Introducing San Salvador Volcano

[BrickFielder]

Introduction

My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles.

Introducing Volcano San Salvador  (El Boquerón and the Quezaltepeque volcanic center)

Reason for Interest

The volcano El Boquerón (Big Mouth) towers above the capital city San Salvador of El Salvador. Historical landslides, earthquakes and eruptions have affected populated areas. This is a volcano which can cause problems without eruption yet in many ways seems so benign.

The Setting

El Salvador is a small central American Pacific Coast country with Honduras to the North and East and Guatemala to the West with an Ocean to the south. The Sierra Madre Mountain range is in the North along the border with Honduras.  Further south is the coastal Mountain range which is split into five groups of active volcano groups. The lowlands along the coast have a tropical climate with mango, coconut and Cashew trees, the plateau between the mountain ranges has a semitropical climate and mountain areas experience more temperate weather with pine and oak. Temperatures typically ranging from 60 to 85 Degrees Centigrade with a wet winter season from May to September. The wet season rain comes from low pressure systems forming in the Pacific often leading to Thunderstorms (often in the afternoon or overnight). The country has frequent earthquakes with a history of significant damage to buildings in the past (1951, 1965, 1982, 1986 and 2001). Volcanic activity occurs periodically with the large VEI6 eruption of llopango in the 5th century most notable. The economy is notable for indigo, coffee, fruit and more recently for surfing. Due to the use of the U.S. Dollar as a currency El Salvador is becoming more Americanized with shopping malls and boutique Hotels beginning to crop up. Parts of El Salvador are troubled by gangs and corruption so it can be risky to visit despite its growing Americanization. El Salvador is known for its gorgeous flowering trees, the Maquilishuat the pink-tufted national tree, the beautiful Roble Colorado with its fuchsia and the Arbol de Fuego a tree with brilliant orange flowers. This is also home for armadillos, anteaters, sloths, ocelots, jaguar, spider monkeys, parrots, parakeets, boa constrictors, turtles, coatis and the national animal the Turquoise-Browed Motmot.

The Tectonic Environment

Most of Central America rests on the relatively motionless Caribbean Plate. The Cocos Plate close offshore in the Pacific is being carried northeast and sub-ducting below the Caribbean Plate. The Pacific relatively dense ocean floor is forced down under the lighter land mass, creating the deep Middle America Trench that lies off the coast of El Salvador. The subduction of the Cocos Plate accounts for the frequency of earthquakes near the coast. As the rocks of the ocean floor are forced down, they melt, and the molten material pours up through weaknesses in the surface rock feeding volcanoes.

The Volcano

The summit area of the volcano just below 6000 feet high is within a national park (Parque El Boquerón). The park is covered with a pine forest with exotic flowers such as cartuchos, hydrangeas, hibiscus, calla lilies and begonias to be seen along the trails through the park giving a sensual overload of fragrances. The park also has a visitor center and trails leading up to the 5km wide crater with four viewing points along about a 50m section of the rim. Within the 558m deep crater is Boqueroncito (Little mouth) a cinder cone from the last eruption which can be visited by those intrepid enough to venture down the steep walls of the crater.

Timeline of the last eruption

On June 7th 1917 after the celebrations of the Eucharist, the population surrounding the volcano of San Salvador felt the devastating effect of a large earthquake at approximately 6:55 pm. This earthquake and the subsequent tremors caused severe damage to nearby towns and villages. At 8:11 pm the Pinar hill on the northern side of the volcano, developed small craters (Boqueroncitos), from which clouds of black smoke rose to the sky. This was followed by a 250m wide and 2km long river of magma that flowed passed the community of Las Granadas.

At around 8:45 pm another slightly lesser earthquake was felt. This earthquake coincided with the start of a new eruption from the craters of “los chintos” specifically from the lower crater “el tronador”. Magma flowed to the north covering a length of 6.5 km and had a width of 100m. This large blanket of magma sliced the railway line from Quetzaltepec to El Sitio del Niño.

