Ecology of the Indonesian Seas Part 1

By Tomas Tomascik and Anmarie J. Mah

The Ecology of the Indonesian Seas distills for the first time the information found in thousands of scholarly works relevant to an understanding of the sustainable use of marine and coastal resources in these islandsmany of them available up to now only in Dutch, German or Indonesian. It is an invaluable tool for government planners, resource managers, ecologists, university students, scuba divers, and all those with an interest in the sea.

The first volume provides a review of the geology, physical oceanography and meteorology of the archipelago. Coral reefs, one of the most important, yet least known ecosystems in Indonesia, are introduced in this volume through discussions of the geologic history of reefs, followed by a review of the major theories of coral reef formation, development and their world distribution. Corals and other reef-associated organisms are then examined. The chapter on foraminifera, major producers of the present and past carbonate deposits, presents some little-known Indonesian assemblages. Next, the authors look at the natural environmental factors that affect coral reef development and survival. A chapter on coral reef growth and development concludes the first volume.

• Language: English
• Publisher: Periplus Editions
• Release date: Feb 5, 2013
• ISBN: 9781462905027

Book preview

Chapter One

Introduction

Situated upon the equator, and bathed by the tepid water of the great tropical oceans, this region enjoys a climate more uniformly hot and moist than almost any other part of the globe, and teams with natural productions which are elsewhere unknown. —WALLACE 1869

WHERE ARE THE INDONESIAN SEAS?

The tropical Indo-Pacific ocean has recently been considered as the 'largest ecological system on earth' extending from the eastern tropical Pacific to the east coast of Africa (Sheppard et al. 1992). This vast area contains much of the world's marine biodiversity including a wide range of marine and coastal environments that support it.

The Southeast Asian waters, and those of New Guinea, biogeographically known as the Indo-Malayan triangle, are located in the middle of the Indo-Pacific, and represent an area of high marine biodiversity. Ekman (1953) considered this area as the faunistic centre from which other regions of the Indo-West Pacific recruited their faunas. Briggs (1974) suggested that the Indo-Malayan triangle with its widely distributed marine fauna covers a broader area within the Indo-Pacific, known as the Indo-Polynesian Province, with the Indo-Malayan triangle as the centre.

Indonesia, derived from the Greek words Indos meaning Indian and nesos meaning islands, is the main part of the Indo-Malayan triangle, stretching roughly from 6° N to 10° S and from 95° E to 142° E. Table 1.1 provides a brief geographic summary of the Indonesian Archipelago. Indonesia occupies a central position of the Indo-Pacific, thus creating a permeable barrier between the Pacific and Indian Oceans and the Asian and Australian continents. With more than 17,500 islands and a coastline in excess of 80,000 km, the Indonesian Archipelago is a storehouse of marine biodiversity. The marine and coastal environment of Indonesia includes a high diversity of ecosystems such as beaches, sand dunes, estuaries, mangroves, coral reefs, seagrass beds, coastal mudflats, tidal forests, algal beds as well as many small island ecosystems. In addition to these critical coastal ecosystems, the Indonesian Archipelago contains vast continental slope areas, abyssal plains and deep oceanic trenches. Each of these marine ecosystems, with its associated habitats, supports a wealth of marine resources. About 78% of Indonesian territory are shallow seas located on the Sunda and Sahul Shelves, separated by the deep Timor, Banda and Flores Seas (fig. 1.1). Most of the present, albeit limited, knowledge of marine life comes from shallow water biota at depths of less than 200 m.

As the world's largest archipelago, Indonesia is positioned in a strategic location with respect to global ocean circulation patterns. The dynamic nature of the archipelagic seas, their interaction with the Pacific and Indian Oceans and the monsoonal climate to a great extent explain the high marine biodiversity of the region. Plate tectonics, and the associated seismicity and volcanism, played, and continue to play, a key role in the geologic evolution of the archipelago. These natural geological processes are the most important long-term factors affecting the physical, chemical and biological processes as well as human populations inhabiting the archipelagic islands as early as the Paleogene.

The movement of the crustal plates is a dynamic process that has important ramifications for the present-day geographic distribution of marine ecosystems and associated biological communities. Since the climate of the archipelago is under the influence of the Asian-Australian monsoon system, the climate exerts a major influence on the large-scale circulation patterns of the intra-archipelagic seas and plays a significant role in the productivity of the coastal and marine systems.

Table 1.1. The geographic summary of the Indonesian Archipelago.

Figure 1.1. Map of the Indonesian Archipelago showing the extent of Sunda and Sahul Shelves as delineated by the 200-m isobath.

HISTORICAL PERSPECTIVE

Studies of Indonesian marine fauna and flora pre-date Linnaeus' Systema Naturae (1758 - 10th edition). The well-known naturalist, G. E. Rumphius (1627-1702), worked on materials collected from Ambon, and other areas of the Moluccas, with some materials likely originating from other parts of the eastern Indonesian seas. Working with the VOC and stationed in Ambon, Moluccas, most of his life, his con tribution to science was monumental. His work was collated in two great works, D'Amboinsche Rariteitkamer (1705) and Herbarium Amboinense (1741-50); the latter work catalogued 1200 species of marine plants (fig. 1.2).

The rich marine diversity of the Indonesian Archipelago has motivated marine scientists to organize numerous expeditions to this fascinating region. Early French expeditions include Physicienne (1817-20); Coquille (1822-25), Astrolabe (1826-29) and Bonite (1836-37). English expeditions during this period included the Beagle (183236) with Charles Darwin, Sulphur (1836-42), and Samarang (1843-46). These were followed by the Challenger Expedition (1873-76), which passed through the eastern part of the archipelago and concentrated on oceanography as did the German expedition Valdivia (1898-99). Challenger's 50 volumes, written over a period of 20 years, made a great contribution to our understanding of the world's oceans. One of the most significant early expeditions to the eastern parts of the Indonesian Archipelago was that of the Siboga (1899-1900).

These early expeditions generated great interest in the Malay Archipelago, not only in the marine environment, but also in terrestrial exploration. Indeed, the period between 1850 and 1900 saw an influx of terrestrial naturalists, collectors and travelers. The works of the great naturalist Sir Alfred R. Wallace (1854-62) contributed greatly to our understanding of the biogeography of the archipelago, and his faunal and floral demarcation known as Wallace's Line was later explained by plate tectonics. In 1842, Pieter Bleeker, a prominent ichthyologist, began working on Atlas Ichthyologique which constituted the first ichthyological research in Indonesia.

It was not until 1904, with the establishment of the Visscherij Station in Pasar Ikan (Jakarta), that marine biology became entrenched in the national development planning. In 1919, the Visscherij Station was given a new mandate for much broader oceanographic research, and was renamed Laboratorium voor het Onderzoek der Zee.

The 1900s began with the German Expedition Planet (1906-07), followed by the highly successful Snellius Expedition (1921-30). However, data from these early expeditions have not been widely available. More recent expeditions were aboard the Russian research vessel Vityaz (1963), the American Vega (1963-64), the Baruna Expedition (1964), the Mariel King Memorial Expedition limited to the Moluccas (1970), the Rumphius Expeditions I-IV (1971, 1973, 1977 and 1980), the Franco-Indonesian Corindon cruises (1981,1983), the Snellius II Expedition (1984-85) and the Franco-Indonesian Karubar Expedition (1991).

