Ocean Resources

In addition to their biological resources, the world’s oceans are rich in minerals and sources of energy that can be exploited by human beings.

► Resources exploited today:

Salt

Extracted from the sea since antiquity, salt was initially only used to preserve and season food. Today, it has become a raw material for the chemicals industry. Its scientific name is sodium chloride. How is it made? As a result of erosion*, the sodium in rocks is carried away by rivers to the sea. Seawater contains many elements such as magnesium and chlorine; sodium combines with chlorine to from sodium chloride. On average, seawater contains 35 grams of salt per kilogram of water. Seawater is evaporated in shallow basins in solar salt ponds to extract the salt. Solar salt ponds are located in regions where the climate is dry and warm, adjacent to the North and Mediterranean seas for example, and also in temperate zones, such as along the Atlantic coastline of France. One third of the world’s salt is extracted from seawater and the rest come from many underground deposits, which are the remains of seas that no longer exist.

 

Salt Field

Metals

Fifty years ago it was believed that mining manganese and other precious metals was going to become the new source of wealth to be taken from the oceans. The floors of the ocean deeps are strewn with multi-metal nodules-stones loaded with many metals. But since then we have learned how to extract these metals easily from the earth, and in great quantity. For the moment, sea water principally supplies us with bromine and magnesium. Bromine is produced from sea salt primarily in the united states, the gulf of Mexico, France and Japan. Magnesium is obtained from the ‘first stages waters’ of solar salt ponds primarily in the gulf of Mexico.

Metals

Metals Classification

Oil and gas

Oil and gas (hydrocarbon*) stored several hundred or several thousand meters beneath the ocean floor are extracted using costly, heavy machinery (platforms, hydro- carbon extraction systems). Two-thirds of the ocean’s hydro-carbon deposits, are deposits are located off the shores of Middle Eastern countries and are especially concentrated in the Persian Gulf. At present, marine hydrocarbon deposits supply 30 percent of  the total volume of oil produced worldwide and 10 percent of the world’s natural gas supply.

Oil

Gas

Fresh water 

Desalination plants transform seawater into fresh, potable water. Now a days use this method to obtain the majority of their water supply. There are two principal ways to remove salt from seawater: either by distilling it, whereby the water is heated until it evaporates and thus separates from the salt, or by filtering it, whereby the water is poured through very fine, highly perfected membranes. Even though distillation is the technique most widely employed today, filtering could become at least as profitable as efficient in less than fifteen years.

Fresh Water

► The Ocean’s new resources:
Tidal energy 

Strong tides are the source of considerable energy. This energy can be converted into electrical power at plants that harness tidal energy to drive turbines and generators. The first type of this plant was built in 1967. But this energy has not been developed as was initially hoped due to competition from nuclear power plants, which are less expensive, and the limited number of sites along the coast where tidal variations are sufficiently great to install tide driven power plants.

Tidal Energy

Wave energy

The energy created by waves is called mechanical energy; it can be harnessed by floating installations placed on the water, or by shore installations. Plants of this type have been installed around the globe, and in particular in Japan, China, India, Norway and Great Britain. Converting this inexhaustible energy supply into electricity is pollution free. There is only one problem: the operating costs for these plants are still very high.

Wave Energy

Hydrothermal source

Hydrothermal Vents: Discoveries from the Deep

In 1977, the French submersible *Chana* made a groundbreaking discovery at a depth of 3,000 meters off the Mexican coast: unusual mineral formations resembling termite mounds. These structures, identified as hydrothermal vents, can reach temperatures exceeding 350°C. Remarkably, diverse life forms, including shellfish and giant worms, thrive in this extreme environment.

Today, research into the ocean’s depths is advancing rapidly. Scientists have found that certain bacteria can flourish in these hot, dark conditions, challenging our understanding of life’s adaptability. As we continue to explore these fascinating ecosystems, we may uncover insights into the origins of life itself, possibly revealing the secrets held by these "super-bacteria.

Drugs

The Potential of Marine-Derived Drugs

The vast ocean is home to over 200,000 species of invertebrates, including fish, mollusks, and crustaceans, as well as various algae. These organisms harbor countless chemical compounds, many of which remain uncharacterized but hold promise for human health. For the past forty years, the pharmaceutical industry has been investigating these marine resources for potential treatments, particularly for cancer.

