Tungurahua

Unterköfler Rene

Tungurahua Volcano, Ecuador

Content

Introduction.................................................................................................................................4Plate tectonics and the Andes.............................................................................................4

The Tungurahua..........................................................................................................................6Made up of three volcanic edifices.....................................................................................6High Risk of hazards.........................................................................................................11Eruptive history.................................................................................................................14Pictures..............................................................................................................................15

Figures:

1 Volcanoes of Ecuador 2 Scheme of the different plate boundaries and their relation to volcanism 3 Volcanic arcs, flat-slab segments and subducted ridges 4 Eruption on the 05.12.06 5 Sketch showing the main periods of Tungurahua development 6 NW–SE topographic profile of the present cone 7 Simplified geological map of the Tungurahua 8 Generalized stratigraphy of Tungurahua volcano 9 A lahar in the La Pampa sector of Tungurahua10 Hazard map for pyroclastic flows (PF) and lahars (L),11 Hazard map showing areas of relative risk12 Ash plume at 09.03.200713 Morphology of the snow and ice-covered summit crater prior current eruption 197414 A dark ash plume 16.11.199915 Pyroclastic flows at 18:14 on the 14. 07.2006

Tabels:

1 Tungurahua factsheet 2 Eruptive history

Titelfigure from: http://upload.wikimedia.org/wikipedia/commons/d/d1/Equador_Tungurahua.JPG)

Tungurahua factsheet:

Country, Region: Ecuador, Tungurahua

Surounding: Cordillera Central of the Andes

Volcano/Eruption Type: Stratovolcano, Strompolian

Volcanic arc: North Volcanic Zone

Last known eruption: 2009 (active)

Summit Elevation: 5023 m

Relief high: 3000 m

Latitude: 1°28'1"S

Longitude: 78°26'30"W

Age of rock: Holocene (Gomes 1994)

Close villages/towns: Riobamba, Banos, Pelileo Nuevo,Pelileo Viejo

Growth rate per year (estimated last 2300 y): 1.5x106 m3/year

Steep slopes of the cone: 25°–35°

Table 1: Tungurahua factsheet

Fig. 1: Volcanoes of Ecuador (Source: http://vulcan.wr.usgs.gov/Volcanoes/Ecuador/Maps/map_ecuador_volcanoes.html)

Introduction

The Tungurahua Volcano one of Ecuadors most active volcanoes is located in the Northern Volcanic Zone of the Andes. According to one theory the name Tungurahua means “Throat of Fire” based on the words tunguri (throat) and rahua (fire) from to the native American language Quechua.

Plate tectonics and the Andes

The whole Lithosphere is divided in parts which move in different directions according to each other. This processes are covered under the name plate tectonics. As shown in the figure bellow we divide between convergent, transform and divergent plate boundaries which are in relation with volcanism.

In case of South American the oceanic Nazca plate and the South American plate forms a convergent plate boundary. Due to the subduction of the oceanic crust the Andes starting to exist in the Jurassic Period and in the Cretaceous Period finally the began to reach their present form by uplifting, faulting, folding of sedimentary and metamorphic rocks of ancient cratons. A craton is an part of the continental crust that has survived the merging and splitting of continents and super continents for at least 500 million years but some of them are even more than 2 billion years old. Besides of the Nazca plate also a part of the Antarctic Plate is forming an destructive zone with the South American plate trough this orogenic events minor to major earthquakes and volcanic eruptions are caused.

The Volcanoes in the Andes range are divided in four volcanic zones (see fig. 3):

The North Volcanic Zone (NVZ): Northern Colombia, Ecuador and northern Peru

The Central Volcanic Zone (CVZ): Southern Peru, Bolivia, northern Chile and Argentina.

The South Volcanic Zone (SVZ): Central Chile until Chile Triple Junction

The Austral Volcanic Zone (AVZ): South of the Chile Triple Junction (subduction of the Antarctic Plate)

The volcanic zones are separated by the Peruvian flat-slab segment, the Pampean flat-slab segment, Patagonian Volcanic Gap and defined by Bucaramanga flat-slab segment in the north.

