Our Planet I. Internal Structure of the Earth: From the results of seismology, we can obtain the picture of the broad structure of the earth as a whole given in the figure below. The three main units are the crust, the mantle and the core. 1. The Crust: The outermost and thinnest layer of the lithosphere is called the crust. The thickness of the crust varies greatly between the ocean basin (as little as 13 km in places) and continents (possibly 30-50 km thick under certain mountains). The specific gravity of the crust ranges from 2.5 to 3.4. The following figure and table indicate the variation of density with depth in the earth. Depth Discontinuity Layer Composition State Density (oC) (km) Upper Crust Granitic Solid 2.7 (SIAL) 17-25 400 Conrad Lower Crust Basaltic Solid 3.0 (SIMA) 32-38 Temperature 600- Mohorovicic 1000 Mantle Olivine Solid 3.3 5.3-8.0 2900 Gutenberg 1500Outer Core NIFE Liquid 9.0-10. 5000 11.5 11.7 5120 Lehmann Inner Core NIFE Solid 10-18 1900-6000 Crustal rocks vary not only in thickness and density, but also in composition. Those beneath the ocean basins are heavier than those which underlie the continents. They have been called SIMA because they are rich in iron, silicon, and magnesium. Therefore, they have a greater specific gravity/density of about 2.9. The material comprising the continental crust appears to occur in two distinct layers. The upper layer is essentially granitic in nature. Since these rocks contain a high percentage of silicon and aluminum, they are often referred to as SIAL. This layer is 16-24 km thick and has a density of about 2.65 to 2.7. The lower layer, also about 16-24 km thick, appears to be composed of simatic rocks similar to those underlying the ocean basins. They are often referred to as SIMA. The boundary of SIAL and SIMA is called "Conrad Discontinuity". Figure below shows the relationship between surface features and crustal structure. The base of the crust is marked by a rather clearly defined break called the "Mohorovicic Discontinuity" or the MOHO. This sharp boundary lies 32-48 beneath the surface. Here the velocities of P and S Waves are somewhat accelerated (from about 5 to 8 km per second), implying change in density in the rocks below the HOMO. 2. The Mantle: Beneath the "Mohorovicic Discontinuity" there is an 2880 km thick intermediate zone called the Mantle. The velocities of P and S waves increase gradually upon entering this zone. Their behaviour suggests that the mantle increase in density with depth. The specific gravity of the rocks in this zone ranges from 3.3 (in the upper part of the mantle) to as much as 8.0 at the bottom. The Upper Mantle is a "Low-Velocity Zone (LVZ)". When P and S waves pass through this layer, the velocity is reduced markedly. Magma is found in this layer. "LVZ" is also called "Asthenosphere". 3. The Core: The earth's core is the region extending downwards from the core-mantle boundary at 2900 km. The boundary is called the "Gutenberg Discontinuity". Generally, core is only 15% of the total volume of the earth, but it is 32% of the total mass of the earth. The core with a diameter of about 3475 km, is very hot (about 3700oC), dense and under tremendous pressure. It has been divided into two parts: an exterior "Outer Core" (Liquid); and an "Inner Core" which is believed to be solid. The outer core is believed to be fluid because the material in this zone will not transmit S waves, and P waves travel at reduced velocity. The outer core is believed to be about 2220 km thick; the materials composing this zone have specific gravities of 12.0 or more. The inner core, with a diameter of approximately 1255 km, is believed to be solid. This is suggested by the fact that there is an abrupt increase in the speed of P waves deep within the core. It has also been suggested that the inner core may be composed largely of nickel and iron (NIFE). These rocks are presumed to be quite heavy; some may have a density of more than 17.0. II. Theory of Continental Drift: 1. Development: In the 17th and 18th centuries, when the outlines of the continents had become sufficiently well known, many scientists, such as Francis Bacon (1620) commented on the similarity of the shaped of the coastlines on either side of the Atlantic. By the end of the 19th century, the geology of the southern continents was sufficiently well known for an Austrian geologist, Eduard Suess, to suggest that Africa, South America, Australia and India were once part of a super-continent, Gondwanaland (after a region in India called Gondwana). In 1910, a German meteorologist, Alfred Wegener, proposed that at the beginning of the Mesozic era (200 million years ago) Gondwanaland was united with Eurasia and North America to form a single super-continent, which he called Pangaea which was surrounded by a primeval sima-floor ocean (Panthalassa). Wegener proposed that this vast continent began to break up at the beginning of the Mesozoic era by a process of lateral crustal movement, forming a northern continent (Laurasia) and a southern continent (Gondwanaland) separated by a long narrow ocean known as Tethys. 2. Evidences To support his theory, Wegener marshalled considerable evidences: i) The fitness of the edges of continents: The real edge of a continent lies somewhere on the continental slope, where the sea bottom falls comparatively rapidly from somewhere near sea level down to the deep ocean floor, about 4000 metres below the surface. The similarity in shape of a contour (500-fathoms) drawn on the continental slope of eastern South America and the same contour drawn on the west African slope is quite striking. ii) Glaciation: All four southern continents reveal signs of a period of large-scale glaciation in Carboniferous-Permian times (250 million years ago) when they were closer together and nearer the South Pole. iii) Evidences from Geology: Similar fauna and flora fossils and geological structures were found in the corresponding continents. For example, coalfields in Appalachians and Europe are of the same carboniferous period. In northern America, Appalachian Mts extends to Newfoundland, stop at Atlantic Ocean, but the extension can be found at the East Atlantic edge in Ireland. iv) The matching of rock layers: Similarity of age, time and sequence of rock layers, eg. the Cretaceous and Jurassic sedimentary basins in West Africa and Brazil. In Lower Cretaceous time (about 135 to 100 m.y.a.) several sedimentary basins formed along the continental margin of Brazil and West Africa. v) Evidence from Paleontology: A plant