Despite significant interest, funding for marine research and pharmacological development often falls short. Organizations like the IOC play a crucial role in supporting governments by sharing the latest techniques and advancements in marine observation.

Currently, intense research into the genetic material of marine species is underway, led by the pharmaceutical and biotechnology sectors. This research is paving the way for exciting discoveries and the development of innovative products that could transform healthcare.

 

Drugs

The ravage of industrial fishing

Industrial Fishing

The ravage of industrial fishing 
  

In Europe, ‘tangle drift nets’ several kilometers in length wreck havoc in the north sea the Atlantic and the Mediterranean .Weighted down at the bottom and held up by buoys at the top, these nets position themselves vertically beneath the surface of the water. They are allowed to drift with the currents and winds waiting for fish to be caught up in them. Sometimes they are attached at one end to a ship to ‘herd’ species towards the coast from the open sea. These nets are immense and much too long! Salmon is fished in the Baltic Sea with nets over 2 km long. The European Union voted to totally prohibit the use of tangle drift nets in community waters as of 1 January 2oo2.

Industrial fishing techniques are devastating. Fishermen not only haul in the species they are looking for but net other fish they don’t want. They throw most of them- either dead or dying- back into the sea. Some of these are endangered species. Between 27 and 35 million tons of fish are ‘wasted’ in this way, meaning nearly one third of all those that are caught! The FAO (Food and Agriculture Organization) of the United Nations has sounded the alarm. It estimates that the majority of fish stocks are being over exploited and that the number of industrial fishing vessels must be lowered by one quarter to return to the levels of abundance and catches of the 1970s.

Some states have already adopted measures forbidding or limiting the right to throw away fish at sea, primarily Iceland, Namibia, Norway and New Zealand. There are ways to fish only those species desired, but the techniques are not as profitable. But without resorting to selective fishing, net sizes could easily be reduced. 

To manage ocean resources, the United Nations Convention on the Low of the sea, also known as the Montego Bay Declaration was signed in 1982 and ratified by 130 states. The Declaration, which came into force twenty  years later, set up exclusive economic zones (EFZ) giving coastal states the exclusive fishing to all that the oceans contain within a370 kilometers (200 nautical miles)limit from their coasts. Of course t he states have also undertaken to protect and exploit the biological resources* of the high seas in such a way that sustainability is ensured. How is this accomplished? By encouraging fisheries to use selective fishing techniques that reduce waste to a minimum during hauls, by monitoring fishing operations, by preserving and restocking marine species endangered by extinction, and by preserving ecologically vulnerable zones. The IOC has also suggested creating protected areas on the high seas in international waters; many countries are opposed to this idea.      

Urbanization and tourism: threats to the littoral

In both industrialized countries and developing countries, such as Brazil, urban growth along the coast has reached its limits. In Europe, the Mediterranean shoreline is covered with constructions over hundreds and hundreds of kilometers, completely blocking the landscape. The sandy soil settles with the weight of buildings and must be held in place by enormous concrete embankments in some locations. Many cities built below sea level are sinking. Both financial and technical means are needed to keep this from happening; means that developing cities do not possess. Another major problem is sewerage water, which is discharged into the sea along the coast, endangering all marine organisms. To this must be added the massive influx of tourists and with them the construction of hotels in the dunes and beach ‘renovation’. Around 100 million tourists congregate on the shores of the Mediterranean every year. And if this trend continues, there may be three to four times more by 2025. This sudden population explosion causes much harm to the marine environment, but tourism also boosts the fragile economies of many countries. Since the tourism is largely dependent on the quality of the environment. 

           Human activity also includes the construction of dams on rivers and destruction of mangroves to make room to grow timber commercially. These practices not only endanger the fragile balance of the marine ecosystem but the human beings who depend on it as well. 

The ravage of industrial fishing

The Global Oceans observing System

Improving understanding of the role the world’s oceans play in climate change has become a priority for the IOC of UNESCO. Together with the world Meteorological Organization (WMO) and other bodies, the IOC set up the global ocean Observing system (GOOS) in 1985.

Little-by-little, an army of satellites, ships and buoys have been deployed to monitor the world’s oceans. They record water temperature, colour and salinity, wave height, current velocity and wind speed at regular intervals. These data are then transferred to super computers on all the continents where they are input into digital models that are used by scientists to simulate the behavior of great expanses of the high seas and coastal regions in order to prepare forecasts.

