Chapter 4 Metamorphic Rocks Metamorphic rocks are rocks that have undergone a change in texture and or mineralogy due to high temperature or pressure or through the action of chemical alteration induced by very hot and chemically aggressive pore water The process of alteration is termed metamorphism and it generally occurs along active plate tectonic boundaries or deep within the Earth s crust some 10 to 30 km below the surface In these areas temperatures can exceed 800 C and pressures can build up to 6 kbars 6000 times atmospheric pressure Although extreme these conditions are not quite enough to actually melt rock Nevertheless the effects of metamorphism can be dramatic and frequently result in the total overprinting or destruction of the rock s original mineralogy and texture 4 1 Types of Metamorphism Geologists recognize several kinds of metamorphism however in the GY 111 laboratory component we can restrict ourselves to the three major types 1 regional 2 contact and 3 cataclastic Figure shows a detailed sketch of deformed and folded rock layers that are typical in regionally metamorphosed areas From LeConte J 1905 Elements of Geology D Appleton and Co New York NY 667p 51 Chapter 4 Metamorphic Rocks 52 4 1 1 Regional Metamorphism The most widespread and easily recognizable metamorphic rocks are those that form through regional metamorphism This is a large scale process involving both heat and directed pressure and generally occurs as part of the mountain building process It is important to recognize that regionally metamorphosed rocks are important elements of all major mountain chains including the Rockies and the Appalachians Rocks generally undergo changes in their texture when they experience regional metamorphism Most of the time this is a completely destructive process and little remains of the rock s original texture Marble which was originally limestone is composed of crystalline calcite The exact variety of limestone from which the marble formed e g oolitic micritic fossiliferous is usually impossible to tell because nothing was preserved of the original texture following metamorphism In rare cases regional metamorphism may not be so pervasive and some of the original texture of a rock may be preserved Metamorphosed sedimentary rocks containing bedding can retain vestiges of sedimentary structures Look for evidence of preserved bedding or cross stratification in some of the samples in your specimen tray 4 1 2 Contact Metamorphism This type of metamorphism as the name implies involves localized alterations induced by contact with an igneous intrusion The major agent of contact metamorphism is heat Temperatures in excess of 800 C can occur when magma intrudes into country rock However unlike regional metamorphism alteration associated with intrusions only affects a very thin rind or halo of country rock surrounding the magma body Original rock textures can be altered but they are seldom destroyed like that which occurs in regional metamorphism Contact metamorphism occurs adjacent to volcanoes in mountain belts and hot spots and along divergent plate boundaries The most common type of contact metamorphic rock is hornfels which is a pretty non descript gray black rock Because it has few if any readily distinguishable characteristics you will probably come to hate this rock if your instructor sees fit to put one in your metamorphic rock tray 4 1 3 Cataclastic Metamorphism This type of metamorphism involves shear force and results when rock bodies slide past one another along faults Tectonic movements build up tremendous pressure but temperature remains low causing the rocks to grind and pulverize one another into Chapter 4 Metamorphic Rocks 53 small bits and pieces Cataclastic metamorphic rocks are usually found in mountain belts where they are associated with regional metamorphism One of the most important types of rock produced through this type of metamorphism is mylonite 4 2 Metamorphic Textures Metamorphic textures are distinguished on the basis of the size shape and orientation of the crystals that comprise the rock Each kind of metamorphism produces its own unique texture or textures Regional metamorphism generally produces rocks that display fine layering or partings collectively termed foliations Foliated rocks contain an abundance of aligned platy constituents such as the mica minerals biotite and muscovite Some foliated rocks also contain large relatively well formed crystals that grew during metamorphism e g garnet or staurolite These crystals are called porphroblasts they are somewhat analogous to phenocrysts in igneous rocks and the rock s texture is said to be porphroblastic Porphroblasts can form during any of the three types of metamorphism but you will encounter them most commonly in the foliated rocks Contact metamorphism and regional metamorphism of rocks that contain few platy minerals e g quartz sandstone or limestone produces rocks that are characterized by fine or coarse interlocking crystals that do not display foliation These are known as non foliated or granular metamorphic rocks and they typically contain an abundance of equidimensional grains such as quartz or calcite Cataclastic metamorphism produces rocks with a porphroclastic texture one dominated by large fragments set in a fine matrix of crushed minerals A complete listing of the metamorphic rocks that you are responsible for in the laboratory component of GY 111 appears in Table 4 1 4 2 1 Foliated Rocks When minerals are exposed to stress e g directed pressure they tend to re orient themselves perpendicular to the direction of stress to offer least resistance to the force Figure 4 1 Platy minerals such as muscovite biotite and chlorite are particularly susceptible to re orientation but pencil like minerals e g amphibole can also be affected In general the more of these minerals that you have in a rock the better the foliation that develops however chemical changes can occur as metamorphism proceeds and it is possible for micas and other minerals to grow at the expense of other Table 4 1 The metamorphic rocks classified according to texture and metamorphic grade Chapter 4 Metamorphic Rocks 54 Chapter 4 Metamorphic Rocks 55 less stable minerals Mineral replacement occurs in a specific order which can be related to the intensity of metamorphism or metamorphic grade of a rock see Section 4 3 Foliated rock is classified on the basis of intensity of foliation degree of mineral separation and