Brava, Cape Verde

Geography

Brava is almost circular with a diameter of up to 11 kilometres and an area of 67 square kilometres. The medium mountainous island is the westernmost of the Ilhas de Sotavento (engl. 'Islands under the Wind') and lies to the west of the island of Fogo. Brava's altitude is barely sufficient to receive precipitation from the trade wind clouds. However, because it lies in the lee of Fogo's volcanic cone, the island is often covered by lee clouds, so evaporation is less and vegetation is somewhat richer. The Cape Verdean man of letters João Rodrigues characterized Brava as "bathed in green and filled with flowers" and in this comparable to Madeira in contrast to many of the more barren islands of the archipelago. The conditions for agriculture are therefore, together with those of Fogo, better than on the other islands of the Sotavento.

The island rises with steep cliffs to a wide plateau, deeply cut by erosion valleys.

The island is more densely populated at the higher altitudes. Here you will also find the island's capital Vila Nova Sintra and the pilgrimage site Nossa Senhora do Monte. On the east coast, the only permanent port on the island is in Furna. Vinagre is the source of an acetic mineral spring that used to be used for spa purposes.

About six kilometres north of the island lie the uninhabited Ilhéus do Rombo, 20 kilometres southwest the submarine Cadamosto seamount closes the archipelago to the west.

The port town of Furna in the northeast of Brava

Geology

Originally a deep sea mountain of volcanic origin, the island was raised above sea level over time. Today, unlike on Fogo, there is no significant volcanic activity on the island anymore, but much stronger seismic activity than on Fogo. The geological evolution of Brava can be divided into three volcanostratigraphic units (from young to old):

• Upper unit
• Middle unit
• Lower unit

The Lower Unit consists of a submarine volcanic succession of Pliocene age (Piacenzian to Gelasian) formed 3 to 2 million years BP. During this time, Brava was in the seamount stage; this part consists of hyaloclastites and pillow lavas interbedded with innumerable cogenetic vein cohorts. Geochemically, these are ankaramites and nephelinites. The Lower Unit was intruded in the Old Pleistocene (1.8 to 1.3 million years BP) by a plutonic alkali rock/carbonatite complex of the Middle Unit, consisting of pyroxenites, ijoliths, nepheline syenites and carbonatites (soevites). Between 1.3 and 0.25 million years BP, these two units were subject to severe erosion; an erosional unconformity formed with the succeeding Upper Unit. In the upper Middle Pleistocene, 250,000 years ago, volcanism resumed after a long dormant period, yielding mainly phonolitic rocks. It is characterized by explosive, phreatomagmatic and magmatic ejecta. Pyroclastic flows (block and ash flows), pyroclastic blankets with surge phenomena and numerous craters formed by phreatic eruptions. Effusive lava flows and lava domes were formed.

A special feature of Brava is the occurrence of intrusive as well as extrusive carbonatites. Intrusive carbonatites are found in the plutonic Middle Unit; they are younger than comparable deposits on the neighbouring islands of Fogo, Santiago and Maio. Extrusively, young Pleistocene to Holocene carbonatites are found in the Upper Unit and are unique to Cape Verde.

The island is thought to have been uplifted from the sea since the seamount stage because of submarine deposited hyaloclastites and pillow lavas and uplifted beaches up to 400 metres above sea level. Estimates give an uplift rate of 0.2 to 0.4 millimetres/year.

Brava is traversed by many faults, lateral thrusts and volcanic/tectonic lineaments that have been instrumental in shaping the evolution of the island. The intrusive veins of the lower unit are orthogonally organized with a predominant east-west orientation. Faults in the two upper units follow predominantly north-northwest to northwest directions.

The spatial arrangement of the faults corresponds more or less to the stress field encountered on the neighbouring island of Fogo. Both islands as well as the Cadamosto seamount and the Ilhéus do Rombo sit on the same substructure, which is supported by 500 to 1000 metres of thick marine sediments at a depth of about 4000 metres. The sediments, in turn, overlie oceanic crust from the Lower Cretaceous, which begins at a depth of about 5300 meters and extends to a depth of 13 kilometers.