Fig. 2: Scheme of the different plate boundaries and their relation to volcanism (source: Encyclopedia of Volcanoes, 1999)

Fig. 3: Volcanic arcs, flat-slab segments

and subducted ridges

(Source: http://en.wikipedia.org/wiki/Andes)

The Tungurahua

The Tungurahua is (1°28'1"S, 78°26'30"W) a steep-sided andesitic-dacitic stratovolcano with an impressing appearance due to a height of 5023 m and an extreme relief of 3200 m compared to the his surrounding. Close villages are Banos (130 000 inhabitants) 5 km to the north, Riobamba (230 000 inhabitants) 30 km to the southwest, Ambato 30 km to the northwest and Patate. Together with other large active volcanoes such as Cotopaxi, Sangay, Antisana and Cayambe, Tungurahua defines the eastern volcanic row in Ecuador, some 35 km behind the volcanic front that lies on the Western Cordillera. According to the global volcanism program this volcano has experienced 16 eruptions since its discovery in 1532 but 5 of them are uncertain (Smithsonian’s Global volcanism program, 2009).

Fig. 4: Eruption on the 05.12.06 (Source: http://mobidic.senderismo.iespana.es/fotosecuador/volcanTungurahua.jpg)

Made up of three volcanic edifices

The first Tungurahua was a 14-km-wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series. Tungurahua II which was mainly composed of acid andesite lava flows younger than 14,000 years BP, was partly destroyed by the last collapse event, 2955 +/- 90 years ago. From this event a large amphitheatre and a ~ 8-km3 debris deposit still remain. The second one, Tungurahua II, collapsed about 3000 years ago and produced another large debris-avalanche deposit and a horseshoe-shaped caldera open to the west. Inside of this caldera the modern glacier-capped

stratovolcano Tungurahua III was constructed which has already rebuilt the cone to about 50% of its precollapse size by the emission of ~ 3 km³ of volcanic products. There have been two recognized construction periods of Tungurahua III (see fig. 8) The first one was form ~ 2300 to ~1400 years BP and produced high rates of lava extrusion and pyroclastic flows occurred. The magma composition remained essentially basic andesite during this period. Within the last ~ 1300 years eruptive episodes mostly only take place once per century and starts generally with lapilli fall and pyroclastic flow activity of different composition, andesite and dacite, and end with more basic andesite lava crater plugs (see fig. 8). This development was observed in three historic eruptions (1773, 1886, 1916-1918). Over the last 2300 years a growth rate of 1.5x106 m³ per year is estimated for the Tungurahua III. (Hall et al. 1999, p 1-2)

Fig. 5: Sketch showing the main periods of Tungurahua development up through the 3000-year-old avalanche event. (source: Hall et al. 1999, p 7)

Fig. 6: NW–SE topographic profile of the present cone showing the avalanche deposit and representative stratigraphic sections in the Chambo valley (Source: Hall et al. 1999, p 9)

Fig. 7: Simplified geological map of the Tungurahua (source: Hall et al. 1999, p 10)

The following figure gives a short impression about the magma composition and an additional overview about eruptive stages, major eruptions and datings.

Fig. 8: Generalized stratigraphy of Tungurahua volcano with special emphasis to the post avalanche Tungurahua concerning magma composition, sequence name/deposits and eruption periods. (Source: Hall et al. 1999, p 15)

High Risk of hazards

Keeping in mind that sector collapse and associated debris avalanche are rare events, the Tungurahua have already experienced that at least two times. Due to the 3000 m high relief and the steep slopes of the current cone a future collapse, even when it is of small volume, could cover an area similar to the one affected by the ~ 3000 year-old avalanche event (see fig. 7).

Besides an avalanche event, lahars are the greatest hazard (see fig. 9) and also remain high long after the eruption has ended. Some of the largest lahars probably result from lake breakouts and Tungurahua’s setting is supporting such a possibility quite well especially due to the two rivers Pastaza and Topo, 30 km downstream from Banos.

Fig. 9: A lahar in the La Pampa sector of Tungurahua showing an active, steep sided erosional channel down the axis of the deposit. Source: http://www.volcano.si.edu/volcanoes/region15/ecuador/tungurah/3208tun3.jpg)

Other hazards are lava flows, pyroclastic flows and associated debris flows. Based on the high velocity of the magmas the lava flows do not reach high velocities and therefore the are not a major hazard for the populations living close to the volcano. Considering historic and prehistoric events it is obvious that pyroclastic flows and lahars triggered by pyroclastic flow or surge activity are the principal hazards and the biggest threat for Banoes and other villages.