Global Ocean Observing System

An observation and research system has been set up in the tropical pacific to forecast the EI Nino phenomenon. It’s called the TOGA programme. All the governments concerned contribute to this programme; they may use the data collected to try to lower the potential effect of either drought or flooding in relation to agriculture, the use of water, management of the fishing industry and all other activities that depend on atmospheric conditions. 

         More important still, the programme has developed models that scientists now use to forecast the coming of EI Nino one year in advance.

Physical Oceanographic

‘EI Nino’ generally occurs around Christmas-its name in fact means ‘the infant Jesus’-along the south American coasts of the pacific. The phenomenon occurs with more or less intensity every three to four years. It is believed to be caused by changes in the circulation of the trade winds. Normally these winds blow from east to west, pushing warm surface water towards Indonesia and Australia, and allowing deeper, colder water to surface along the south American coast. But sometimes the trade winds are not very strong or even blow in the wrong direction. The warm water is driven towards South America and raises the temperature of the ocean. The result is the giving off of heat and humidity that provokes storms and torrential rains in normally very arid regions.

This is called; opera-tional oceanography’. The forecasts are available to be both public and private users. Weather services use them to forecast the weather and save lives, for example by identifying a hurricane in formation. Some scientists estimate that the sea level rises by 2.5 mm per year due to global warming and that this could cause the flooding of inhabited zones and agricultural areas.

Oceans and carbon

Another way oceans influence climate is by storing carbon. This atom* enters into the composition of many molecules*, including carbon dioxide (CO2). This gas is naturally present in the atmosphere, but human activity industry, automobile traffic and the burning if wood, coal, natural gas and fuel oil have increased its production. In addition to carbon dioxide, these activities also give up other gases, such as methane (produced by rice growing and cattle-raising for instance) and ‘Chlorofluorocarbons’ (CFCs). All these gases contribute to the enhanced greenhouse effect that is threatening to disturb the climate of our planet. 

Global Carbon Cycle

Trapping carbon dioxide in the world’s oceans

Oceans are the biggest potential reservoir for storing the CO2 produced by human activity and for keeping it from entering the atmosphere. This may be an effective way to reduce the consequences of too much CO2. The IOC is closely monitoring initiatives aimed at developing a CO2 trap. Two techniques are currently being tested: a injecting this gas directly into the ocean bottom and b helping plankton, which feed on CO2, to grow in specific areas of the ocean (by adding iron dust). But where these ‘traps’ should be set, and above all, making sure that these techniques will not disturb oceanic ecosystems are major questions that must first be answered.

Solar Radiation

 Due to global warming, the sea level is rising; if this continues, the sea level will be around one miter higher in 100 years. The Maldives islands in the Indian Ocean will be wiped off the map. Here again the world’s oceans play an essential role because they naturally absorb great quantities of CO2: approximately 18 billion tones per year. Algae consume part of this amount and the remainder falls to the sea floor. But this absorption capacity has its limits. In 100 to 200 years it will be saturated and will be saturated and will mo longer be able to compensate for the CO2 emissions caused by human activity.

         The average temperature on earth is already rising. Without efforts to lower greenhouse gas emissions, experts predict an increase in temperature of from 2 to 5ºC by the end of the twenty-first century.     

Oceans and human beings

More than half the world’s inhabitants live near seacoasts today at a distance of less than 200 kilometers from the water. In 2025, this will be the case for more then three-quarters of the world population, in other words for 6.3 billion persons! Most of them don’t live in seaside resorts but are concentrated in enormous cities (metropolises) built on the coast. These city dwellers are often faced with poverty and some of them have less than two US dollars a day to live on.

    
           If so many people crowd next to the shore it’s because that’s where the resources needed to sustain life are found in other words primarily fishing. But this poses a majors problem because as the world population continues to grow and is concentrated in coastal areas it will increasingly depend on ocean resources.

Sea Fish

UNESCO’S proposal for sustainable development and for lowering poverty the coming decade, emphasis that both an economically viable way of life and the riches of the ocean must be mad accessible to all, as well as an environment that is safer and healthier for coastal population.  

  

♦ Fishing, a vital food source

Fishing is the most widespread economic activity of the world’s oceans. If fishing practices were better controlled this industry could feed more people and provide more jobs, especially in developing countries. But today, most fishing zones are being fished out. This is not surprising: 90 million tons of fish are caught each year! To arrive at this figure, fishermen haul in fish of any size and any age, including the very young, and those during their reproductive phase. Species don’t have enough time to reproduce and their population decrease more and more each year. 

