Texture In metamorphic rocks individual minerals may or may not be bounded by crystal faces. 3.20. Metamorphic rocks may be classified on the basis of foliation into two types. Figure 7.7 shows an example of this effect. matamorphic rocks that have mineral grains with a random, interlocking texture. The light coloured bands are due to quartz and Feldspar, while the dark coloured bands are due to the presence of Ferro-magnesium minerals. The major differences between foliated and nonfoliated metamorphic rocks are in the areas of texture, appearance and the type of pressure applied during recrystallization. It is a process in which pressure is the dominating factor. Under greater heat and pressure, as metamorphic minerals such as mica and feldspar begin to form, strain orients them in layers. Metamorphic rocks exhibit a variety of textures. Marble is extensively used in modern buildings also for the decorative purpose in columns, staircases and floors. Metamorphic rocks form when sedimentary, igneous, or pre-existing metamorphic rocks are changed by heat, pressure, and chemically reactive waters. This group includes the rock that cannot be split into thin sheets. There are two main types of metamorphic rocks: Foliation in geology refers to a repetitive layering in metamorphic rocks. They are not made from molten rock – rocks that do melt form igneous rocks instead. A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as cataclasis.Cataclasis involves the granulation, crushing, or milling of the original rock, then rigid-body rotation and translation of mineral grains or aggregates before lithification. This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. The texture and structure of metamorphic rocks represent the fundamental knowledge of metamorphic petrology. These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith. Different types of rocks and minerals can form non-foliated metamorphic rocks. Metamorphic textures and mineralogy develop progressively over several hundreds of feet/meters of drilling. 3.22. Isograds that parallel major structures suggest some cause-and-effect relationship between the structural and metamorphic development of the region. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … Image Guidelines 4. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism. Vast areas of the ancient continental shields are covered by thick sequences of horizontal sedimentary rock layers. As the water run… Regional means that the metamorphic conditions occur over large geographic areas and include differential (or shearing) stresses, which help to form the layered structure known as foliation. This group includes the rock that can split into thin sheets. Such features include folds, nappes, and faults with a wide variety of geometries. Another model proposes that fluids circulating along the fault zone caused retrograde metamorphism and thus reset the rocks located nearest to the fault to lower temperatures. 2. Metamorphism associated with nappes (large recumbent folds) in the Alps and the Appalachians provides strong evidence that the tectonic transport of rocks typically occurs at rates faster than those of thermal equilibration—in other words, that the nappes can transport hot rocks for large distances without significant cooling. These conditions recrystallize the sand grains and the silica cement that binds them together. Types of Metamorphic Rocks. Copyright 9. Engineering Uses. The pressure applied to the reforming rock causes the differences in the way the rock looks once recrystallized and determines whether it will be foliated or nonfoliated. Although neither of these models provides an adequate explanation for the entire length of the Main Central Thrust, they both emphasize the significant control that structural features can exert on the development of metamorphic rocks. Before uploading and sharing your knowledge on this site, please read the following pages: 1. During metamorphism, new minerals grow, with different sizes, shapes, and orientations than those of the original minerals. These rocks typically cooled very slowly, allowing the minerals within them to form large, uniform crystals that are tightly packed together. The structures of the metamorphic rocks are: The slaty structure is also called slaty cleavage. The metamorphic rocks are extensively used as building stones. A related rock… ; Examples of hydrous minerals that occur in low grade metamorphic rocks: Navigate parenthood with the help of the Raising Curious Learners podcast. Classification of Metamorphic Rocks 2. It is considered to be an excellent building material for important monumental, historical and architectural buildings. Magma metamorphic rock forming here heat Pressure from surface rocks 4. This is the rock name to remember when you find a hard, nondescript rock that looks like it … Argillite. Low-grade metamorphism takes place at temperatures between about 200 to 320 o C, and relatively low pressure. After crustal uplift, these areas became continental surfaces. A rock possessing gneissose structure exhibits a pronounced appearance in which light and dark coloured band alternate as in Fig. • This can greatly affect the hardness, texture or layer patterns of the rocks. Content Filtration 6. In many cases, the correlation of metamorphic isograds and their position in the structure implies a genetic relationship between the two. After reading this article you will learn about:- 1. Metamorphic rocks form when high temperatures and pressure act on a rock to alter its physical and chemical properties (metamorphism means 'to change form'). Granulose (even grained) structure is produced due to the predominance of equigranular minerals such as quartz, feldspar, pyroxenes and calcite as shown in Fig. Each of these will be discussed in turn, then we will summarize how metamorphic rocks are classified. The flaky minerals are absent or present only in small amount on breaking a granulose rock producing a rough fracture surface. Several explanations have been proposed to account for this anomalous distribution of temperature with depth. Nappe formation is a major process of crustal thickening during continent-continent collision; emplacement of the nappes results in burial and heating of the underlying rocks. It is formed by the parallel arrangement of flat, tabular, elongated or flaky minerals, such as Muscovite, Biotite, Chlorite, talc and Hornblende as shown in Fig. