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Volcanic activity since 1700 AD has killed over 260,000 people, destroyed entire cities and forests, and disrupted local economies for months to years. Scientists have estimated that in the year 2000, the population at risk from volcanoes had increased to approximately 500 million, which is comparable to the entire world’s population at the beginning of the 17th century. One major volcanic hazard is the landslide or debris avalanche deposit.
Nicaragua’s Volcan Mombacho (Figures 1-3) rises 1400m above the western shores of Lake Nicaragua (Cocibolca) on a basement of Quaternary ignimbrite of the Las Sierras Formation. Conspicuous debris avalanche deposits on two sides of the volcano provide evidence for recent flank failure (slope collapse). One deposit, below a well-defined collapse scar, forms the remarkable Las Isletas (Figures 4-7), an archipelago of 365 small islands, and the arcuate Peninsula de Aseses, a full 10 km away from Mombacho The 20,000-year-old Apoyo pumice underlying the deposits gives a maximum age to the flank failure, landslide and resulting creation of Las Isletas.
Debris Avalanche Deposits (DAD)
Unlike ordinary mountains, which are formed by slow uplift and erosion, volcanoes are constructed rapidly. As a consequence, many are liable to massive flank failures, leading to devastating debris avalanches (e.g., Mount St. Helens, 1980). Such failures occur worldwide about once every 25 years, and even small ones can present a major hazard – particularly if far-reaching tsunamis are generated.
Volcanoes are susceptible to landslides because they are composed of layers of weak, fragmented volcanic rocks that tower above the surrounding terrain. Further, some of these rocks have been altered to soft, slippery, clay minerals by hot, acidic ground water inside the volcano.
Most volcanic failures are generated by magmatic intrusion (flowing lava) and flank spreading. Volcanoes subjected to hydrothermal activity develop weak cores as a result of internal alteration and increased core pressures. The slopes of the volcano respond to internal weakening by initially deforming slowly, but due to gravity’s effect on the steeper upper portions, may then collapse catastrophically.
However, Volcan Mombacho in Nicaragua experienced a previously unrecognized type of failure, one triggered by sub-volcanic basement spreading (lateral displacement). Notably, collapses related to basement spreading do not require that the volcano be magmaticaly active, and thus flank failure may pose a significant risk for all volcanoes, even inactive ones.
Ejecta
Pyroclastic rocks (ignimbrites) are the results of volcanic explosions; that is, they are fragmental pieces of rock, whether they are minerals, crystals or glass, ejected from the vent. Pyroclastics take two forms:
(1) Pyroclastic fall deposits are those that have traveled through the air as some kind of projectile during a volcanic eruption.
(2) Pyroclastic flow deposits. The majority of these form when hot fragmenting material made buoyant by hot gas begins to flow as a fluid (fluidization). This occurs when the hot gases accompanying the ejecta, together with trapped air and gas being released by the ejecta as it vesiculates, forms an air cushion around each particle preventing it from coming into contact with adjacent particles. Thus, the whole mass behaves like a fluid with low viscosity, enabling it to travel great distances down slopes (and often up slope as well).
Tephra is a general term for all the fragmental material erupted explosively from a volcano – ranging from fine dust (ash) to car-sized blocks. Tephra-fallout layers have two special features:
(1) They are erupted over a very short time, geologically speaking, usually a matter of only hours or days to a few weeks.
(2) They can be spread widely over land and sea to form a thin blanket that has the same age wherever it occurs.
Once identified by geochemical analysis, a tephra layer provides a marker bed for an ‘instant’ in time, that instant being the time of eruption that produced the layer.
Evidence for explosive activity in the form of tephra layers and pyroclastic flow deposits is absent from Las Isletas, further emphasizing flank failure can occur without magmatic activity.
Isletas
Las Isletas are located 3 km south east of Granada. Volcanic in origin, most of the islets are covered with unusual vegetation and are rich with bird life. San Pablo islet has a restored 19th century fort while a nearby islet has a howler monkey colony. Many of the islets are privately owned and inhabited. Boat trips to Las Isletas leave daily from Acese Port or Cabana Amarilla Port at the south end of Malecion, Granada.
To claim this EarthCache you must
1. Take and post a picture of you and your GPSr at the posted coordinates and e-mail me the answers to the following questions:
2. What is the predominant color of the beach sand and why?
3. What is the predominant mineral found in ignimbrite?
4. Differentiate between a debris avalanche deposit and a lahar.
Do NOT post your answers on your log, encrypted or otherwise.
DO NOT LOG AS A FIND UNTIL YOU HAVE A PICTURE READY TO POST AT THE TIME OF LOGGING A FIND FOR THIS EC. Logs with no photo of the actual cacher logging the find or failure to answer required questions through e-mail will result in a log deletion without notice. Exceptions will be considered if you contact me first.
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