called Glossopteris was widely grown in South America, South Africa, Australia, India and Antarctica at Carboniferous-Permian. The seeds of Glossopteris was too big to transport by wind from continent to continent. On the other hand, a ancient animal called Lystrosaurus was found in South Africa, South America and Antarctica at Permian. Lystrosaurus could not swim. For these reasons, continents may be joined together at the period of Carboniferous and Permian. vi) Evidence from Climatlogy: Coalfields are found in Antarctica. It is believed that the ancient climate of Antarctica was very warm and humid for vegetation growth. However, A huge ice-cap has been covering Antarctica now. So, the location of Antarctica is not fixed but moved. vii) Palaeomagnetism: Igneous rocks when cooled retain a certain magnetisation, the result of the presence of grains of substances containing iron which were aligned and permanently magnetized in accordance with the direction of the earth's magnetic field at that time. This 'fossil magnetism' thus provides a record by which the position of the magnetic poles at various times may be located. The figure below shows the polar wandering curves for South America and Africa. It shows that when plotted on the best-fit reconstruction, the Carboniferous, Permian and Triassic poles lie within overlapping circles of error. None of them have overlapping circles of error in their present-day positions. The palaeo-latitudes of Permian and Carboniferous time place the southern parts of both continents within the Antarctic circle of that time, that is within 23.5 degrees of the pole. The Triassic poles from Africa and South America place both continents forty degrees closer to the equator. There are as yet no reliable data for the Jurassic pole of South America, but the Cretaceous poles are first pole on the best-fit reconstruction that do not have overlapping circles of error. Such a separation suggests the continents moved apart in the Triassic to Cretaceous time interval. IV. The Hypothesis of Sea-Floor Spreading: While palaeomagnetic directions have been instrumental in 'proving' continental drift, they also form the critical evidence in favour of spreading. Just over twenty years ago, the ocean floors were still regarded as the oldest and most dormant parts of the Earth. By 1960, however, improved techniques had enabled scientists to examine, for the first time, the detailed topography of large areas of ocean floors. For the ocean floors were revealed not entirely as dull, flat regions but as possessing "mountain ranges" comparable with the most impressive mountainous regions on the continents. It later emerged that these ranges, or "ridges" were not isolated features but formed part of a world-wide and almost continuous network, and that, furthermore, they were seismically active. From this quickly developed the idea that the ocean floors, far from being dormant and old, were, in parts, the most active and youngest regions of the Earth. The hypothesis was first formulated by Harry Hess in 1960. Hess suggested that, despite their great age and apparent permanency, continents have been, and are being, passively drifted apart and together on the backs of mantle-wide convection cells. In contrast the ocean floors are young and ephemeral features of the Earth's surface, constantly being regenerated at ridge crests and destroyed in the trench systems. From consideration of the earlier ideas regarding the age of the initiation of drift in the Atlantic area (based on the geologic record of the surrounding continental margins) Hess suggested that the sea-floor might be spreading at a rate of approximately 1 cm per year per ridge flank. The occurrence of earthquakes along the crest of the mid-ocean ridge system, the dearth of sediments at ridge crests and the active volcanic islands associated with the crest of the Mid-Atlantic Ridge are all readily explained by Hess's model. Moreover, the ocean basins as a whole contain a remarkably thin veneer of sediments and small number of sea mounts if recent rates of accumulation and formation are extrapolated over the whole of geologic time. In 1960 Hess was also able to state that no material greater than about 100 million years in age had ever been recovered from the deep ocean floor or truly oceanic islands. The flow of heat through the ocean floor from the Earth's interior was first determined in the 1950's. Results obtained revealed that the heat flow through the ocean floor is in general comparable to that determined previously for the continents. Over the mid-ocean ridges, however, it is, in places, several times the emplacement of hot mantle-derived material in the vicinity of ridge crests. Moreover, for most latitudes and orientations of ridge crests the magnetic anomalies (disturbances) are roughly symmetrical about the ridge axis. V. The Theory of Convection Currents: Subcrustal convection within the earth was suggested by William Hopkins in 1839. Osmond Fisher in his "Physics of the Earth's Crust" of 1881 pointed out that the frictional drag on the underside of the crust would be expected to promote mountain building along the margins of the continents, where two systems of currents approached one another and turned downwards. He also recognized that "The existence of convection currents beneath the cooled crust of the earth at once furnishes a means of obtaining those local increments of temperature which in some form or another appear to be needful in order to explain volcanic phenomena. The theory was later developed by A. Holmes which provides a support the theory of continental drift, and helps in the development of the theory of plate tectonics in the 1970's. The suggestion has been made that radioactive heat is responsible for the energy required, which sets up convection currents moving (at an estimated rate of 1 cm per year) in a series of "cell" throughout the upper part of the mantle and the simatic layer. These currents rise under the oceans, then move out horizontally in either direction, taking with them the "continental rafts" of the lighter sialic material in opposite directions, and causing them to drift apart at present rates of about 2.5 cm a year. The table below shows the estimated rate of crustal movements. These lateral crustal movements would result in tensional cracks in the ocean floor midway between the continents, such as the Mid-Atlantic Ridge, the East African Rift Valley and the San Andreas Fault. Areas Movements Continental Drift (average over 200 m.y.) 2-4 