Another spectacular phenomena of nature are volcanic manifestations. Generally, when we refer to volcanic activity, we associate it with huge amounts of fire or ignited materials that burst violently onto the surface of the earth. That image is valid for only one type of volcanic manifestation, which we will soon see. Before reviewing in more detail “volcanism”, in relation to natural disasters, we will attempt to develop a concrete definition that will serve as a frame of reference.
Volcanic activity or volcanism includes all of the phenomena by which the magma and its components come from the deep interior of the earth and arrive at the surface, whether in a gaseous, liquid or solid form, usually, with some type of observable thermal manifestation.
Volcanism includes a range of manifestations, forms and volcanic products, such as: the emanations (fumaroles, sulphuric compounds, and others), that are gaseous manifestations; and hot water springs, enriched with minerals that dominate hydrothermal sources (spas, geysers and others).
There is also a type of volcanism called intrusive volcanism, which can affect surfaces over several thousand square kilometers. In this manifestation, the ascent of the magma is extraordinarily slow (centimeters per year), and it never “spills” itself onto the surface due to its slow ascent, it completely solidifies. Therefore it is not spectacular in the short time, as its effect is based on elevating the materials on top of it. It is discovered only when the erosive processes eliminate the materials that it elevated.
Almost all of these manifestations have benefits for mankind, in areas such as: mining, medicine, energy resources, and others.
We wanted to give a brief overview of volcanic manifestations in order to improve the initial idea of volcanoes and because the objective of this number of biocoenosis is to better understand phenomena that cause natural disasters. In the following pages our discussion will be confined to volcanism defined as extrusive volcanism. That is, the type of manifestation where magma or its components arrive on the surface with some degree of violence and high temperatures, through a central conduit or fissure.
WHAT IS MAGMA?
In order to comprehensively understand the mechanics and the origin of volcanic materials, it is necessary to know about the material that causes the eruption, magma.
We know that rocks are a mineral aggregate among the most important of which is silica. Silica, as well as other minerals, are poor conductors of heat. As the heat is accumulated, it eventually fuses rocks with the aid of water vapor at high pressure. This is a very hot natural liquid, formed under the Earth’s surface in depths from 10 to 700 km. Initially it is composed of rock minerals as well as gases that due to the intense pressure are liquefied into the same solution. These gases, as well as other components, will volatilize when the magma begins to ascend. They, along with the water vapor that is generated, play a very important role in the mechanics and the characteristics of volcanic eruptions.
Several theories exist about the energy or heat source that permits the formation of magma. Most probably there is not a single explanation for the origin of all volcanic eruptions. It is important to succinctly mention the most important energy or heat sources.
a) The mechanics of the plates. Energy generated from the shock and scraping of the two plates between the oceanic ridges that is extremely volcanic. In this second case, there seems to be a very narrow link with the presence of a convection current in the ascending part of the magma that would contribute the heat that originates from the earth’s interior.
b) Radioactivity. All rocks have some degree of radioactive elements. When exposed to high temperatures and internal pressures, the natural process of atomic decomposition could be accelerated, freeing sufficient energy to fuse rocks, thereby producing magma.
c) Thickness of the rock columns. It is based on the enormous pressure exerted by the crust, which has a thickness of a little more than 60 km. The weight of all of those rocks, on occasions, would be able to fuse those at the bottom, thereby producing magma.
Magmas differ in their viscosity, which depends on the temperature of its components, as well as the proportion of volatile elements and water vapor.
As was indicated, the presence of these last two components is very important, as the magma ascends, in part from its own expansion of the bodies as the temperature rises. However, the fundamental reason is the presence of abundant amounts of water vapor which makes the volcano behave similar to a hydraulic pressure machine. Magma once it arrives on the surface of the earth is called lava.
ERUPTIONS
Although we will soon see that all volcanic eruptions are not the same, we will try to indicate here those aspects that are more or less common.
According to what we have already seen, magma will ascend when provided with certain favorable conditions, in particular pressure and temperature, as well as the presence of abundant gaseous elements. As magma does not exist in a permanent form under a volcano, eruptions are also not a permanent phenomenon. Rather they represent small moments in the normal life of a volcano.
