Seamounts grow through volcanic activity to great heights on the sea floor over millions of years. They are found in all of the oceans and reach heights of 1000 to 4000 metres. Smaller seamounts are also called knolls.
A coating on the rocks
Cobalt crusts are rock-hard, metallic layers that form on the flanks of submarine volcanoes, called seamounts. Similar to manganese nodules, these crusts form over millions of years as metal compounds in the water are precipitated.
As with manganese nodules, deposition occurs very slowly. Crusts grow 1 to 5 millimetres per million years, which is even slower than nodules. Depending upon the concentration of metal compounds in the sea water, crusts with different thicknesses have formed in different ocean regions. On some seamounts they are only 2 centimetres thick, while in the richest areas thicknesses can be up to 26 centimetres. Because the cobalt crusts are firmly attached to the rocky substrate, they cannot simply be picked up from the bottom like manganese nodules. They will have to be laboriously separated and removed from the underlying rocks.
It has been estimated that there are over 33,000 seamounts worldwide. The exact number is not known. Around 57 per cent are located in the Pacific. The Pacific is thus the most important cobalt crust region in the world.
The western Pacific is of particular interest. The world’s oldest seamounts were formed here during the Jurassic period around 150 million years ago. Accordingly, many metallic compounds were deposited here over a long period of time to form comparatively thick crusts. This area, around 3000 kilometres southwest of Japan, is called the Prime Crust Zone (PCZ). The amount of crust in the PCZ is estimated to total 7.5 billion tonnes.
- 2.18 > Manganese nodules and cobalt crusts contain primarily manganese and iron. Because iron is plentiful in land deposits, it is not a key factor in marine mining. For the other elements making up lower weight per cents of the deposits, however, there are great differences to occurrences on land. In the manganese nodules nickel and copper predominate, while in cobalt crusts cobalt, nickel and rare earth elements are more significant.
A metal-rich crust
Like manganese nodules, cobalt crusts also represent a very large metal resource in the sea. As the name suggests, the crusts contain a relatively large amount of cobalt compared to deposits on land and to manganese nodules. The largest share of metals in the cobalt crusts, however, consists of manganese and iron. The crusts are often more precisely referred to as “cobalt-rich ferromanganese crusts”. Tellurium is also comparatively abundant in cobalt crusts. Tellurium is necessary particularly for the production of highly efficient thin-film photovoltaic cells.
In absolute terms the crusts of the Prime Crust Zone do not contain as much manganese as the manganese nodules of the Clarion-Clipperton Zone. However, the quantities of manganese in the PCZ are still almost 3 times greater than the economically minable amounts on land today. Furthermore, in the southern area of the PCZ, comparatively high contents of rare earth elements are found in the crusts.
- 2.19 > Cobalt crusts occur in different ocean regions than manganese nod-ules. Each of these resources has its own especially abundant regions. The most important cobalt crust area is the Prime Crust Zone (PCZ) in the western Pacific. The area of greatest manganese nodule concentration is the Clarion-Clipperton Zone (CCZ).
- 2.20 > Cobalt crusts are especially abundant in the western Pacific within a region the size of Europe, called the Prime Crust Zone (PCZ). When compared to deposits on land and to the manganese nodule area of the Clarion-Clipperton Zone (CCZ), it is notable that the occurrence of cobalt and tellurium in particular are comparatively large in the PCZ, with amounts exceeding both the land deposits and those in the CCZ.
Strong currents around seamounts
Cobalt crusts form on all exposed rock surfaces on undersea rises, particularly on seamounts and knolls. Seamounts act somewhat like gigantic stirring rods in the sea to produce large eddies. Nutrients or other materials that rain down from the sea surface or that are transported by ocean currents are often trapped by these eddies at the seamounts. These can include metallic compounds that are deposited on the rocks. An important precondition for the formation of cobalt crusts is that the rock and the growing crusts remain free from sediments. This condition is met at the seamounts and other elevated areas. Currents carry the fine sediments away and keep the rocks and crusts exposed.
