The planet we live on: The beginnings of the Earth Sciences

Figure 1.51: Slate, a low-grade metamorphic rock, formed by the metamorphism of mudstoneor shale. <strong>The</strong> new slaty cleavage can be seen in this photo.ducing a high-grade metamorphic rock, called gneiss (Figure 1.54). Gneiss can also beformed by the high-grade metamorphism of granite.When mudstones are regionally metamorphosed, the metamorphic sequence is from slate,to schist to gneiss. When limestones are regionally metamorphosed, marble is formed, asit is when limestones are thermally metamorphosed (Figure 1.55). In regionally metamorphosedmarbles, the calcite crystals can be aligned, allowing them to be distinguished frommarbles produced by thermal metamorphism. Similarly, regional metamorphism of puresandstone forms metaquartzite that can have aligned grains, helping us to distinguishit from thermally-metamorphosed metaquartzite (Figure 1.56). You can simulate howmetamorphic rocks are formed using the http://http://www.earthlearningidea.comactivity, ‘Metamorphism - that’s Greek for ‘change of shape’, isn’t it?: What changes can<strong>we</strong> expect when rocks are put under great pressure in the Earth?’.Regionally metamorphosed areas are the roots of mountain chains that have become exposedby the erosion of the rocks above. You can trace the progression of metamorphism,if the margins of the region are composed of mudstones, limestones and sandstones, containingfossils. Moving inward, broad regions of slate are found, sometimes with deformedfossils. <strong>The</strong>se are follo<strong>we</strong>d by schist zones, where any fossils have been destroyed, andfinally zones of gneiss. Meanwhile, limestones become marbles, progressively destroyingfossils, while sandstones become metaquartzites. So as you move in from the margins,original structures and fossils are progressively lost, the crystal size of the minerals becomeslarger, and the rocks tend to become more compact and tougher.36