Once the magma is created, the volatile materials must be able to exert significant pressure towards the surface, much like a piston. This pressure allows the magma to break through and between the materials that are on top it and therefore, ascent. This ascent takes advantage of areas caused by preexisting fractures. In the case of volcanoes these are called “chimneys” (principal and secondary). Chimneys are always totally or partially obstructed by magma that solidified from previous eruptions. The internal pressure then causes new fractures that are perceived as small earthquakes around the volcano or as perceivable noises; the penetration of the magma in the volcano will cause deformations, and the volcano swells and deflates. An increase in temperature in preexisting liquid and gaseous manifestations, as well as an increase of gas discharge can also be detected.
Even when the eruption process has begun, the eruption itself does not occur until the internal pressure is sufficient to launch the superior part of the magma column that obstructed the chimney. When this takes place a boom is heard and a great amount of materials are projected into the air. The materials include: lava, pyroclastics and magma from previous eruptions; lava and pyroclastics from the current eruption; gases and diverse amounts of water vapor.
The finest pyroclastics (such as sand and ash), along with the volatile materials, can be propelled more than 10,000 meters above the volcano and by action of the wind can be distributed up to tens of kilometers of distance.
As well, due to the violence of the eruption and the abundant presence of water vapor, cumulonimbus clouds can be formed (storm clouds) on the volcano, along with intense precipitations accompanied by lightning and thunder. This phenomenon can have diverse peculiarities, its duration and form will depend on the permanence of the internal pressure and energy.
WHAT LAUNCHES THE VOLCANIC ERUPTIONS?
Previously, it was indicated that the characteristics of an eruption depend, to a great extent, on the existence of gases and water vapor.
In fact, the presence of these components is used as one of the criteria to classify eruptions in the following manner:
a) Effusive eruptions: there are relatively small amounts of gases and water vapor, the lava therefore spills out of the volcano in torrents and although there are small explosions, lava dominates. Its viscosity will depend on the temperature; it has been seen to cross several kilometers, if its temperature is higher than 1000 ºC. They are typified by the eruptions called the “Hawaiian” type.
b) Explosive eruptions: due to a significant presence of gases and water vapor these eruptions are characterized by explosions and the expulsion of abundant amounts of pyroclastic materials, although lava can also be present. According to the relative explosion to lava three types of eruptions can be distinguished in this group: Estromboliana, Vulcaniaca and Peleana
c) Phreatic eruptions: water and particularly water vapor clearly dominate over all other components. Although small emissions of pyroclastics and lava can also be present, this group is characterized by enormous vapor clouds (mixed with gases and ashes) that, due to their form during the emission process, are called feathers.
A great variety of materials are discharged in a volcanic eruption; the following are the most important ones:
a) Lava: the midpoint for rock fusion is 750 ºC therefore when magma reaches the surface with temperatures superior to 750 ºC, lava will be able to spill onto the surface. If lava is close to 730 ºC it will be virtually solidified, thus its ability to flow will be very limited and it will be deposited in places very near its emission. Depending on the temperature at expulsion as well as the grade of the slope and the topographic characteristics, the speed of lava is very variable and can be from a few meters per hour up to approximately 20 km per hour. Whenever lava spills appear in the form of tongues they could remain with high temperatures for up to several months after the eruption.
b) Pyroclastics: All the volcanic materials that are discharged in a solid state, generally by explosions, are included in this category. These materials are anywhere from finely crushed particles, such as dust and volcanic ash that, carried by the wind, can reach enormous distances, to huge boulders, that can weigh several tons and after following a parabolic path they are deposited relatively near the crater.
The dust, ash, and river and volcanic sands are products of the pulverization of lava from particularly violent explosions. If they are still in an incandescent state when they fall, they stick and form a compact mass called a volcanic cover.
The boulders are solid materials that do not break on impact. Here gravel, pebbles, Iapillis and the boulders themselves are located. With the exception of boulders, they are small rocks that generally, do not exceed 5 cm, but a considerable amount of these materials, as well as of the finer materials previously indicated, can cause enormous destruction.
“Volcanic bombs” are of an intermediate and variable size. They are pieces of lava that are discharged spinning and in a doughy state and therefore are converted into an almond shape (stretched in the ends) and a resultant aerodynamic form.
c) Gases and water vapor: important gas emissions occur in all eruptions. Sulfurous components are dominant, but there are also noble gas discharges and ones of the simplest form such as hydrogen. At times, some of these gases make the eruptions even more spectacular due to the combustions that take place; as their concentrations upon emission are highly dangerous.