Figure 1.2. One of the first illustrations of a marine alga from eastern Indonesia shown in Herbarium Amboinense (1750).

Drawing by Mickey Meyer.

Considering the huge amounts of information collected by the various expeditions, it is surprising that the marine biodiversity of the Indonesian Archipelago remains largely unknown and generally unavailable to the public. The results of many expeditions are frequently reported as parts of other works, or they are published several decades following the expedition. For example, it was not until 1982 that information on the majid crabs collected during the 1929-30 Siboga Expedition and the 1970 Mariel King Memorial Expedition in the Moluccas was published (Griffin and Tranter 1982). Forest (1987) cited specimens of pagurid crabs collected in the Makassar Strait during the Corindon II Expedition in his discussion of the Polychelidae from other parts of the world. Specimens from these Indonesian cruises made valuable contributions to world biogeography and taxonomy.

Today the main centre for oceanographic research is at P30 (Pusat Penelitian dan Pengembangan Oseanologi) located in Ancol, Jakarta, which is the successor of the Lembaga Oseanologi Nasional (LON), with a new mandate that focuses on basic oceanographic research as well as on applied research in all fields of marine sciences, from engineering to taxonomy.

OBJECTIVES OF THE BOOK

The main objective of The Ecology of the Indonesian Seas is to introduce Indonesian students to the fascinating marine environment of the Indonesian Archipelago. The book has been written as a teaching text suitable for undergraduate courses, government agencies and the interested public, and is an introduction to the marine ecology of the Indonesian Archipelago, one of the world's most dynamic marine and coastal regions.

This book provides an overview of a number of coastal and marine environments of the Indonesian Archipelago, with a focus on ecological processes. However, the book primarily focuses on shallow-water coastal ecosystems, particularly coral reefs. The influence of human populations on the marine and coastal resources and their supporting ecosystems are also examined.

MAIN THEMES

The overall theme of the book is to touch upon our current knowledge, of the ecology of the archipelagic seas in view of its geologic history, the physico-chemical and biological environments, and human interactions. Marine ecology is viewed as a study of the interrelations between living organisms, including human populations, and their environment. The chapter on the physical oceanography and climate of the region illustrates the geographic complexity of the archipelago, and the key role of the monsoons in the overall physical characteristics of the marine environment. Major global climate changes associated with glaciation had a pronounced influence on the distribution of terrestrial biota; however, this influence is less clearly exhibited in the marine realm, where few barriers to dispersion exist. The book places great emphasis on the geologic history of the archipelago, since the diversity of marine ecosystems and associated biological diversity are a function of geologic history. Specifically, plate tectonics, seismicity and volcanism of the region directly influence the physical distribution of landmasses and submarine topography, which in turn influence the overall circulation patterns of the archipelagic seas and the climate itself. Recent advances in physical oceanography, through cooperative research studies with international scientific organizations, have highlighted the key role of the Indonesian Throughflow, a major transport of Pacific water masses into the Indian Ocean through the archipelagic basins.

The dynamic nature of the archipelago, in terms of its geology and climate, directly influences the inputs of nutrients and trace elements into the seas through volcanic activity, sedimentation, and in recent history, large amounts of soil eroded from areas under human influence.

The sections on biological communities collate much information on the various marine communities, but more importantly they point to serious gaps in our knowledge. While the need for applied marine research is clearly recognized, this book points out that a serious lack of basic research is now making advances in applied fields very difficult. A more balanced approach is required if the government's sustainable-use policies of marine and coastal resources and the conservation of biodiversity are to be successfully implemented.

Marine resources, both flora and fauna, have been exploited for human consumption or other needs, since the early history of mankind. Fish, crustaceans, molluscs and marine mammals are appreciated and widely consumed as a source of animal protein, or as a delicacy, by many cultures throughout the world. Coral reef fish and other reef inhabitants displayed in the world's public aquariums attract millions of people, and are also enjoyed by people in the privacy of their own homes.

In Indonesia, marine resources are a valuable export commodity. Corals, shells, and seaweeds are exploited as industrial raw materials. People are now looking to the sea for bioactive substances for medical and pharmaceutical purposes.

Scientists know very little about the loss of genetic and species resources from marine environments. Marine species, or even populations, have disappeared in historical times. Major disturbances to marine ecosystems could lead to the loss of genetic diversity. Marine, and especially coastal areas, are affected by human activities onshore and inland.

A solid knowledge and understanding of the marine and coastal environments of the Indonesian Archipelago must form the foundation for effective management and conservation of the marine and coastal resources. It is the objective of this book to provide the initial tools from which Indonesia's most precious asset, its people, may manage and conserve for future generations, its second-most valuable asset, the Indonesian seas.

Chapter Two

Geology

Geology teaches us that the surface of the land and the distribution of land and water is everywhere slowly changing. It further teaches us that the forms of life which inhabit that surface have, during every period of which we possess any record, been also slowly changing.—WALLACE 1869

INTRODUCTION

The world changes, but to most of us, our Earth is the symbol of stability. Viewed from the standpoint of a human life span, and indeed generations of humans, the foundations of Earth remain unchanged. However, on a geological time scale, the earth has undergone radical transformations from its birth to present. These slow but ongoing changes are modifying the surface of the earth, reshaping the continents and ocean basins. More rapid changes are occurring on the earth's surface as a result of human activity. Examples of anthropogenic actions which have caused major modifications to the earth's topography include the construction of dams, open-pit mining, and rice cultivation. Flannery (1994) suggests that the 60,000 years of human occupation in Australia have impacted the Australian continent to such an extent that virtually all of the continent's ecosystems are in some sense man-made. In Indonesia, Javanese and Balinese rice cultivation have had a major impact on the ecology of those islands (see Whitten et al. 1996), as had open-pit mining on the islands of Sulawesi and Irian Jaya. But human impacts on the Indonesian Archipelago are the topic of a later chapter. Of interest to us now is the land beneath our feet, its birth and evolution. Several theories evolved during the 1900s leading to the now accepted theory of plate tectonics. However, prior to discussing plate tectonics and its role in shaping the Indonesian Archipelago, we will briefly review the structure of the earth.

STRUCTURE OF THE EARTH

The earth has a layered structure comprising the core, mantle, and crust (fig. 2.1). The core is divided into a solid inner core and a nickel-iron outer core. The outer core is liquid, and it is the circulation within the outer core that generates the earth's magnetic field. This point will be significant when we later discuss magnetic anomalies.

Figure 2.1. The earth's layered structure. Cross-section through one hemisphere of the earth showing the inner core (1), the outer core (2), the lower mantle (3), the upper mantle (4), and the crust (5). The inner core is approximately 2414 km in diameter. The outer core is 2253 km thick, as is the lower mantle, the upper mantle is roughly 644 km in thickness, and the crust varies from 3.2 km under parts of the oceans to 120 km thick beneath mountains. Figure not to scale.

