VOLCANISM

INTRODUCTION

A volcano is the vent through which molten rock material and associated gases pass upward from the earth’s interior onto the earth’s surface. The term volcano also refers to the mountain built by these materials. Strictly speaking, the volcano is defined in terms of the existence of the vent not the piling up of the material. The volcano becomes active the moment gases start to come out of the vent. Some active vents never discharge anything but gases and thus do not affect the landscape in any significant way. (Figures from GOH)

VOLCANISM

The movement of hot, liquid rock below and above the crust is called Volcanism.

FORMATION OF VOLCANOES

Beneath its cool outer crust, the earth is glowing hot. Scientists believe that temperatures inside the earth keep increasing with depth. For every kilometer beneath the surface, the temperature increases about 30oC. The temperature may reach 5,000oC or more at the core.
Tremendous heat below the surface causes rock to melt into Magma. Magma is hot liquid rock below the surface. Magma seems to form at 40 to 60 kilometers below the surface. Below that depth great pressures prevent rock from melting. Thus Magma forms in the lower crust and upper mantle. Melting takes place in “Magma Chambers”, areas where heat builds up or pressure is lowered.
When solid rock melts into magma, it expands. It also becomes less dense than surrounding rocks. Because of its lower density, magma rises up from the chamber through cracks in solid rock. It forces the cracks to widen. In time, magma breaks through the surface and erupts. When magma reaches the surface, it is released as lava. Volcanoes build up where lava spews up (gushes out) through cracks in the crust.

PRODUCTS OF VOLCANOES

The complex and variable products of volcanoes include liquids, solids and gases. Usually the gases and molten liquid are mixed together when they leave the vent and separate in the atmosphere. In some cases the gases are mixed with solids. The key process of volcanic activity is the separation and escape of gases from their associated liquids and solids.

Volcanic Liquids

The liquid material is the most important and is known as the Lava. This is nothing but molten rock and is composed of minerals. Accordingly, as it consists of larger or lesser proportion of Silica, lava is said to be of two types:

a. Acid Lava
It is light in color and weight, and usually melts at a relatively higher temperature. It is a thick fluid and moves slowly.

b. Basic or Basaltic Lava
It is heavier in weight and darker in color. A relatively lower temperature is required to melt it and being thinner in texture or more liquid it flows at a faster speed.

Volcanic Solids

The dust particles and cinder (ash) fragments constitute the solid material thrown out by the volcanoes. Solid material is only thrown out during explosive eruption. The solid material is derived partly from the cooling of the lava fragments in the air, partly from the blowing off of the lava crust plugging the crater and partly from the breaking off of the rim of the crater itself.

a. Volcanic Dust
The very fine particles thrown out by the volcanoes are known as the volcanic dust. It is gray in color and very light in weight.

b. Lapilli
Small lava fragments that rain on the rim of the volcano are known as the Lapilli or Scoriae (volcanic ash / remains of what is burned) or Pumice Lones. These are as big as finger joints and usually angular in shape.

c. Cinder
The solid particles that are smaller than lapille are known as volcanic ash or cinder. They are like peas.

d. Breccia
When the solid angular fragments are bigger than finger joints, they are known as Breccia and fall like projectiles.

e. Volcanic Bomb
Sometimes the lava thrown into the air solidifies into a round mass before falling to the earth; this is known as volcanic bomb. This solid material is most common when volcanism is explosive and violent.

Volcanic Gases
Liquid and solid material of volcanoes is usually accompanied with a large amount of gases. As soon as the eruption has taken place, a dark cloud of smoke rises and mounts above the crater. From the very shape of this cloud, it is known as Cauliflower Cloud. This cloud consists of several types of gases, the chief among them being steam or vapor. Next in importance are the inflammable gases like Hydrogen, CO2, and SO2. Ammonium Chloride is also found in sufficient quantity. These gases seldom produce any visible effect on the volcano itself but they get dissipated in the atmosphere and effect the rocks that lie in the neighborhood.