cm/year Renewal of the Pacific floor 3-4 cm/year Elongation of Hawaiian Chain (3000 km in 100 m.y.) 3 cm/year Maximum transport of Atlantic Islands form Ridge 3 cm/year San Andreas fault (present displacement rate) 5 cm/year Great Alpine fault, N.Z. (present displacement rate) 2.5 cm/year Magna Fossa fault, Japan (160 km in 4-8 m.y.) 2-4 cm/year Algerian faults (180 km in 6-12 m.y.) 1.5-3 cm/year VI. The Theory of Plate Tectonics: This great, unifying concept draws sea-floor spreading, continental drift, crustal structures and world patterns of seismic and volcanic activity together as aspects of one coherent picture. Theory of Plate Tectonics was developed. It is proposed that the entire surface of the Earth comprises a series of internally rigid, but relatively thin plates. Although the size of the plates is variable, much of the earth's present surface is occupied by half a dozen or so large plates. These plates are continuously in motion both with respect to each other and to the Earth's axis of rotation. Virtually all seismicity, vulcanicity and tectonic activity is associated with different motion between adjacent plates. These plates consist of the earth crust and upper mantle forming a strong, rigid earth shell called the lithosphere. Beneath the lithosphere lies the soft, weak layer (Asthenosphere). There are three types of plate margin: 1. Constructive Margin: During the spreading process, which occurs at oceanic ridges, new crust is created and moves away from the ridge along with the underlying, uppermost mantle. The newly generated crust and its upper mantle is effectively welded to the plate's trailing edge. Thus a ridge represents a zone along which two plates are in motion away from each other; yet they do not separate, because new material is continuously added to the rear of each. (eg. Mid-ocean Ridges) Constructive Plate Margin and Destructive Plate Margin 2. Destructive Margin: It occurs at the deep ocean trenches or fold mountain belts where two plates approach each other and one slips down under the margin of the other at an angle of about 45o. This zone is known as the subduction zone. (eg. Himalayas, Java Trench) 3. Conservative Margin: These are margins at which the plates neither gain nor lose surface area, but simply past each other. However, due to the friction arising from the lateral movement of the plates, earthquakes are usually caused. (eg. San Andreas Fault, Rockies Mts.) The mechanism for the plate movement is very complex and not yet very clear. Nevertheless, it is generally thought to be related to the circulation of convection current which is driven by the heat derived from the radioactive elements in the asthenosphere. Ascending current may locate at the constructive plate margin and drags the plates apart while the descending current may located at the destructive plate margin and pulls the plates to be plunged into the mantle. Folding and Fold Mountains All major mountain chains are 'fold-belts', known also as 'orogenic belts'. They are characterized by very complex folding, plutonic and volcanic rocks of various types and very thick sequences of sedimentary rocks, often 10000 m thick or more. 1. Features and Characteristics of Fold Mountain Areas: A study of the present fold mountain belts reveals the following characteristics: a. There are parallel belts of fold mountains separated by intermountain plateau where the sedimentary strata are much less intensely folded. For example, between the Rockies and other fold mountains of North America is found the Colorado and other plateaux; between the Himalayas and the Kunlun is the Qinzang Gaoyuan. b. They are composed of enormous thickness of sedimentary rock strata (8000 to 10000 m) of materials that are today found in continental shelf. But when the core of fold mountains has been exposed, igneous and metamorphic rocks are also found. c. Young fold mountain zones represent lines of weakness of the earth's crust where most of the world's recently active volcanoes lie and where the greater majority of the earthquakes originate. d. The sedimentary strata have been compressed into various kinds of folded structures. The degree of folding and the types of folded structure formed depends on the nature and intensity of the compression. Folds are structures in which the primary surfaces of reference are bent or distorted without loss of essential continuity. Three geometrical varieties of folds can be distinguished, anticlines, synclines and monoclines. An anticline is an arch in which the two sides or limbs dip outwards away from one another. A syncline is a fold in which the limbs dip towards one another. A monocline is a simple step-like flexure in which more or less horizontal beds locally assume a dip in one direction and then flatten out again. The elements making up a fold are illustrated in the figure below. In a single bed, the two sides or limbs meet at the fold-axis or hinge-line which is the of maximum curvature. The axial plane is the locus of the fold-axis of successive layers, it is the place at which the two limbs meet. The crest of an anticline is the highest point of the fold with reference to any particular bed; it is parallel to the fold-axis. Similarly the trough of a syncline is the bottom of the fold with reference to a given bed. The fold-axis may be horizontal or it may be inclines, in which case the axis and the fold are said to plunge or to pitch. The plunge is measured in degrees from the horizontal in a vertical plane, while the pitch is given by the angle between the fold-axis and the strike of the axial plane. The profile of a fold is its form as seen in a plane perpendicular to the axis. Simple/ Symmetrical Fold: The fold is upright and symmetrical when the axial plane is vertical and the two limbs dip at similar angles. Asymmetrical Fold: Where on limb dips more steeply than the other, the axial plane is inclined and the fold is asymmetrical. Overturned/ Over Fold: With increasing asymmetry. The middle limb connecting an anticline with its adjacent syncline becomes inverted and the fold is said to be overturned. Recumbent Fold: Where the axial plane is roughly horizontal, the fold is said to be recumbent . In structures of recumbent fold most of the middle limb may be sheared out altogether. Overthrust Fold: Further development of the structure then results in forward movement of the rocks of the upper limb along the plane of shearing. The latter has become a thrust plane and the structure an Overthrust Fold. The sheet of rocks that has moved forward along the thrust plane is referred to as a nappe. Anticlinorium & Synclinorium: Moreover, a complex of folds of different orders is called an anticlinorium, if the whole appears to have been arched upwards; or a synclinorium if the generalized form is that of a depression. 