The mantle is mostly solid, consisting of dark, heavy rocks which are rich in iron-magnesium silicates such as olivine and pyroxene (Rhodes 1991). The asthenosphere is part of the upper mantle. It is known as the zone of mobility, since it is nearly molten (Gross 1990) with convection movements and isostatic adjustments occurring within this layer. Magmas (molten rock) may also be generated (Bates and Jackson 1980). The lithosphere includes the crust and part of the upper mantle. It is 50-100 km thick. The lithosphere floats on the asthenosphere.

The crust, or what we know as the earth's surface, forms die upper portion of the lithosphere. There are two types of crust: continental and oceanic. Continental crust is generally 30-40 km thick, but may be 120 km thick beneath mountains and is rich in aluminum and silica. Oceanic crust is 3-7 km thick and rich in magnesium and iron. Continental crust is thicker but less dense than oceanic crust, so it floats higher in the asthenosphere than oceanic crust. This is an example of isostasy, an equilibrium condition comparable to floating. However, there are two different concepts of the mechanism of isostasy: the Airy hypothesis and the Pratt hypothesis (fig. 2.2).

The Airy hypothesis postulates an equilibrium of crustal blocks of the same density but of different thickness (i.e., topographically higher mountains are of the same density as other crustal blocks, but have greater mass and deeper roots) (Bates and Jackson 1980). This is an example of floatational equilibrium (Spencer 1972) and can be compared to blocks of wood floating in water.

Figure 2.2. Isostasy: The Airy hypothesis and the Pratt hypothesis. The Airy hypothesis (A) is based on the premise that the crustal blocks are of equal density but the roots are at different levels. The Pratt hypothesis (B) assumes that the blocks are of different densities but the differences are compensated at a certain depth. The Heiskanen hypothesis (C) is a combination of the Airy and Pratt hypotheses.

Spencer 1972, Kingston 1988.

The Pratt hypothesis postulates an equilibrium of crustal blocks of different densities (i.e., topographically higher mountains are less dense than topographically lower units such as the ocean floor, while the depth of crustal material is everywhere the same) (Bates and Jackson 1980). The depth at which the effects of the different densities are balanced (i.e., the level to which all the blocks sink) is the level of compensation.

The lesser-known Heiskanen hypothesis combines assumptions from Airy and Pratt to account for mountain roots, and the variations in crustal densities. It is currently thought that neither the Airy nor the Pratt hypothesis can fully explain the isostatic equilibrium which exists.

The lithosphere is broken into several pieces or plates and it is the movement of these plates which is known as plate tectonics.

Mantle Convection

Mantle convection is responsible for the movement of the lithospheric plates. Since the earth's crust is a part of the mantle, it is carried piggyback, with the mantle's movements. The mantle is warmed at the core-mantle boundary, and then slowly rises to the surface where it cools and sinks again.

Cooling of the mantle takes place through several processes:

volcanic eruptions at mid-ocean ridges and hot spots;

seawater circulation through new crust;

heat conduction through the ocean floor (Gross 1990).

Once cooled, the thickened and denser lithospheric plates are eventually drawn down through subduction trenches, coming to rest on the core-mantle boundary. Over a period of millions of years, the plates will warm and decrease in density until they are sufficiently buoyant to rise through the mantle and begin the cycle again. Images of rising and sinking mantle can be seen using seismic tomography which is similar to computerized axial tomography or the CAT scans used in the field of medicine to image human organs (Gross 1990).

Hot Spots. Some spots on the core-mantle boundary are anomalously hot. At these hot spots, plumes of molten rock rise through the mantle and form volcanoes on the surface. Hot spots do not move with the overlying mantle and lithospheric plates. They remain in the same location for tens of millions of years, recording the movement of the overlying plate with a chain of volcanoes (Gross 1990). The Hawaiian islands and the Emperor Seamounts were formed over a hot spot which is currently found to the southeast of the Hawaiian islands. Loihi is still submerged, but will eventually join the chain of volcanic islands. There are no hot spots in the Indonesian Archipelago.

Hot spots are also believed to be responsible for the formation of several aseismic ridges such as the Ninety-east Ridge in the Indian Ocean, and the Walvis and Rio Grande Rises in the South Atlantic (Brown et al. 1989).

Plate Tectonics

The Beginnings of the Theory of Plate Tectonics.   The theory of continental drift (or more appropriately, continental displacement) was first proposed in 1915 by Alfred Wegner after he observed the jigsaw puzzle fit of the eastern coast of South America with the west coast of Africa. However, he was not the first to observe this intriguing fact, for Alexander von Humbolt, the naturalist-explorer, had made a similar observation in 1801 (Seibold and Berger 1982). Wegner's book, The Origin of Continents and Oceans, was originally published in 1915 in German, followed by the English translation in 1924. Wegner contended that the similarity of the rocks and fossils of the two coasts supported his theory. He theorized that all the continents were once part of a single continent known as Pangaea. The continents moved apart by the displacement of large plates of continental (sialic) crust, moving freely across a substratum of oceanic (simatic) crust (Bates and Jackson 1980). However his hypothesis was not widely accepted by the scientific community. The major objections were due to his proposed mechanisms. Wegner suggested that the continents were rigid plates moving through the ocean basins. The motions were driven by the variations in the gravitational attraction of the earth's equatorial bulge and the westward drift due to the attractions of the Sun and the Moon (Gross 1990). In addition, Wegner's fossil evidence was not definitive.

Prior to the first mapping of the ocean floor in the 1950s, it was believed that the ocean basins and continents were stable features of the earth. Robert Dietz, in 1961, published a paper entitled Continent and Ocean Basin Evolution by Spreading of the Sea Floor introducing the term sea-floor spreading. Based on the ocean mapping data, Harry H. Hess, in 1962, hypothesized in History of Ocean Basins, that the earth's outer surface was in motion, causing continents to fragment, move and create new ocean basins. Hess also proposed that new oceanic crust was being formed at mid-ocean ridges by volcanic activity, and destroyed in trenches (Gross 1990). J. Tuzo Wilson in his 1965 paper, A New Class of Faults and Their Bearing on Continental Drift, elaborated on the ideas, strengthening the concept with the addition of a new type of plate boundary, transform faults.

Figure 2.3. Earth's magnetic poles. Current magnetic orientation is normal with the north and south magnetic poles close to the north and south geographic poles respectively. During periods of reversed polarity, the north and south poles are interchanged.

Magnetic Anomalies.   Magnetic anomalies on the ocean floor were first noted during submarine detection activities. However, it was not until 1963 that the striped pattern parallel to mid-ocean ridges could be. explained. Drummond Matthews and Fred Vine postulated that the patterns represented reversals in the earth's magnetic field. As new oceanic crust is extruded from mid-ocean ridges, the minerals in the cooling rock are aligned in accordance with the existing magnetic orientation. Each stripe represents a section of the ocean floor formed during a particular magnetic orientation, with the adjacent stripe indicative of a different magnetic orientation or magnetic reversal. At present, the magnetic orientation is normal (i.e., the north and south magnetic poles are relatively close to the north and south geographic poles) (fig. 2.3). The.alternating bands of rock with different magnetic orientations create the distinctive striped pattern we call magnetic anomalies (fig. 2.4).

