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Here is a compilation of essays on ‘Volcanoes’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Volcanoes’ especially written for school and college students.
Essay on Volcanoes
Essay Contents:
- Essay on the Meaning of Volcanoes
- Essay on the Formation of Volcanoes
- Essay on the Arisen of Volcanoes
- Essay on the General Course of an Volcanic Eruption
- Essay on Volcanic Deposits in the British Islands
- Essay on the Distribution of Volcanoes
- Essay on the Effects of Volcanic Activity
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Essay # 1. Meaning of Volcanoes:
Sometimes the molten rock, ash, steam and other gases find their way to the surface of the earth through some vents or openings. These ejected materials accumulate around the vent and give rise to a volcanic cone or a hill. The conical hill along with the vent is known as volcano.
The mouth of a volcano is funnel shaped and hollow. This funnel-shaped hole is called the crater and the opening through which molten rock materials come out from inside is called the pipe or vent.
Fuji Yama in Japan, Visuvius in Italy, Cotopaxi in Equador and Barren Island are good examples of volcanic mountains.
The ejection of materials through the crater is called volcanic eruption. Volcanoes are generally classified into three types. These are active, dormant and extinct.
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The active volcanoes generally erupt fairly frequently and are of two types, namely, permanent and intermittent:
(a) Permanent volcanoes:
The volcanoes that eject molten materials continuously are called permanent volcanoes.
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(b) Intermittent volcanoes:
These are those volcanoes in which an eruption occurs at the end of a certain period of time. Stromboli of Italy and Etna of Sicily are the examples of active volcanoes.
A volcano which erupted sometime in the past, and is now passive, but may erupt again, is called a dormant volcano. Fuji Yama of Japan and Vesuvius of Italy are the dormant volcanoes.
Extinct volcanoes are those whose activity has not been known to us during the historical time. Kilimanjaro of Africa and Chimborazo of Equador are the examples of extinct volcanoes.
Sometimes it is difficult to say whether a volcano is extinct or dormant. For example, the Vesuvius, the Krakatoa, Mt. Unzen of Japan and Mt. Pintubo of Philippines once thought to be extinct became active recently.
Essay # 2. Formation of Volcanoes:
The actual causes of the formation of volcanoes are not yet definitely known.
The probable causes of their origin are given below:
(a) Difference of Pressure on the Surface:
Pressure below the earth’s surface increases with depth. The interior of the earth is under high pressure. It is at the same time very hot. Because of high pressure and high temperature the materials of the earth’s interior are in the viscous state. Owing to the release of pressure from the top portion for some reason, the viscous materials get expanded. There is then a natural tendency for molten materials to make an upward drive through the fissures or weak zones of the earth. By this way a volcano may be formed.
(b) Chemical Reaction:
There are some radioactive minerals in the interior of the earth. They emit heat continuously and as a result of this, the other materials get expanded and create enormous pressure. Then the underlying expands and molten materials may come out through some fissures forming volcanoes.
(c) Percolation of Water to Earth’s Interior:
Besides those two causes, a volcano may be caused when water percolating through the surface meets the molten materials. In this case water turns into steam of great volume. Steam, lava, and other materials may then come out to the surface forming a volcano.
Essay # 3. Arisen of Volcanoes:
It is a well-known fact that in all the northern and central part of Europe at the present time there are no active volcanoes. But, the geologist professes to be perfectly certain that there were, during many of the past geological periods, many volcanoes in our island. Before we can understand how he has come to this conclusion we must, according to Hutton’s method, find out how any known volcano has arisen and what traces it has left after it has become extinct.
Vesuvius:
The nearest European volcano to our islands is Vesuvius, and it is fortunate that we know a great deal about Vesuvius during the last two thousand years. The Neapolitan volcanoes extend from Vesuvius on the east to the Islands of Procida and Ischia on the west.
Before the Christian era, as far back as any tradition is known, there seems to have been no activity at Vesuvius. But a Greek colony, which settled on Ischia about 380 B.C., had to abandon the island owing to an eruption, and there were traditions of volcanic eruptions driving earlier colonists away as well.
Since the date above mentioned there has been complete cessation of vulcanicity in that island except at one time, when there was an emission of lava. On the mainland to the west of Naples there was Lake Avernus, occupying the inside of an ancient crater, where probably poisonous gases were emitted, if what Lucretius tells us is true, that birds could not fly over it without being stifled.
Thus we see that just before and during the beginning of the Christian epoch there was some evidence that the volcanic forces, which had in far-off ages built up high mountains near the Bay of Naples, were not quite extinct.
