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Natural Hazards in Bangladesh
RODINTA, GONDWANA AND ASIA
of these locations there has been a geologically immediate response to the entry of continental lithosphere into the subduction system. Collision of a relatively minor intra-oceanic island arc, the Luzon arc, with the continental margin of Asia has given rise to near 4 km of topographic relief in Taiwan over the past 5 Ma. The deformation front is propagating across the leading edge of the continental margin of eastern China just as the last of the remaining oceanic lithosphere is being subducted and arc magmatism is ceasing. In northern parts of Taiwan where collision has finished, orogenic collapse with attendant extrusion parallel to the axis of the collision belt is now well advanced and topographic relief is actively being reduced. Prevailing models for the India - Asia collision, the greatest on-going collision event in existence, suggest that collision between two major continental fragments at 55 Ma was not followed by mountain-building orogenesis for 20 million years or longer. One might ask why this has not previously been
561
thought to be highly anomalous. Either existing models that invoke India-Asia collision at 55 Ma are wrong, collision occurred at a different time and/or a different collisional event is recorded. We suggest that two different tectonic events are recognisable. Features related to each event need to be interpreted separately and no collisional continuum should be assumed. One event occurred at around 55 Ma, another at around 30 - 25 Ma. Collision of India with an intra-oceanic island arc at the end of the Cretaceous, removed a south-facing convergent plate boundary from within Tethys. Recognition of this event provides an explanation for the slowdown in convergence between India and Asia at 55 Ma. This intra-oceanic island arc is now preserved within the India-Asia suture zone. Continent-continent collision between India and Asia did not occur until the end of the Oligocene and was met by an immediate response in the form of orogenesis.
Natural Hazards in Bangladesh Afia Akhtar Geological Survey of Bangladesh, Dhaka, Bangladesh Natural hazards like Nor’Wester tropical cyclone and flood are most common in Bangladesh. These events occur almost every year and cause loss of life and properties, the magnitude of which depends on how devastating the events are. Bangladesh, a small land having 147570sq.km. and a developing country, lies between 20° 34’to 26O38’north latitude and 88O01’ to 92O41’east longitude. There are three distinct climatic patterns in Bangladesh: a) the summer with average temperature from 21OC to 34OC, b) the winter temperature from l l ° C to 29OC and c) the rainy season having average rainfall from 1194mm to 3454mm. But, more specifically, Bangladesh is characteristically influenced by a tropical monsoon climate and demarcated by two main seasons, the dry and the wet, the period of which ranges from November to March and J u n e to October respectively. April and May, these two months is the pre-monsoon period when violent thunderstorms along with hailstorms known as ‘Kal Baisakhi’ / ‘Tufan’in Bangla or ‘Nor’Westers Cyclone’ in English, happens in Bangladesh frequently, the effects of which are sometimes very severe. The Tropic of cancer passes across the middle part of Bangladesh and as a result it should be an arid country. But the fact is that, the southwesterly monsoon brings heavy rainfall to Bangladesh and makes it a wet country, and it is because of the Himalayan mountain ranges on the north of the country. The part of Sylhet of NE Bangladesh and specially the Cherapunji of Assam (India), just north of Bangladesh are the highest rainfall areas of the world, the annual average rainfall is more than 5080mm. A total length of about 24140 km. of the country is covered by a network of three big rivers - the Ganges, the Brahmaputra, the Meghna, forming a complex river system. And the location of Bangladesh is just on the lower reaches or the confluence of these three mighty river systems. The origins Gondwma Research, V. 4, No. 4, 2UUl
of all major rivers are northwest, north and northeast of Bangladesh and flows towards south to the Bay of Bengal. The total catchment area of three river system is about fourteen times of total land of Bangladesh. Moreover, almost the entire Bangladesh is a part of Bengal Basin, the second biggest delta of the world. Water from snow melting of the Himalayas which lies in the north and runoff or water flow from this vast catchment areas along with local monsoon heavy rainfall, altogether generate a tremendous volume of water which flows southwards over Bangladesh to the Bay of Bengal. Some times, the river systems of Bangladesh are unable to accommodate these huge volumes of water and as a result, the heavy downpour and water flow from upper reaches inundates many parts of Bangladesh and causes floods. Northern Bangladesh naturally suffers more due to this heavy flow of water and consequently river erosion is another common hazardous phenomenon in this area, which engulfs agricultural lands and houses and retard the national economic growth greatly. The effect of tropical cyclone is catastrophic but more severe especially in the coastal or southern part of the country along the Bay of Bengal, as the rise of sea surface temperature is the main reason for the formation of the cyclone. Not only that, Bangladesh is also highly vulnerable to earthquake hazard, as it is just adjacent to the seismically active Himalayas and the Arakan-Yoma hill ranges and in between two tectonic plates - the Indian plate and the Burma sub-plate. This region falls within one of the world’s three major seismic or earthquake belts - the Mediterranean and Trans Asiatic or Eurasian Belt. The overall geographical position and geological condition of Bangladesh is such that it is a hazard-prone area. Being a developing country, it is not always possible for Bangladesh to
562
RODINIA, GONDWANA AND ASIA ~~~~~~~~
take adequate mitigation measures to cope with these natural hazards properly. To cope with this acute situation, there should be societal motivation to acquire geoscientific knowledge and develop adequate means to protect life and properties through
proper land use planning, construction of disaster proof houses, growth of vegetation and so on. In this regards, public awareness should be created and combiried afford from public, private and government sectors should be incorporated.
