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Metamorphic rocks or metamorphites
(rocks changed due to heat and/or pressure)

If rocks are expo­sed to strong heat or pres­su­re or even both at the same time, then they will be chan­ged, regard­less of their ori­gi­nal com­po­si­ti­on. The che­mi­stry remains the same, as the­re is not­hing added or remo­ved, but the avail­ab­le mate­ri­al is re-orga­nis­ed to form new crys­tals of dif­fe­rent mine­rals, in the same way as ovn heat can turn dough into cake (or coal…). This pro­cess is cal­led meta­mor­pho­sis, and the resul­ting rocks are meta­mor­phi­tes or meta­mor­phic rocks. For the result, the ori­gi­nal kind of rock plays an important role, alt­hough it can be dif­fi­cult to deter­mi­ne if meta­mor­pho­sis has been strong.

If hot mag­ma intru­des into cold rocks, then the­se will beco­me qui­te hot. They will recrystal­li­se in the con­ta­ct zone, which is often obvious on both sides of basaltic intru­si­ons.

Dark intrusion in grey granitic rocks with light-grey, slightly metamorphosed contact zone (thickness of intrusion ca. 5 centimetres), Antarctic Peninsula

Dark intru­si­on in grey gra­ni­tic rocks with light-grey, slight­ly meta­mor­pho­sed con­ta­ct zone (thic­kness of intru­si­on ca. 5 cen­ti­me­tres), Ant­arc­tic Pen­in­su­la.

If rocks get bet­ween col­li­ding pla­tes, then they will expe­ri­ence extre­me pres­su­re and thus meta­mor­pho­se. A typi­cal rock for such a situa­ti­on is gneiss. With other words: when con­ti­nents col­li­de and moun­tain chains form, then under the moun­tains, in depths of several kilo­me­tres, the­re will be a wide gneiss area. 

Gneiss (Kan­ger­lus­suaq, West Green­land)

Gneiss (Kangerlussuaq, West Greenland)

When the moun­tains are even­tual­ly worn down by ero­si­on, the gneiss core will be expo­sed at the sur­face and tell the geo­lo­gist that moun­tains must once have been the­re, alt­hough the land­s­cape may have been ero­ded down to the level of table-flat low­lands at sea level.

Should such a flat low­land area expe­ri­ence uplift again, the result will be a high pla­teau to begin with. This will then be dis­sec­ted by ero­si­on: gla­ciers and rivers crea­te val­ley which incise into the pla­teau. Moun­tains will remain bet­ween the val­leys for a while. 

High plateau more than 1000 metres high, dissected by glacial valleys and fjords. Western Milne Land

High pla­teau more than 1000 metres high, dis­sec­ted by gla­cial val­leys and fjords.
Wes­tern Mil­ne Land.

Geo­lo­gi­cal­ly, such a recy­cled pla­teau is a moun­tain chain in two ways: the gneiss core of the old moun­tains, which have been ero­ded for a long time and which now gives wit­ness of an anci­ent moun­tain chain. This will usual­ly be cal­led ‘fold belt’. And then the young moun­tain chain, which is one in a stric­ter sen­se with moun­tain peaks, val­leys etc, ever­ything made up of old, ‘recy­cled’ rocks. This is an important princip­le in geo­lo­gy: ever­ything will be recy­cled again and again, not­hing real­ly disap­pears, rocks just chan­ge their appare­an­ce, loca­ti­on, com­po­si­ti­on etc. The geo­lo­gi­cal sys­tem ‘Earth’ works in cir­cles: the rock cir­cle. Mol­ten rock comes to the sur­face to cool down and form lava. This is then ero­ded and depo­si­ted again some­whe­re else as a sedi­ment. Teco­nic pro­ces­ses bring it back into the deep lay­ers of the crust, whe­re it can be meta­mor­pho­sed, then it is gneiss. This can final­ly melt and the who­le pro­cess can start all over again, with one of its count­less varia­ti­ons.

Most rocks which we see expo­sed at the sur­face have gone through this rock cycle alrea­dy many times. Some­ti­mes, the youn­gest pro­cess which affec­ted the rocks is the only one which left its traces, as older ones were wiped out at the same time, but some­ti­mes rocks have pre­ser­ved traces of several genera­ti­ons of rock-forming pro­ces­ses and thus make it pos­si­ble to recon­struct Earth histo­ry.

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