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Turrialba bulletin, 4-8 January 2010 11 January 2010

Posted by admin in activity reports, Costa Rica, eruptions, Turrialba.
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The Observatorio Vulcanológico y Sismológico de Costa Rica (Ovsicori) has released a bulletin covering activity at Turrialba volcano between 4 and 8 January 2010 (PDF), including the eruption of 5 January 2010, which is described as follows:

Around 14:00 on 5 January 2010 ashfall originating from the western crater of Turrialba volcano was reported in several localities around the volcano (La Central, La Pastora, La Esperanza, among others) and in more distant locations such as the Irazú Volcano National Park, Aguacaliente de Cartago, Tres Ríos, Curridabat and Desamparados. This eruption, which is another manifestation of the degassing of the volcano, consitutes the first eruption of ash from Turrialba in many decades.

The bulletin notes that during 2009 seismic activity at Turrialba was characterized by the ‘almost constant occurrence of LP [long period] type earthquakes’ associated with gas (mainly water vapour) movements within the volcano, and that this activity continued into the first few days of 2010 with an average of 10 earthquakes per day. This pattern of behaviour changed on 4 January 2010:

On 4 January at 10:57 (local time) there was a phreatic explosion (caused by the decompression of steam at high temperatures) of a large scale at the volcano (greater than those occurring during 2009) followed by another three explosions of lesser magnitude at an interval of 10 minutes. These explosions were produced by the opening of ducts for the escape of pressurized gas. The explosions were followed by a tremor of large amplitude that was maintained for more than 45 minutes. The tremors resulted from the movement of fluids through restricted ducts, in this case associated with the expulsion of gas and ash or fine particles swept along by the expelled gas. The tremor persisted but its amplitude decreased towards 15:20 as several smaller explosions took place followed by almost continuous tremor, sometimes spasmodic, reaching large amplitudes at 23:30 on 4 January and 10:45 on 5 January. From 15:30 on 5 January the tremor is recorded continuously and at low amplitudes.

This low-amplitude tremor continued until 16:58 on 8 January; since that time the tremor has ended and the volcano has returned to the same LP type earthquakes that were recorded before 4 January, ‘so it is possible to be sure that activity at the volcano has returned to the levels of before the phreatic eruption’.

Ovsicori scientists carried out a field visit on 6 January to assess ashfall and collect ash samples. They found ‘a very thin layer of ash’, not uniform in distribution, across and area between 8 km and 15 km WSW of the volcano. Analysis of the ash revealed that ‘the ashes erupted by Turrialba volcano between 5 and 6 January are almost [pH] neutral, i.e. of a nature between very slightly acidic (mildly corrosive) and very slightly alkaline (mildly caustic)’. The low acidity of the lava ‘suggests that so far there has been no involvement of lava in the ash eruptions or additional inputs of new magmatic gases’.

The bulletin offers the following interpretation of current activity at Turrialba, and the prospects for the next few days and weeks:

The eruptive activity that began on 5 January represents a process at a superficial level in the volcano, consisting of the rapid heating and vaporization of a body of subterranean water followed by sudden decompression through the fracture system of Turrialba. The discharge of steam and gases through the fumaroles on this occasion has had much more vigorous results than those observed in previous years. The heat source is provided by the intrusion of a magmatic body at depth … The absence of juvenile magmatic material in the recently erupted ash and its pH indicate that the groundwater has not come into contact with the magma intruded at depth but has been mainly heated by the conduction of heat through the rocks within the volcano.

In summary, at present there are no characteristic signs indicating the presence of magma at shallow levels in the volcano, so the occurrence of an eruption of magmatic nature in the near future (the next few days or weeks) can be ruled out. However, it is not possible to rule out the occurrence of new phreatic events (similar to those seen in recent days).

For all our coverage of Turrialba: Turrialba « The Volcanism Blog.

Information
Global Volcanism Program: Turrialba – summary information for Turrialba (1405-07=)
Observatorio Vulcanológico y Sismológico de Costa Rica – Ovsicori website
Volcán Turrialba – information from Ovsicori
Monitoreo Volcanico (Red Sismológica Nacional) – volcano monitoring updates from the Costa Rican national seismological network

The Volcanism Blog