2. Formation of Fold Mountains: According to the plate tectonics theory, fold mountain originates where plates of crust converge. Orogeny occurs where a block of continental crust is carried into collision with another crustal plate. The full development of an orogenic belt requires collision between two plates, with subduction of one under the one and a continental block at or near the edge of at least one of the plates. Firstly, the land surface is being actively eroded supplying a large amount of sediments. There is an accumulation of great depths of sediments in a large depression called a geosyncline under an ocean. Later the plates on each side of the sediment zone move towards each other producing a great compressional force. The movement of the plates towards each other crumples the sedimentary deposits. As the sedimentary rocks are folded up, fold mountains are formed. While this is happening, the hot magma of the mantle may also flow out to the earth's crust forming volcanoes. Besides, this is sometimes accompanied by faulting. A good example is the Himalayas along the boundary between the Eurasian Plate and the Australian Plate. A special case is that of continent-continent collision, eg. the closing of the Tethys Sea and the formation of the Himalayas by the collision of India with the Asian continent in early Tertiary time. The figure below shows the formation of the Himalayas by the collision of plates. Faulting and Its Associated Landforms 1. Faults: A fault is a fracture of dislocation in the earth's crust along which there has been displacement of the rocks on one side relative to those on the other. The movement of the rocks on a fault may have been in any direction, vertical or horizontal or some combination of these. The surface of the break along which the movement has taken place is the fault-plane; a fault-zone comprises a group of such surfaces. The fault-plane may be vertical, inclined or gently undulating and its inclination is recorded as a dip, the angle between the fault-plane and the horizontal. Hade is the angle between the fault-plane and the vertical. The horizontal trend of a fault is its strike. If the fault-plane is inclined, the upper side and the rocks which lie above it are referred to as the hanging-wall; those below it are the foot-wall. The intersection of a fault-plane with the ground surface is known as the fault-line. The total movement on one side of a fault relative to that on the other, in the fault-plane, is termed the slip or displacement; the vertical component of the slip is the throw; the horizontal component in the strike direction of the fault-plane is usually referred to as the shift and the other horizontal component of the slip, at right angles to the strike, is the heave. The walls of a clean-cut fault may be polished by friction between the moving blocks, and traversed by grooves or striations . Such surfaces are called slickensides. Major faults are generally accompanied by a great deal of grinding and crushing of the wall rocks, and there may be a more or less broad fracture belt, known as a fault zone, consisting of lenticles and sheets of shattered rock. The latter is called a fault breccia when it is largely made up of angular fragments. Faults may be divided into several categories in relation to the movements that have taken place on them. A normal fault is generally inclined at an angle between 45o and the vertical. The rocks abutting against the fault on its upper face or hanging wall are displaced downwards relative to those against the lower face or footwall. The terms 'downthrow' and 'upthrow' for the two sides are purely relative. Normal faults involve an extension of the faulted beds. Regions that are divided by faults in to relatively elevated or depressed blocks are said to be block faulted. The face of the earth today is diversified by many boldly preserved topographic features which are primarily due to vertical movements involving normal faulting. Such movements have been unusually active during late geological time, and they still are, right down to the present time. fault scarps exposed at the surface are being gradually worn back by erosion, but long ages must elapse before such features as the block mountains illustrated in the figure below are levelled out. Reverse or thrust fault is inlined at an angle between 45o and the horizontal. The beds on the upper side are displaced up the fault plane relative to those below. Shortening of the faulted area is thus involved and the operation of compression seems to the implied. The celebrated Moine and associated thrusts of the North-west Highlands, where they mark the north-western boundary of the Caledonian orogenic belt in Scotland, are the classic examples of low-angle thrust faults. Tear faults or wrench faults are formed where the movement was dominantly horizontal and are often called transcurrent or strike-slip faults. The best known of all wrench faults is the great San Andreas 'Rift' of California, which has a known length of over 1350 km. Table 5 indicates that the average rate of movement during the last 140 million years has been 4 km per million years. But Pleistocene and present-day rates are much higher. 2. Block Fault mountains: Regions which have been divided by faulting into relatively elevated or depressed blocks are said to be block faulted. The upstanding fault blocks, which may be small plateaux or long ridge-like block mountains, are called horsts (block mountains). A long fault trough, forming a tectonic valley bordered by parallel fault scarps, is known as a rift valley. Between the horsts of the Vosges and the Black Forest the Rhine flows through a rift valley. The river occupies the valley, but did not excavate it. The most renowned system of rift valleys, however, is that which traverses the East African plateaux from River Zambezi to the Red Sea and beyond. Cross-section of East African Rift Valley Volcanism and the Earth's Crust Vulcanicity is the process by which matter is transferred from the earth's interior and erupted on to its surface. The oldest rock exposed on the surface of the earth were produced by volcanic activity. Volcanoes are undoubtedly one of the greatest natural hazards to life on this planet. Below the crust, there