Sea floor produced now and during other periods of normal magnetic orientation show strong or positive magnetic values. During periods of reversed magnetic fields, the north and south magnetic poles are reversed, and the rocks record a weak or negative pattern. Reversals of the magnetic field occur approximately every hundred thousand to a few million years. It has been estimated that in the past 76 million years, 171 magnetic field reversals have occurred (Beiser and Krauskoff 1975). Why these reversals take place is not yet known.

Figure 2.4. Magnetic anomalies. A schematic portrayal of how the distinctive striped pattern of ocean floor magnetic anomalies originates. The sea floor presently being formed along the mid-ocean ridge is of normal magnetic orientation. Earlier episodes of normal magnetic fields are marked by the other shaded bands. The white bands indicate sea floor formed during periods of reverse polarity. Note the symmetry of the magnetic anomalies on either side of the mid-ocean ridge.

Modified after Ross 1977, and Gross 1990.

Not all oceanic crust displays magnetic anomalies. During a period of the earth's history, 80-120 million years ago, magnetic reversals did not occur. As a result, no magnetic anomalies are present in rocks formed during that interval. Deep burial and intense heat may also erase the pattern of magnetic anomalies (Gross 1990).

Magnetic anomalies are also a means to map the age and rate of spreading of the ocean floor. By correlating the pattern of oceanic magnetic anomalies with the pattern observed in rocks of known ages on land, we can determine the ages of the various sections of ocean crust, and estimate the rate of sea-floor spreading.

Spreading rates range from less than one centimetre per year on the Mid-Atlantic Ridge, to 16 centimetres per year on the East Pacific Rise. In all cases, the youngest crust is found in a band straddling the spreading ridge, with increasingly older crust on each side of the ridge as the distance from the ridge crest increases. The faster the spreading rate, the thicker the central band of young crust. The oldest oceanic crust, estimated to be 190 million years old, is found in the North Pacific near Asia, and along the margins of the North and South Atlantic. It is believed that approximately half the ocean floor is less than 80 million years old (Gross 1990).

Further support for Wegner's Pangaea theory was provided by Sir Edward Bullard in 1965. Using a common depth contour as the edges of all the continents, Bullard was able to piece together the continents to form a supercontinent resembling Pangaea. There were some areas of overlap where relatively new features such as coral reefs and river deltas had developed, but overall the fit was good enough to support Wegner and the Pangaea concept (Gross 1990).

Plate Tectonics -The Parts and the Processes. Plate tectonics is the movement of the earth's lithospheric plates (composed of the upper mantle and crust) on the asthenosphere. There are seven major plates and many small plates. The major plates are: Pacific, Indo-Australian, North American, South American, Eurasian, African, and Antarctic. Smaller plates include the Philippine Plate, China Plate, Gorda Plate, Cocos Plate, Nazca Plate, Caribbean Plate, Scotia Plate, Arabian Plate, Iranian Plate, Hellenic Plate, and Juan de Fuca Plate (fig. 2.5).

Plate Boundaries.   All plates are in contact with several other plates. There are three types of plate boundaries:

a) divergent or constructive margins

b) convergent or destructive margins

c) conservative boundaries or transform faults.

Divergent or Constructive Margins.   Divergent margins are also known as Atlantic, passive, aseismic or constructive margins. Divergent margins develop when continents rift apart and form new ocean basins. As a result, continental crust and the adjacent oceanic crust are part of the same plate. As the rift widens, the continental margin grows further from the spreading centre and closer to the stable interior. Micro-continents may form if these pieces of continental crust are isolated as a result of rifting or other plate movements (Brown et al. 1989). The actual cause of a divergent margin is not fully understood, but it is thought to begin with the development of crustal stretching, extensional faults, rift basins, and possibly regional uplift and volcanism initiating sea-floor spreading (Hutchinson 1992).

A divergent margin, as shown in figure 2.6, generally features the following physical characteristics:

a) a continental shelf, gently sloping (average gradient of 0.1°) from the shore and extending to a depth not exceeding 130 m; may be 1500 m wide

b) a steep (average gradient of 4°) continental slope, 20 to 100 km wide, continues from the continental shelf to a depth of 4000 to 5000 m, and may feature submarine canyons

c) the continental rise, (average gradient of 1 °), may be up to 600 km wide and leads into the abyssal plains (Hutchinson 1992; Brown et al. 1989).

Spreading ridges or spreading centres are the site of new crust formation and occur at divergent or constructive margins. As new crust cools and moves away from the spreading ridge, it increases in density and thickness. New crust is only a few kilometres thick, while old crust reaches thicknesses of 150 km. With increasing density, the crust sinks deeper into the asthenosphere from a depth of 2500 m to 6000 m as it ages (Gross 1990). Two extensively studied examples of early stage continental rifting are the northern Red Sea and the Salton Trough in Baja, California (Hutchinson 1992). Active spreading centres in this region are found in the Andaman Sea, Aru Trough, Bismarck Sea, and the Woodlark Basin (Petroconsultants Australasia 1991).

Figure 2.5. Tectonic plates of the world. Simplified world map showing the major plates and some minor plates.

After Brown et al. 1989, Gross 1990, and Ganeri 1994.

Figure 2.6. Divergent margins. A generalized cross-section showing the main features of divergent margins: (1) continental shelf, (2) continental slope, (3) continental rise, (4) continental margin, (5) abyssal plain, (6) oceanic ridge. Not to scale.

Modified after Brown et al. 1989, and Hutchinson 1992.

Convergent or Destructive Margins.   Convergent margins are also known as active, seismic, Pacific-type, or destructive margins. On land, visible signs of convergent margins are active volcanoes, frequent earthquakes, island arcs, and young mountains (Gross 1990). Below the sea surface, subduction trenches mark locations of convergent or destructive margins.

Subduction Zones.   Most subduction zones involve the subduction of oceanic crust beneath lower density continental crust. Oceanic crust is destroyed as it is drawn into the mantle. Earthquakes occur as the plates drag past each other. Deep- and intermediate-focus earthquakes (100-700 km below the surface) are indicative of subduction zones (Gross 1990). General features of a subduction zone are: trench, accretionary wedge and fore-arc ridge, fore-arc basin, volcanic arc (island arc), and back-arc basin (fig. 2.7). The trench is where the descending plate first contacts the material from the overriding plate as it heads towards the mantle. The accretionary wedge forms when sediments and other materials are scraped from the subducting plate and accumulate (like the action of a snowplow pushing the snow ahead to clear a path), at the front of the overriding plate (Hamilton 1989). It is composed of deformed rocks. The fore-arc ridge is the summit of the accretionary wedge. The fore-arc basin is found landward of the accretionary wedge and contains relatively less-deformed sediments (Hutchinson 1992). The frontal arc located between the accretionary wedge and the volcanic arc is a zone of uplift and deformation. The volcanic arc is a zone of active igneous activity (Hutchinson 1992). The back-arc region is found behind the volcanic arc and may contain marginal (back-arc) basins or ancient, inactive arcs.