On the 9th of June the main crater began to erupt. During the afternoon the water that had once formed the lake in the crater slowly began to evaporated, forming a thick cloud of smoke. By the 28th the water had completely disappeared and there were explosions that sent up columns of hot mud and ash. The ashes expelled from the eruption fell over many nearby towns and villages.

In November 1917 the eruption that began on the 7 of June came to an end. The most devastating event being the initial earthquake which destroyed most of the buildings nearby with later ash fall further destroying buildings. It was reported that although the death toll was relatively low that a large percentage of houses in San Salvador were uninhabitable, water supplies were severely damaged, and the entire business section destroyed by fire.

Most Recent Activity

On the 22nd of Mar 2012 t the Mexican 7.4 magnitude earthquake near Acapulco might have affected the currently dormant San Salvador volcano by triggering an earthquake swarm under the volcano. The swarm consisted of 25 quakes, including 8 quakes between magnitude 2.2 and 3.3. The quakes were located at shallow depths between 0.5 and 5.8 km under the volcano.

During October 2020 at least nine people were killed after torrential rain caused a landslide on the northeast flank of the volcano. The landslide tore a 4 km path through the village of Los Angelitos, and Nejapa where 60 homes were buried and 75 damaged.

 

Volcano History

Eruptions have typically been flank eruptions along two or three fault zones with most recent ones being along a N40W fault zone through the central volcano. Some of these flank eruption craters are “La Joya”, “Boqueroncillo”, “Los Chintos” and “El Playón”. The last eruption south of the main caldera was 1700 years ago with most recent activity to the North West furthest from the city of San Salvador.

One historic eruption of San Salvador involved a caldera collapse and left deposits up to 4m deep up to 4km away and lighter deposits up to 18km. A further major eruption and caldera collapse in 1200 AD covered an area of 300km2 with ash.

In 1976, a bulldozer operator leveling ground uncovered a building covered in volcanic ash. The building was part of a peasant community of Mayans later found to be to be 1400 years old. Dan Miller, a USGS geologist determined the ash came for a hydro magmatic explosion crater approximately 400 m. away. The village was covered by more than 4m of volcanic material. Sediments suggest that the eruption was explosive and hurled material at a high velocity.

Discussion Ideas

For me the most disturbing and perhaps overlooked hazard is the simple risk of water supply damage and fire risk to a city from a volcanic earthquake.  There also seems to be a lot of communications equipment on the volcano which is a bit worrying. Perhaps you have a different view and the risks are minimal or different? Maybe you want to explore some of the things I skipped over a bit, like deforestation, El Salvador  gangs, civil wars, history, llopango, wildlife, trees and flowers.

 

[swebby]

Nice idea BrickFielder. 

The 1200 eruption may have been a VEI4, a repetition of which would be big enough to cause major problems for San Salvador. 

Vesuvius 79AD was VEI5 - ten times as big, but the Soufrière Hills volcano on Montserrat destroyed the capital of the island, the town of Plymouth, with a VEI3 back in 1995, so it is clear that the El Boquerón system could cause significant problems.  I use those two examples as Pompeii is about the same distance from the summit of Mt Vesuvius as the centre of San Salvador is from it's name sake (6 miles) and the town of Plymouth was about the same distance (2.5 miles) from the summit of Soufrière Hills as the large suburbs of San Salvador.

background info link

 

Global Volcanism Program | San Salvador

VOLCANO.SI.EDU

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The massive compound San Salvador volcano dominates the landscape W of El Salvador's capital city of San Salvador. The dominantly andesitic Boquerón stratovolcano has grown within a...

El Salvador itself appears to have developed greatly from the periods of unrest in the 70/80's, but i doubt it would be able to cope by itself if it faced significant disruption to San Salvador from a minor to moderate eruption, especially if it happened in the summer rainy season?

In the grand scheme of things San Salvador volcano is far from being on the list of very worrying volcanoes. I see the alert level for Taal in the Philippines has been raised recently.