Vesuvius, which we now recognise as a modern cone within an ancient one, the remains of the latter being called Monte Somma, was, up to the first century, a single regular truncated cone. Plutarch writes of its old crater having steep cliffs and being covered with wild vines. The flanks of the mountain were of a rich soil and were covered with fertile fields stretching down towards the busy towns of Pompeii and Herculaneum.
An especially violent shock on the 5th of February A.D. 63 gave warning of a recrudescence of vulcanicity. Pompeii was badly damaged, many buildings being thrown down. But the inhabitants were rich and at once set about restoring what was damaged. After sixteen years most of the houses were in good repair, and at least two damaged temples had been rebuilt. In August A.D. 79 the great catastrophe occurred, and when the Forum was dug out a few years ago, it was seen that the west side of the colonnade, which had been almost completed when the earthquake of A.D. 63 threw it down, had been only partly rebuilt, and the area was strewn with blocks which were being made ready for the rebuilding by the stone masons.
On August 24 the elder Pliny, who was commander of the Roman fleet, went with his ships to rescue the people at the foot of Vesuvius from danger. Arriving too late he went to Stabiae where, on the next morning, he was suffocated by fumes..
His nephew, the younger Pliny, tells the story of what he saw in a letter to Tacitus. It is easy to follow the course of events. A column of vapour ascended from Vesuvius and then spread out on all sides. This caused intense darkness, which was lit up at times by flashes of lightning.
Volcanic ashes, as the fragments blown out from a volcano are called, fell on ships many miles away, earthquake shocks were frequent, and the sea went back, leaving marine animals high and dry.
Pompeii and Herculaneum:
But the ashes did not only fall on ships and the earthquakes were only, as it were, premonitory warnings of what was to follow. For at last a series of gigantic explosions blew one side of Monte Somma into the air and the enormous mass of material on falling buried up the prosperous cities of Pompeii and Herculaneum.
On the west side the descending ashes mingled with torrents of rain and so gave rise to a huge stream of mud, which poured down upon Herculaneum and sealed it up, on the south side, driven by the wind, the ashes spread out like a cloud and covered Pompeii. From this cloud there descended first broken fragments of pumice about the size of a walnut, which rained down to a depth of about 9 feet, then came fine ash wet with rain and settled for a depth of some 6 or 7 feet over the doomed town.
When the sun set the storm of ashes had ceased, but by that time Herculaneum had vanished and only the roofs of the houses of Pompeii projected above the surface. As far as we can tell, no lava flowed from the mountain during this period. But after this year other periods of activity occurred, and in 1036 we learn that a lava stream did make its way down the mountain-side.
Other times of activity were in the years 1049, 1138, 1306, 1500, and 1631, though the activity in 1500 was very slight. During the long pause before 1631 there occurred in the Phlegraean Fields to the west an event of some importance. In 1538 a new mountain was suddenly formed; it was named Monte Nuovo, and good accounts of its formation have come down to us.
Monte Nuovo:
After several shocks during September 28, we are told that very early on the 29th flames of fire were seen, and shortly after the earth burst open and ashes and pumice were blown out. Later on the inhabitants of the neighbouring town fled in terror to Naples, and it is stated that the sea had retired and left multitudes of fish to die on the shore.
The eruption continued for two days and nights, and on the third day, signs of activity having ceased, several people went up the new hill which was over 400 feet high and 8000 feet round its base. When they looked down into the crater they saw a boiling cauldron. Some days afterwards fresh explosions took place, but with decreasing activity.
No lava was extruded during the eruption, and the whole mountain was built up of large and small fragments which were blown up into the air by explosions and fell close to the orifice. No further volcanic action has occurred here since the time of the formation of the hill and it is now completely covered with grass.
For nearly a century after the birth of Monte Nuovo Vesuvius continued tranquil, and there was no violent eruption for nearly five hundred years. At the bottom of its crater cattle grazed, and its sides were covered with bushes. But in 1631 these grassy flats and woods were blown into the air and seven streams of lava poured down from the mountain and overwhelmed several villages. From the seventeenth century to the present day there has been a constant series of eruptions and no great convulsion.
In fact, the behaviour of the district is very much like that of a safety valve; when there is an opening the forces below cause more or less quiet extrusion of molten rock, but if the opening gets plugged up then the activity ceases, it may be for many years, but there comes a time when the imprisoned forces get sufficiently great to blow away the overlying tract which had sealed up any opening that had before existed.