Water vapor is the other element that, whether in large of small amounts, is present in all eruptions. Water vapor creates collective clouds and precipitations, sometimes, of great proportions.
d) Other types of emissions: among which are: pumice stone; ardent clouds which are formed by high density incandescent ash emissions, which tend to slide through the natural channels of the volcano, reaching speeds of more than 100 km/h. This speed along with the elevated temperatures and high concentration of gases, creates an enormous destructive power; mud avalanches, which also have an enormous destructive power.
VOLCANIC FORMS
We have seen that the superficial part of our planet is very thin, thus many authors call it the “skin” of the Earth.
Following this analogy, volcanoes can be considered the pores of the skin with many diverse forms and sizes. This agrees with the studied complex processes that form them.
Not all eruptive activity is analyzed by means of a volcano, as it can also happen through cracks. The first type is called the “central type” and the second is referred to as the “fissural type”.
Studies indicate that the eruptive activity normally begins through a crack with volcanic materials piling up around it, taking the shape of one or several volcanoes. Volcanoes can measure from hundreds of meters high if the activity has been brief and not very intense, to various kilometers high, if the intensity has been significant and over thousands of years.
Basically, two large types of volcanic forms can be distinguished:
a) a shield: (such as an inverted plate), this corresponds to volcanoes constructed mainly by successive lava layers that where very fluid when expelled; and
b) a cone that including several types, corresponds to a volcano constructed mainly by pyroclastics and highly viscous lava emissions.
Another way to classify volcanoes is according to their activity or lack thereof. According to this, volcanoes are considered active when there is information (not necessarily written) that shows that they have been active in historical times. By elimination, all the rest are considered to be inactive, turned off, sleeping or extinct volcanoes.
Experience has shown us that eruptive activity can begin, at times, on certain land masses that were originally considered to be hills, peaks, and others which were not suspected to be volcanic structures. For this reason, it is recommendable to refer to volcanoes vs. latent volcanism in order to distinguish from those that do not have proof of activity in historical times.
WHERE DO VOLCANIC ERUPTIONS TAKE PLACE?
Volcanic eruptions do not originate randomly or in any place on the earth’s surface. According to what we have seen, there must be a series of factors in order that, in agreement with some of the mechanisms already indicated, the magma is generated and sufficient pressure is reached in order to allow the magma to ascend to the earth’s surface.
According to this, there are some areas, from the ocean floor to “dry” land, that present characteristics favorable for eruptive phenomena. These correspond basically to flat lengths in which:
a) The plates are colliding (compression) or
b) They are breaking and moving away (distension).
Among the first example are: the Circle of Fire of the Pacific (including Central America); the axis of tertiary mountain systems of Central and Southern Europe, that extend until the sector of Balochistan, in Asia; and the Great African Rift (fracture), that extends from the Dead Sea to Madagascar.
Among the second we have: the meso-Pacific ridge; the Atlantic ridge, the ridges of Western and Eastern Indian region. It is in these sectors, where almost all of the active volcanism on our planet is found. It is very important for the inhabitants of these areas, to adequately know and understand the phenomena, as they form part of their natural landscape.
EFECTS OF ERUPCIONS
Given the range of eruptions and volcanic products, the effects are very diverse. They can even be transformed into beneficial effects over time.
In general, the sector most affected by an eruption corresponds to the area near the eruption itself; the effects diminish quickly as the distance from the center of eruption increases.
It can be said that the areas that occupy the volcanoes are areas of permanent potential risk.
The main effects of the volcanic eruptions can be classified in the following manner:
a) Lava materials. A lava current is like a slow flowing river that tends to creep towards low areas and preexisting channels. Due to the high temperatures (minimum 750°C), it causes incandescence and combustion in objects that are up to 100 m away. During its advance large bubbles burst splashing lava. These splashes can reach several tens of meters. In its passage the lava flow causes total destruction of anything in its course. Nevertheless, after several decades it will serve as mother rock for the formation of new soils in the area.
b) Pyroclastic materials. These stones of all sizes are emitted in an incandescent state and normally accumulate around the volcano, concentrating around the crater. However, depending on the speed and direction of the wind, as well as on the direction in which the crater cap ruptured, they can reach great distances. If the eruption is sufficiently violent, ashes can be sent as far as the stratosphere to be distributed over the entire globe as a thin film, which is able to lower the earth’s temperature.
