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

Figure 1.50: Mudstone metamorphosed in a metamorphic aureole, showing the new randomlyorientated metamorphic minerals.Rocks containing randomly orientated metamorphic minerals must have been producedby thermal metamorphism, but metamorphic rocks with aligned minerals are producedduring regional metamorphism. Under the high temperatures and great pressures causedby tectonic episodes, when new minerals crystallise and original minerals recrystallise,they do so at right angles to the pressures affecting the rocks. So the new and recrystallisedold minerals become lined up and parallel to each other. If the new minerals are flat platyminerals, as many new regional metamorphic minerals are, then the new rocks which formwill develop a new metamorphic layering, called foliation.When mudstone or shale are regionally metamorphosed, the clay minerals recrystalliseinto very fine grained new metamorphic minerals. <strong>The</strong> new rock develops a slaty foliation(cleavage) and is called slate, whilst any fossils in the original rock are either deformed ordestroyed. Because of the alignment of the new minerals, slates can easily be split alongthe foliation, which is called slaty cleavage (Figure 1.51). This is why slates can besplit into the thin waterproof sheets used to make roofs for buildings. Since the directionof the new slaty cleavage is often different from the direction of the original bedding inthe sedimentary rocks, <strong>we</strong> can sometimes see both the bedding and the cleavage in slates,running in different directions (Figure 1.52). See how ‘fossils’ can be deformed by pressureusing the http://http://www.earthlearningidea.com activity, ‘Squeezed out of shape:detecting the distortion after rocks have been affected by Earth movements’.Slate is a low-grade metamorphic rock, since it is formed at relatively low temperaturesand pressures. As metamorphic pressures and temperatures increase, the new metamorphicminerals in the slates grow in size and some new minerals, like garnets, can form,producing a new coarser-foliated, medium-grade metamorphic rock, called schist (Figure1.53). Continued metamorphism causes the minerals to separate into foliated bands pro-35