Cobalt crusts are found at water depths from 600 to 7000 metres. Studies at seamounts have shown that the thickest crusts and those richest in resources are located on the upper areas of the seamount slopes, where currents are most active. On the average these lie in water depths of 800 to 2500 metres, near the oxygen minimum zone. Analyses also show that the crusts between 800 and 2200 metres have the highest cobalt contents. Researchers do not know the reason for this.
Like a sponge, or the activated charcoal used as a filter in aquariums, cobalt crusts are very porous. Thanks to the many micrometre-sized pores, the crusts have a large internal surface area. In the same way that pollutants are trapped in the pores of an activated charcoal filter, metal compounds are deposited on the large surface areas of the crusts. Because the dissolved metals occur at very low concentrations in sea water, growth of the crusts requires very long periods of time. The crusts are mainly formed through the deposition of iron oxide-hydroxide [FeO(OH)] and manganese oxide (vernadite, MnO₂). The other metals are deposited with the iron oxide-hydroxide and vernadite on the crust surfaces rather like hitchhikers. The reason is that, in the ocean, various metal ions attach themselves to the iron oxide-hydroxide and vernadite molecules in the water. Iron oxide-hydroxide is slightly positively charged and thus attracts negatively charged ions such as molybdate (MoO42-). Vernadite, on the other hand, is slightly negatively charged and attracts positively charged ions such as cobalt (Co2+), copper (Cu2+) or nickel (Ni2+).
Incidentally, most of the metal ions contained in sea water originate from land. Over time they are washed out of the rocks and transported by rivers to the oceans. Iron and manganese, however, usually enter the ocean through volcanic sources on the sea floor called hydrothermal seeps.
Crust mining in sovereign territory?
Manganese nodules and cobalt crusts are of equal interest for future marine mining because they contain traces of many industrially important metals that, because of the immense tonnage of the deposits, are of economic interest. But there are important differences with regard to the exploration and future mining of the crusts. One of these, for example, is the legal situation. In contrast to manganese nodules, most of the richest crust occurrences are not found in the international waters of the high seas but in the Exclusive Economic Zones (EEZs) of various island nations. Thus the International Seabed Authority (ISA) will not be responsible for determining the conditions for future mining there. Rather, the respective local governments will have jurisdiction. However, to date no country has presented concrete plans.
For those crust deposits in international waters, on the other hand, a binding system of regulations has recently been established. In July 2012 the ISA adopted internationally binding regulations for the exploration of such crust occurrences in regions of the high seas. It is true that China, Japan and the Russian Federation at that time had already submitted working plans to the ISA for future exploration in the international waters of the western Pacific, but the council and the assembly of the seabed authority first have to approve these. The working plans specify what basic information the countries want to collect in the upcoming years, including taking samples from the sea floor and analyses of the crusts, depth measurements or studies of faunal assemblages.
Problematic thickness measurements
The exploration of cobalt crusts is also fundamentally different from the manganese nodule situation in some technical aspects. Manganese nodules can be brought quickly and easily on board with a box corer, similar to a backhoe, and then sampled to measure the metal content, for example. Furthermore, the nodules are relatively evenly distributed over the sea floor. This allows relatively straightforward assessment of the deposits by photos and video recordings, particularly with respect to the size of the nodules. Sampling and measurements of the thickness of cobalt crusts, however, are much more difficult because rock boulders have to be torn or drilled out. Local thickness differences are poorly constrained and the spot sampling is extremely time-consuming and expensive.
- 2.21 > Many metal ions end up in the cobalt crusts as “hitchhikers”. In the water, the metal ions attach themselves to iron oxide-hydroxide and vernadite molecules, and are then deposited onto the porous surfaces with them.
- Instruments that could be pulled through the water near the bottom to accurately measure the crust thickness while passing over would be much more efficient. This would allow large areas to be studied in a relatively short time. Scientists are therefore working to refine high-resolution acoustic instruments. These send sound waves into the sea floor and record the reflected signals, then calculate the layered structure of the subbottom. This kind of apparatus is standard technology in exploration for other resources on the sea floor. However, instruments precise enough to measure cobalt crust thicknesses to the nearest centimetre and to distinguish them from the underlying rocks are not yet available. >