is material at times becomes liquid and thus may be able to rise to the surface of the earth through conveniently placed fissures and pipes or vents. Despite the high temperatures at depth this material is usually solid because of the great pressure exerted by the superior masses of rock. At times, a local increase of heat, combined almost certainly with a reduction in pressure causes this material together, in all probability, with the basal layers of the crustal SIMA to become molten rock or magma. As the magma rises the decreasing pressure releases some of the included gases and when the surface of the earth is reached much of the original gas has been lost. The molten rock is the lava which unlike the original magma is relatively poorly charged with gas. Whilst access of magma to the continental surfaces and ocean floors results in volcanic activity the rising magma may penetrate only to a limited extent into the earth's crust, chilling, after engulfing some of the crustal rock, when its movement is arrested. All such activity, whether taking place within the crust or on its surface, is known as vulcanicity. There are two types of vulcanicity: Intrusive and Extrusive vulcanicity. 1. Intrusive Forms of Vulcanicity: The results of the forcing into the earth's crust of molten magma depend on firstly its degree of fluidity, and secondly the character of the planes of weakness, such as joints and faults, or cracks and fissures in the crests of anticlines along which it can penetrate. A mobile magma flows farther, as a thin sheet, while a more viscous magma solidifies rapidly in a dome-shaped or lenticular mass. The main intrusive forms are shown in the figure : a. Dykes: These are formed when the magma has risen through near-vertical fissures, solidifying to form 'walls' of rock cutting discordantly across the bedding planes of the country rock. Where affected by denudation, the dyke may either stand up as a wall (where the dyke-rock is harder than the country rock), or it may be worn away to form a long narrow ditch-lick depression. b. Sills: Horizontal sheets of rock solidify from magma which has been ejected concordantly between bedding planes; they may be of any thickness and extend for many square kms. In North America the Palisades extend for over 80 km along the western bank of the Hudson River near New York; this sill is probably 300 m thick. c. Laccoliths: These features are produced where tongue-like lateral intrusions of viscous magma have forced the overlying strata into a dome. In its simplest form the magma solidifies as a cake-like mass, but often there are subsidiary laccoliths around the main one, or several laccoliths may be formed one above another, resulting in a 'cedar-tree' laccolith. d. Batholiths: Large masses of rock occur in the heart of mountain ranges, formed by deep-seated movements on an enormous scale, so that the masses of magma cooled slowly to form large-crystalled rocks such as granite, later exposed by prolonged denudation as massive upland areas. The edges of a batholith descend steeply to unknown depths, while the country rocks with which the intrusions have come into contact are often metamorphosed by thermal contact. Intrusive rocks can be classified into Hypabyssal (Intermediate) and Plutonic. Dykes, Sills and Laccoliths are hypabyssal, but Batholiths is plutonic. 2. Extrusive Forms of Vulcanicity: A volcano consists of vent or opening at the surface of the crust through which material is forced in an eruption. This may accumulate around the vent to form a hill, or it may flow widely over the country rock as an extensive level sheet. The resultant landforms depend largely on the nature of the material ejected, which varies in different cases and different stages in an individual eruption. There are three types of extrusive materials. a. Gases: Gases emitted during the course of an eruption include gaseous compounds of sulphur and hydrogen with carbon dioxide, but most are dissipated directly into the atmosphere. Stream (about 50-70%) is possibly derived from surface water such as in a crater-lake or from the sea, but most probably originates from the water to be found in magma. Stream is the most important factor affecting the eruption. b. Solid: When an eruption is accompanied by a series of explosions, solid materials are ejected, known generally as Pyroclastic Debris/ Pyroclasts or Tephra. These may include fragments of the country rock disrupted when the pipe was blown through the crust, angular fragments of solidified lava from a previous eruption that had cooled in the pipe, and finer materials such as scoria, pumice, cinders (lapilli), dust and ash. The coarser fragments form breccia, while the finer materials may be loosely cemented as tuffs. Sometimes small amounts of liquid magma are thrown into the air and solidify before reaching the ground in globular masses known as 'volcanic bombs'. Other masses remain liquid until they hit the ground, when they spatter and congeal, forming small spatter cones, 3-6 m high, as around Sunset Crater in Arizona. c. Liquid: Usually the most important product of an eruption is lava, the molten magma which reaches the surface. The form of a volcanic cone depends to a large extent upon the nature of this lava, and so to some extent does the nature of the eruption. Some lavas contain much silica (SiO2), i.e. acid lavas, with a high melting-point; they are very viscous, solidify rapidly and so do not flow far. Acid lavas build high, steep-sided cones, and they may solidify in the vent and cause recurrent explosive eruption. Where, on the other hand, the lava is relatively poor in silica and rich in iron and magnesium minerals (basic lava), it has a lower melting-point and will flow readily for a considerable distance before solidifying. Such a lava tends to produce a much flatter cone of great diameter, and as the flow from the vent is unchecked and widespread, the eruption is quiet, without much explosive activity. The mobility of the lava also varies with the amount of gas dissolved within its mass; in Hawaii lavas containing gas have remained mobile at a temperature of 850oC, while others without much gas content have solidified at 120oC. 3. Types of Volcanoes: Volcanoes are built by the eruption of molten rock and heated gases under pressure from a relatively small pipe, or vent, leading from a magma reservoir at depth. Both explosive and quiet types of eruption occur, the forms built differing for the two types. Volcanoes of explosive eruption are cinder cones and composite