Fig. 10: Hazard map for pyroclastic flows (PF) and lahars (L), showing the main runouts followed by historic flows. 1 = high hazard for directed blasts and PF over the total area and L in the valleys; 2 = minor hazard area for PF; 3 = high hazard for L in the lower Pastaza valley (source: Hall et al. 1999, p 19)

At the moment the Tungurahu is still active and lahars, ash fall, ash plumes, explosions and incandescence has already noticed this year. The following generalized hazard map (fig. 11) shows the areas of relative risk. In the highest potential risk zone lies the city of Banoes while the town Patate is located in the zone of lowest potential risk. The risk of Lahars continue also hundreds of kilometres outside off the map towards E along the downstream the Rio Pastaza.

Fig. 11: Hazard map showing areas of relative risk, highest risk for the dark shaded area and for the stream to the east is a high risk for lahars (source: http://www.volcano.si.edu/volcanoes/region15/ecuador/tungurah/2607tun5.jpg)

Eruptive history

The following list covers only eruptive events from the last 300 years and a few additional ones with hight Volcanic Explosive Index (VEI) a complete list is available at http://www.volcano.si.edu/.

Start date Stop date Dating Technique

VEI Eruptive Characteristics

05.10.1999 continuing Historical Records

3? Central vent eruption Explosive eruption Pyroclastic flow(s) Lava flow(s) Lava lake eruption (?) Fatalities Damage (land, property, etc.)

Mudflow(s) (lahars) Evacuation

06.05.1993 06.05.1993 Historical Records

1? Central vent eruption Phreatic explosion(s)

03.03.1916 01.12.1925+/- 30 days

Historical Records

4 Central vent eruption Explosive eruption Pyroclastic flow(s) Phreatic explosion(s) Lava flow(s) Damage (land, property, etc.)

Mudflow(s) (lahars)11.01.1886 1888

+/- 1 yearHistorical Records

4 Central vent eruption Explosive eruption Pyroclastic flow(s) Lava flow(s) Fatalities Damage (land, property, etc.)

Mudflow(s) (lahars)Lava Volume of 8.9x107 m3

04.02.1773 07.1773 (?) Historical Records

3 Central vent eruption Explosive eruption Pyroclastic flow(s) Lava flow(s) Damage (land, property, etc.)

Mudflow(s) (lahars) EvacuationLava Volume of 1.0x 108 m3

730 AD +/- 200 years

Unknown Radiocarbon 4 Central vent eruption Explosive eruption Pyroclastic flow (s)Tephra Volume9.5x 107 m3

1010 BC +/- 100 years

Unknown Radiocarbon 5 Central vent eruption Explosive eruption Pyroclastic flow(s) Lava flow(s) Fatalities Damage (land, property, etc.)

Mudflow(s) (lahars) Debris avalanche(s)Tephra Volume 1.3x109 m3Collapse of Tungurahua II edifice

7750 BC (?) Unknown Radiocarbon 4 Central vent eruption Explosive eruption Pyroclastic flow(s)Tephra Volume > 1x108 m3

Table 2: Eruptive History (source: http://www.volcano.si.edu/world/volcano.cfm?vnum=1502-08=&volpage=erupt)

Pictures

Fig. 12: Ash plume at 09.03.2007 (source: http://www.volcano.si.edu/world/volcano.cfm?vnum=1502-08=&volpage=var)

Fig. 13: Morphology of the snow and ice-covered summit crater prior current eruption 1974 (source: http://www.volcano.si.edu/world/volcano.cfm?vnum=1502-08=&volpage=var)

Fig. 14: A dark ash plume 16.11.1999 (source: http://www.volcano.si.edu/world/volcano.cfm?vnum=1502-08=&volpage=var)

Fig. 15: Pyroclastic flows at 18:14 on the 14. 07.2006 (source: http://www.volcano.si.edu/world/volcano.cfm?vnum=1502-08=&volpage=var)

References:

Minard L. Hall, Claude Robin, Bernardo Beate, Patricia Mothes, Michel Monzier, 1999, Tungurahua Volcano, Ecuador: structure, eruptive history and hazards

Haraldur Sigurdsson, Bruce F. Houghton, Stephen R. McNutt, et al. 1999, Encyclopedia of Volcanoes

Smithsonian’s Global volcanisim program, 2009, http://www.volcano.si.edu/world/volcano.cfm?vnum=1502-08=

Wikipedia, 2009, http://en.wikipedia.org/wiki/Tungurahua