“The rate of weathering of dunite massifs in the tropical zone can be quantified, or at least a minimum rate of weathering can be firmly established. The first example is the dunite massif of Conakry/Guinea. This dunite occupies the entire peninsula on which Conakry, the capital of Guinea, is situated. It has an approximate length of 50 km and an average width of 5 km. Over its entire surface, it is covered by a thick lateritic weathering crust, which is very clearly visible as a purplish red area on satellite pictures (see Fig. 7.5). This lateritic crust, which is the iron-rich insoluble red residue of the dunite after deep tropical weathering, contains virtually no silica, magnesium or calcium oxides which were completely leached out during the weathering process [19]. These components make up around 90% of the original dunite. This means that 1 m of laterite is equivalent to 10 m of dunite, or even more if the remaining components of the laterite were not completely immobile but have also been leached to some extent. The same author presents evidence that iron has in fact been fairly mobile and was partially leached out as well, which means that 1 m of laterite is equivalent to more than 10 m of dunite. The weathering crust has a thickness between 30 and 100 m. The age of the dunite (that is to say the time at which this dunite intrusion formed) has been determined as 195 million years. From these data, it is simple to calculate the minimum rate of weathering as follows: 50 m of laterite is equivalent to 500 m of dunite, 500 m (= 500 million microns) divided by 195 million years is 2.6 mm/year. This is already ten times faster than deduced from laboratory experiments, but the real rate of weathering must have been considerably faster. The rock is an intrusion. That means it was emplaced between rocks at some depth and covered by other rocks, which had to be removed first by erosion before the dunite became exposed and could start its weathering process. If the dunite intrusion has taken place at 2km depth, it would take 100 million years before the dunite massif was entirely laid bare by erosion at an estimated erosion rate of the order of 1–2 cm/1,000 years [20]. This is the average erosion rate for all continents. This correction alone more than

doubles the calculated rate of weathering. That is not the only positive correction that must be made. In more recent times, the weathering process, under such a thick weathering crust, has virtually come to a standstill, as the thick laterite crust effectively shields the underlying rock from further interaction with the atmosphere. This shortens again the time span over which weathering was active, and thereby increases the rate of weathering.

-R.D. Schuiling, Geoengineering Responses to Climate Change P154