cones; those formed by relatively quiet outflow of lava are lava domes. Quiet eruption of lava, if issuing from extensive cracks, of fissures, in sufficient quantities may make great plains or plateaux of lava. a. Cinder Cones: Smallest of the volcanoes are the cinder cones, built entirely of 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 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. Large pieces up to several tons in weight are volcanic bombs; they may be somewhat plastic when ejected. Smaller pieces, a fraction of an inch up to an inch or two in size, are cinders; these make up the bulk of the cinder cone. Still finer particles are termed ash and volcanic dust. The ash falls like snow upon the ground within a few miles of the eruption. Finer dust is carried by winds to distant regions and may settle out only after years of drifting in the atmosphere. Cinder cones rarely grow to more than 150 to 300 m in height. Growth is rapid. Monte Nuovo, 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. Cinder cones usually occur in groups, often many dozens in an area of a few tens of square miles. b. Basic Lava Cones/ Shield Volcanoes: A very important type of volcano is the lava dome or shield volcano. The best examples are from the 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 up to 4000 m above sea level, but including the basal portion lying below sea level they are more than twice that high. In width they range from 16 to 80 km at sea level and up to 160 km wide at the submerged base. Lava domes are built by repeated outpourings of lava. Explosive behaviour and emission of fragments are not important. The lava, which in the Hawaiian lava domes is of a dark basaltic type, is highly fluid and travels far down the low slopes, which do not usually exceed 4o or 5o. c. Acid Lava Cones: When explosive activity is absent, and the lava flows quickly from the vent, building a volcano. Viscous acid lava does not flow far and it produces a steep dome, with convex sides, eg. Puys d' Auvergne. Some of the more highly viscous lavas may be squeezed out slowly to form great 'spines', called volcanic plug. The vent of the volcano was solidified by lava, and the country rock were removed. The exposed vent is also called volcanic plug. d. 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. 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. Among the outstanding examples of recently formed composite volcanoes are Fujiyama in Japan, Mayon in the Philippines. Height of several thousand feet and slopes of 20o to 30o are characteristic. 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 south into the East Indies and New Zealand. There is also an important Mediterranean group, which includes active volcanoes of Italy and Sicily. The eruption of large composite volcanoes is usually accompanied by explosive issue of steam, cinders, bombs and ash, and by lava flows. The crater may change form rapidly, both from demolition of the upper part and from new accumulation. e. Calderas: One of the most catastrophic of natural phenomena is a volcanic explosive so violent as to destory the entire central portion of the volcano. There remains only a great depression, 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 island in Indonesia, exploded in 1883, leaving a great caldera. It is estimated that 75 cubic km 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. 4. Dormant and Extinct Volcanoes: An active volcano is one which is definitely known to have eruption periodically in historic times. However, some volcanoes may be described as dormant when a renewal of eruption activity is possible. Vesuvius had been dormant so long before its eruption of A.D. 79 that it was thought to be extinct. Other volcanoes may definitely be regarded as extinct, since they were formed in long-past geological times, and are situated in areas with no sign of any volcanic activity. They may retain their original forms, as the miniature cones of Auvergne, but for the most part many ancient cones have been destroyed beyond recognition through long continued denudation. 5. Distribution of Volcanoes: A large part of the world has not experienced volcanic activity during the most recent periods of geological time; there are some 520 known active volcanoes, but many thousands of extinct ones. The main volcanic peaks are indicated on the figure above; it is evident that there is a close relationship between their location and the major lines of weakness in the earth's crust. The most striking development is around the basin of the Pacific Ocean - the 'Pacific Ring of Fire', where two-thirds of the world's volcanoes occur. Most are associated with fold mountain ranges or faulted blocks. A chain of volcanoes nearly 3,200 km in length can be traced through southern Alaska, the Alaska Peninsula and the Aleutian Islands, one of the most volcanically active parts of the world. There are 80 active volcanoes, ten calderas over 1.6 km in diameter, and since 1760, 225 major eruptions have been recorded. In South America most of the highest peaks are volcanoes rising from the folded ranges, including Aconcagua (7021 m), which though extinct, is the world's loftiest volcano, and Guayatiri (6060 m), which erupted in 1959. On the Asiatic margins of the Pacific, volcanoes are strung along the island arcs; the most famous is snow-capped Fujiyama, rising to 3776 m with 24 km of the sea. Another line runs through the East Indies towards New Zealand; the island arcs of the western Pacific, which are often on the edge of submarine trenches, have hundreds of volcanoes, while many of the more remote islands (Hawaii, Tonga, Samoa) are volcanic cones rising from the ocean floor. In Africa volcanoes are to be found along the line of the East African rift-valley; Mount Kenya (5195 m) and Kilimanjaro (5889 m) are probably only recently extinct. The Alpine-Himalayan belt of folding is not associated to the same extent with active volcanoes, except in the central Mediterranean, where they are probably related to geologically recent movements of subsidence. Farther east, in Asia Minor and to the south of the Caspian, are many extinct cones. The Himalayas form the most striking exception to the general occurrence of volcanoes along zones of recent folding, for none is to be found there. Conversely, volcanic activity is widespread in Iceland, where there has been no recent folding of the crust. Several of the Atlantic islands, notably along the line of the Mid-Atlantic Ridge, have volcanoes; a peak on Tristan da Cunha erupted in October 1961, and the Azores and Canaries have experienced eruptions within historic times. The West Indian arcs have many indications of past vulcanicity, as well as a few active cones. 