Metamorphism and Metamorphic Rocks

[...]... enclosing rocks Is an important transporting medium for ions Fluids and Metamorphism (4)   When intergranular fluids are absent, metamorphic reactions are very slow When pressure increases due to burial of a rock, and as metamorphism proceeds, the amount of pore space decreases and the intergranular fluid is slowly squeezed from the rock Fluids and Metamorphism (5)  Any fluid that escapes during metamorphism. .. a vein Metamorphic changes that occur while temperatures and pressures are rising (usu In presence of intergranular fluid) are termed prograde metamorphic effects (prograde metamorphism) Figure B8.2 Fluids and Metamorphism (6)  Metamorphic changes that occur as temperature and pressure are declining (and usually after much of the intergranular fluid has been expelled) are called retrograde metamorphic. .. effects (retrograde metamorphism) Role of Time in Metamorphism   Coarse-grained rocks are the products of long sustained metamorphic conditions (possibly over millions of years) at high temperatures and pressures Fine-grained rocks are products of lower temperatures, lower pressures or, in some cases, short reaction times The Upper And Lower Limits Of Metamorphism   At the lower end, metamorphism occurs... result in various colors Metamorphism of Sandstone   Quartzite is the metamorphic derivative of quartz It is derived from quartz sandstone by filling of the spaces between the original grains with silica and by recrystallization of the entire mass (large grains grow and small are dissolved) Types of Metamorphism (1) There are four types of metamorphism:  Cataclastic metamorphism   In/ around... occurs in sedimentary and igneous rocks that are subjected to temperatures greater than about 100oC, usually under pressures of hundreds of atmospheres, caused by the weight of a few thousand meters of overlying rock At the upper end, metamorphism ceases to occur at temperatures that melt rock How Rocks Respond To Temperature and Pressure Change In Metamorphism  Lower-grade Metamorphism: Slaty Cleavage... deformation) Contact metamorphism  Contact with magma: dominated by recrystallization Types of Metamorphism (2)  Burial metamorphism   Deep burial by overlying rocks: dominated by recrystallization (aided by water) Regional metamorphism  Both mechanical deformation and chemical recrystallization Contact Metamorphism (1)   Occurs when bodies of hot magma intrude into cool rocks of the crust Mechanical... sedimentary rock being metamorphosed Higher-grade Metamorphism: Schistosity   At intermediate and high grades of metamorphism, grain size increases Foliation in coarse-grained metamorphic rocks is called schistosity (the parallel arrangement of coarse grains of the sheet-structure minerals) Figure 8.5 Figure 8.6 Mineral Assemblage Change   As temperature and pressure rise, one mineral assemblage evolves... lenticular parts of sandstone/quartzite beds dismembered by “sub-horizontal” extension/stretching) & max Figure 8.3 stress/pressure: “sub-vertical”: schistosity of mudstone Fluids and Metamorphism (1)   Sedimentary rocks have open spaces between their grains filled by a watery intergranular fluid This fluid:     Is never pure water Always contains small amounts of dissolved gases and salts Contains... amphibolites, but is not pronounced because micas and chlorites are usually absent At the highest grade of metamorphism, amphibole is replaced by pyroxene and an indistinctly foliated rock called a granulite develops Greenschist Folded quartz vein in a greenschist (or greenstone) (green colour caused by chlorite) Metamorphism of Limestone  Marble is the metamorphic derivative of limestone     Coarsely... of minerals such as quartz and feldspar Figure 8.8: KNOW THIS WELL Figure 8.9 KNOW THIS WELL: Fig A Metamorphism of Basalt   Greenschist (= metamorphosed mafic volcanic rock) has pronounced foliation like phyllite, but also a very distinctive green color because of its chlorite content Amphibolite and granulite   When greenschist is subjected to intermediate-grade metamorphism, amphibole replaces . 1 Metamorphism and Metamorphic Rocks 2 What is Metamorphism? (1)  Metamorphism is the change in form that happens in Earth’s crustal. rocks in response to changes in temperature and pressure. 3 What is Metamorphism? (2)  There are six major factors that control metamorphism:  Chemical composition  Temperature change  Pressure. regional thermal metamorphism 7 Temperature And Pressure (2)  Low-grade metamorphism: at low temperatures c.a. 100oC to 500oC, and at relatively low pressures.  High-grade metamorphism: at