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Branches, Geology, Metamorphic Rocks, Petrology, Rocks. compression. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. Report a Violation 10. One model suggests the fault transported hot Asian rocks over cooler Indian rocks, which caused cooling of the Asian rocks in the vicinity of the fault. The rock having Schistose structure has a tendency to split readily into flakes, leaves or thin slabs. Without careful examination by the Geologist, and the recognition of the subtle changes in mineralogy and texture, much time and money can be wasted by drilling past the economic basement. These rocks are identified by their minerals and texture. The metamorphic rocks are foliated and non-foliated as well. Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. The need for stability may cause the structure of minerals to rearra… Quartzite is made of metamorphosed sandstone, while marble is formed from limestone that was metamorphosed. It is a process in which temperature is the dominating factor. Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock. Gneissose Structure and Layers • Rocks having alternate light and dark layers. It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Cataclastic texture: It is produced under stress and in absence of high temperature, whereby rocks … What type of stress formed the structure shown in this figure? For example, isograd patterns that are cut by faults clearly indicate that metamorphism predated fault displacement, whereas isograd sequences that overprint structural discontinuities imply the reverse. In arid climates, vegetation is sparse, and weathered rock and soil cover much of the land surface. The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks. a rock with metamorphic minerals but no foliation or lineation. Disclaimer 8. During metamorphism, protolith chemistry is mildly changed by increased temperature (heat), a type of pressur… The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. Cataclastic - Hornfelsic - Granoblastic - Schistose Metamorphic rock - Metamorphic rock - Structural features: Metamorphic rocks are often intimately related to large-scale (kilometres of tens of kilometres) structural features of Earth. Structures in Metamorphic Rocks 3. Non-foliated metamorphic rocks do not have a platy or sheet-like structure. Rocks are formed on Earth as igneous, sedimentary, or metamorphic rocks. The most important non-foliated rock is marble. The structures of the metamorphic rocks are: (i) Slaty structure, (ii) Schistose structure, (iii) Gneissose structure, and (iv) Granulose structure. Isograd distributions associated with nappe structures can be either normal or inverted, depending on the relative rates of nappe emplacement and heat transfer. Metamorphism occurs when solid rock changes in composition and/or texture without the mineral crystals melting, which is how igneous rock is generated. These strata were deposited in shallow inland seas at various times in the 600 million years following the end of Precambrian time. Uploader Agreement, Term Paper on Metamorphic Rocks | Geology, List of Important Metamorphic Rocks | Geology, Classification of Rocks | Petrology | Geology, Classification of Metamorphic Rocks | Geography, Remote Sensing: Definition and Applications | Topography | Geography, International Tourism in India: Introduction, History, Trends, Opportunities and Future, Forestry: Definition, Branches, Costs, Programme and Conclusion | Geography, Contribution of Russia to World Geography (In Hindi), French Scholars and their Contribution to Geography in Hindi. Texture and structure of metamorphic rocks. Metamorphic rocks are classified on the basis of metamorphism into three types. In metamorphic rocks some or all of the minerals in the original rock are replaced, atom by atom, to form new minerals. Metamorphic rock, any rock that results from the alteration of preexisting rocks in response to changing conditions, such as variations in temperature, pressure, and mechanical stress, and the addition or subtraction of chemical components. The rock possessing slaty cleavage has a unique property of splitting into thin sheets. ... metamorphic rocks that contain distinct layers with parallel, flat, or elongated minerals. In this video, you will get to know about the different types and structure of metamorphic rocks. Metamorphic rocks are formed from other rocks that are changed because of heat or pressure. There are several ways that non-foliated rocks can be produced. For example, one of the major structural features in the Himalayan mountain belt is the Main Central Thrust, a thrust fault that runs for hundreds of kilometres from east to west and was responsible for the transportation of rocks belonging to the Eurasian Plate southward over those of the Indian Plate.
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