6. Minor Volcanic Forms: A variety of minor volcanic forms can be distinguished, usually though not necessarily associated with volcanoes approaching extinction. a. Solfatara, Fumarole and Mofette: A solfatara is a volcano which only emits steam and gas. The floor of this crater is comparatively cool, and one can walk about on it, although here and there are pools of boiling water, while jets of water vapour or puffs of sulphurous gas are emitted periodically. The term solfatara is commonly limited to cases where the gas emitted are sulphurous, while the name fumarole is applied to emissions of steam and other gases. Perhaps the most famous example of fumarole is in the valley of Ten Thousand Smokes in Alaska. The special name of mofette distinguishes vents which emit carbon dioxide; examples are to be found in the Phlegraean Fields, in Auvergne and in Java. b. Hot Springs and Geysers: In some areas associated with past or present volcanic activity, the chief product is hot water. It may flow out continuously as a thermal spring, containing mineral substances in solution or suspension, which may be deposited round the edge of the surface pool in the form of crusts of travertine (calcium carbonate) or of siliceous sinter (geysertie). Several thousand hot springs are known in Iceland. Where hot water is ejected with considerable force accompanied by steam, an intermittent paroxysmal fountain occurs, known as a geyser, as in Iceland, Yellowstone National Park, and the North Island of New Zealand. Some erupt periodically at regular intervals, others more spasmodically . The cause is complex, but it is due to superheating far down in the pipe of the geyser; the temperature of the water at depth increases, but convection cannot easily take place up the column of water because of its length. The water at depth is heated beyond 100oC because of the pressure of the column above, and its ultimate sudden conversion into superheated steam causes the water in the upper part of the pipe to be violently emitted. Cooler water flows into the pipe, and the heat increase begins again. One of the most famous geysers is 'Old Faithful', in Yellowstone, which has an average eruption interval of about 65 minutes, ranging between 33 and 95 minutes. It shows 50-100 cubic metres of near-boiling water, with steam, 40-60 m into the air, lasting from 2 to 5 minutes. Ocean Ridges, Ocean Trenches and Island Arcs 1. Ocean Ridges: These are the main features rising above the abyssal plains. They connect through all the oceans to form a worldwide feature nearly 60,000 km long. Each ocean ridge has its own distinctive features, but all are composed solely of the basaltic lavas typical of ocean-floors; there is no mixture of sedimentary, igneous and metamorphic rocks characteristic of the continental areas. The ocean ridges are formed by fissure eruptions with a fairly uniform rate of molten rock emission. In combination with fracturing along the central rift valley sides this leads to the formation of the parallel ridge system. Iceland is a particular interesting place in this context, since it is built astride the Mid-Atlantic Ridge system. It is formed mostly of lava flows, and volcanic activity, including hot springs, is a constant feature of the island's environment. Great fissures cross the island and basaltic material wells up along these at temperatures of 1200oC; it cools and blocks the fissure, forming wall-like intrusions, or dykes. At times volcanic cones are formed after explosive activity. Other features of ocean ridges indicate that there is a flow of energy along their length. Shallow earthquakes are located commonly beneath the centres of ridges, with their foci at 25-35 km depth. Since earthquakes take lace where stresses are relieved inside the earth, and are known to occur where movements of molten rock take lace beneath a volcano prior to eruption, it can be inferred that similar movements are taking lace beneath the ridges. This is confirmed by the subsurface temperature pattern (the figure below), which suggests a strong heat flow concentrated at the ridge and giving rise to the melting of rock material beneath the crust. 2. Ocean Trenches: The deepest parts of the oceans are elongated troughs descending to depths of over 10,000 metres. They are hundreds of kilometres across and thousands of kilometres long. Sediments accumulating on the trench floors are relatively thin today. Nearly all the trenches occur around the margins of the Pacific Ocean, and an arc of volcanic islands is commonly present on the continental side of the trenches. The trenches are interpreted as the lines along which the ocean-floor, manufactured at the ocean ridges and spreading out towards the continents, plunges beneath lower density continental or ocean-floor rocks at the subduction zone to a depth where the rock materials are fused, resulting in volcanic and earthquake activity. 3. Island Arcs: Island arcs are often related closely to fold mountain ranges. They are found today mostly around the western margin of the Pacific Ocean and in the northeast of the Indian Ocean. They form where a section of the ocean-floor is subducted in the ocean basin away from a continent, i.e. where ocean-floor crust is on either side of the convergent plate boundary. Japan is the largest area of land formed in this way, and is a mountainous country. Mount Fuji reaches nearly 4,000 m, and several other peaks on Honshu top 3,000 m. They are all volcanic in origin. The island of Honshu is largely a pile of basalt and andesite lying between the Japan Trench and the Sea of Japan. The Pacific Ocean floor is subducted at the trench, a hypothesis which is supported by the greatest volume of volcanic rocks being erupted over western Japan during the last 2 million years. It seems that the subducted ocean-floor becomes mobilized as magma when it reaches 120 km depth, and this is why the east of the island experiences little volcanic activity compared with the west. The evidence of the double metamorphic belts suggests that Japan has been formed by the driging together of two island arc masses at different times, and just to the south of Japan today are the two separated arcs of the Philippines and Marianas. The Sea of Japan now forms an effective sediment trap for material eroded from Asia as well as from western Japan. On