Sea Fish Archive

Cooked Fish

Industries fishing techniques can be held responsible for over fishing And they also harm coastal fisheries. Poorly equipped, local fishermen work the waters close to the shoreline where the fish, which are victims of both over fishing and higher concentrations of pollution than in the open sea, are becoming increasingly scarce. In the southern part of the globe the inhabitants of many fishing villages have barely enough fish to eat. It is consequently nearly impossible for them to sell their catch.

Oceans and Pollution

River Pollution

For a long time humans believed that the immensity of the oceans could absorb waste without any consequences. We now know that everything we throw into the ocean modifies its equilibrium: nothing is lost. Oil spills, the proliferation of algae in the Mediterranean, the disappearance of marine species…..we are aware today of the harm caused by pollution, which comes from two main sources.

           Pollution from the continents represents 70 percent of marine pollution. Sewage water industry waste (metals and phosphates), chemical, waste, loaded with pesticides and fertilizers. Once they’ve been used for crops, fertilizers are carried by run-of to rivers, which then transport them to the sea.

Polluted Pond Water

       The second main source is pollution resulting from maritime transport and from oil transport in particular. Everyone knows about oil tanker accidents-Erika and prestige being the most recent-but offshore oil rigs also cause the sadly famous ‘oil spills’. Soiled beaches, oil-smeared birds, uneatable fish and shellfish: the damage they cause is horrendous. But on a worldwide scale, there is a for more serious form of pollution than oil spills from tanker accidents, and that is the deliberate discharge of oil, which as a result constantly degrades the marine environment. The principal guilty parties are the oil tankers that empty their cargo in to the sea. This is called ‘degassing’ (degasification). Despite the creation of surveillance systems and the many measures taken to prohibit this practice, oil tanker are not always very scrupulous* and degas on the high seas clandestinely*. In all approximately 600,000 tons of hydrocarbons are discharged into the oceans every single year! A Large part of it is ‘digested’ by  them: oil dissolves slowly in water and falls to the deeps to never rise again. But that doesn’t keep it from seriously harming marine fauna, and this in turn diminishes the resources of developing countries, which are very dependent on ocean products. In addition to hydrocarbons Other aggressive pollutants also contaminate the oceans, notably waste from chemical products transported by ship that industrialized countries do not want to dispose of themselves. 

Factory Polluted the River

The result is that this cargo is sent to far away country that are less strict about regulation, often to parts of Africa or South America. Thus these are the places that are the most exposed to Pollution hazards. 

 

Targeting Pollution

Plastic Pollution

In 1995, 108 governments and the European Commission working within the scope of the united Nation adopted the ‘Programmer of world action for the protection of marine environments against pollution caused by terrestrial activation.’ All of this countries are truing to lower the emission of pollutants at their source, being aware that ways this is accomplished are: by improving technical processes and closely monitoring industrial discharges; by limiting in vulnerable zones such as Brittany in France, the use of fertilizers responsible for the proliferation of microscopic algae, and by building or improving waste water treatment plants. These countries are also striving to improve human habitats in coastal areas, and to develop the coastline while respecting its natural features.

Humans are directly affected by pollution  

The type of pollution that is most threatening to human being is the discharge of sewage water. First it contaminates bathers, then fish and shellfish, and those who eat them in turn. Cholera and hepatitis epidemics caused through eating contaminated food are becoming a frequent occurrence, especially in Latin America, around the Mediterranean and in Southeast Asia.

            Just having a swim in polluted water is far from being harmless.  According to the world Health Organization (WHO), this is the cause of nearly 250 million cases of intestinal flu each year as well as of respiratory illnesses. Generally speaking, one bather out of twenty risks becoming ill after taking a dip in theses!

Plastic Pollution Video:

Sustainable development

Climate change

Sustainable development is a way of meeting the needs of present day generations without compromising the capacity of future generations to meet their needs, for instance by improving the quality of water and the environment, by minimizing the differences in the standard of living throughout the world, and by changing certain ways of living and consuming. UNESCO is helping to open up an avenue of sustainable development focused on human beings and based on respecting the rights of human and democratic principles, on solidarity, dignity, sharing and equality.