Impacts of Overpopulationon Natural Environment with Special Reference to Bangladesh, a Developing Country in Gondwana Terrain Afia Akhtar and M. Nurul Hasan Geological Survey of Bangladesh, Segunbagicha, Dhaka, Bangladesh -1
1
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Sumval of living beings and sustainable development of a society depend on maintenance of healthy and hygienic environmental ecosystem. And sustainability of envlronmental ecosystem can only be achieved when there is a balance between population growth and availability of natural resources. Since at the end of the 20thcentury, enmronmental problems have been getting more and more attention than any other issues but still no fruitful solution has been arrived at. Impacts of ever-growing population vary depending upon whether a country is developed or developing. Poverty, illiteracy, unemployment, crime and pollution are the signs of overpopulation in comparison to the earth's resources. More people need more houses and more industrial and farm products. As a result, more and more forestlands are being used for urbanization, industrialization and agricultural purposes, which affect the environment and climatic
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condition greatly. Increasing population and development activities generate a huge volume of waste products and sewage, which pollute the environment. Pollution is one of the most serious environmental problems that the world is facing today. To minimize environmental pollution and to eradicate poverty, illiteracy and crime, population control is therefore a must. Bangladesh, having a land resource of about 147570 sq. km,is a developing country and its population density (about 850 per sq. km) is extremely high, whereas its resources are meager. Population control, application of proper geoscientific knowledge to minimize environmental degradation, and effective means to explore and exploit natural resources in a sustainable manner to meet the growing demand of exploding population will help to reshape the country's socio-economicinfrastructure and maintain a balanced healthy environmental ecosystem. ~
~~~~~~~~~~~~~
Enriched Sr-Nd Isotopic Characteristics of Amphibolites in the Oki-Dogo Island, Japan:Evidence for Crust-Mantle Interaction by Deeply Subducted Continental Crust Yoji Arakawal, Tsugio Kouta', Yasuhide Kanda' and Hiroshi Amakawa2
'
Department of Geology, Faculty of Education, Saitama University, Urawa, Saitama 338 - 8570, Japan Ocean Research Institute, University of Tokyo, Minamidai, Nakanoku, Tokyo 164 - 8639, fapan
The Oki metamorphic rocks in the Oki-Dogo Island, western Japan, comprise mainly of high grade gneisses, amphibolite and granites. Petrological work has shown that the rocks were affected by regional metamorphism of amphibolite facies, with granulite facies in part (Hoshino, 1979). Suzuki and Adachi (1994) carried out CHIME age determination of monazite and zircon grains within the paragneisses and proposed a model that the protolith ages of the metamorphic rocks were Carboniferous-Permian and later experienced a metamorphic event at 250 Ma. In spite of many studies on the metamorphic rocks of Oki-Dogo Island, their origin and tectonic evolution have not fully been discussed. In this study we present a geochemical and Sr-Nd isotopic study of the amphibolites and focus our attention on their highly enriched isotopic signatures.