the Pacific Ocean side a wedge of flysch-type sediment is accumulating. Thus the volcanic rocks form the first land, but the atmospheric processes act on these rocks and produce sedimentary rocks. It would seem that the trench develops as plate descent begins. Ocean-floor crust together with any covering sediment is thrust beneath more ocean-floor crust, giving rise to rocks metamorphosed under high pressure conditions on the continental side of the trench. When the descending plate reaches over 100 km in depth partial melting takes place, magma rises to form a pile of volcanic rocks and the island arc begins to rise. The heat generated in this way results in further metamorphism as a result of combined heat and pressure. The greater the depth reached by the plates, the more intense the volcanic activity and the farther the magma has to ascend. This affects the type of lava produced: basalts are poured out first, followed by andesites which become increasingly alkaline and are erupted farther from the trench. Earthquakes Earthquakes, natural vibrations within the earth's crust, provide indisputable evidence that crustal movements are still taking place today. Some of these earth tremors are quite violent and are responsible for large-scale death and destruction. Most, however, are too small to be felt by man and must be detected by means of delicate recording instruments called seismographs. 1. Seismic Waves: The investigation of earthquakes and the transmission of earthquake waves through the earth is known as Seismology. An earthquake shock generates elastic vibrations or 'waves'. which move out in all directions from the point of origin, or focus, of the earthquake. There are four kinds of seismic waves: a. Primary or compressional waves (P waves) consisting of longitudinal vibrations which give an oscillatory movement to particles in the direction in which the waves are propagated. These waves propagate very rapidly through both solids and liquids and are usually the first indication that an earthquake has occurred. The speed of P-waves varies and is in the region of 5.5 km/s in the crust, 8 km/s just below the crust and about 13.5 km/s in the lower portion of the mantle. b. Secondary, Shear- or Distortional waves (S waves) which are transverse vibrations with an oscillatory movement at right angles to their path. The P waves and S waves are body waves travelling through the earth. The S waves travel at only about 60% of the speed of P waves. However, their amplitude and period are almost double those of P waves. S waves require a rigid medium for propagation and cannot propagate through the liquid core. P waves and S waves are body waves. c. Love waves (L waves) are waves of long period which travel around the periphery of the earth. These are surface waves. d. Rayleigh waves consist of a retrograde elliptical motions similar to wind-driven ocean waves. Like L waves, Rayleigh waves are also surface waves. However, L waves propagate at about 4.5 km/s while Rayleigh waves are about 10% slower. A large proportion of the earthquake energy is carried by the L waves and the Rayleigh waves. The P waves have the greatest velocity and are the first to arrive at a recording station at a given distance from the focus; the S waves are a little slower, and the L waves the slowest of the three groups. But the L waves have the greatest amplitude and are those that do the most damage. 2. Distribution: Although earthquakes may occur at any place over the entire earth, most of them originate in areas of crustal unrest and are associated with mountain-building movements. Earthquakes, like volcanoes, occur in rather well-defined seismic belts. About 80% of the world's earthquakes originate in the Circum-Pacific belt of young mountain ranges and chains of volcanic mountains. This belt extends from Chile along the western borders of South and North America, northward to the Aleutians, Alaska, Japan, the Philippines, Indonesia, New Zealand, and certain Pacific islands. The second major seismic belts, the Mediterranean and Trans-Asiatic belt, extends from the Caribbean area through the Himalayas and Alps and includes Spain, Italy, Greece, and northern India. Approximately 15% of the earth's seismic energy is released in the Mediterranean and Trans-Asiatic zone; the remaining 5% is released in other parts of the world. 3. Earthquake and Plate Tectonic: The occurrence of earthquake is considered to be highly related with the location of plate margins. The greatest intensity of earthquake is found along the destructive plate margins where oceanic plates undergo subduction such as the volcanic arcs and trenches of the Pacific Ocean Basin. Intense pressure is built up along the zone of collision and stress and strain relieved by sudden fault slippages generate devastating earthquake. Constructive plate margins and conservative plate margins also experience seismic activities though the intensity is generally much smaller in comparison with those along the destructive plate margins. Typical examples are the San Andreas Fault which locates along the Pacific Plate and the America Plate in California and the Mid-Atlantic Oceanic Ridge. 4. Effects of Earthquake: The destructive effects of earthquakes are familiar to almost everyone : shattered buildings, displaced roads and railways, collapsed bridges, great cracks in the ground and changes in sea level. The loss of life accompanying a major earthquake may be staggering. For example, an earthquake that occurred in north central China in 1556 is said to have killed 830,000 people. Because earthquakes normally occur rapidly and unexpectedly, there is little time for precautionary measures. The death toll may also be raised by complicating factors such as disease, flood, fire and famine. In addition, an earthquake will usually produce numerous geologic changes in an area. These include landslides, avalanche and mudflows, disruption of ground-water circulation, and sunken and fissured ground. Earthquakes that occur beneath the ocean often generate great waves of water called tsunamis. These waves, which have been known to be as much as 60 m high and to travel at speeds of up to 800 km/hr, are capable of producing tremendous destruction. One such wave, associated with the Lisbon earthquake of 1755, attained an estimated height of 15 m and washed inland for more than 1 km. Another, occurring north of Tokyo along the Pacific coast of Japan in 1896, reaching heights of up to 30 m and was responsible for the deaths of more than 27,000 people.