AN INTERESTING FEATURE

On the very edge of the plateau scarp near Níjar is a fascinating geological feature. If you explore the Google map its form is very clear. Looked at from above it  resembles a volcanic crater but its petrology differs. It is well known to many who go to collect garnets (right), so=called ‘black diamonds’ so numerous that they discolour the loose tufa sands on which they lie. On the southern side which adjoins the edge of the plateau escaping water has carved a ravine and perhaps a saddle (see below). The rim is an almost geometrical circle of limestone from which this basin descends. The crater itself consists of tufa debris.

There are small crystals of such transparency as one rarely finds in other volcanic regions of the Cape (above). Most of the quartz elsewhere, for example, is opaline or opaque often additionally coloured by minerals like citrine, amethyst, and jasper. They don’t occur here in the quantity of the garnets but are thinly scattered around the same areas. They look like fragments of best Bohemian crystal , though there is also some opaline citrine. There much Muscovite debris (right), which results from lower grade metamorphosis (or so I read). Garnets are embedded in this which is also an aluminium based rock, as well as in tufa containing fragments of volcanic glass (below). Small crystal size indicates rapid cooling in water.

My petrology reference tells me that garnets are produced from aluminium silicate at high temperature and might occur in zones of both intermediate metamorphosis and high grade. They do occur elsewhere in the Cape, such as the Cala de los Garnejos, there amongst magma rocks. But nowhere are they a numerous as here. The largest of these is 1 cm across, apparently as large as they get. So no doubt about the volcanic origins of this bowl. However, it did seem to me on reflection that it was perhaps not that simple!

This suggests to me a situation with widely variable temperature zones. Add to this the almost smooth, taking erosion into account, limey capping, the complex seems similar to many areas where geysers erupt frequently. In the bottom of such a basin superheated water would have been ejected at high pressure from fissures reaching down to the remnants of a basolith and even when not actually erupting would emit fluids at very high temperatures.

So the idea emerged that resulted in the sketch below. That the basin was filled with water, perhaps even a lake at some stage, is suggested by the eroded ravine that discharges into the Campohermoso plain, with another possible spillage over the shallower saddle, though that might have been due to wave erosion since it faces south west into what would most probably have been an arm of the sea. This doesn’t explain how the hollow was first generated! And it could also have been a normal small eruption!

RAVINE

SADDLE

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Above: garnets in profusion amongst tufa debris

Right: the limestone of the crust forming the perimeter of the basin, a fine powdery kind of mineral that does not appear otherwise layered but homogeneous, as though laid in a single operational period (geologically speaking).