The Earth’s crust is made up of numerous continental plates that are in permanent motion. They move a few centimetres each year. This continental drift means that plates are veering away from each other in some places. At these plate boundaries the Earth’s crust is splitting apart. Fresh magma is continuously being extruded from the fissures and, over time, it piles up on the ocean floor to form large undersea ridges.
The sea floor – humankind’s resource repositoryThe oceans hold a veritable treasure trove of valuable resources. Sand and gravel, oil and gas have been extracted from the sea for many years. In addition, minerals transported by erosion from the continents to the coastal areas are mined from the shallow shelf and beach areas. These include diamonds off the coasts of South Africa and Namibia as well as deposits of tin, titanium and gold along the shores of Africa, Asia and South America.
Efforts to expand ocean mining into deep-sea waters have recently begun. The major focus is on manganese nodules, which are usually located at depths below 4000 metres, gas hydrates (located between 350 and 5000 metres), and cobalt crusts along the flanks of undersea mountain ranges (between 1000 and 3000 metres), as well as massive sulphides and the sulphide muds that form in areas of volcanic activity near the plate boundaries, at depths of 500 to 4000 metres.
Back in the early 1980s there was great commercial interest in manganese nodules and cobalt crusts. This initial euphoria over marine mining led to the International Seabed Authority (ISA) being established in Jamaica, and the United Nations Convention on the Law of the Sea (UNCLOS) being signed in 1982 – the “constitution for the seas”. Since entering into force in 1994, this major convention has formed the basis for signatories’ legal rights to use the marine resources on the sea floor outside national territorial waters (Chapter 10).
- After that, however, the industrial countries lost interest in resources. For one thing, prices dropped – making it no longer profitable to retrieve the accretions from the deep sea and utilize the metals they contained. Also, new onshore deposits were discovered, which were cheaper to exploit. The present resurgence of interest is due to the sharp increase in resource prices and attendant rise in profitability of the exploration business, and in particular to strong economic growth in countries like China and India which purchase large quantities of metal on world markets.
Even the latest economic crisis is not expected to slow this trend for long. The industrial and emerging countries’ geopolitical interests in safeguarding their supplies of resources also play a role. In light of the increasing demand for resources, those countries which have no reserves of their own are seeking to assert extraterritorial claims in the oceans.
Manganese nodulesCovering huge areas of the deep sea with masses of up to 75 kilograms per square metre, manganese nodules are lumps of minerals ranging in size from a potato to a head of lettuce. They are composed mainly of manganese, iron, silicates and hydroxides, and they grow around a crystalline nucleus at a rate of only about one to 3 millimetres per million years. The chemical elements are precipitated from seawater or originate in the pore waters of the underlying sediments. The greatest densities of nodules occur off the west coast of Mexico (in the Clarion-Clipperton Zone), in the Peru Basin, and the Indian Ocean. In the Clarion-Clipperton Zone the manganese nodules lie on the deep-sea sediments covering an area of at least 9 million square kilometres – an area the size of Europe. Their concentration in this area can probably be attributed to an increased input of manganese-rich minerals through the sediments released from the interior of the Earth at the East Pacific Rise by hydrothermal activity – that is, released from within the Earth by warm-water seeps on the sea floor and distributed over a large area by deep ocean currents.
- 7.4 > The sea floor contains extensive resources. They are concentrated in certain regions depending on how they were formed.
- Cross-section view of a manganese nodule: Over millions of years, minerals are deposited around a nucleus.
- Manganese nodules are composed primarily of manganese and iron. The elements of economic interest, including cobalt, copper and nickel, are present in lower concentrations and make up a total of around 3.0 per cent by weight. In addition there are traces of other significant elements such as platinum or tellurium that are important in industry for various high-tech products.
