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Oceanography - Part 2
10.0 Tsunami
The seismic waves, travelling through the ocean and sea water, result into high sea waves which are known as tsunamis. 'Tsunami' is a Japanese word which has been universally adopted to describe the seismic sea waves. These waves are capable of causing considerable destruction in certain coastal areas, especially where submarine earthquakes occur. Once a tsunami is generated, its steepness (ratio of height to length) is extremely low. This lack of steepness, combined with wave's very long period (5 to 6 minutes), enables it to pass unnoticed beneath ship at sea. As the seismic sea wave approaches shore, the situation changes rapidly and often dramatically. While the period of the wave remains constant, velocity drops, and wave height greatly increases. As the crest arrives at coast, the water surges ashore in the manner of a very high, fast tide. In the vicinity of the epicenter of the earthquake, the tsunami waves can reach a height of over 30 metres (100 feet).
The tsunami travels at a speed of 100 to 150 km per hour which may pick up to 650 to 950 km per hour. Likewise, it may travel considerable distance. The tsunamis occur in or around the oceanic trenches. They are more frequent in the Pacific ocean. The other oceans like the Atlantic ocean and the Indian ocean, are, however, not free from the occurrence of seismic waves. On 1st April, 1946, a fracture along the Aleutian Trench, generated a tsunami that quickly engulfed the Scotch-Cap lighthouse in Unimark Island in the Aleutian Trench. The lighthouse was completely destroyed, and five coastguards working in the lighthouse were died.
Some of the greatest tsunamis of the recent past include the Japan tsunami (11 March 2011) which killed more than 25,000 people in and around Sendai city and ravaged the Fukushima nuclear plant. The nuclear radiation alarmed the population of Japan, China, North and South Koreas and as far as U.S.A. and Canada. Banda Aceh (Sumatra) seismic wave which occurred on 26th December, 2004 in the Sunda Trench and killed over two lakh people in the neighbouring countries. The impact of the tsunami was quite serious in Andaman and Nicobar Islands and the Tamil Nadu coast of India. Coastal areas of the distant countries like Sri Lanka, Maldives, Oman, and Somalia were also adversely affected resulting into human casualties and loss to property.
In 1703, at Awa (Japan) one lakh people died because of seismic waves of serious magnitude. On 27th August, 1883, the explosive volcanic: eruption of Krakatoa in Indonesia generated 35 Meter high waves that destroyed 163 villages and killed more than 36,000 people. The Lisbon city of Portugal was inundated by tsunami (over 15 metres in height) in the earthquake of 1755 in which over 60,000 people were killed.
Since 1948, an International Tsunami Warning Network has been in operation around the Pacific Ocean to alert the residents to possible danger. Its headquarters is at Honolulu. Much success has, however, not been achieved as the occurrence of earthquakes is sudden and the speed of the tsunami may be around one thousand km per hour. Tsunamis are thus a great natural hazard.
11.0 ENVIRONMENTAL ISSUES
With the increasing pressure of population and international trade, the sea-routes have become crowded. The excessive utilisation of ocean resources can result in the accidental (or unintentional) release of harmful substances. The marine pollution change the quality of the water or affect the physical and biological environment. The major causes of marine pollution are
(i) volcanic eruption, (ii) oil-pollution-natural seeps, offshore-drilling, near shore refining, and accidents of tankers, (iii) spill of refined oil, (iv) dumping of wastes, etc.
The international sea-routes are very polluted. Out of all the oceans, the North Atlantic Ocean, Arabian Sea, Red Sea, Mediterranean Sea, North Sea, Baltic Sea, Strait of Malacca, and China Sea are the most polluted regions of the world.
Environmental change is a continuous process that has been in operation since the Earth came into existence about 4600 million years ago. Earlier, there was a belief that man is the product of the Earth, but now human beings are affecting the environment significantly.
Man’s activities - like dumping garbage at sea, oil spills in the oceans, nuclear leakage from Japan (Fukushima) etc. - are causing great damage to the ecosystems.
Some major environmental issues the ocean is facing are:
Ocean warming: Higher temperatures in the ocean, caused by rising global temperatures as a result of climate change, have been detected at depths of more than 3,000m. Evidence suggests that warming ocean temperatures are responsible for increasing the intensity of tropical cyclones, as well as disrupting global fisheries by causing valuable fish stocks to migrate towards cooler waters nearer the poles. Warmer waters are also a major threat to coral reefs, making them both more vulnerable to bleaching and other damage, and more susceptible to the effects of acidification. Some researchers have indicated that at 1.7°C above pre-industrial temperatures, all warm-water coral reefs will be bleached, and by 2.5°C they will be extinct.
Ocean acidification: The oceans have absorbed approximately 30% of the carbon emissions since the Industrial Revolution. It has acted as a giant buffer, helping to cushion the effects of climate change. But this has come at a huge cost to fundamental ocean chemistry and ecosystems. Due to increasing carbon dioxide in the atmosphere the ocean has experienced a 30% decrease in its mean pH levels. At current rates, pH will drop by up to 200% more by 2100, a rate of change ten times faster than anything else suffered by the ocean for 55 million years, and which will reduce the ocean's capacity to absorb carbon in the future, and threaten the food security of communities reliant on vulnerable species of shellfish.
Unsustainable use of marine resources: According to FAO estimates , 85% of fish reserves are fully exploited, over exploited, depleted, or recovering from depletion. Additional climate-related fishing losses are being concentrated in tropical least-developed countries, many of them in Africa and South-East Asia, further effecting fishing communities. Illegal, unreported, and unregulated (IUU) fishing is responsible for the loss of between 11 million and 26 million tonnes of unaccounted for fish, out of a total world capture of approximately 80 million tonnes. Destructive fishing practices - including dragging nets the size of football fields across the sea bed - are causing tremendous damage to breeding, nursery and fishing habitats for marine life. Studies estimate that up to 73 million sharks are killed every year to supply the fin trade. The population of some species has reduced by 70 to 80 percent.