Figure 2.7. General features of a convergent margin are: (1) active trench, (2) accretionary wedge, (3) fore-arc ridge, (4) fore-arc basin, (5) frontal arc, (6) volcanic arc, (7) back-arc region.

Modified after Curray 1989, and Hutchinson 1992.

The Benioff zone, also known as the Benioff seismic zone, is a seismic zone where earthquake foci cluster. An earthquake focus is the point within the earth which is the centre of an earthquake (Bates and Jackson 1980). The Benioff zone stretches downward from the oceanic trench, dipping toward the continents usually at an angle of 45°.

The Scenario Involving Old Subducting Crust.   When the subducting or descending oceanic plate is old and therefore dense, it descends as a steeply dipping slab. This is a common scenario in the western Pacific basin where the oldest oceanic crust occurs (fig. 2.8).

Subduction of the oceanic crust may also be accompanied by back-arc spreading and basin formation as is the case in the area of the Mariana Trench (Gross 1990). According to Hutchinson (1992), subduction involving old oceanic crust is low-stress, and commonly features a small accretionary wedge, few large earthquakes, igneous rocks with a narrow basaltic compositional range, a steeply dipping descending slab, and a well-developed back-arc basin.

The Scenario Involving Young Subducting Crust.   When the subducting plate is composed of relatively young, still-buoyant crust, it descends as a shallow-dipping slab (fig. 2.9). The east coast of the Pacific (i.e., the west coast of North and South America) is an example of where this type of shallow subduction is taking place. Active volcanoes and young mountains line the coast. The mountains are formed from material scraped off the subducting plate (Gross 1990). Subduction involving young oceanic crust is classified by Hutchinson (1992) as high-stress, and features a large accretionary prism, large shallow earthquakes, igneous rocks of varied composition, and a shallow-dipping plate.

Figure 2.8. Subduction involving old oceanic crust. The main features to note are: (1) steeply dipping oceanic plate, (2) deep trough with little accumulation of sediments or a small accretionary wedge on the upper plate, (3) active island arc, (4) well-developed back-arc basin and, (5) extinct arc. (Few large earthquakes.)

Modified after Gross 1990, Curray 1989, and Hutchinson 1992.

When material is scraped off an oceanic plate and accretes to a continental plate, it is known as an ophiolite. Ophiolites are a specific type of exotic terrane which is any fragment of continental or oceanic plate welded onto a continent. Ophiolites are often rich sources of sulphide minerals formed at spreading centres, and are mined for copper, lead, zinc, and silver (Gross 1990).

Subduction is presently occurring off the coasts of Sumatra, Java, along the East Sunda Arc, Banda, Seram, Sangihe, North Sulawesi, Cotabato/West Sangihe, and Halmahera (Petroconsultants Australasia 1991). The trenches where subduction is taking place are known as active trenches, as opposed to inactive trenches where subduction has ceased.

Collision of Two Continental Plates.   Collision of two continental plates, does not produce a subduction scenario because both plates are composed of fairly buoyant crust. Instead, overriding and uplift can occur with one plate being folded and thrust upon the other (Gross 1990). This is what happened when the Indian Plate collided with the Eurasian Plate resulting in the formation of the Himalaya Mountains. Collisions are also taking place in Sulawesi, and Timor-Tanimbar (Petroconsultants Australasia 1991).

Subduction Involving Two Oceanic Plates.   When one oceanic plate is subducted beneath another oceanic plate, a trench is present, and a volcanic island arc forms on the overriding plate. On the opposite side of the island arc, away from the trench, commonly a marginal or back-arc basin forms as a result of sea-floor spreading.

Figure 2.9. Subduction involving young oceanic crust. The main features to note are: (1) shallowly-dipping oceanic plate, (2) relatively shallow trench, (3) large accretionary wedge, (4) fore-arc basin, (5) active volcanoes. Earthquakes are generally large and shallow.

Modified after Gross 1990, and Hutchinson 1992.

Conservative Boundaries or Transform Faults.   The third type of plate boundary involves lateral movement with transform faults (fig. 2.10). Transform faults are a type of strike-slip fault in which the plates slide horizontally past each other with oceanic crust neither created nor destroyed (Roberts 1989). Many transform faults are associated with mid-ocean ridges and are seismically active. They run perpendicular to the line of the ridge, offsetting the ridge, and terminate where they meet the ends of the two offset ridge segments which they connect (Brown et al. 1989). The extension of a transform fault beyond those points is known as a. fracture zone, and is characterized by no relative sideways motion, and low seismicity (i.e., small earthquakes) as it is located within a single plate. Examples of transform faults in Indonesia are in Sumatra and Sorong (Petroconsultants Australasia 1991). One of the world's best-known examples of a transform fault is the San Andreas Fault in California, U.S A.

Figure 2.10. Transform faults. The ridges of the earth are marked by a series of transform faults which offset the ridges resulting in the irregular outlines. Movement along the faults during seafloor spreading produces shallow earthquakes marking the active section of the fracture zone. Beyond the active area the fracture zone features steep ridges and valleys. (1) Mid-Atlantic Ridge, (2) Southwest Indian Ridge, (3) Indian Ridge, (4) Southeast Indian Ridge, (5) Ninety-east Ridge, (6) Java Trench.

Modified after Ganeri 1994, Earth 1992, and Gross 1990.

Earthquakes

Rocks, like people, can be affected by stress. Stress builds up in rocks as lithospheric plates move past each other. When the stress is greater than the strength of the rocks, the rocks are strained, then fail (i.e., fracture along zones of weakness), faults are formed, and the energy is released in the form of an earthquake (Keller 1979). Although plates are often described as sliding past another plate, the movement is not smooth and there is much friction where the plates meet. Movement along faults produces seismic waves to depths of 700 km (Ross 1977), and in the near-surface of the earth.

An earthquake may be described as a sudden motion of the earth caused by faulting or volcanic activity. As many as a million earthquakes each year are recorded on seismographs, with most escaping the notice of human populations.

The centre of an earthquake is called the focus or hypocentre. The focus is the point deep within the earth of the initial rupture and where the strain energy is first converted to elastic wave energy (Bates and Jackson 1980). Shallow-focus earthquakes are those with a focus at 70 km or shallower. Deep-focus earthquakes begin below 300 km (Kingston 1988). The point on the earth's surface above the focus is called the epicentre. The epicentre is generally located directly over the focus, except when the earthquake originates deep within a subduction zone.

The Mercalli Intensity Scale.   The Mercalli intensity scale, devised in 1902, is an arbitrary scale of earthquake intensity, ranging from I (detected only by instruments) to XII (almost total destruction). Its adaptation to urban conditions is known as the modified Mercalli scale (Bates and Jackson 1980).

Modified Mercalli scale (from Whitten 1972, and Spencer 1972):

Instrumental. Detected only by seismographs.

Feeble. Noticed by only a few persons at rest. Delicately suspended objects may swing.

Slight. Resembles vibrations caused by heavy vehicle traffic.