The amphibolites show tholeiitic to alkaline chemical affinity, and are classified into two groups (Group I and 11). Group I amphibolite is characterized by low SiO, (44-47 Yo) and high TiO, and P,O,, while Group I1 amphibolite shows high SiO, (4748 YO)and low TiO, and P,O, contents. The Group I amphibolites have high Zr/Y (4-12), Nb/Y (0.3-1.1) and Ti/Y (400-900) and fall in the category of within-plate basalt (WPB). The Group I1 amphibolites, on the contrary, indicate low values of Zr/Y, Nb/Y and Ti/Y, and are classified into volcanic arc basalt (VAB) and MORB (one sample). Chondrite normalized rare earth element (REE) patterns are clearly distinct between two groups. Group I samples yield LREE enriched pattern (La/Yb(N)=4.69.0), while Group I1 samples show relatively flat patterns (La/Yb(N)=1.7-3.8). One sample from Group I1 with MORB signature presents La/Yb(N) of 0.8. G o n d w a n a Research, V. 4,No.4,2001
of these locations there has been a geologically immediate response to the entry of continental lithosphere into the subduction system. Collision of a relatively minor intra-oceanic island arc, the Luzon arc, with the continental margin of Asia has given rise to near 4 km of topographic relief in Taiwan over the past 5 Ma. The deformation front is propagating across the leading edge of the continental margin of eastern China just as the last of the remaining oceanic lithosphere is being subducted and arc magmatism is ceasing. In northern parts of Taiwan where collision has finished, orogenic collapse with attendant extrusion parallel to the axis of the collision belt is now well advanced and topographic relief is actively being reduced. Prevailing models for the India - Asia collision, the greatest on-going collision event in existence, suggest that collision between two major continental fragments at 55 Ma was not followed by mountain-building orogenesis for 20 million years or longer. One might ask why this has not previously been
561
thought to be highly anomalous. Either existing models that invoke India-Asia collision at 55 Ma are wrong, collision occurred at a different time and/or a different collisional event is recorded. We suggest that two different tectonic events are recognisable. Features related to each event need to be interpreted separately and no collisional continuum should be assumed. One event occurred at around 55 Ma, another at around 30 - 25 Ma. Collision of India with an intra-oceanic island arc at the end of the Cretaceous, removed a south-facing convergent plate boundary from within Tethys. Recognition of this event provides an explanation for the slowdown in convergence between India and Asia at 55 Ma. This intra-oceanic island arc is now preserved within the India-Asia suture zone. Continent-continent collision between India and Asia did not occur until the end of the Oligocene and was met by an immediate response in the form of orogenesis.
Natural Hazards in Bangladesh Afia Akhtar Geological Survey of Bangladesh, Dhaka, Bangladesh Natural hazards like Nor’Wester tropical cyclone and flood are most common in Bangladesh. These events occur almost every year and cause loss of life and properties, the magnitude of which depends on how devastating the events are. Bangladesh, a small land having 147570sq.km. and a developing country, lies between 20° 34’to 26O38’north latitude and 88O01’ to 92O41’east longitude. There are three distinct climatic patterns in Bangladesh: a) the summer with average temperature from 21OC to 34OC, b) the winter temperature from l l ° C to 29OC and c) the rainy season having average rainfall from 1194mm to 3454mm. But, more specifically, Bangladesh is characteristically influenced by a tropical monsoon climate and demarcated by two main seasons, the dry and the wet, the period of which ranges from November to March and J u n e to October respectively. April and May, these two months is the pre-monsoon period when violent thunderstorms along with hailstorms known as ‘Kal Baisakhi’ / ‘Tufan’in Bangla or ‘Nor’Westers Cyclone’ in English, happens in Bangladesh frequently, the effects of which are sometimes very severe. The Tropic of cancer passes across the middle part of Bangladesh and as a result it should be an arid country. But the fact is that, the southwesterly monsoon brings heavy rainfall to Bangladesh and makes it a wet country, and it is because of the Himalayan mountain ranges on the north of the country. The part of Sylhet of NE Bangladesh and specially the Cherapunji of Assam (India), just north of Bangladesh are the highest rainfall areas of the world, the annual average rainfall is more than 5080mm. A total length of about 24140 km. of the country is covered by a network of three big rivers - the Ganges, the Brahmaputra, the Meghna, forming a complex river system. And the location of Bangladesh is just on the lower reaches or the confluence of these three mighty river systems. The origins Gondwma Research, V. 4, No. 4, 2UUl
of all major rivers are northwest, north and northeast of Bangladesh and flows towards south to the Bay of Bengal. The total catchment area of three river system is about fourteen times of total land of Bangladesh. Moreover, almost the entire Bangladesh is a part of Bengal Basin, the second biggest delta of the world. Water from snow melting of the Himalayas which lies in the north and runoff or water flow from this vast catchment areas along with local monsoon heavy rainfall, altogether generate a tremendous volume of water which flows southwards over Bangladesh to the Bay of Bengal. Some times, the river systems of Bangladesh are unable to accommodate these huge volumes of water and as a result, the heavy downpour and water flow from upper reaches inundates many parts of Bangladesh and causes floods. Northern Bangladesh naturally suffers more due to this heavy flow of water and consequently river erosion is another common hazardous phenomenon in this area, which engulfs agricultural lands and houses and retard the national economic growth greatly. The effect of tropical cyclone is catastrophic but more severe especially in the coastal or southern part of the country along the Bay of Bengal, as the rise of sea surface temperature is the main reason for the formation of the cyclone. Not only that, Bangladesh is also highly vulnerable to earthquake hazard, as it is just adjacent to the seismically active Himalayas and the Arakan-Yoma hill ranges and in between two tectonic plates - the Indian plate and the Burma sub-plate. This region falls within one of the world’s three major seismic or earthquake belts - the Mediterranean and Trans Asiatic or Eurasian Belt. The overall geographical position and geological condition of Bangladesh is such that it is a hazard-prone area. Being a developing country, it is not always possible for Bangladesh to
562
RODINIA, GONDWANA AND ASIA ~~~~~~~~
take adequate mitigation measures to cope with these natural hazards properly. To cope with this acute situation, there should be societal motivation to acquire geoscientific knowledge and develop adequate means to protect life and properties through
proper land use planning, construction of disaster proof houses, growth of vegetation and so on. In this regards, public awareness should be created and combiried afford from public, private and government sectors should be incorporated.