1987. The Brundtland Commission defined for the first time the concept of sustainable development before the United Nations.

1992. Government met in Rio de Janeiro (Brazil) for the first Earth Summit (United Nations Conference on the Environment and Development). The summit focused on the key question of how to reconcile the demands of the present day with expectations for the future. There were many debates and alarming observations concerning the current state of our planet and the likely evolution of living conditions. To set the problems aright, more than 150 countries worked on a common programme for the twenty first century, called the ‘Action 21 Programme’. Since then this programme has served as the basis for measuring the progress of the made in change in question.

Carbon

Sea Pathfinder

Warm Shallow Water

2002. From 26 August through  to 4 September in Johannesburg, South Africa, 129 States, including some 100 Chiefs if State and 60,000 delegates, met under the aegis* of the United Nations fir the World Summit on Sustainable Development, also called ‘Rio+10’.Particioants reported on the projects that are needed right now. Despite the reluctance of rich countries to commit to figures and dates, this second Earth Summit accomplished at least two objectives: it made a collective decision to lower greenhouse gas emissions by 5.2 percent within the next 10 years, and many States become aware of the urgency of remedying the ills of our planet. Healthy oceans and coastal regions are vital to sustainable development and poverty reduction. This is why, as mentioned earlier, the Action 21 Programme has an entire chapter devoted to the world’s oceans. Their well-being is essential to the survival of living things and to the cultural, social and economic prosperity of human population. To safeguard our survival or re-establish it we must learn how to manage the world’s oceans and coastlines, basing our decisions on scientific knowledge. This concerns all the countries of the world because:    

One country alone cannot study and monitor the world’s oceans on a continuous basis but it should at least be able to tackle the problems that touch its littoral. Subsequent to extensive urban growth in the 1970s, Brazilian cities such as Sao Paulo and its surrounding areas still have major air and water pollution problems. Their beaches are suffering today because the most polluting industries have been relocated on the littoral. All countries depend either directly or indirectly on the world’s oceans for survival and each must thus assume responsibility for its current condition. In addition, countries for which the economy and living conditions depend directly on the ocean must be provided with the means to solve problems on the local leave.

Wind and currents

Wind Power

The sun heats the upper layer if the world’s oceans to a depth of 20 to 30 meters. In doing so it causes the evaporation of enormous quantities of water, which escapes into the air in the form of water vapour. This transfer od heat between the ocean and atmosphere, which is really a transfer of energy*, causes the air to stir, forming winds. Winds are responsible for creating some of the currents that keep ocean water moving. The three types of ocean currents are surface currents, density-driven or thermohaline currents and slope currents.  

                                                                             

Wind Making Current

                    The action of the wind on the water’s surface creates surface currents. But these don’t move just any which way. Blowing along the equator, the trade winds create east-west current. When they reach the continents these currents run along their edges. Due to the spinning of the earth, currents that meet continents are deviated to the right in the northern hemisphere and to the left in the southern hemisphere (Carioles effect), creating the Gulf Stream in the North Atlantic and the Australian current in the pacific.

            Density-driven or thermohaline currents are deep currents driven by differences in seawater density, caused by variation in salinity and temperature. These currents are vary cold (around 0°C) and they are much slower than surface currents. It takes several centuries for them to circle the globe.

                                                     

Antarctic Ocean Circulation

                                                                 

    Step-1

                                                                         

Step-2

In 1992, governments from around the world met in Rio de Janerio for the United Nation conference on the Environment and development. This marked the first debate on the question of how to satisfy current needs without harming those of future generations (sustainable development). The conference drafted a very thick report called the Action 21 programme; chapter 17 is devoted in its entirety to the world’s oceans, reminding us that the ocean is indispensable for life.

             The last noticeable of the three currents are the slope currents. They can be explained by the fact that the ocean’s surface is not flat but sloped 2 to 3 meters. This slope increases in the presence of atmospheric low-pressure systems. Currents are created to compensate for these differences in an attempt to make the sea level. The ocean currents and the drain in your bathtub circulate water in the same direction because of the spinning of the earth. In the northern hemisphere water swirls clockwise; in the southern hemisphere it swirls counter-clockwise!