Moderate. Felt by most people. May waken some. Rocking of free-standing objects. Walls crack. Sensation like heavy truck striking building.

Rather strong. Sleepers awakened. Bells ring. Widely felt.

Strong. Felt by all. Trees sway. Some damage from overturning and falling of objects.

Very strong. General alarm. Everyone runs outdoors. Damage is negligible in buildings of good design and construction. Slight to moderate damage in well-built ordinary structures.

Destructive. Fall of factory smoke stacks, columns, monuments, and walls. Heavy furniture overturned.

Ruinous. Ground begins to crack. Houses collapse. Underground pipes break.

Disastrous. Ground badly cracked. Many buildings destroyed. Some landslides. Rails bent. Water splashed over banks.

Very disastrous. Few buildings remain standing. Bridges and railways destroyed. Broad fissures in the ground.

Catastrophic. Total destruction. Waves seen on ground surface. Objects thrown into the air.

The modified Mercalli intensity scale is based on effects which can be observed, and refers to the violence of the earthquake motion (Spencer 1972). However, this scale is difficult to apply for accurate, quantitative, worldwide comparisons of earthquakes. In 1935, the Richter magnitude scale was devised.

The Richter Magnitude Scale.   The Richter magnitude scale, more commonly used today, and generally referred to as the Richter scale, is a logarithmic scale, based on the amount of energy released at the focus, as recorded by a seismograph. A seismograph records earthquake waves. The amplitude of the largest wave determines the magnitude (Keller 1979). An earthquake of magnitude 6 produces a displacement on a seismograph 10 times larger than does a magnitude of 5. A magnitude 5 earthquake releases approximately 10²¹ ergs of energy which is equivalent to the first atomic bomb detonated in 1945, or 20,000 tons of TNT (Spencer 1972). However, although the total energy released may be comparable, its mode of dispersal produces very different effects depending on whether it was released in a highly concentrated form as in the atomic bomb, or widely dispersed as in an earthquake. Table 2.1 provides a qualitative description of the Richter scale.

Effects of Earthquakes.   In addition to the well-known disastrous effects of earthquakes on population centres, earthquakes affect marine environments (see section on coral reefs and natural disturbances). The primary effects of earthquakes are violent ground/substrate motion, surface rupture, and permanent substrate displacement of a metre or more (Keller 1979). Secondary effects maybe divided into short-term events such as landslides, tsunamis and floods. Long-range effects are regional subsidence or emergence of landmasses.

Distribution of Earthquakes.   Active seismicity marks plate boundaries. The zones of seismicity tend to be narrow when associated with mid-oceanic spreading centres, and strike-slip faults. Wider zones occur above subducting plates, and in those parts of continents undergoing distributed extensional, strike-slip, and compressional deformation (Hamilton 1979). Earthquakes of a magnitude greater than 8.0 occur primarily along subducting plate boundaries, and less frequently in continental strike-slip and compressional deformation situations.

Distribution of Earthquakes Worldwide.   The earth can be divided into 10 regions based on seismic activity:

The circum-Pacific belt contains most of the shallow- and intermediate-depth earthquakes, and almost all deep-focus earthquakes;

The Alpine belt of Europe and Asia, containing the Alps and Himalayas, is the other significant zone of shallow and intermediate-depth earthquakes;

The Pamir-Baikal zone of central Asia;

The Atlantic-Arctic belt;

The belt of the central Indian Ocean;

Rift zones, notably those of east Africa;

A wide triangular active area in eastern Asia, between the Alpine belt and the Pamir-Baikal zone;

Minor seismic areas, usually in regions of older mountain building;

The central basin of the northern Pacific Ocean; almost nonseismic except for the Hawaiian islands;

The stable central shields of the continents, also nearly nonseismic.

Distribution of Earthquakes in Indonesia.   Indonesia, located within the famed Ring of Fire, is frequently hit by earthquakes covering a range of magnitudes. Approximately 10% of the world's seismicity occurs in the Indonesian Archipelago. Katili (1985) gives a brief summary of the distribution of Indonesia's shallow, intermediate, and deep earthquakes (fig. 2.11).

Table 2.1. Scales of magnitude on the Richter scale.

Figure 2.11. Distribution of shallow and deep earthquake epicentres in Indonesia.

After Katili 1985, and Hamilton 1974.

Indonesia's shallow-focus earthquakes most commonly occur above the subducting plate boundaries of the Java-Sumatra Trench and the Banda Trench, within the active collision zone between Sulawesi and Halmahera, and associated with transcurrent faults (i.e., the Great Sumatran fault zone, the Palu-Koro-Matano fault zone, the Gorontalo fault zone, and the Sorong fault zone).

The intermediate-depth (focus depth of 100-300 km) earthquakes are generally spread along the whole volcanic arc divided in the middle by the axis of active volcanoes in Sumatra, Java, the Lesser Sunda Islands, Sulawesi and Halmahera (Katili 1985).

The deep earthquakes (focus depths of 500-800 km) are clustered along an east-west trending belt from the Java Sea to the Banda Sea, and a north-south trending belt from Sulawesi to Mindanao.

VOLCANOES

To many, the mere mention of Indonesia brings to mind the archipelagic nation's most famous volcanoes, Krakatau and Tambora. Krakatau, immortalized by Hollywood's movie industry, although geographically misplaced in the film Krakatau, East of Java, ranked fourth in the world's greatest historic eruptions (Hutchison 1982). The 1815 eruption of Tambora on the island of Sumbawa was the most violent explosion of recorded history, yet is relatively unknown outside geological circles. Indonesia's two most famous eruptions will be discussed in greater detail after an introduction to volcanoes and volcanic activity.

The Shapes of Volcanoes

Volcanoes come in several shapes and sizes. Volcanoes show a wide variety of forms, depending largely upon the composition of the erupted material and hence the style of eruption (Thorpe and Brown 1985). The three main shapes are cinder cones, shield volcanoes and composite or stratovolcanoes.

Several factors determine the shape of a volcano:

Land surface;

Type and nature of material ejected (i.e., viscous lava and cinder build up steep cones, while more fluid lava flows further away from the vent and results in wide-based mountains);

Forcefulness of the eruption;

Duration of the activity;

Eruptive history of the volcano.

Cinder Cones.   Cinder cones are built when pyroclastics, such as cinders, and ash, pile up around the vent. A crater normally surrounds the vent. The slopes are generally steep (greater than 10°) and symmetrical. These volcanoes are usually basaltic or andesitic. The magma contains a relatively high gas content, resulting in higher explosivity (van Bemmelen 1949) than cumulo-volcanoes (volcanic domes) or lava shields (shield volcanoes of the basaltic type). Examples: Lamongan, East Java and Vesuvius, Italy.

Shield Volcanoes.   Shield volcanoes feature the largest cones. This type of volcano has a broad, shield, or low-slope profile. Its diameter may be between 100 and 200 km (Thorpe and Brown 1985). Shield volcanoes represent the largest discrete volcanic form, but they are rarely preserved in the geological record, since they occur mainly as oceanic islands (Thorpe and Brown 1985). They are produced by eruption of low-viscosity lava from either a central vent, fissures or parasitic vents. The flow generally consists of very fluid basaltic lava or rhyolitic ash flows (Bates and Jackson 1980). The gas content of the magma is low. They are also sometimes referred to as lava domes. Examples: Sukadana, South Sumatra and Mauna Loa, Hawaii.

