cassoni tube at Mt. Etna.

Xenoliths are one of the most important pieces of rock to come from volcanoes. They are fragments of material which are not part of an area of molten/solidified magma within a magma chamber or conduit. The area xenoliths come from is also known as the country rock. Xenoliths are important because of where they come from, they give us an insight into the rocks present deep into the Earth's interior and into those at shallower levels as well. Common xenoliths can include the rock basin underneath a volcano, to crystals sourced from the mantle - such as the green olivine and the black pyroxene.

The picture below gives an example of a sedimentary xenolith found in a 2001 lava flow on Mt. Etna. It is from the base of Mt. Etna and tells us something about where the magma came from and whether it was part of the central conduit system or not. In this case the magma is from an eccentric eruption which rose quickly, enabling xenoliths to be present.

Thought I would share this image of a panorama I created of the Rifugio Sapienza area which was threatened by lava flows in 2001. On the far right is the scoria cone Upper Silvestri. In the far background is La Montagnolla, with the 2001 lavas above the building.

The question of whether there was active volcanism on Mercury has been disputed for decades, with scientists failing to produce working hypotheses for volcanism. The reason for this scepticism is that there were assumed to be a lack of volatiles beneath the surface of Mercury. Volatiles (such as CO, CO2, H2O, SO2, H2S) are needed for an eruption to become explosive, if these volatiles are allowed to exsolve from magma (via reduced pressure as magma rises) and are present in sufficient quantities, gas bubbles of volatiles can nucleate and the process of fragmentation can occur, which will give rise to explosive eruptions. Mercury has a low atmospheric pressure - close to 0 and at lower atmospheric pressures fewer volatiles are needed to create explosive eruptions. However it is not as simple as this, with some volatiles deemed as more 'important and efficient' when considering eruption processes. For example CO2 is less efficient as it tends to exsolve at greater depths, whilst H20 is the most efficient. For a number of reasons it is unlikely that H20 is present within the interior of Mercury. Any Mercurian eruptions are therefore likely to be smaller in size (Hawaiian, Strombolian, Vulcanian) or limited to effusive (non-explosive) eruptions. There are several theories as to which volatiles are present within the Mercurian inter  

With the recent flybys of the MESSENGER probe interesting aspects of the surface have been definitively identified as volcanic in origin.  Images of Mercury have shown evidence of a variety of volcanic features. Within the Caloris basin, wide and relatively flat shield volcanoes (comparable to the ones seen on the Moon) have been discussed by Kerber et al. (2009) with evidence of pyroclastic deposits surrounding one of these shield volcanoes. These formations provide more than just evidence of volcanism on Mercury but also that volatiles are (were) in existence within the interior. The pyroclastic deposits are hypothesised to have been formed by the equivalent of a Hawaiian lava fountain eruption. The shield volcanoes were formed by effusive volcanism in the form of lava flows. There is also further evidence of volcanism from satellite imagery in several of the Mercurian plains. Pit craters have been identified due to their steeper walls than the ones formed by impact craters. This provides some evidence of magma chambers, it is surmised that these features formed in a similar manner to caldera collapse on Earth. 

Mercury orbits the sun at an average of ~58 million km and has a radius of ~2400 km.

With the entry into orbit of MESSENGER the volcanic history of Mercury will start to unravel! Still to look forward to over the next couple of weeks in my brief planetary volcanism discussions - Venus, Mars, Io and a discussion of cryo-volcanism!

Journal articles read in relation to this topic and which may be of interest:

Kerber et al. (2009). Explosive volcanic eruptions on Mercury: Eruption conditions, magma volatile content, and implications for interior volatile abundances. Earth and Planetary Science Letters 285, pp. 263–271

Head et al. (2009). Volcanism on Mercury: Evidence from the first MESSENGER flyby for extrusive and explosive activity and the volcanic origin of plains. Earth and Planetary Science Letters 285, pp. 227–242

Gillis-Davis et al. (2009). Pit-floor craters on Mercury: Evidence of near-surface igneous activity. Earth and Planetary Science Letters 285, pp. 243–250

In other volcano news, Etna is continuing its activity, with strombolian/hawaiian activity occurring over the past few days. Ruapehu (New Zealand) sustains its signs of unrest with increased CO2 degassing and maintained high crater lake temperatures.

Its taken me a while to get up and running but will now hopefully start to post more regularly.