TYPES OF VOLCANOES
The volcanoes of the earth have different shapes and sizes. Their shapes and sizes depend upon the material that has been thrown out and has gone into their formation. Invariably all the volcanoes have a conical shape with the crater fixed as a bowl at the apex of the cone. According to the material that has gone into the formation of a volcanic cone, they may be said to be of the following three main types:

1. Cinder Cones
Smallest of the volcanoes are the cinder cones, built entirely of the pieces of solidified lava thrown from a central vent. They form where a high proportion of gas in the molten rock causes it to froth (fine bubbles) into a bubbly mass and to be ejected from a vent with great violence. The froth breaks up into small fragments which solidify as they are ejected and fall as solid particles near the vent. [Strahler 508]
Cinder cones rarely grow to more than 150 to 300 m in height. Growth is rapid. Monte Nuovo, near Naples, Italy, grew to a height of 120 m in the first week of its existence. The angle of slope of a recently formed cinder cone ranges between 26o and 30o. So loose is the material that it absorbs heavy rain without permitting surface runoff. Erosion is thus delayed until weathering produces a soil which fills the interstices (cracks).
Cinder cones may erupt in almost any conceivable topographical location, on ridges, on slopes, and in valleys. Cinder cones usually occur in groups, often many dozens in an area of a few tens of square miles. They sometimes show an alignment parallel with fault lines in the underlying rock.

2. Composite Volcanoes
Most of the world’s great volcanoes are composite cones. They are built of layers of cinder and ash alternating with layers of lava, and for this reason have been called Strato-Volcanoes by some writers. The steep-sided form is governed by the angle at which the cinder and ash stands, whereas the lava layers provide strength and bulk to the volcano (Strahler 508). Among the outstanding examples of recently formed Composite Volcanoes are Fujiyama in Japan, Mayon in Philippines, Mt. Hood in Oregon, and Shishaldin in the Aleutians. Other famous ones, less perfectly formed, are Vesuvius, Etna and Stromboli in Italy and Sicily. Heights of several thousand feet and slopes of 20o to 30o are characteristics. Many composite volcanoes lie in a great belt, the Circum-Pacific Ring, extending from the Andes in South America, through the Cascades and the Aleutians, into Japan; thence (from there) south into the East Indies, and New Zealand.
The eruption of large composite volcanoes is usually accompanied by explosive issue of steam, cinders, bombs, ash and by lava flows. The crater may change form rapidly, both from demolition of the upper part and from new accumulation. (Strahler 509)

3. Lava Dome or Shield Volcanoes
A very important type of volcano differing greatly in form from other types, is the Lava Dome or Shield Volcano. The best examples are from Hawaiian Islands, which consist entirely of lava domes.
Lava domes are characterized by gently rising, smooth slopes which tend to flatten near the top, producing a broad-topped volcano. The Hawaiian Domes range to elevations upto 4,000 m above sea level. In width they range from 16 to 80 km at sea level and upto 160 km wide at the submerged base.
Lava domes, as the name implies, are built by repeated out pouring of lava. Explosive behavior and emission of fragments are not important as they are for cinder cones and composite cones. The lava is highly fluid and travels far down the low slopes, which do not usually exceed 4o or 5o.
Instead of the explosion crater, lava domes have a wide, steep-sided central depression or sink, which may be 2-3 km or more wide and several hundred feet deep. These large depressions are a type of Caldera produced by subsidence (ebb) accompanying the removal of molten lava from beneath. Caldera is the large opening with a flat floor formed due to violent explosion destroying the crater. (Gupta 262-263).
The lava domes are quite common in Germany, Central France and the Pacific Ocean Region. (Strahler 513)

TYPES OF VOLCANIC ERUPTIONS

According to the force with which the materials are thrown out, eruptions are said to be of three types:

1. Fissure Eruptions
These are those eruptions in which lava quietly wells-up through numerous fissures and spreads over large portions of the earth. There is no violence in it. The lava comes up and floods the country so that in the course of years the lava may grow to a thickness of several hundred feet. The Deccan region of India and the Snake River Region of USA are typical examples where such eruptions took place. It were such eruptions that took place in the early days when the earth was cooling. At that time the earth’s crust was thin. Hence magma could easily break through parallel fissures by melting the surface or the crust. The lava that issued forth through such eruptions spreads over and obliterates (levels) large valleys and mountains.

2. Tranquil or Effusive Eruptions
These are those eruptions that belong to the second stage of the cooling of the earth. The crust had cooled and its outer boundary had become sufficiently thick so that it was not possible for the magma to break thorough at any point. The eruptions took place at selected sites through deep fault planes and round the orifice (outlet) was formed a lava dome. In these also, there is no violence. The magma is seething in the channel and then bubbles up like froth (lather) or scum (foam). The volcanoes of Hawaii, Samoa, and Iceland are of this type.