The actual mining process does not present any major technological problems because the nodules can be collected fairly easily from the surface of the sea floor. Excavation tests as early as 1978 were successful in transporting manganese nodules up to the sea surface. But before large-scale mining of the nodules can be carried out there are still questions that need to be answered. For one, neither the density of nodule occurrence nor the variability of the metal content is accurately known. In addition, recent investigations show that the deep seabed is not as flat as it was thought to be 30 years ago. The presence of numerous volcanic elevations limits the size of the areas that can be mined.
Furthermore, the excavation of manganese nodules would considerably disturb parts of the seabed. The projected impact would affect about 120 square kilometres of ocean floor per year, an area the size of the city of Kiel. Huge amounts of sediment, water, and countless organisms would be dug up with the nodules, and the destruction of the deep-sea habitat would be substantial. It is not yet known how, or even whether, repopulation of the excavated areas would occur.
Since 2001 several permits have been issued to governmental institutions by the ISA to survey manganese fields. These are not for actual mining but for a detailed initial investigation of the potential mining areas. In 2006 Germany also secured the rights to a 150,000 square kilometre area – twice the size of Bavaria – for a period of 15 years. Last year, for the first time, industrial companies also submitted applications for the exploration of manganese nodule fields in the open sea in cooperation with developing countries (Kingdom of Tonga, Republic of Nauru).
Cobalt crustsCobalt crusts form at depths of 1000 to 3000 metres on the flanks of submarine volcanoes, and therefore usually occur in regions with high volcanic activity such as the territorial waters around the island states of the South Pacific. The crusts accumulate when manganese, iron and a wide array of trace metals dissolved in the water (cobalt, copper, nickel, and platinum) are deposited on the volcanic substrates.
Their growth rates are comparable to those of manganese nodules. The cobalt crusts also contain relatively small amounts of the economically important resources. Literally tonnes of raw material have to be excavated in order to obtain significant amounts of the metals. However, the content of cobalt (up to 2 per cent) and platinum (up to 0.0001 per cent) is somewhat higher than in manganese nodules. Extracting cobalt from the ocean is of particular interest because it is found on land in only a few countries (Congo, Zaire, Russia, Australia and China), some of which are politically unstable. Alternative marine prospects could reduce our dependence on supplies from these countries.
Technologically, the mining of cobalt crusts is much more complex than manganese nodules. For one, it is critical that only the crust is removed, and not the underlying volcanic rocks. In addition, the slopes of the volcanoes are very ragged and steep, which makes the use of excavation equipment more difficult. It is therefore not surprising that cobalt crust mining is only at the conceptual stage at present. Cobalt crust mining would also have a significant impact on the benthic organisms. It is therefore vital that prior environmental impact studies are carried out. In most cases monitoring by the International Seabed Authority (ISA) is not possible because many cobalt occurrences are located within the territorial waters of various countries.
- 7.5 > Massive sulphides form at black smokers – hot springs on the sea floor with temperatures approaching 400 degrees Celsius. These vents discharge minerals from the Earth’s interior, forming chimneys that rise to several metres above the seabed. Black smokers are also unique habitats.
Massive sulphidesThe third resource under discussion is a sulphur-rich ore that originates at “black smokers”. These occurrences of massive sulphides form at submarine plate boundaries, where an exchange of heat and elements occurs between rocks in the Earth’s crust and the ocean due to the interaction of volcanic activity with seawater. Cold seawater penetrates through cracks in the sea floor down to depths of several kilometres. Near heat sources such as magma chambers, the seawater is heated to temperatures exceeding 400 degrees Celsius. Upon warming, the water rises rapidly again and is extruded back into the sea. These hydrothermal solutions transport metals dissolved from the rocks and magma, which are then deposited on the sea floor and accumulate in layers. This is how the massive sulphides and the characteristic chimneys (“black smokers”) are produced.
These were first discovered in 1978 at the East Pacific Rise. For a long time it was thought that massive sulphides with mining potential were only formed on mid-ocean ridges, because the volcanic activity and heat production here are especially intense. But since then more than 200 occurrences worldwide have been identified. Experts even estimate that 500 to 1000 large occurrences may exist on the sea floor. But there are also great differences in size. Most occurrences are only a few metres in diameter and the amount of material present is negligible.
