Introducing Ilopango Caldera

[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 Ilopango Caldera

Reason for Interest
The caldera is the result of an eruption 10 times the size of the St Helen's eruption which potentially changed the worlds climate and affected millions worldwide. When we focus on big events we tend to lose sight of other smaller hazards and for me there is one to watch out for here. Whilst another big eruption is unlikely a resumption of activity could make the lakes water more poisonous affecting roughly 3 million people who use it as a source.

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. 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 Americanised 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 Americanisation. 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 subducting 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 Caldera
The scenic 8 x 11 km Ilopango caldera, filled by one of El Salvador's largest lakes, has a scalloped 150-500 m high rim and reaches 300m deep. The caldera, which lies immediately east of the capital city of San Salvador, is strongly controlled by regional faults of the central Salvador graben(rift). Four major explosive eruptions during the late Pleistocene and Holocene produced extensive pyroclastic flows and deposits that blanket much of El Salvador. The latest collapse resulted from the massive Terra Blanca Joven eruption devastated early Mayan cities. Post caldera eruptions formed a series of  lava domes within the lake and near its shore. The Islas Quemadas, a group of low islets in the center of the lake that mark the summit of a largely submerged lava dome were formed in 1879-80 during the only historical eruption of Ilopango. More than 200,000 inhabitants live in the drainage basin of Ilopango lake, El Salvador, even though high concentrations of boron and arsenic make this water unsuitable for human consumption. The Rio Desagüe drains water from Lake Ilopango into the Jiboa River which then goes south into the Pacific Ocean. The river does not appear natural and the names in the area support the idea that the Maya created the river by blasting a channel using gunpowder. I have a reputation for being a site for UFO visitations and many visit me specifically to see UFO's. 

Past Eruptions
The Islas Quemadas dome in Lake Ilopango, El Salvador, was extruded during December 1879 to March 1880. The eruption took place in six distinct periods, five of which began near fortnightly minima in the amplitude of the semidiurnal solid earth tide. An earthquake swarm preceded dome extrusion. 

The Tierra Blanca Joven eruption
In the summer of 536 AD a mysterious cloud appeared over the Mediterranean basin and local climate cooled for more than a decade causing crop failures and widespread famine. Researchers say there were two volcanic eruptions with the first in 535 or 536 in the northern hemisphere and another in 539 or 540 in the tropics that altered the world climate. Suspicion has been that the 540 eruption was due to the Ilopango eruption in El Salvador although  although other candidates are El Chichón in Mexico or the Haruna eruption in Japan.

 

There are problems with the argument that Ilopango Tierra Blanca Joven eruption was responsible for this climate change period. The first is the research by Victoria Smith of Oxford University who analyzed an ice core recovered from Greenland and also carried out radiocarbon measurements from a charred tree found in the TBJ ash deposits and accurately dated the eruption to 431 CE. The next issue is the examination of the ice core GISP2 from Greenland which showed high levels of tropical sea water fossils in the ice for the 540 period. From examination of the Colle Gnifetti Glacier ice core in the Swiss Alps researchers found particles which resembled volcanic rocks from Iceland and dated them to 536 AD.

Other research by Robert Dull however identified that there were 8 phases of eruption by Ilopango which were close together so a single year for the eruption might be a stretch. This research also looked at three tree samples which gave mixed results. The first showed an earlier date and the second two suggested an eruption around 540 AD. 

Recent Activity
 Ilopango has an active hydrothermal system and Divers have reported increasing seepage of hot water in the south of the lake near the Cerro Los Patos island. The area between the Islas Quemadas and the lake’s southeast margin also experienced the most frequent seismicity of any lake sector.

Discussion
The arguments about the source of the 536 to 540 climate changing eruption will go on, but we should not let that distract us from other hazards, like hydrothermal activity increases affecting the water quality. Just watch out for the UFO's though.