Krakatoa:
Probably the most violent volcanic eruption of recent times occurred in the Islands of Krakatoa in August 1883. The Royal Society appointed a committee to investigate the eruption, and the report of this committee is a very remarkable production and stands for all time as an example of what such a report should be.
Krakatoa is an island lying between Java and Sumatra. Although prior to the catastrophe this island had attracted but little attention, it lies in the heart of the great focus of volcanic activity in the world at the present time. Java itself contains forty-nine great volcanic mountains, and from it a chain of volcanoes is continued to the east and to the west.
Krakatoa did not show the regular conical shape of a volcano, but was only a part of a huge crater of which other smaller islands close by were also fragments.
The volcano of which these islands are the remnant must have been formerly one of gigantic dimensions, probably some twenty-five miles in circumference at the sea-level and over 10,000 feet in height. It was built up almost entirely of lava, and rested on beds of post-Tertiary age.
At some unknown date the central mass of the volcano was blown away, and only irregular portions of its crater- ring were left. Then occurred quiet eruptions at the bottom of the crater, causing the formation of small cones which filled up the older crater in parts, and raised these above sea-level. At one edge of the old crater eruptions built up the cone of Krakatoa from lava extrusions and outbursts of ashes.
In historic times the lateral cone of Krakatoa and the other land which was part of the original volcano were seen to be covered with luxuriant vegetation, and the products of its forests attracted the inhabitants of neighbouring islands.
The first eruption recorded as having taken place at Krakatoa is in 1680, though tradition points to former ones having occurred while man lived in the neighbourhood. The eruption was of a moderate type and the island soon recovered its former state, and the vegetation which had been destroyed once more spread and for two hundred years the volcanic forces lay dormant.
In 1880 earthquakes were frequent, and in May 1883 the inhabitants of the town of Batavia, 100 miles away from Krakatoa, heard booming sounds, and for many hours their doors and windows were rattled. On the next day ashes fell on Java and Sumatra at places opposite to Krakatoa, and later a column of steam from Krakatoa showed the position of the disturbance.
The height of the cloud of dust above the island was estimated to be seven miles, and falls of dust were noticed three hundred miles away. This eruption soon decreased in intensity.
An excursion party from Batavia landed on the island and found some depth of fine ash scattered over it, and many” trees had been stripped of their leaves and branches by the fall of this deposit.
On August 11 another visit to Krakatoa showed that the forests had been completely destroyed, and that the fine ash covered the surface of the ground near the shore to a depth of 20 inches. Some fourteen foci of eruption were seen, from which steam and dust were being sent out.
This recrudescence of activity finally culminated on August 26 and 27 in a series of tremendous explosions. During these days three European ships were actually in the Straits in which Krakatoa lies, and as they escaped destruction those on them have been able to give a record of their observations.
The course of the events which happened seem to have been as follows- During the night of the 26th white-hot fragments of lava were blown out and rolled down the mountain-sides, smaller cinders were blown out to sea, and finer ashes, remaining in the air, caused intense darkness in the early morning.
From sunset till midnight there was a continuous roar which moderated in the early morning. Each explosive outburst of steam blew off the crust which formed on the lava and left the white-hot molten material to throw its light on to the overhanging pall of ash, and so lit up the scene for miles around.
But now the somewhat peculiar situation of the volcano made itself felt. It lies close to the sea-level, and so the removal of so much material by the constant explosions allowed the waters of the sea to make their way into the heated mass of lava. This must have chilled and frozen much of the molten rock.
Some flashing into steam of the water no doubt occurred, and this gave rise to waves in the sea which travelled to the shores of Java and Sumatra, and were noted on the evening of the 26th and through the night.
The general result of this action of the sea-water was the closing of the safety-valve while the forces below it were still active, and finally there resulted four tremendous explosions between 5.30 a.m. and 10.52 a.m. on the morning of the 27th.
What happened in this space of time to the island itself was that the whole of the northern and lower portion of the island was blown into the air, while a large part of the cone which existed there also vanished. Where the island had risen to heights of between 300 and 400 feet was now in some places more than 1000 feet below sea-level.
These terrible explosions caused huge waves to travel away from the centre of destruction and, rushing up on to the coasts of Java and Sumatra, stranded all vessels near the shore, devastated towns and villages, destroyed two lighthouses, and caused the deaths of over 36,000 people.