The larger sized boulders (weighing several tons) generally fall near the crater; however they can roll down slopes causing small avalanches. The smallest materials can accumulate in significant amounts to cover roofs of various types of buildings; to block drains, ditches and channels. In particular, the ashes and the volcanic dust can cause serious respiratory problems, especially in people with preexisting ailments or respiratory difficulties and when there is prolonged exposure it can manifest effects up to various years after the event.
When there is a large amount of ash that covers great areas, it can cause the death of the vegetative layer and crops as well as animals that have their habitat there. If, in addition, precipitation occurs, the ash is transformed into a type of very viscous paste that prevents the movement of cattle and can cause their resultant death. After a few years however, the ash that covered the area transforms into an element that rejuvenates the soil, with the incorporation of the constituent mineral elements of the volcanic ash.
c) Gaseous materials. All of the gaseous emissions are dangerous and many are lethal, depending on the type of eruption, particularly two of them: the ardent clouds and the ignimbritas. The first type is generally emitted by a volcano; the second, through cracks. The effects however, are increased by the fact that their high density makes them behave as fluids, where they can reach great speeds when they find favorable slopes, impeding precautionary measures taken in the emergency.
d) Other materials. Correspond to mud avalanches that form another group, they do not originate directly from the eruption, according to what we will be seen in the corresponding section.
THE PREDICTION OF VOLCANIC ERUPTIONS
To date, there is no technique that allows us to exactly predict the occurrence of a volcanic eruption. However, many years ago, volcanologists observatories were installed in diverse volcanoes around the world, equipped with a series of instruments that have been collecting valuable information and have allowed, in some cases, to predict an eruption exactly, up to the moment that the activity in the surface starts and the location that the materials will be emitted from.
In 1974, Costa Rica began to systematically work in this field. Currently, there is a network of sound-ranging instruments that pursue the mentioned objective.
In any case, it is very important to point out that volcanoes have individual behaviors, and it is therefore necessary to permanently study each one, more or less in the same way that a doctor has a clinical file for each patient, as, although there are certainly some characteristics common to all volcanoes, there are yet others that are unique to each one. It is also important to research volcanoes pasts, in order to determine the characteristics that typify them. With this information and other knowledge it would be possible to elaborate, for example, volcanic risk maps, determining for each area the possible effects that the eruptions can cause.
In any case, research has been able to establish a series of manifestations that precede a volcanic eruption and which allow the population and the authorities to take appropriate measures. The most significant manifestations are:
a)Tremors in the volcano and the environs, with a gradual increase in intensity (they become stronger), and frequency (the time between each tremor diminishes). This is a result of the slow ascent of magma. It can begin weeks or even months before the actual eruptive process arrives.
b)Perceivable underground noises in the area of the volcano. These are also due to the ascent of the magma. Sometimes they are like rumblings and, in other occasions, like dry cracks of something breaking.
c) Cracking of frost, ice or snow if present, until it melts.
d) Increase in water temperature in the springs of the volcano or surrounding area. Some water flows can dry up or increase in temperature to a point that it is possible to observe some degree of evaporation. The increase in the ground temperature can kill the vegetation layer.
e) Abnormal behavior of animals when they perceive vibrations that are not detected by humans.
f) Start of fumaroles emissions, or, if they already existed, an increase in the intensity of the flow and a change in their normal coloration.
g) Swellings and deformations in the volcano; at times, the entire volcano is affected and it is possible to measure and to determine the pulsations by means of an instrument called a tiltmeter. In a lesser degree it is possible to perceive changes in slopes in some sectors of the volcano and small hills can even be formed.
h) Lastly, close to the eruptive climax, small explosions and ash emissions are produced, these increase in intensity and frequency.
It is important to clarify that all of these factors do not need to take place in order to presume that an eruption will take place, as at times, only one or two factors appear. Therefore, the moment that it is verified that some of the factors occurred, warning should be given to the pertinent authorities immediately.
References
Cevo Guzmán, Juan Humberto. Volcanic manifestations. In: Biocenosis; p. 40-7. 1997
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