Turrialba bulletin, 4-8 January 2010 11 January 2010

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

Costa Rica: eruption at Turrialba 6 January 2010

There have been signs of increased activity at Turrialba volcano in Costa Rica for some time – a rise in gas emissions, temperature increases around the crater, new cracks in the summit area – and now comes news that there has been a small eruption, the first at Turrialba since 1866. The Costa Rican geological service Ovsicori have issued a bulletin on the eruption (PDF), which reports as follows:

On 5 January 2010 at 14:48 local time an inhabitant of the La Central area located to the south-west of Turrialba volcano reported hearing a strong eruption of Turrialba, much later people living near the volcano reported ashfall. Also, officials of the Parque Nacional Volcán Irazú reported the presence of ash in vehicles parked in the car park inside the national park. Additionally, residents of Turrialba, Tres Ríos and owners of properties on the slopes of Turrialba volcano reported ashfall.

From mid-December 2009 until 4 January 2010, LP-type earthquakes (low frequency) had been predominantly registered, with a significant decrease of volcanic tremor. On 4 January 2010 there was a significant increase in volcanic tremor, both in duration of registration and in the amplitude of the signal. Coinciding with the increase in the recording of tremor there was a significant decrease in LP-type earthquakes. In the early hours of Wednesday 6 January an OVSICORI-UNA team will go to the area to make observations and collect data and information to carry out relevant studies.

A green (preventative) alert was issued by the authorities for the area around Turrialba, and about 20 people were evacuated from the volcano’s slopes.

The volcanic deposits reported as falling in villages to the south-west of Turrialba up to 3 km from the crater were described by Ovsicori as ‘white powder’. Eliécer Duarte of Ovsicori told La Nación that ‘this is not ash. Ash is black, glassy, sharp, and causes much damage’. Duarte said that the material produced by this eruption could represent ‘a stage preceding the eruption of juvenile material’, indicating that the volcano ‘now has the capacity to erupt much heavier material into the atmosphere’. The suggestion is that the material erupted so far is pulverized old rock rather than fresh magmatic material. However, local inhabitants have insisted that the material is ‘black and sandy. A Red Cross official in one affected village has reported sulphurous smells and ashfall, and says that there are ‘black spots’ on the plastic sheeting covering crops in local plantations. The National Emergencies Commission (CNE), meanwhile, describes Turrialba’s activity in a press release as ‘a minor ash eruption with a little acid rain’.

Overall, the Costa Rican authorities seem to be regarding this event as a minor eruption, but a possible precursor of larger-scale activity to come.

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

News
Ovsicori confirma erupción de sedimento blanco del Turrialba – La Nación, 5 January 2010
Alerta en Costa Rica por erupción de ceniza en el volcán Turrialba – EFE, 5 January 2010
Vecinos insisten en que cayó ceniza en poblados de Oreamuno y Cartago – La Nación, 5 January 2010
Tras 145 años, despierta el volcán Turrialba – El Informador, 5 January 2010
Costa Rican volcano erupts after long silence – Reuters, 5 January 2010
Families evacuate Turrialba area as volcano acts up – Tico Times, 5 January 2010

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