Impacts of Overpopulationon Natural Environment with Special Reference to Bangladesh, a Developing Country in Gondwana Terrain Afia Akhtar and M. Nurul Hasan Geological Survey of Bangladesh, Segunbagicha, Dhaka, Bangladesh -1
1
1
1
I
I
x
Sumval of living beings and sustainable development of a society depend on maintenance of healthy and hygienic environmental ecosystem. And sustainability of envlronmental ecosystem can only be achieved when there is a balance between population growth and availability of natural resources. Since at the end of the 20thcentury, enmronmental problems have been getting more and more attention than any other issues but still no fruitful solution has been arrived at. Impacts of ever-growing population vary depending upon whether a country is developed or developing. Poverty, illiteracy, unemployment, crime and pollution are the signs of overpopulation in comparison to the earth's resources. More people need more houses and more industrial and farm products. As a result, more and more forestlands are being used for urbanization, industrialization and agricultural purposes, which affect the environment and climatic
, ll_..",
,,....,
..."."""xIII""".~"""".
_lll__
... .-."
""_
..,
.....
"
"".,."",l"l
""
...,,
condition greatly. Increasing population and development activities generate a huge volume of waste products and sewage, which pollute the environment. Pollution is one of the most serious environmental problems that the world is facing today. To minimize environmental pollution and to eradicate poverty, illiteracy and crime, population control is therefore a must. Bangladesh, having a land resource of about 147570 sq. km,is a developing country and its population density (about 850 per sq. km) is extremely high, whereas its resources are meager. Population control, application of proper geoscientific knowledge to minimize environmental degradation, and effective means to explore and exploit natural resources in a sustainable manner to meet the growing demand of exploding population will help to reshape the country's socio-economicinfrastructure and maintain a balanced healthy environmental ecosystem. ~
~~~~~~~~~~~~~
Enriched Sr-Nd Isotopic Characteristics of Amphibolites in the Oki-Dogo Island, Japan:Evidence for Crust-Mantle Interaction by Deeply Subducted Continental Crust Yoji Arakawal, Tsugio Kouta', Yasuhide Kanda' and Hiroshi Amakawa2
'
Department of Geology, Faculty of Education, Saitama University, Urawa, Saitama 338 - 8570, Japan Ocean Research Institute, University of Tokyo, Minamidai, Nakanoku, Tokyo 164 - 8639, fapan
The Oki metamorphic rocks in the Oki-Dogo Island, western Japan, comprise mainly of high grade gneisses, amphibolite and granites. Petrological work has shown that the rocks were affected by regional metamorphism of amphibolite facies, with granulite facies in part (Hoshino, 1979). Suzuki and Adachi (1994) carried out CHIME age determination of monazite and zircon grains within the paragneisses and proposed a model that the protolith ages of the metamorphic rocks were Carboniferous-Permian and later experienced a metamorphic event at 250 Ma. In spite of many studies on the metamorphic rocks of Oki-Dogo Island, their origin and tectonic evolution have not fully been discussed. In this study we present a geochemical and Sr-Nd isotopic study of the amphibolites and focus our attention on their highly enriched isotopic signatures.
The amphibolites show tholeiitic to alkaline chemical affinity, and are classified into two groups (Group I and 11). Group I amphibolite is characterized by low SiO, (44-47 Yo) and high TiO, and P,O,, while Group I1 amphibolite shows high SiO, (4748 YO)and low TiO, and P,O, contents. The Group I amphibolites have high Zr/Y (4-12), Nb/Y (0.3-1.1) and Ti/Y (400-900) and fall in the category of within-plate basalt (WPB). The Group I1 amphibolites, on the contrary, indicate low values of Zr/Y, Nb/Y and Ti/Y, and are classified into volcanic arc basalt (VAB) and MORB (one sample). Chondrite normalized rare earth element (REE) patterns are clearly distinct between two groups. Group I samples yield LREE enriched pattern (La/Yb(N)=4.69.0), while Group I1 samples show relatively flat patterns (La/Yb(N)=1.7-3.8). One sample from Group I1 with MORB signature presents La/Yb(N) of 0.8. G o n d w a n a Research, V. 4,No.4,2001