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Another document of the volcanic origin of Sal and Capo Verde.
Details of Cape Verde
The emergence of the Cape Verde Islands was an interaction between volcanic activity and erosion. The islands are the summit of a huge submarine mountain landscape that emerged from the so-called Ridge of Cape Verde. The Ridge of Cape Verde is an eastern extension of the Mid-Atlantic Ridge and separates the southern cape verde basin from the northern Canary Basin.
The submarine volcanic activity in the Mid-Atlantic, which ultimately led to the emergence of the Cape Verde archipelago, started at the turn of the Cretaceous to Tertiary because of the divergence of the continents Africa and the America following the opening of the southern Atlantic. The islands are the tops of vast volcanoes under the surface of the sea, whose base are giant shield volcanoes. These shield volcanoes rose already in the Jurassic and sit in several thousand meters below on the bottom of the Atlantic.
The volcanic peaks rising over the sea are constantly exposed to erosion since their creation .The loose volcanic rocks were and still are grinded down due to wind and rain,. Impressive are the deep, mostly dry erosionvalleys (Ribeiras), which can be torrential rivers with rain.
On the shores of the sea contributes to permanent changes in morphology, where sediments can be deposited (accumulation) or removed (erosion, abrasion). For the deposition of sediments also provides the wind, which distributes the sands of the Sahara particularly on the eastern islands .
On the eastern islands of the volcanic activity ended much earlier, so that by the progressive erosion the volcanic structure has been largely exhausted and flat, desert-like islands emerged. On the island of Fogo, however, the volcanic activity lasts until today. The Pico de Fogo last broke in 1995.
Details of Sal
The island of Sal, in view of its close proximity to the African continent, has a climate very similar to other zones with the same latitude. It is a very arid island, and although volcanic in origin, constant erosion throughout its history has made it almost flat. The highest point is Monte Vermelho with a height of 406 m. From north to south, it is about 3O km long but does not exceed more then 12 km in width. The overall surface area is 216 km2.
Just north of the capital Espargos lies on the regular very dry and barren island the oasis Terra Boa.
Here the locals build cabbage, beans, corn and various other vegetables.
North of Terra Boa the landscape is abruptly dry again. Here you regulary can watch fata morganas.
And the hill, you are standing in front of, often seems to stand in a see.
The stones of the vulcano
Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or gray.
On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007). The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarse) groundmass are generally referred to as dolerite (also called diabase) or gabbro.
During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in a special formation. The topology of the lateral shapes of these formation can broadly be classed as a random cellular network.
With this in mind you have do some observations to log the cache :
Send me the name of the "hill" with the estimated high
Descibe the stones the hill consists of
Take a picture with you with your GPS in your hand
E-mail me the answer and the image to camel_f(at)gmx.de before logging the cache. I will get back to you with the permission to do so.
Additional Hints
(Decrypt)
CYRNFR ABGR GUNG V RKCRPG PNPUREF GB QB RKNZVANGVBAF BA FVGR, ABG SEBZ 5 ZVYRF NJNL !