But not only were sea-waves produced, but there were also air-waves. Some were so rapid that they produced sounds even at a place 3080 miles away, others were larger and broke in windows and cracked walls a hundred miles away at Batavia, where also lamps were thrown down, gas- jets extinguished, and a gasometer leaped out of its well.
Other air-waves were traced in their course by barometers at various places scattered widely over the surface of the world, and so we know that they travelled several times round the globe.
At Greenwich the depressions of the barometer due to these air-waves were recorded on every day between noon on August 27 to early in the morning of September 1. During the whole of August 27 eruptive action seems to have continued. Three vessels remained beating about in the Straits near the island, and their crews were busy for hours shovelling off their decks the volcanic dust which descended.
The finer particles of dust, however, floated still farther away. Between 7 and 10 in the morning the sky began to darken at Batavia, soon after lamps had to be lit, and a descent of fine dust began about 10.30 and lasted for nearly an hour, when complete darkness came on, the heavy dust- rain continuing till 1, and a less heavy precipitation till 3 P.M.
Complete darkness was experienced still farther to the east at a place one hundred and fifty miles from the volcano.
What happened during the days succeeding August 27 is doubtful; there may have been several small eruptions, but we know that on October 10 there was an explosion of some magnitude, and a large sea-wave. The commotion in the sea due to the cataclysmic convulsion of August 27 travelled far and wide.
Its terrible effects on the neighbouring islands of Java and Sumatra have already been noticed, and, of course, as it passed away from the centre of disturbance it gradually lost its intensity, still, tide-gauges and eye-observations tell us that to the west it was noticed up both the coasts of India, along the south of Arabia, round the south coast of Africa, to the east coast of Central America and Terra del Fuego, while it also spread to the north and reached the coast of France and was recorded at Devonport and Portland.
To the east it spread to Australia and New Zealand, to Alaska and San Francisco.
In Australia sudden rises of 5 and 6 feet were recorded on the west coast; on the shores of Java the rise was one of about 50 feet, while in the English Channel it was only barely noticeable. But if the effect on the sea near England was so slight there was another effect so pronounced that it must have forced itself on the attention of almost everyone in our islands.
The coarser particles which were shot into the air fell close to the island, finer ones rained down, as we have noted, a hundred miles away in Java, but still finer ones, shot up into the air to enormous heights by the terrific explosions, were carried in the upper regions of the atmosphere all round the globe. One effect of these fine particles floating in the air was to give most extraordinary sunset effects on clear evenings.
The writer very vividly remembers some of them he saw in Kent. The brilliant, almost lurid glows in the western sky lasted long after the usual sunset effects would have passed away, and were far brighter than the normal sunset colours.
These splendid sunset effects were noted in Australia, in India, and throughout Europe and America.
In Surrey particularly fine sunset colourings were noted on September 8, and on several occasions between that date and early November. On November 9 the effect was most wonderful and magnificent. Afterglows were recorded throughout November, December, and January, but during February and March the coloration decreased, and after March no peculiar illumination was observed.
Essay # 4. General Course of an Volcanic Eruption:
The general course of a volcanic eruption becomes plain when we consider such cases as those of Vesuvius, Monte Nuovo, and Krakatoa.
A district may have been quiescent for centuries, but then from some cause the earth’s internal heat produces stresses at some point in the crust below that district and the crust gives. Such a crack even though it be exceedingly small in amount causes a tremor, and things on the surface are shaken. It is rare that any actual Crack appears on the actual surface of the ground, but this does occur sometimes.
The molten rock which is pressed up in some cases finds its way along these cracks and may eventually solidify in them without ever reaching the surface; these rock masses are spoken of as Intrusions. At other times water makes its way down to the heated area and, flashing into steam, produces an explosion.
This, if powerful enough, may blow off a huge amount of the superincumbent crust and cause vast damage, but if an opening has once been drilled the explosions are less violent and merely cause the ejection of blocks of stone or masses of molten rock. These may either fall back into the crater or be scattered over the mountain-sides round it.
Such mountains as Monte Nuovo and many of the so-called cinder cones of Central France have been formed in this way and show no other signs of volcanic activity. But in other cases molten rock wells up the central crater and pours over or breaches its cone and flows down its sides. In this way we get lava streams, and these, owing to subsequent periods of explosive violence, may get covered by volcanic ashes.
But the story of a volcano is not quite so simple as this, for as the volcano rises higher the pressure needed to force the column of molten rock up to near its summit must grow greater, and so it often happens that the internal forces find it easier to produce an opening on the sides of the mountain and so get formed those lateral cones, which in some cases are of such frequent occurrence. These may be merely cinder cones or they may pour out molten rock.