The Oceans: Balancing our climate

                                                                                                                

Greenhouse Effect Photo

The world’s oceans play an important role in regulating world climate. So much so that they have been referred to as weather’s ‘steering wheel’. They act in several ways. First of all, oceans store heat when it is too hot either during the summer or and during the day and then release heat in another season or at night. But oceans are not just storage tanks. They are also vehicles: currents carry heat in great quantities from hot regions to cold regions. And when the heart returns the atmosphere, marine breezes push it towards the continents. This is why it is often warmer closer to the ocean than farther inland. The Gulf Stream, for instance, carries warm water from the tropics and warms up the west coats of Europe. This is the reason the climate in western Europe is much milder than in central Europe.

                                                                   

Sunlight Effect

                                                                 

Sea warm water

                                                                 

Atmospheric

                                                                   

Ice Land animal

                                                                   

Weather Testing

         Rainfall is also directly linked to the oceans. The evaporation of seawater fills the air with water vapour and this creates clouds. The amount of evaporation controls could frequency and size. In the tropics evaporation is so grate during the day that when the sun sets and the air cools down, storm clouds form and cause short and violent downpours. Around 380,000 cubic kilometers of water evaporate from the surface of seas and oceans each year and only one-quarter of this amount falls on the continents. The relationship between the world’s oceans and the atmosphere is extremely complex: winds and currents criss-cross and cause all kinds of climatic variations, from simple sea breezes to terrible hurricanes, which can be devastating. One such variation is called ‘EI Nino’. This large scale oceanic phenomenon occurs in the pacific and upsets the climate over that entire region of the globe, sometimes affecting even greater areas.

The Oceans: A Short Introduction

The Earth often called the ‘blue planet’. Seen from space, it looks blue because of all the water on its surface. Three quarters of the globe are covered with seas and oceans, representing 361 million square kilometers of salt water! The world’s seas and oceans contain practically all the water on the planet, the rest is mostly fresh water stored in the form of ice in polar ice caps* and glaciers and river water.

            The marine environment is vast complex and hard to access. We don’t know much about it and studying this milieu* is both costly and complicated. And yet the world’s oceans play a vital role in the planetary environment; we must learn more about them in order to protect it.

The world’s oceans and adjacent seas and all the organic and inorganic natural resources they contain are indispensable to the survival of our planet as we know it today. Safeguarding* the air we breath, the water we drink, the food we eat and the climate we live in depends on the oceans.’ This was the declaration made by UNESCO’S intergovernmental oceanographic commission (IOC) at the world summit for Sustainable Development held in Johannesburg in September 2002.

                                                       

Topography

Snail 

 

Tortise

                                                             

Sea weed

                                                                     

  Sea Snake

 

Their beginnings:

How were the oceans created? In order to answer this, we must go back four and a half billion years to the earliest ages of the earth. There were many active volcanoes at the time, spewing out thousands of tons of magma. This gooey, burning hot liquid rock contained enormous quantities of gas that created the earth’s first atmosphere known as the primitive atmosphere, and which was full of steam. With the cooling of the earth, the water vapor condensed and became. Dilution rains fell to the ground and accumulated in the low-lying areas of the land, creating ponds and lakes and, little by little, the world’s first seas and oceans. Over time the composition of these bodies of water changed, but not their quantity, which has always remained the same since the very beginning and represents one and a half billion cubic kilometers of water!

                                                                                                                                         

 

Sea Fish Video 

Salty water:

In the beginning the sea water was fresh water. During hundreds of million years since sea water came into being it has been continually loaded with mineral salts and biological matter. These substances are carried by rivers and result from the decomposition of rocks beneath the sea and from gases that escape from the Earth’s crust when volcanoes erupt.

Ocean floors:

Ocean floors are not flat but have highly contrasted reliefs. Because the oceanic crust is cut up into pieces, ocean floors are mountainous and their shapes change. In places the crust fissures and forms what is called the mid oceanic ridges. Ranging from 1 to 4 kilometers height and almost as wide, these enormous marine mountain chains stretch 60,000 kilometers over the globe like the seams of a baseball. Magma rich in minerals from the depths of the Earth spills out on to the ocean floor at these ridges, continually creating new oceanic crust. In some locations the magma spews out violently in the form of an undersea volcanic eruption. In other spots, the ocean floor disappears when a portion of oceanic crust slides into one of the deep sea trenches, whose depth varies from 6,000 to 11,000 meters.

     Just imagine for a second that the oceans have been drained. We would be able to see a landscape of mountain chains, plains, volcanoes and very deep valleys.