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1. The basement: The Precambrian Shield

The Precambrian Shield

Lar­ge parts of cen­tral and south Green­land are made up by an old ‘shield’, a plat­form of rocks, which has been sta­ble for a long time, but had pre­vious­ly gone through a long, com­plex histo­ry. A num­ber of events, all of them inde­ed ‘world­shaking’, have left their traces. Several times con­ti­nents col­li­ded and alte­red all rocks bet­ween them due to the extre­me pres­su­re. Sedi­ments were depo­si­ted and vol­ca­noes erup­ted. All the­se rocks were later expo­sed to lar­ge pres­su­re when they got bet­ween col­li­ding tec­to­nic pla­tes and were then defor­med, bent and bro­ken, and meta­mor­pho­sed, and all this even several times through Earth histo­ry.

Stron­gly meta­mor­pho­sed Pre­cam­bri­an rocks near Kan­ger­lus­suaq (West Green­land)

Strongly metamorphosed Precambrian rocks near Kangerlussuaq (West Greenland)

This com­plex mix­tu­re of dif­fe­rent, main­ly meta­mor­phic rocks then beca­me sta­ble and has lar­ge­ly remai­ned unch­an­ged sin­ce then. Rivers and gla­ciers have alte­red the shape at the sur­face, but the rocks as such have not chan­ged. Thus, they pro­vi­de a solid base for ever­ything that has hap­pend more recent­ly. Such a solid base is ter­med ‘base­ment’ or, if it is very old and lar­ge, ‘shield’. Ever­ything which is youn­ger and sub­se­quent­ly most­ly depo­si­ted on top of the base­ment, is cal­led cover rock to descri­be the basic struc­tu­re, at least time-wise.

Most parts of Green­land con­sist of such an old shield or are at least under­lain by one. Pre­cam­bri­an rocks are also expo­sed in the inner­most part of the Scores­by­sund, even though only in a qui­te limi­ted area, at least Pre­cam­bri­an rocks that have not been affec­ted by the Cale­do­ni­an oro­ge­ny. The Pre­cam­bri­an rocks are most­ly gra­ni­tes and gneis­ses. The com­plex histo­ry which has led to their ori­gin can part­ly be recon­struc­ted. Lar­ge-sca­le tec­to­nic events such as the col­li­si­on of con­ti­nents, which have chan­ged rocks stron­gly and over a wide area, have hap­pen­ed at least three times: 2.6 bil­li­on years ago (Archae­an), 1.9 bil­li­on years ago (lower Pro­ter­o­zoic) and 1 bil­li­on years ago (midd­le Pro­ter­o­zoic).

With some expe­ri­ence you can some­ti­mes see immedia­te­ly that rocks have such a com­plex histo­ry which invol­ved several pha­ses of move­ment, pres­su­re, heat etc. This is a bit more com­pli­ca­ted and thus bey­ond the scope of this litt­le over­view; for detail­ed results, even pro­fes­sio­nal geo­lo­gists need to work for qui­te some time both in the field and in the labo­ra­to­ry, espe­cial­ly to get abso­lu­te ages as tho­se ones given abo­ve, which can of cour­se not direct­ly be read in the land­s­cape.

Stron­gly defor­med sedi­ment sam­ple with small veins (ver­ti­cal). Three steps are necessa­ry to form such a rock: first­ly depo­si­ti­on of the ori­gi­nal sedi­ment, second­ly com­pres­si­ve tec­to­nics for defor­ma­ti­on, third­ly intru­si­on of veins (which are not defor­med and thus youn­ger than the defor­ma­ti­on). Size ca. 20 cm (left-right).

Yalour rocks 2

In the Scores­by­sund, rocks not alte­red sin­ce the Pre­cam­bri­an are expo­sed only in the inner­most Ves­t­fjord and Nord­vest­fjord in so-cal­led ‘tec­to­nic win­dows’. The­se are holes in cover rocks which other­wi­se cover the Pre­cam­bri­an base­ment. In wide are­as of the cen­tral Scores­by­sund, the­re are Pre­cam­bri­an rocks which have been affec­ted by the Cale­do­ni­an oro­ge­ny.

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last modification: 2013-10-12 · copyright: Rolf Stange
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