The wind which prevails during an eruption sends the ashes this way or that, and so the deposit of volcanic detritus round a volcano is not so continuous as an aqueous sediment. Lavas and ashes may be mixed up, and the lava stream may end off with great abruptness both at its lower end and along its sides. Moreover, when the whole mass has become firm, subsequent shocks may crack it, and into those cracks molten rock may be squeezed, giving rise to what are called dykes.
In general, therefore, beds which owe their formation to igneous activity do not show anything like the same regularity that is seen in the case of aqueous sediments. Moreover, as a rule, volcanic detritus encloses no life, for though the living things, such as the inhabitants of Pompeii, may get killed on the surface of the earth when it has ashes poured down on to it, yet the mass of the ash above contains no life of the period. It may, however, contain blocks of rocks which enclose fossils of their own age, and so be useful in giving something of a clue to the age of the ash.
Fossiliferous Ash Beds:
But there is another possibility; some ash may fall into the sea, and then as it settles it will entomb some of the life of the period, and successive eruptions may build up a great thickness of rock in which afterwards fossils in plenty may be found. Such a deposit will often be in part like a terrestrial ash bed, and in part like a marine sediment, and these volcanic deposits are the most hopeful ones in which to search for fossils.
When denudation gets to work on a volcano it is obvious that loose ash beds will be scoured away with rapidity, and so when the denuding agents get either to the solid plug of frozen lava in the vent, or to streams of lava down the sides, or to the dyke intrusions, these hard rocks will be left as prominences above the softer and more easily eroded beds.
The discussion of vulcanicity is, however, incomplete unless one considers the origin of the molten rock; here we cannot appeal to active or modern volcanoes, as in their cases we can only see surface phenomena. But, after ages of denudation, ancient volcanoes have been dissected, and their roots, as it were, disclosed.
We then find that there was a large mass of molten rock below them which no doubt was compressed at different times and squeezed upwards. These huge masses of liquid material ultimately froze, and so have arisen the enormous masses of granite so well known in parts of our islands. Such a rock is spoken of as plutonic to distinguish it from a molten rock which solidified on the earth’s surface and is called volcanic.
If one asks what caused the pressure one must remember that our earth has a hot interior, as proved by every coal-mine and deep boring, and a cooled crust, and that as the earth slowly gives out its heat it shrinks, and the crust settles on to the shrinking nucleus. This settling down must give rise to enormous stresses, and it is no wonder that cracks appear and molten material is squeezed up them.
Fissure Eruptions:
But though the Vesuvian type of eruption is the one nearest to our doors and the one most commonly found at present on the earth, if we go to Iceland we shall find another and a very different type of eruption allied to what is called the Fissure type.
In Iceland, though there are volcanoes of the Vesuvian type, the most common eruptions take place from a chain of volcanoes arranged along a large fissure. Small ash cones may be built up at points on the fissure and the position of actual eruption moves on so that the rings of the cones intersect, but at times flows of lava pour out from the fissure without any cones at all, flood over the surrounding country, fill up the valleys, and instead of an undulating plain produce a more or less level lava desert. Such was the course of events in 1783 when one of the greatest extrusions of lava in modern times occurred in Iceland.
When such an outpouring takes place, the rivers which afterwards arise cut their way into the lavas and gradually remove them from the older rocks which they overlie or rest against. Ultimately the lava plain may become a plateau scored by streams that wind over it.
This type of volcanic action differs from the Vesuvian type in that it possesses few, if any, ash beds, and in its occurring with great regularity over a vast area.
The fissure type of rock being pre-eminently a lava there seems little hope of finding fossil evidence of its age. But when we remember that extrusion is not continuous and that rivers make their way over the plateau and form gravels, and that these gravels may be covered by a subsequent flow, we see that it is possible for fossil evidence to be found even amongst the lavas of the fissure type. Where the centre of volcanic activity lay beneath the sea the beds due to the eruption will get covered by marine deposits and so evidence will accumulate to give the age of the eruption, if in after times the sea-floor is raised above sea-level.
Essay # 5. Volcanic Deposits in the British Islands:
Now when we examine the rocks of our islands we find repeated evidence that in various periods there existed centres of vulcanicity in many places. The Pre-Cambrian rocks of Charnwood in Leicestershire, of Shropshire, of the Malverns and elsewhere contain undoubted beds of volcanic ash, while when we come to the succeeding Primary rocks we find abundant proof that volcanic action took place during their formation.