3. Explosive or Central Eruption
This type of eruption is characterized by great tremor and violence so that fearful clouds are thrown up in the sky followed by a rain of angular fragments and then the jetting out of the lava. Such eruptions cause earthquakes and produce a fearful atmosphere all around. But they are not so important in shaping the relief of the earth. The bulk effects are also not very significant. Such grand eruptions that are accompanied with lightening, thunder, rumbling, and explosions are very common in Mount Vulcano. Therefore they are known as Vulcanian Eruptions. (Gupta 262)
These represent the third stage in the cooling of the earth. When the crust of the earth became sufficiently thick, it was not possible to break through even marked lines of weakness. Hence the magma and gases go on being accumulated inside till their force becomes so much that they blow off the plug and shoot out with great violence. Mount Etna of Sicily, Vesuvius of Italy, and Fujiyama of Japan are typical examples.

ENVIRONMENTAL ASPECTS OF VOLCANOES

Volcanic eruptions count among the earth’s greatest natural disasters. Wholesale (massive) loss of life and destruction of towns and cities are frequent in the history of peoples who live near active volcanoes. Loss occurs principally from sweeping clouds of incandescent (glowing from heat) gases that descend the volcano slopes like great avalanches; from lava flows whose relentless (ruthless) advance engulfs whole cities; from the descent of showers of ash, cinders, and bombs; from violent earthquakes associated with the volcanic activity; and from mud flows of volcanic ash saturated by heavy rain. For habitations along low lying coasts, there is the additional perils (risk) of great seismic sea waves, generated by submerged earth faults.
The surfaces of lava flows and volcanoes remain barren and sterile (infertile) for long periods after their formation. Certain types of lava surface are extremely rough and difficult to traverse (cross); the Spaniards who encountered such terrain in the southwestern United States named it Malpais (bad ground). Most volcanic in time produce highly fertile soils that are extensively cultivated. (Strahler 514)
Volcanic ash may have remarkably beneficial effect upon the productivity of soil where the ash fall is relatively light. The eruption of Sunset Crater, near Flagstaff, Arizona, in 1065 AD, spread a layer of sandy volcanic ash over the barren reddish soil of the surrounding region and caused it to become highly productive because of the moisture-conserving effect of the ash, which acted as a mulch (moisture trapping sheet) in the semi-arid climate.
Young and mature volcanoes possess most of the natural resources of rugged (irregular, hilly, rocky) mountains of other types. Steep slopes prevent extensive agriculture, although providing valuable timber resources. Thus the San Francisco Mountains, a group of maturely dissected volcanoes in Northern Arizona, are clothed in what is perhaps the finest known Western Yellow Pine Forest (ponderosa pine).
As scenic features of great beauty, attracting a heavy tourist trade, few landforms outrank volcanoes. National parks have been made of Mt. Rainier, Mt. Lassen, and Crater Lake in the Cascade Range. Mt. Vesuvius and Fujiyama also attract many visitors.
Mineral resources, particularly the metallic ores, are conspicuously lacking in volcanoes and lava flows, unless later geologic events have resulted in the injection or diffusion of ore minerals into the volcanic rocks. The gas-bubble cavities in some ancient lava have become filled with copper or other ores. The famed Kimberlite Rock of South Africa, source of diamonds is the pipe of an ancient volcano.
As a source of crushed rock for concrete aggregate or railroad ballast, and other engineering purposes, lava rock is often extensively used. Thus, the ancient lava layers that make up the Watchung Ridges of Northern New Jersey have in places been virtually leveled in quarrying operations continued over several decades.

DISTRIBUTION OF VOLCANOES
Volcanoes are not scattered irregularly over the globe. Most of those that are now active lie within certain well defined belts and by far the greater part of the earth has been free from volcanic action since man appeared upon its surface. These belts coincide to a large extent with the belts of crumpling (wrinkle), which have formed the great mountain ranges of the present day, but the coincidence is not complete. There are, for instance, no volcanoes in the Himalayas, and on the other hand there is no sign of recent folding in Iceland. The chief volcanoes of the world are found in following belts:

1. Circum Pacific Ring
This belt consists of the western coasts of the Americas, the Kuerile Islands, Japan, the Philippines, Indonesia and New Zealand. At the present day, it is upon the borders of the Pacific that volcanic activity reaches its maximum development. A line of great volcanoes may be traced up the Andes and through Central America and Mexico. In the United States and Canada, there are a few active vents, but in the ranges of the west, there are many which have not been long extinct. Living volcanoes reappear in Alaska and the line is continued through the Aleutian Islands, Kamchatka [Atlas 30-T4], the Kuril Islands [Atlas 30-S6], Japan, Formosa [Atlas 30-Q7], and the Philippines to the Molucca Group [Atlas 31-R10].
Another belt of volcanic activity runs through Sumatra, Java and the Sunda Islands, [Atlas 30] generally Barren Island in the Andaman and some extinct volcanoes in Burma mark the northwesterly termination of the belt, while towards the east it is continued, with several interruptions, through New Guinea [Atlas 40-E6], the Solomon Islands [Atlas 40-F6], the New Hebrides [Atlas 40-H7], and New Zealand to Mount Erebus on the Antarctic Continent.
The islands in the midst of the Pacific are all either volcanic or made of coral, and in many of the groups eruptions still take place. There are active volcanoes, for instance, in the Sandwich Islands, the Tonga Islands, and the Samoa Group [Atlas 40-J7]. The Fiji Islands are an example of a group which is of volcanic origin, but in which volcanic action has now ceased.