Composite Cones (Stratovolcanoes).   Stratovolcanoes, or composite cones, have a central vent from which lavas as well as pyroclastics are expelled. This type of volcano is composed of alternating layers of lava and pyroclastics, resulting from the prolonged activity of a central vent causing the formation of bedded-volcanoes (van Bemmelen 1949).

Stratovolcanoes are the most common shape for andesitic volcanoes. They have steep, irregular conical forms with diameters of 10-40 km. The volcano often has a shape combining cinder and shield volcanoes. There are often many dikes and sills. The lava, viscous and acidic, is generally restricted to the area of the volcano, but the pyroclastic material may be transported by wind more than 1000 km (Thorpe and Brown 1985). These deposits are often used by stratigraphers as markers or reference points. Examples: Merapi, Central Java and Fujiyama, Japan.

Composite volcanoes clearly show a zonation of volcanic products which may be divided into the central, proximal arid distal zones with increasing distance from the central vent (Thorpe and Brown 1985).

The central zone is located within 2 km of the central vent. It is characterized by lava conduits. The volcanic products associated with this zone are coarse, poorly-sorted pyroclastic materials which have been deposited close to the vent.

The proximal zone, situated 5-15 km from the central vent, contains a higher proportion of lava flows and a variety of pyroclastic flow deposits.

The distal zone is found beyond the proximal zone, and consists of pyroclastic flow deposits associated with fine air-fall deposits dispersed by the wind away from the volcano.

Other Volcanic Landforms

Caldera.   A caldera is a large, basin-shaped volcanic depression, more or less circular, the diameter of which is many times greater than that of the included vent or vents (Bates and Jackson 1980). There are two main types of volcanic calderas depending on the mode of formation (i.e., collapse caldera, and explosion caldera). A collapse caldera is formed when the top of the magma chamber collapses into a void or cavity created by the removal of magma either by large volume eruptions of lava or pyroclastics, by subterranean withdrawal of magma, or contraction of magma as it cools and crystallizes. The top of the volcano collapses into the void (subsidence phenomena) forming an enclosed or partially enclosed depression called a caldera which maybe tens of kilometres across. Most calderas are of this type (Bates and Jackson 1980). An explosion caldera is formed by the explosive removal of the upper part of a volcanic cone. This type of caldera is extremely rare, and is small in size according to Bates and Jackson (1980).

Plateau Basalts.   Plateau basalts are the most extensive volcanic landform, with the basaltic lavas covering areas up to 10⁵ km² (Thorpe and Brown 1985). The lava originates from cracks or fissures (i.e., fissure eruptions) rather than from a central vent (Rhodes 1991). Plateau basalts erupted in rapid succession over vast areas and have, at times, flooded sectors of the earth's surface on a regional scale (Bates and Jackson 1980). Examples are known from India, Iceland and the United States of America where the Columbia River Plateau has an estimated volume of 417,000 km covering an area greater than 250,000 km (Spencer 1972). These lavas range in age from Eocene (40-60 million years old) to Pleistocene and Recent (i.e., less than 2 million years old).

Volcanic Domes.   Volcanic domes are dome-shaped, bulbous masses of hardened lava. They form above and around volcanic vents. When a volcanic dome forms on the side of, or close to a larger volcanic cone, it is known as a parasitic volcano. Parasitic volcanoes are generally 1-2 km diameter, created by a single, short-lived eruption (Thorpe and Brown 1985).

Lahar.   Lahar is an Indonesian word which has been adopted by geologists worldwide to describe mudflows which contain debris and angular blocks mostly of volcanic origin (van Bemmelen 1949). The transporting medium is a mixture of cool (<100°C) liquid water and gas. The transported materials include poorly-sorted large volcanic fragments in a finer ash-size matrix (Thorpe and Brown 1985). Some of the materials may be derived directly from magma reaching the surface or juvenile materials (Bates and Jackson 1980). According to Thorpe and Brown (1985), eruptions of volcanic material through lakes, below ice or during heavy tropical rain may generate lahars composed largely or entirely of juvenile volcanic material.

Van Bemmelen (1949) described normal lahars or cold mudflows as not exclusively volcanic. They originate by heavy rainfall on slopes covered with loose material or by earthquakes (e.g., Benkulen 1933) and are typical for tropical volcanoes (e.g., Merapi). They were also known as rain lahars but may more correctly be termed debris flows or slumps. These debris flows are especially common where deposits form in shallow water conditions subject to volcanic instability and seismic activity (Thorpe and Brown 1985).

Hot mudflows are caused by the emptying of a crater lake through crater wall collapse or explosion. Under these circumstances, the lake water is mixed with hot volcanic material (e.g., Kelud, East Java 1811, 1826, 1835, 1848, 1864, 1901, and 1919) (van Bemmelen 1949).

Lahars tend to be deposited in low-lying land close to a volcano, although lahars have been found up to 300 km from their source (Thorpe and Brown 1985). Rain lahars are typical of tropical volcanoes and influence the outline of the cones (van Bemmelen 1949).

The shape of a volcano is obviously a reflection of the volcanic activity which is in turn determined by several physical factors.

Factors Determining the Types

of Volcanic Activity

Several factors determine the type of volcanic activity:

temperature, composition, fluidity/viscosity, and pressure build-up of the magma

types of release conduits or openings through which the magma may be released (i.e., summit craters, parasitic craters, radial fissures, regional fissures)

where the eruption occurs (i.e., submarine, sublacustrine or under a lake, subglacial, or subaerial).

Eruptions are generally classified as either explosive, lava or mixed eruptions.

An explosive eruption is an eruption that is characterized by the energetic ejection of pyroclastic material (Bates and Jackson 1980). There are two main types of explosive eruptions. Incandescent eruptions describe episodes involving glowing pyroclastic material such as ash flows, and Nuée ardentes. Phreatic explosions involve the explosion of steam, mud and other incandescent (non-glowing) materials.

A lava eruption is characterized by the emission of lava, generally free of explosions.

A mixed eruption includes both the emission of lava and the explosive ejection of pyroclasts.

Specific types of volcanic eruptions have been named after type localities (e.g., Hawaiian, Strombolian, Plinian, Pelean, Vulcanian, or Katmaian type eruptions).

Types of Volcanic Eruptions.

HAWAIIAN TYPE.   The Hawaiian type eruption is characteristic of shield volcanoes. The height of the eruptive column is very low (i.e., materials are not sent high into the atmosphere). The area affected by ash fall is less than 0.1 km² and explosive episodes are rare. Fissures on the sides of the volcano, are the conduits from which the highly fluid basaltic lava is extruded (Bates and Jackson 1980). Example: Mauna Loa, Hawaii.