North Wales is one of the districts which must again and again have included volcanoes throughout Cambrian and Ordovician times, while on the other side of. St. George’s Channel Ordovician lavas and ashes are found to the north of Dublin and in the Waterford area. Nearer the centre of Ireland such rocks are seen close to Kildare, and in the west, in Galway and Mayo, extensive coarse and fine ash deposits and lavas have been proved to be of Ordovician age.
In South Wales Ordovician lavas and ashes are seen forming Cader Idris, and they occur in Shropshire, in the Lake District, and in Ayrshire. The Ordovician period, in fact, was one, as far as our islands are concerned, of almost universal volcanic activity. It will be noticed that much of our mountainous country owes its ruggedness and its height to the Ordovician volcanic rocks, which by their hardness have been able to withstand denuding forces.
After this widespread outburst of volcanic forces came the Silurian period, and for long it was thought that there was then a complete cessation of the ejectment of ashes and the outflow of lavas. Comparatively recently, however, it has been shown that Silurian lavas and ashes are to be seen in Kerry in Ireland, to the north of Bristol, and in the Mendips, and in South-west Wales. But elsewhere, as far as we know, throughout our islands deposition of marine sediment went on quietly throughout Silurian times.
After Silurian times there came a period of uplift from Central England northwards, and the formation of a series of large sea lochs in which the Old Red Sandstone was laid down.
The tract of land stretching north from the Cheviots contained many active volcanoes. In South Devon also there was much lava poured out and ash accumulated near what must have been the northern margin of a great ocean stretching southwards.
In South Scotland many of the plugs of igneous rocks which filled up old vents in Old Red Sandstone times have proved much harder to wear down than the surrounding ash cones, and so have remained forming hills standing up somewhat abruptly amongst softer strata. This volcanic tract of Argyllshire is continued to the west in Ulster.
To the south of this line volcanoes of Old Red Sandstone age occurred in the Pentland Hills, where numerous “necks ” of this Age are found. These form the western end of a long line of active volcanoes which stretches westwards to the Ayrshire coast.
The upper Old Red Sandstone beds but rarely enclose evidence of igneous activity, but in Carboniferous times once more the subterranean forces burst into vigorous action.
Carboniferous volcanoes were most abundant in Scotland, and they persisted from the lowest beds to the basement beds of the coal-measures. They were distributed over the central valley from the south of Kintyre to the Firth of Forth.
In England Carboniferous volcanic rocks are seen in Derbyshire, in the Mendips, and near Weston-super-Mare. In the Isle of Man there are the relics of a group of Carboniferous vents, but in Ireland the only evidence of activity during this period is in King’s County and near Limerick.
In Scotland the predominant type of outflow of lava is of the plateau type, but the cinder-cone type of volcano is seen abundantly in the Firth of Forth region and forms the regular type seen elsewhere in the British Isles.
The plateau type of eruption built up enormous sheets of igneous rock which now form the prominent escarpments of many hills in South Scotland, and the vents up which the molten rock rose were plugged up by the frozen lava, and now remain as numerous prominent hills rising abruptly above the softer sandstone around them. Instances of these hills are North Berwick Law and the Bass Rock.
Eventually, but before the close of the Carboniferous period, the plateau extrusions were submerged and buried under the Carboniferous limestone series, and then a new type of eruption began, the cinder-cone type. The relics of these cones are abundant in the Carboniferous beds of Scotland.
After a very considerable time earth movements ridged up the coal-measures and formed a series of inland seas, and in the Permian period we find feeble and short-lived volcanoes in Ayrshire, in East Fife, and in South Devon. In Ayrshire there are several volcanic necks, which descend vertically through the surrounding rocks and form vertical columns of volcanic material. This material is usually a coarse ash, but in some cases molten rock has risen up the vent.
In East Fife, along the shore near St. Andrews and Elie, there are abundant small necks piercing the Carboniferous strata. That they are older than the beds they pierce is proved by the blocks which fill them being of the same mineral character as the Carboniferous beds, while they also contain the same fossils.
At Largo a coarse volcanic ash lies unconformably on the ridges of the newest Carboniferous beds, and though no Permian sedimentary beds are known in this district, these volcanic beds, being post- Carboniferous, are considered to be of Permian age.
In East Fife some sixty necks can be seen, and the larger ones form conspicuous hills such as Largo Law.