2. Eurasian Belt
This belt consists of Italy and the eastern Mediterranean Region. It spreads through Caucasia, Armenia, Persia, Baluchistan, Burma and finally terminates in the East Indies or Indonesia [Atlas 30-H5 to P]. The great belt of folding which runs from west to east across Europe and Asia is also associated with volcanic activity. In Italy and the neighboring islands Vesuvius, Etna, Stromboli (lava dome, pipe type crater), and Vulcano (lava dome, funnel type crater) [Atlas 22-F4,5] are still active. Santoria in the Greciah Archipelago has been the scene of many out bursts. Farther east, there are numerous volcanoes of gigantic size, but they are either extinct or in the solfatara stage. Ararat [Atlas 30-H5], for example, and many other mountains in Armenia and Asia Minor are volcanic. Elburz [Atlas 30-J6] and Kazbek in the Caucasus and Demavend [Atlas 22-J6] in south of the Caspian are also old volcanoes. In the region where the boundaries of Iran, Afghanistan and Baluchistan meet, there are several cones of considerable size and one or two of them emit steam and other gases. In the time of Humboldt, active volcanoes were said to exist in the great mountain chains of Central Asia.

3. Atlantic Belt
This belt consists of West Indies, and the Islands of Eastern Atlantic from Iceland to St. Helena [Atlas 36-C7] which are all of volcanic origin.
A shorter line of volcanoes, also associated with recent folding of the earth’s crust, occurs in the West Indies, where the Lesser Antilles [Atlas 18-D1] are largely volcanic. Iceland is the last surviving remnant of a great volcanic area which in earlier times extended from Greenland to the north of Iceland. The Azores [Atlas 36-A2], Madeira [Atlas 36-A2], Cape Verde Islands [Atlas 36-B4], and Canary Islands [Atlas 36-B3] are all volcanic, but the volcanoes for the most part are now extinct.

4. Africa
In Africa there are a few volcanic centers. In the Cameroon there was an eruption in 1909. A considerable number of volcanoes lie in or near the great Rift Valley in Uganda [Atlas 36-G6], which extends from Jordan down the Red Sea and through the east of Africa. Kenya and Kilimanjaro [Atlas 36-G6] are volcanoes. A small cone south of Lake Rudolf has recently been in eruption and also one or two south of Lake Nyanza, Kenya. There are many volcanic cones in Madagascar. The Grand Comoro Islands [Atlas 36-H7] has been active several times since the island were discovered. In Reunion Island (France) the Piton de la Foumaise is still frequently in eruption.

5. Indian Ocean
Mauritius and man other islands in the Indian Ocean are volcanic but now extinct. Far to the south of Kerguelen Island there are still signs of activity.

VOLCANIC LANDFORMS

Introduction

The popular concept of a volcano is a conical mountain with a circular base and a pointed top. Indeed, many volcanoes have built features matching this image. However, depending upon the nature of the products and how they accumulate, volcanoes build a variety of landforms. These volcanic landforms include:

Volcanic Plains and Volcanic Plateaus
Flat sheets of extrusive igneous rocks are called volcanic plains and volcanic plateaus. These features are formed from extremely fluid shape mafic lava (consisting of silicates rich in iron and magnesium) and or from mixtures of particles dispersed in volcanic gases that were highly mobile and flowed with great speed as if they were liquid. If their lava issued from fissures, they formed on the earth’s surface through fissures eruptions.
Volcanic plains and volcanic plateaus are usually regional in extent. A volcanic plain or volcanic plateau can cover more than 250,000 square kilometers. After thousands of individual eruptions, the thickness of the layers of extrusive rock may reach about a kilometer. Thus, the volume of extrusive igneous rock underlying a volcanic plain or a volcanic plateau can reach 400,000 cubic kilometers.
The difference between a volcanic plain and a volcanic plateau is the height of the surface compared to the surrounding countryside. A volcanic plain is a low-lying area in which the volcanic rocks usually are thin. A volcanic plateau is a high standing area usually built by the accumulation of thick sheets of extrusive igneous rocks.

