22 Oct MINING OF SEA BED SAND: ANOTHER POINT OF VIEW.
MINING OF SEA BED SAND: ANOTHER POINT OF VIEW.
The ingenuity of man always had motivated him to explore the nature to locate usable commodities for the sustenance of humanity and hence of civilization. Recent idea of gathering construction grade fine aggregate or sand from the seabed in the continental shelf is one of those. It is not at all a new idea any way. We all now know that several island nations as well as others have been gathering sand for application in the concrete or cement mortar. In this article I will examine briefly the feasibility of the proposition from a geological stand point – most often ignored point. In a very short time, the public mindset in Kerala got sharply divided into one of the two camps, viz., the protagonists and the antagonists.
Any construction using cement concrete and steel calls in what are called in industry circles, as coarse and fine aggregates. As far as the former is concerned, there is no great confusion as to the types or sources, by the sheer knowledge that natural processes of weathering of rocks and erosion and transport do not produce it. Thus, obvious and uncontroversial recourse had been and is from a rock-crushing-sizing plant. Contrary to this, the fine aggregate (= commercially designated as sand) used to be sourced and gathered from the modern and ancient river channels, shallow sea bed in the inner shelf as well as provinces away from the riverine or shelf sources, from the crusher tailings (after screening). With out a geo-scientific knowledge of origin of sea sand, its mining scenario may run into sentiment and emotion ridden realms and unrealities.
Truly, quantities of sand sized quartz (a primary constituent of commonest surface rocks like gneisses and granite with a share of only 30% by volume and a mineral of chemical and physical neutrality to cement) is under transport in the stream channels, from the erosion rocks and soils (disaggregated by chemical weathering). Further, this sand has many temporary storages like in the channels as sand bars of various types, in the flood plains, in the abandoned channels etc.
Unfortunately not only that we exhausted the all of stream channel sand as result of the construction boom in the last decades, but ecologically and physically killed all the rivers themselves for ever (in human terms), by the continuing the sanction to gather sand to the local bodies under the three tier panchayats. River sources its sand immediately from the soil only, and majority of us do not know that a meter of soil needs at least a million years to form, and in an in situ soil, sand sized quartz is just about 20 to 30 percent. Hence the rate of sand production is an extremely slow process.
In addition, sand that reaches the sea bed is again worked by the wave and tide generated currents and gradually transported beach ward to be deposited there sometimes permanently. Like in many beaches of Kerala, sand that accumulated in the beach face is eroded and shifted massively to the inshore to be stored in the in shore as longshore submarine bars, and to be slowly re-deposited in the beach any way.
Due to the large scale fluctuations of sea level, in the Pleistocene, the shorelines of the continents including that of India stood several 10s meters ( for India 100 m.) below the present shoreline, forcing the rivers draining the continents to traverse through longer courses (along the modern continental shelf) and depositing the sand there. This scenario is unfortunately not in the knowledge base of lay citizens or the non-specialists – hence, all the hue and cry fouling the air.
Data – State of the art
. The most productive shelf region of Kerala, is rather vast and stands at 22400 Km2 (length 560 Km and width of shelf ~ 40Km) or 20,000 Km2 – to strike conservative figure. As a marine geologist, I say that there exists a considerably large quantum of reliable data with the NIO, MW of GSI and some others, indicating the earlier lower stands of sea level and hence of the past shoreline elevations. Mapping of the sandy sediment in the seabed can be taken up by shallow geophysical and geological work, in order to isolate potential areas for commercial dredging. The dredged can be primarily washed in seawater, and later on further washed in fresh water and not exactly drinking standard water. More over the special cement that would accommodate mildly salty sand will go along way in reducing the water use by this process.
We often ignore, vastness of the resources of the continental shelf – living and non-living and renewable to varying degrees (including sea bed sediment of a decameter or two in depth), which is so large in proportion to the land based resources, due to the very sheer size of the sea bed. We have been gathering large quantities of the living resources of the oceanic realm to complement our (sea) food needs. Large hue and cry is expressly unheard of about this issue, as it supports and contributes to the gastronomic needs and also to the GDP. On top of this we also believe in the ability of the species to regenerate.
I might state that the sandy-mud or the gravelly-sandy-mud (containing black minerals in certain parts off Kerala, and certain other continents) covering sea bed – a non-living resource in the conventional sense- is only forming at very low rate day, but were supplied to the seabed by the west flowing rivers – large and small draining the continental-scape – in the Quaternary or say during the last 1.8 million yr. BP. Therefore it is wise to be able to keep an inventory of all the viable sources of the raw materials useful to mankind. This is only commonsensical any way. The question of express exploitation comes only as the close second.
With the geological background of the sea bed, rate of soil formation and erosion and transport process, weathering of rocks, rock composition and hence nature of sediment cover, some volume estimates essential to fully appreciate the science of sea sand mining. Given a land area of 38863 Km2 for Kerala, it is possible to estimate the size of the sand content (in the sea bed sediment), for a source slab of uniform thickness of 1.0 m., and quartz sand content of 30% and Sp.Gr. of 2.65. By weathering and erosion, this slab ought to yield quartz laden sediment headed to the sea to the west.
Estimated sand content of this slab is (30.89 e12 Tons) 30 Trillion Tons By the way the same slab also would have an ilmenite (Sp.Gr. of 4.2 and abundance of 1.0%) content of (1.632 e12) or say 1.5 Trillion tons.
These estimates consider the land-river-sea system as a closed system (which is more or less true), uniform character for the basement rocks and expressly no rate of erosion. If we allow the entire Pleistocene to be the period of erosion, it is well over 1.8 Million years – more than what is essential for the formation of 1.0 M of soil. This exercise is carried out only to broaden the realities of the sand content of the sea bed.
Certainly post depositional sediment transport by currents generated by waves and tides would have further redistributed the sand with on the sea bed at preferred sites and forms to be discovered by mapping by appropriate tools available to marine geoscientists to day.
Unfortunately, some data or facts (and not the science per se) of geo-science are very simple to be appreciated by the non-specialist and the layman alike. Most of the time such people, become highly opinionated just for the fun of being one (?). In the case of sea sand also we face such a dilemma. Obviously, these are truly proven to be true by sea bed mapping and exploration, and then one can finally provide an alternate source for the sand starved construction industry.