In Devonshire lavas and volcanic ashes are found near Exeter, but, as in Ayrshire, there is no thick accumulation of the ashes. In Devon, however, the volcanic activity was far feebler than in Scotland.
The long story of Igneous activity in our islands now comes to an end for an enormous length of time. It is to be noticed that a region of vulcanicity frequently continues to be one throughout the Primary period. Thus in South Scotland we have volcanoes in Cambrian and Ordovician times, then came a rest during Silurian times, which was followed by a renewed outburst on a gigantic scale in the Old Red Sandstone and Carboniferous periods; this activity then became feebler, and though we do find evidences of volcanic action there in Permian times, the volcanic forces were evidently dying out.
After this for untold ages throughout the whole of the Secondary period came silence. No explosions rent the ground, no faulting caused earthquakes, there was no ejectment of lava, the Jurassic period passed and was succeeded by the Cretaceous, and bed after bed was laid down over the greater part of our island. Time after time parts of the land were submerged or were upraised, but of all the enormous thickness of rock which accumulated not one inch is of volcanic origin.
Then we enter Tertiary times, and then once more the subterranean forces got to work and produced, effects on a grander scale than had ever before been witnessed in our island region.
The Tertiary Fissure-Eruptions:
The type of eruption which was characteristic of this Tertiary period of activity was the fissure type; very occasionally there was a small amount of crater building, as is proved by the occurrence of ashes, but the mass of igneous rock extruded was through almost innumerable fissures.
The tract of ground covered by these lavas was immense, and though the outpouring took place in Tertiary times the effect of marine and surface denudation has been so pronounced that the vast original plateau is now represented by mere scattered fragments above sea-level. The chief places where it is to be seen are in Antrim, in Staffa and Iona, in Mull and in Skye, but the plateau extended northwards to the Faroe Islands, and the Iceland volcanoes of to-day are the sole remaining places where the eruptive forces still effect the extrusion of molten rock as they did in days gone by.
The lava is of the type called a basalt, an almost black rock and one which frequently gives rise to a columnar structure, as is well seen in Staffa. This basalt rests on chalk in the Antrim district, but in Skye is found on much older rocks.
This basalt, which builds up huge thicknesses of rock in this north-western area, must not be imagined as coming out in one continuous flow. There were pauses between extrusions, and during those pauses rivers flowed over the plateaus, cut channels in them and produced beds formed of rolled blocks of the lava and of other rocks brought down from the hills; then more molten rock flowed out and buried these channels and their beds of conglomerate or gravel.
Thus we find in amongst the lavas water-formed beds, and some of them in Antrim and in the islands off the coast of Scotland have yielded plant remains, leaves and wood and a portion of a fresh-water fish.
After the building up of vast thicknesses of rock by repeated outwellings of lava there came a later period when igneous rock of a somewhat similar nature was intruded in sheets amongst the lavas. This rock withstands the action of the weather better than the softer basalts, and so as the basalts are now as a rule more or less horizontal, the two sets of rocks produce a very definite type of scenery.
The hills which have been carved out of them are flat-topped, and the sides are ringed with a series of horizontal scarps with almost vertical edges, these being formed by the hard intrusions. As the basalts on weathering form fertile ground, the slopes below the dark cliffs of intrusive rock are commonly covered by rich green grass.
Such scenery is well seen along the west coast of Skye and any one sailing up the west coast of Scotland will again and again see instances of it.
A consideration of the height to which the basalt plateau now rises enables one to see what an enormous amount of denudation has gone on since the Tertiary time, during which the lavas were poured out.
Originally the plateau extended from the island of Mull to the mainland to the north-east, where the promontories of Morven and Ardnamurchan are now seen. But between Mull and the mainland the Sound of Mull now exists twenty miles long and two miles broad. From the deepest part of the Sound to the top of the plateau in Mull is nearly 4000 feet, and the huge mass of basalt which used to occupy and cover the present Sound has been completely washed away.
Elsewhere the same story is told, and we must imagine that in Tertiary times a huge plateau of Basalt stretched from Antrim up between the present west coast of Scotland and the Hebrides, and beyond to the Faroe Islands.
From this general consideration of the history of British vulcanicity throughout past geological ages certain general facts become obvious.
The regions of vulcanicity in our islands all lie to the west of a line from the most south-easterly point of Scotland to Exeter. In this western region volcanic activity has persisted from the very earliest times of which we have any knowledge down to Tertiary times. Not only is this general persistence to be noted, but also the fact that particular portions of the volcanic region have been the sites of recrudescence of action again and again.