So far only a few massive sulphide occurrences which are of economic interest due to their size and composition are known. While the black smokers along the East Pacific Rise and in the central Atlantic produce sulphides comprising predominantly ironrich sulphur compounds – which are not worth considering for deep-sea mining – the occurrences in the southwest Pacific contain greater amounts of copper, zinc and gold. They are also located in comparatively shallow water (less than 2000 metres) and lie within the exclusive economic zones of nations near them (Chapter 10), which makes the possible mining more technologically and politically feasible. This is because a country can decide for itself with respect to the mining of marine resources within its own exclusive economic zone. The deep sea floor outside these sovereignty limits, however, is overseen by the International Seabed Authority (ISA; Chapter 10).
Extra Info explore the sea floor in the search for resources.
- Present mining scenarios primarily envision the exploitation of cooled, inactive massive sulphide occurrences that are only sparsely populated by living organisms. Active black smokers are rejected for the time being because most of them contain only comparatively minor amounts of resources. Furthermore, because of the nutrient rich waters rising from below, they provide an important habitat for numerous, and in part, endemic organisms.
The largest known sulphide occurrence is located in the Red Sea, where tectonic forces are pulling Africa and the Arabian Peninsula apart. Here, the sulphides are not associated with black smokers, but appear in the form of ironrich ore muds with high contents of copper, zinc and gold. This occurrence, at a water depth of about 2000 metres, was discovered in the 1960s. Because of its muddy consistency, it appears that these deposits will not prove problematic to mine, and this was successfully tested in the 1980s.
- Of the three sea floor resources discussed here, massive sulphides are the least abundant in terms of total volume, but they are of particular interest because of their high resource content. Some mining companies have already obtained exploration licences in national waters, and are advancing the technology for prospecting and extraction. In May 2010 the ISA even has granted one exploration licence in the Indian Ocean to China. So far only permits for research have been granted for the deep sea.
In the near future the mining of copper and gold from massive sulphides is likely to commence off the coasts of Papua New Guinea and New Zealand. Mining operations had been planned to start this year, but due to the present economic recession, major metal and mining companies have experienced a decline in turnover in spite of the relatively high prices of gold, and the projects were postponed at short notice. But a recovery of the metal market is expected for the future. The companies will therefore soon be able to proceed with their plans.
The future of marine miningOf the three resource types waiting to be extracted from the deep sea, the mining of massive sulphides in the exclusive economic zones (200 nautical miles) of west Pacific nations (Papua New Guinea) seems to be most feasible at present. Despite the latest economic crisis, production could start in the next few years. Because of their relatively high content of valuable metals, the mining of massive sulphides may be profitable for some companies. But the metal content of the global massive sulphides is lower than that of the ore deposits on land. It is therefore unlikely that the marine mining of massive sulphides will have a significant impact on the global resource supply.
Manganese nodules and cobalt crusts present quite different prospects. The amounts of copper, cobalt and nickel they contain could without doubt rival the occurrences on land. In fact, the total cobalt is significantly more than in all the known deposits on land. About 70,000 tonnes of cobalt are presently mined on land each year and the worldwide supply is estimated at about 15 million tonnes. By comparison, a total of about 1000 million tonnes of cobalt is estimated to be contained in the marine manganese nodules and cobalt crusts.
In spite of these immense resources, sea floor mining will only be able to compete with the substantial deposits presently available on land if there is sufficient demand and metal prices are correspondingly high. Furthermore, the excavation technology has yet to be developed.
The serious technological difficulties in separating the crusts from the substrate, combined with the problems presented by the uneven sea floor surface, further reduce the economic potential of the cobalt crusts for the present. Therefore, it seems that marine mining of cobalt crusts should not be anticipated any time soon.