Individual places and topics of interest
Ronda peridotite
Outcropping between Marbella and Ronda is a range of peaks called the Sierra Bermeja. They are composed of a rock rarely seen at surface called peridotite, which here is several kilometres thick. In fact this area is the world's largest outcrop of this type of rock.
Peridotite is the dominant rock type of the upper Mantle - the lithospheric Mantle that sits below the Continental Crust. It originates naturally at depths greater than about 70km - hence its rarity at surface. Studies of this particular body of peridotite show that at some stage it has been at depths greater than 150km [Platt et al, 2013] ! It was exhumed and thrust to surface most likely during the Miocene as part of the collisional process described in Chapter 3 and is associated with the Alpujarrides thrust sheet. Close up, fresh peridotite is a beautiful green-black colour and is composed chiefly of the minerals olivine - (Mg,Fe)SiO4 and pyroxene -(Mg,Fe,Ca,Na)Si2O6 whose coarse crystals are easily visible in hand specimens. Olivine however is amongst the most easily weathered minerals and weathers to something called iddingsite – which is actually a rock type rather than a mineral, consisting of remnant olivine, clay minerals, iron oxide, and ferrihydrate. It is the high iron content in iddingsite that gives the hills their overall brown, orange and red coloured hues, beautiful in the setting sun. You can see and sample many examples of the fresh and weathered rock on the road and trail up to Los Reales peak. Even though diamonds are found in a form of peridotite called kimberlite, it is not true that the Sierra Bermeja contains diamonds, as I have heard talked about. However, there are 'pseudomorphs' of graphite in the form of diamond crystals. Pseudomorphs (meaning "false form") are where one mineral replaces another and takes on the form of the mineral being replaced, instead of its own normal form. So it does look like parts of the peridotite did once contain diamonds! Peridotite also forms the bulk of the nearby ranges Sierra Palmitera, Sierra Real, and Sierra Alpujata - NW and NE of Marbella. |