In the south-west of England we find volcanoes in Silurian, in Old Red Sandstone, in Carboniferous, and in Permian times.
In the south of Scotland we find plenty of evidence of vulcanicity all through the Ordovician period, in the Old Red Sandstone, in Carboniferous, and in Permian times, while there are also plenty of dykes there originally full of molten rock which rose up the cracks in the Tertiary period.
But not only are certain well-marked areas again and again the scenes of volcanic action, others are quite as conspicuous for the absence of that action.
The Central Highlands of Scotland, though they abut on to the Old Red Sandstone, Carboniferous, and Permian volcanic areas to the south, themselves contain no traces of vents. The southern uplands, almost surrounded by volcanic rocks of various ages, are themselves perfectly free of any opening through which either lavas or ashes were sent out.
There seem to be, therefore, certain regions which are regions of weakness through which the volcanic forces were able to make openings to the surface, and these regions remained weak even though at one time or another those forces failed to continue ejecting matter through them.
Another point to be noticed is that we have no evidence of any slackening of the forces which produce volcanic action, and a further point is that the types of vulcanicity now known as existing at the present day have been seen again and again during the history of the world.
The volcanoes such as Vesuvius can be matched in the Primary period, during which, for instance, material in our English Lake District was accumulated to a depth of some 8000 feet.
The small ash-cones of the Neapolitan area are strikingly alike to the Tertiary cones of the Puy de Dôme district in France, and to the small cones of the Carboniferous period as seen in Scotland.
Lastly, the modern eruptions of Iceland resemble the Tertiary fissure-eruptions of North-east Ireland and West Scotland, and the Carboniferous fissure-eruptions of South Scotland.
Although the Tertiary outburst proves that as a whole the volcanic forces had by that time suffered no diminution, yet when one traces the history of volcanic action it is seen to have waxed and waned.
The enormous outpouring of lavas and ejection of ashes in Ordovician times was followed by an almost complete cessation of volcanic action in the following Silurian period. The succeeding Old Red Sandstone period gives evidence of a vulcanicity somewhat less pronounced than that of Ordovician times, and is succeeded by smaller outpourings in the Carboniferous period, while the following Permian volcanoes are comparatively few and unimportant.
Then for the whole of the Secondary period there was a complete absence of any vulcanicity at all. Then in the older times of the Tertiary age came a period of great violence, which died down, and from that time to this the sound of a volcanic outburst has been unheard in our islands. But the history of our islands shows that from such a long spell of quiescence we cannot draw the conclusion that those sounds will never be heard again.
The time may come when Scotland, or Wales, or the Midland district of England will be deluged with lava, or their lands desolated by downfalls of volcanic ash, when Glasgow or Birmingham may repeat the story of Pompeii, and future ages, by excavation at those places, may learn of the civilisation of the twentieth century.
Essay # 6. Distribution of Volcanoes:
Almost all the active volcanoes are in the young folded mountain region, and the fault zones of Africa. There are more than 1000 volcanoes in the world and out of these about 500 are active or dormant.
These are distributed in three belts:
(а) Circum-Pacific Belt:
There are 403 active volcanoes in the belt bordering Pacific Ocean. This belt is known as the Fiery Ring of the Pacific. Some important volcanoes like May on, Fuji Yama and Cotopaxi are in this belt.
(b) Mid-World Mountain Belt:
This belt is extended from the Mediterranean Alps to the Himalayan region. Visuvius, Stromboli, Barren Island, Krakatoa, etc., are some of the best known volcanoes of this belt. There are 83 active volcanoes in this belt.
(c) African Rift Valley Belt:
This belt goes from Palestine in the north to Malagasy Island, through the East African region. The Kilimanjaro in Tanzania is a well-known dormant volcano of this belt.
Over and above these, some scattered volcanoes occur in some islands of the Pacific, Atlantic and Indian Oceans.
Essay # 7. Effects of Volcanic Activity:
Like earthquake, volcanic activity also changes the features of the surface of the earth. These changes are both destructive and constructive.
Great calamities take place due to volcanic activity. Often volcanic eruptions bury many beautiful towns. For example, Harculium and Pompii, the two beautiful Italian towns were completely buried by the erupted materials of Visuvius. Volcanic eruptions may be violent if it originates under sea. It causes strong waves which are highly destructive to life and settlements of coastal regions.
On the other hand, volcanoes may have some good effects. It sometimes creates extensive basaltic plateaus. Generally, the volcanic region is rich in minerals and the volcanic soils are fertile.