GRIPPY GABBRO EARTHCACHE EarthCache
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Walking and scrambling on the Black Cuillin in Skye used to be especially tricky because of the frequent mists and poor visibility and the fact that the nature of the rock made compass bearings unreliable. With the advent of GPS these problems have disappeared but the peaks are still daunting with their extreme ruggedness and awesome drops. A large part of the Main Ridge is made of gabbro rock and one of the things that draws people to the area (as well as the spectacular scenery) is the superb grip of most of the rock. Ragged garments, scratched boots and fingers bear mute testimony to the roughness of Skye gabbro which enables sure-footed ascents of most of the peaks. This Earthcache is about the nature of the rocks and how the mountains on Skye formed.
Igneous rocks are formed by the cooling and solidification of hot molten material called magma. This can occur rapidly above ground, as when lava pours out of a volcano (Extrusive rocks), or more slowly underground (Intrusive rocks).
Rapid cooling produces fine-grained rocks such as Basalt but slower cooling of magma with the same chemical composition produces the rock types called Dolerite and Gabbro. Gabbro is a coarse-grained rock in which crystals are clearly visible to the naked eye. This is because it solidified in large masses deep underground and because of this it is sometimes called an Abyssal or Plutonic rock (after Pluto, the god of the lower regions).
Granite is formed in a similar way to gabbro but from magma with a different chemical composition. The 3 main crystalline components of both rock types are
i) quartz (ie a glass-like mineral which is pure silica),
ii) felspar (ie silicates, which are silica combined with oxides of other elements) and iii) mica. (a lustrous & flexible form of potassium aluminium silicate)
In granite the oxides are acidic and total amount of silica can be 70 – 80% whilst gabbro consists of basic (ie alkaline) oxides and the total amount of silica is lower, often less than 50%. Overall, gabbro looks a much darker rock than granite.
The Cuillin we see today were built by a series of volcanic eruptions which began 70 million years ago. A widespread up-welling of lava through many cracks and fissures in the gneiss and schist rocks in the area started the process off. These lavas spread over the surface and solidified into sheets 10 – 40 metres thick, separated by layers of ash and slag, until a layer of basalt up to 350 metres deep stretched from Ireland to Greenland and Scandinavia. The next stage was a second welling up of basic lava but this time as laccoliths (domes) deep beneath the surface. These cooled slowly under the weight of the basalt and became the gabbro which forms so much of the main ridge. Millions of years of erosion then stripped most of the original covering of basalt to reveal the gabbro. Layers of peat blankets the areas of weathered basalt but the gabbro weathers to a rocky debris that little vegetation can colonise. Progress across the gabbro is intermittently interrupted by sudden ravines or fissures where the dykes of basaltic lava have weathered away.
The co-ordinates would put you at the top of the Inaccessible Pinnacle (In Pin), the second highest and most photographed point on Skye. You don't have to go there but you have to get nearby - to the domed summit of Sgurr Dearg (which used to be the Munro and now isn't even a Munro Top!)
Surprisingly the highest peaks on Skye are not “Grippy Gabbro” but basalt which usually weathers more easily. How can this be ? Well some layers of solid basalt rock became enveloped in the molten laccoliths and were baked hard, smooth and brittle. Because of the extra hardening, these survive the blanket of gabbro when exposed to weathering, unlike the later basalt intrusions.
To log this Earthcache post a picture of yourself with the In Pin in the background and e mail to tell me:
1) Your estimate of how much higher the Inaccessible Pinnacle is than the surrounding ground
2) Whether you think gabbro is the result of fast, slow or very slow cooling.
3) Which of the 3 main minerals is more abundant in gabbro than granite.
4) The name of one more mineral that is frequently present in Gabbro.
You should be able to answer (1) on site, (2) & possibly(3) from the information given and (3) & (4) from a reference book or internet search.
Historical Note:-
Amazingly the Black Cuillin on Skye were only ‘discovered’ in the early 1800’s. In fact the first scramble on record was by a local forester, Duncan Macintyre, and the Reverend Lesingham Smith in 1835. The former climbed the first difficult peak (Sgurr Nan Gillean) with the scientist James Forbes on 7 July 1836 and the final climbing problems on the Main Ridge were not solved until the late 1890’s.
Additional Hints
(Decrypt)
Vaibyirf fperr
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