Welded Tuff
In many eruptions of felsic lava (silicates consisting of quartz and the feldspar), the continuity of the liquid phase is disrupted by the violently escaping gases. As a result, hot blebs (pockets) and clots (lumps) of lava form a gas-and-particle mixture known as a Tephra Flow. In many ways, tephra flows behave as if they were very fluid, fast-moving lava flows. They can spread many kilometers from their vents. Tephra flows have been clocked at speeds of 30 to 100 km/hr.
The temperatures within hot tephra flows may exceed 1000oC; the flows char (burn) trees and wooden buildings and may even melt glass and metals. Tephra flows tend to build up flat-lying layers of particles that may be still so hot, when deposited, that they melt together as they settle forming rock known as Welded Tuff.

Volcanic Shields
Repeated, quiet eruptions of highly fluid mafic lava from a circular vent or from a rift zone may create a broad, gently sloping conical mound of volcanic rock. Because of their resemblance to the shields of ancient warriors, these features have been named volcanic shields. Although the term volcanic shields was first applied to examples in Iceland, the largest examples are the Hawaiian Islands and other Mid Pacific Islands. (Strahler 514)

Volcanic Cones
The most familiar of volcanic landforms are volcanic cones. several varieties are recognized depending on the materials composing them:

Tephra Cones
These consist of fine grained, usually uniformly sized tephra that were ejected from a circular volcanic vent, fell back as solids, and were piled up surrounding the vent. The angle of the sloping sides of nearly every tephra cone in the world is about 30o. Tephra cones rarely are higher than about 450m. This height is usually attained over a period of several months, but it may be reached in as short a time as a few weeks.

Spatter Cones
Eruptions of small clots of liquid lava (spatter/splash) may build small, steep sided spatter cones. Rarely exceeding 30 m in height, a spatter cone looks as though it were built by someone standing in a deep hole and hurling blobs of wet cement up onto the ground. The hot droplets of spatter tend to stick to one another and become welded together. This yields a structure that is more rigid than a tephra cone. Welded spatter may form walls that are nearly vertical.

Complex Volcanic Cones
These are composed of sloping layers of tephra and reinforcing layer of extrusive igneous rock. In these cones, lava from the central vent pushes outward through the sides of the cones as flank eruptions. Eventually, the cone’s sides may be so strengthened by the sheets of igneous rock that it is possible for lava to rise to the top of the cone and to issue forth as summit eruptions.

Composite Volcanic Cones
These are combinations of complex cones built atop volcanic shields. The tephra cone could be the beginning stage in the construction of a composite cone. If the tephra cones were eventually reinforced to become a large complex cone and if layers of highly fluid lava issuing at the base accumulated to great thickness and formed a volcanic shield, the result would be a composite cone. Great conical peaks, such as Fuji in Japan, Vesuvius in Italy, Rainier in Washington, Mayon in the Philippines, and Shasta in California, all famed for their beauty and symmetry, are composite volcanic cones. The secret of such symmetry is eruption through a single central vent. If a second vent becomes activated or if flank eruptions should accumulate, secondary cones may form and destroy the symmetry.

Crater
A circular volcanic orifice having a diameter less than 1.5 km is a Crater. It is opened up by a concentrated jet of hot gases escaping upward through bedrock. Many craters are situated at the apexes of volcanic cones. such craters may enlarge as a result of explosive eruptions or as a result of collapse following the withdrawal of lava from the cone.

Calderas
(Strahler 509) One of the most catastrophic of natural phenomenon is a volcanic explosion so violent as to destroy the entire central portion of the volcano. There remains only a great central depression, termed a Caldera. A portion of the upper part of the volcano is blown outward in fragments, whereas most of the mass subsides into the ground beneath the volcano.
Krakatoa, a volcanic Indonesia, exploded in 1883, leaving a great caldera. It is estimated that 75 km3 of rock disappeared during the explosion. Great seismic sea waves, or tsunamis, generated by the explosion killed many thousands of persons living on low coastal areas of Java and Sumatra.
A caldera formed on an oceanic island may create an enclosed circular body of water that could be used as a harbor for ships. Calderas differ from crater not only in size but also in their relationship to the life histories of volcanoes. Craters are active during most eruptive cycles. The world’s largest modern caldera is at Mt. Aso, Japan; it measures 16 by 22 kilometers. Another large caldera is Morogoro in Tanzania, Africa, which is 18 by 16 kilometers; its floor lies 600 meters lower than its rim. A caldera of Pleistocene age, measuring 68 by 45 diameters, has been identified at Yellowstone National Park.