Rise in sea levels: Sea-level rise threatens the existence of some island nations and coastal cities, and could cause vast areas of land currently used for food production to become inundated. The pace of global mean sea level rise is accelerating: levels rose by approximately 1.8mm per year over the last 50 years, but doubled to 3.1mm per year in the 1990s, and were 2.5mm per year in the period 2003-2007. Estimates vary, but there is a growing consensus that mean levels could rise by over 2 metres before 2100 if further temperature increases stimulate the complex feedback loops which govern the patterns of polar ice melt. Summer sea ice in the Arctic has been decreasing by 7.4% per decade since 1978.
Multiple Stressors: Over 40% of marine ecosystems are already simultaneously facing several of the major pressures outlined above. These interconnected issues are placing the very chemical and thermodynamic foundations of the ocean in jeopardy, and increasing the risk of catastrophic outcomes - such as a mass extinction of vulnerable marine species. The cumulative effect of overlapping threats is a key reason why their impacts are being observed at a faster rate than previously predicted. It is therefore paramount that they be addressed together and across all scales from local to global, instead of on an issue-specific basis as is the norm today.
Climate change: Rising emissions from fossil fuels is the direct cause of the global stressors impacting on the ocean - acidification, warming and sea-level rise, the effects of which are already exacerbating and accelerating the impacts of other threats such as pollution and overfishing.
12.0 STRATEGIC IMPORTANCE OF OCEANS
12.1 Pacific Ocean
The Pacific is a major contributor to the world economy, particularly to those nations its waters directly touch. It provides low-cost sea transportation between East and West, extensive fishing grounds, offshore oil and gas fields, minerals, and sand & gravel for the construction industry.
12.2 The Atlantic Ocean
The North Atlantic trade lane is one of the most important in the global shipping industry. Upgradation of key port infrastructures within Europe are continually upgraded to reflect the demands of shipping across this corridor.
The most strategically important infrastructure assets on this trade lane is the Panama Canal, which provides access between the Atlantic and Pacific Oceans, creating an all-water link between the Far East and the East Coast of North America. The Panama Canal is currently being widened and deepened to accommodate much larger vessels than those which can currently transit the canal. When completed in 2015, the canal will be able to accommodate container ships of around 13,000teu in size, nearly three times larger than the maximum vessel that can transit it today.
The Atlantic Gateway is an ambitious strategy with the aim of creating 250,000 jobs (140,000 directly related to Atlantic Gateway projects). With a planned investment of £14bn the vision set out above a reality. Such developments also link with the European strategy for investment in key low carbon rail and canal transport networks.
12.3 The Indian Ocean
The sea lanes in the Indian Ocean are considered among the most strategically important in the world-according to the Journal of the Indian Ocean Region, more than 80 percent of the world's seaborne trade in oil transits through Indian Ocean choke points, with 40 percent passing through the Strait of Hormuz, 35 percent through the Strait of Malacca and 8 percent through the Bab el-Mandab Strait.
But it's not just about sea lanes and trade. More than half the world's armed conflicts are presently located in the Indian Ocean region, while the waters are also home to continually evolving strategic developments including the competing rises of China and India, potential nuclear confrontation between India and Pakistan, the US interventions in Iraq and Afghanistan, Islamist terrorism, growing incidence of piracy in and around the Horn of Africa,and management of diminishing fishery resources.
13.0 FORMATION OF ARCHIPELAGOS IN SOUTH ASIA
Like mountains and other land formations, archipelagos are formed in part by tectonic activity. When underwater volcanoes, or hot spots, allow magma (liquid rock) to seep out in the sea, rock formations are created under the water. As more and more magma is released, the rock formations eventually peak out over the surface of the water, creating an island.
The Malay archipelago situated between mainland south east Asia and Australia is a group of almost 25,000 islands. It is the largest archipelago areawise. In terms of plate tectonic setting the Indonesian archipelago is situated in the triple junction of the three major plates, which are-the Indo-Australian, the Eurasian and the Pacific Plates. The interaction of the three major plates creates a complex tectonic especially in the plate boundary that is situated on Eastern Indonesia. The subduction of the Indian oceanic plate beneath the Eurasian continental plate formed the volcanic arc in western Indonesia, one of the most seismically active areas on the planet with a long history of powerful eruptions and earthquakes. This chain of active volcanoes formed Sumatra, Java, Bali, and Nusa Tenggara islands, most of which, particularly Java and Bali, emerged within the last 2-3 million years. The Pacific and Australian plate movements controlled the tectonics of the eastern portion of Indonesia.
The islands of Lombok and Sumbawa lie in the central portion of the Sunda Arc. The oldest exposed rocks are Miocene, suggesting that subduction and volcanism began considerably later than in Java and Sumatra to the west, where there are abundant volcanic and intrusive rocks of Late Mesozoic age.
The territory of the Philippines is composed of many island arcs formed by several incidents of subduction. The island arcs are collectively called Philippines island arc system. Each major Philippine island had a complex natural history. With the exception of Palawan, Mindoro and Romblon, most of the Philippine islands are considered to have been parts of island arcs formed at the southern edge of the Philippine Sea plate millions of years ago. As part of the Philippine Sea plate, the islands moved northward as the plate rotated clockwise. These roving islands, known as the Philippine Mobile Belt, eventually collided with the Sundaland. The collision resulted, among others, in a series of subductions around Philippine archipelago.
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