Pacaya Volcano

Pacaya Volcano (2560m) is located 30km south of Guatemala City center and directly south of the large lake-filled Amatitlan Caldera. From 1965-1975 and since 1990, Pacaya has almost permanently been in eruption, displaying primarily strombolian activity together with the extrusion of aa lava flows from summit or flank regions of MacKenney cone. Occasionally, larger explosive eruptions have been witnessed which shall be discussed later. MacKenney cone has formed in the horseshoe-shaped depression left by collapse of over half a cubic km of the W-SW sector of the volcanic edifice around 1100 years ago. This sector collapse produced a 25km long avalanche deposit extending onto the pacific coastal plane. Lava flows from MacKenney cone finally overflowed the E-NE wall of the depression in 2006 and have covered the old trail to the cone, making access more difficult.

Nighttime view of Pacaya's MacKenney Cone. Summit incandescence and lava flows from shield on flank are visible. Dec. 2007	View further to the left relative to adjacent image. In background from right to left: Agua, Acatenango and Fuego volcanoes.

MacKenney Cone viewed from main trail in 2005	Similar view to picture on left in 2007. A new lava shield has formed on left flank and lava flows have reached trail during 2006 and 2007.

View from summit to Cerro Grande lava dome in 2005. Flank of collapse scar is still evident in foreground.	Nighttime image showing similar area as image on left but in Dec.2007. Note that lava flows have completely filled the area between MacKenney Cone and the collapse scar.

Since the sector collapse exposed an up to 200m thick cross-section of the previous structure, it has been possible to paleomagnetically date the most recent deposits of pre-collapse Pacaya. This revealed that activity of Pacaya in the last couple of thousand years has been strongly episodic, involving emplacement of thick deposits of basaltic lava over periods of under 300 years in length followed by inactive phases lasting from 300-500 years (Conway et al., Bull. Volc. 55(1-2), p.25-32 (1992)). Looking further back into the history of Pacaya one sees 3 further distinct phases in its construction (Bardintzeff and Deniel, Bull. Volc. 54(4), p.267-283 (1992)). Little remains visible of the Phase I structure which is largely eroded or buried. Phase II was largely effusive and resulted in the emplacement of a large cone structure around 500000 years ago. Phase III was a largely extrusive phase, resulting in emplacement of several andesito-dacitic lava domes approx. 160000 years ago. This include Cerro Grande, the second highest structure in the complex. Phase II and IV (present) lavas are chemically similar and may have also been partially mixed into the Phase III lavas by some form of connection between the basaltic and dacitic magma chambers.

Mild strombolian activity from active summit crater of Pacaya, Dec. 2007	Mild strombolian activity from active summit crater of Pacaya, Dec. 2007

Inner wall of summit crater lit up by incandescent vents. Agua volcano in background at top left.	Inside of summit crater containing at least 2 active vents, Dec. 2007

Cone with fumarolic deposits on summit of Pacaya, 2005	Vent of cone, 2005	Vent of cone, 2005. Reported to show incandescence at night.

Whilst activity at Pacaya usually has limited impact on the surrounding communities on the flanks of the volcano, a repeat of the flank collapse event would today have catastrophic consequences. Small collapses of the summit of MacKenney cone have been witnessed in 1993 and 1995, leading to a 3.5km hot avalanche and a similar smaller event burning vegetation and damaging a radio station on Cerro Chino, respectively. Further, as the size of MacKenney cone increases, eruptions from the cone will be less constrained by the walls of the collapse depression which until 2006 prevented lava flows extending beyond its perimeter. This increases the risk of hot avalanches, pyroclastic flows and lava flows to the communities on the N flank of the volcano. Also, reactivation of the Cerro Chino crater (last active 1846) would present a risk to these communities. This forest-covered crater is usually passed unnoticed when climbing from San Francisco and is located outside the NW perimeter of the collapse depression. Presently, the most common risk to these communities is the impact of ballistics from larger explosive eruptions. In 1986, for example, 250g ballistics damaged roofs and killed livestock in Calderas (3km from MacKenney cone). In 1989 and 1991, over 1m diammeter blocks were thrown over a km from the crater and smaller blocks were distributed over a wide area. The same happened in 1998 in an eruption which lead to closure of the Airport of Guatemala City due to ashfall. The airport was in fact closed twice that year due to ashfall and again in 2000. The amount of ash involved was minimal, yet it can have a severe impact on aircraft engines and hence even a mm of ash can lead to airport closure (this is a common occurrence at Catania airport during eruptions of Etna volcano). Luckily, larger eruptions are often preceeded by increases in seismicity so that surrounding communities can be warned. Further, visible increases in fumarolic activity have sometimes been reported in the minutes leading up to larger explosions. Large explosive eruptions often involve episodes of intense lava fountaining. Up to 800m high lava fountains were observed on 16 Jan 2000 and less high fountains have been observed on several previous occasions.

On May 27-28, 2010, powerful activity from a new vent south of the summit of Pacaya resulted in heavy ashfall in Guatemala City causing closure of the International Airport. Volcanic bombs and lapilli hailed down on surrounding communities (several of which were evacuated), causing the death a local journalist. In the days following this violent explosive episode, a nearly 100m wide lava flow was emitted from a vent about 1.5 km south of the new crater and progressed over 3 kilometers, cutting roads and burying several properties, such as large parts of Finca El Chupadero including the pineapple plantation and animal housings (resulting in some livestock loss).

Lava flows from lava shield on N flank of MacKenney Cone, Dec. 2007	Wild horse infront of lava flows during the night.

Source of lava flow shown on left in image above.	Looking at the flow source from above it.

Lava flow descending down flank and gradually cooling	Front end of lava flow. Lava is partially solidified and moves slowly.

Animation of advancing lava flow front (about 10x normal speed)

Since the security situation around Pacaya has improved and the volcano has been included in a National Park structure, tourism has significantly increased. Until 2006, when activity permitted, it was possible to climb from San Francisco de Sales to the summit of MacKenney cone in 2 hours. The old path has now been covered by thick aa flows and a new trail to the summit starts from the most N part of the lava overflow. Only the lower part of the trail is however clearly marked (situation Dec. 2007), so one ends up scrambling through steep and loose aa fields. Orientation can easily be lost in poor conditions. Activity can often be observed from the NW flank of the collapse depression which is a popular camping site. Shuttle buses to San Francisco can easily be arranged from Antigua. Tourist infrastructure is not present in the immediately surrounding communities.

Further volcanoes in Guatemala are introduced in the sections on Santiaguito/Santa Maria and Fuego volcanoes.

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