STROMBOLIAN TYPE.   The Strombolian type eruption has a moderate eruptive column height of up to 1 km. Ash falls within a 5 km² radius. The fluid, basaltic lava spurts, fountain-like, from a central crater. The Stromboli Volcano on the Lipari Islands, Italy, typifies this type of eruption pattern.

PLINIAN TYPE.   The Plinian type of eruption features a towering eruptive column reaching up to 20 km. Nuées ardentes are common. The area affected by ash fall may be as much as 1000 km². Unlike the previous two eruptive types which were cone-building, Plinian eruptions are sheet-building. Plinian eruptions are characterized by explosive eruptions in which a steady, turbulent stream of fragmented magma and magmatic gas is released at a high velocity from a vent (Bates and Jackson 1980). Examples: Tambora, and Krakatau.

PELEAN TYPE.   The Pelean type eruption is characterized by gaseous clouds (nuées ardentes) and/or the development of volcanic domes. Example: Mount Pelee, Martinique.

VULCANIAN/VESUVIAN/VULCANO TYPE.   The Vulcanian type eruption, also known as the Vesuvian or Vulcano type, is characterized by the explosive expulsion of new lava fragments. Although the fragments are glowing when they leave the vent, they are too solid or viscous to be altered in shape while airborne. Material expelled includes blocks and ash. Example: Mount Vesuvius, Italy.

KATMAIAN TYPE. The Katmaian type eruption describes the violent, explosive release of vast amounts of pumice and ash, followed by an ash flow and extensive fumarole activity. Fumaroles are holes or vents through which gases and vapors escape. They are characterized by quiet emission and sulphur deposition. Fumaroles are much more common than geysers, but less of a tourist attraction. They may be situated along fissures or in clusters known as a fumarole field. An Indonesian example is Wurlali Volcano crater, Damar Island, Banda Sea. Another example is Mount Katmai, Alaska, which includes the Valley of Ten Thousand Smokes, an area which, as its name suggests, is rich in fumaroles.

PAROXYSMAL ERUPTION.   A paroxysmal eruption is an eruption of the Katmaian, Pelean, Plinian or Vulcanian type.

PHREATIC ERUPTION.   A phreatic eruption involves the explosion of steam, mud, or other material that is not incandescent. It is caused by the heating and consequent expansion of groundwater due to an underlying igneous heat source (Bates and Jackson 1980). The greatest phreatic eruption in recorded history was in 1933 at Suoh, South Sumatra (Hutchison 1989b).

Products of Volcanic Activity

The materials which emanate during volcanic activity fall into one of three groups; lava, gases, and pyroclastics (solid fragmental materials).

Pyroclastics.   Pyroclastic describes clastic rock material which has formed by volcanic explosion or aerial expulsion from a volcanic vent (Bates and Jackson 1980). Pyroclastic rocks are composed of materials fragmented by explosive activity (Thorpe and Brown 1985). Pyroclastics are generally classified according to size and shape. Pyroclastics also include partially fluidized mixtures of particles and gases that travel up to 150 m/s and have internal temperatures of up to 600°C. Pyroclastics are denser than the atmosphere but may be of comparable density to seawater (Sigurdsson et al. 1991).

Pyroclastic Classification,

GENETIC CLASSIFICATION.   Pyroclastics may be separated into two categories on the basis of how they are deposited, or their genetic classification.

Air-fall deposits result when pyroclastic material is erupted into the atmosphere and settles out downwind from the vent. Fine-grained pyroclastic material may travel more than 1000 km (Thorpe and Brown 1985).

Pyroclastic flows result from transport of solid fragments of volcanic rock in a fluid (gas or liquid) matrix away from the volcano. Unlike the air-fall deposits which move up and away, pyroclastic flows travel close to the ground, moving away from the vent laterally. Pyroclastic flows may travel up to 100 km. Since the flows may have volumes of up to 3000 km³, the shields or plateaux created by such flow deposits may cover areas of 10⁵ - 10⁶ km² (Thorpe and Brown 1985). These plateaux are more likely to be preserved in the geologic record than basaltic plateaux, since the pyroclastic plateaux tend to be found in inland continental areas distant from destructive margins.

Nuée ardente is a French term meaning glowing cloud, which aptly describes the swiftly flowing, turbulent, often incandescent, gaseous cloud which contains ash and other pyroclastics in its lower section (Bates and Jackson 1980). Up to several hundred thousand cubic metres of red-hot blocks may be associated with the cloud of hot air and ashes. This glowing cloud is extremely destructive, as was the case in 1902 when the eruption of Mount Pelee released a nuée ardente that wiped out the town of St. Pierre, Martinique.

LITHOLOGICAL CLASSIFICATION.   Pyroclastics may be classified according to type of material and grain size, or a lithological classification.

Blocks are the largest in size, with diameters greater than 64 mm, and may weigh several tons. Blocks are ejected in a solid state.

Bombs are also 64 mm in diameter or larger, but unlike blocks, bombs are ejected while still viscous, so they are shaped as they travel through the air. Bombs, therefore, tend to be somewhat rounded, although they exist in a variety of shapes and have similarly diverse internal structures ranging from vesicular to hollow.

Lapilli fragments range in size from 2 to 64 mm. A lapillus does not have a characteristic shape, and may be viscous or solid when it hits the ground.

Ash consists of fine, unconsolidated pyroclastic material, with a diameter size range of 1/16- 2 mm.

Dust is fine volcanic ash, less than 1/16 mm in size.

Pumice is a light-coloured, porous, glassy, highly vesicular rock which is formed when gases bubble through a highly viscous rhyolitic lava. This rock is of such low density that it floats on water.

Scoria is similar to pumice (i.e., a vesicular volcanic rock), but contains magnesium and iron and is heavier and darker. Scoria is generally formed from andesitic or basaltic lava.

Tuffs are rocks composed of compacted, consolidated volcanic ash or dust.

Agglomerate is a pyroclastic rock consisting of medium- to large-sized pyroclastic fragments embedded in a finer-grained matrix of volcanic ash.

Ignimbrite is a pyroclastic rock formed by the eruption of an ash flow. It is composed of crystals and rock fragments lying in a matrix of glass shards (Roberts 1989). Ignimbrites are essentially consolidated ash flows and nuées ardentes. They may be massive and continuous over long distances resembling lava flows.

From the discussion above, it is apparent that volcanoes produce more than lava. Thorpe and Brown (1985) point out that all volcanoes erupt both lava and solid pyroclastics, although the proportions vary depending on the type of volcano. For example, basaltic volcanoes such as those in Hawaii generally expel 80% lava and only 20% pyroclastics. The volcanoes of island arcs and active continental margins tend to erupt less than 10% andesitic lava, and more than 90% pyroclastics. The quantities of pyroclastic rocks, especially on island arcs and continental margins, may be underestimated because they are more easily eroded and wind-dispersed than solid lava (Thorpe and Brown 1985).

Lava.   Lava includes both molten extrusive rock and its solidifed product (Thorpe and Brown 1985). It is erupted at temperatures ranging between 900° and 1200°C (Rhodes 1991). Three main forms have been described; aa or blocky lava, pahoehoe or ropy lava, and pillow lava.

Aa, or blocky