Geography: Formation of islands, different types of rocks

Lewis Gneiss

Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss.

Gneiss is a common and widely distributed type of rock formed by metamorphic processes from pre-existing formations (igneous or sedimentary rocks).

These some of the oldest rocks in Europe and some of the oldest in the world, having been formed in the Precambrian "super-eon", up to 3 billion years ago.

In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland (west of the Moine Thrust) and on the islands of Coll and Tiree.
These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist and intruded by later basaltic dykes and granite magma.

The gneiss's delicate pink colours are exposed throughout the islands.

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Source: http://en.wikipedia.org/wiki/Gneiss

HEBRIDEAN TERRANE

The Hebridean Terrane forms the western most strip of mainland Scotland, most of the Inner Hebrides and all of the Outer Hebrides.


Similar rocks are also thought to be present on Shetland and have been proved west and north of the Outer Hebrides by BGS shallow boreholes and hydrocarbon exploration wells.

The full extent of this terrane to the west is obscured by the effects of Mesozoic rifting.

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Source: http://en.wikipedia.org/wiki/Gneiss




GREEN BELT

Understanding the nature and origin of greenstone belts is the most fruitful way of studying Archaean geological history. Greenstone belts are basically metamorphosed volcanic belts.

The name comes from the green hue imparted by the colour of the metamorphic minerals within the mafic rocks. Chlorite, actinolite and other green amphiboles are the typical green minerals.

Greenstone belts have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes.

Greenstones, aside from containing basalts, also give rise to several types of metamorphic rocks which are used synonymously with 'metabasalt' et cetera; greenschist, whiteschist and blueschist are all terms spawned from the study of greenstone belts.

The rock is derived from basalt, gabbro or similar rocks containing sodium-rich plagioclase feldspar, chlorite, epidote and quartz.

Oceanic basalts in the vicinity of mid-ocean ridges typically exhibit sub-greenschist alteration. The greenstone belts of the various archean cratons are commonly altered to the greenschist facies.

 

A form of chlorite schist was popular in prehistoric Native American communities for the production of axes and celts, as well as ornamental items.

Temperatures of approximately 400 to 500 °C and depths of about 8 to 50 kilometers are the typical envelope of greenschist facies rocks. The equilibrium mineral assemblage of rocks subjected to greenschist facies conditions depends on primary rock composition.

Basalt: chlorite + actinolite + albite +/- epidote
Ultramafic: chlorite + serpentine +/- talc +/- tremolite +/- diopside +/- brucite
Pelites: quartz +/- albite +/- k-feldspar +/- chlorite, muscovite, garnet, pyrophyllite +/- graphite
Calc-silicates: calcite +/- dolomite +/- quartz +/- micas, scapolite, wollastonite, etc


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Source: http://en.wikipedia.org/wiki/Greenschist