STRUCTURE of METAMORPHIC ROCKS By- Essence of Geology

METAMORPHIC ROCKS Slate Phyllite Metamorphic rocks are defined as the rocks those have formed through the processes of metamorphism on the pre existing rocks ( i.e. primary rocks) under the effect of changing temperature, pressure and chemically active fluid. The phenomena through which the metamorphic rocks are formed is called as Metamorphism. Thus metamorphism is a process involving changes in mineral composition and/or microstructure of a rock, dominantly in solid state under the effect of temperature, pressure and chemically active fluid. Slate Phyllite

TEXTURE and STRUCTURE Textures are small scale features in a rock that are penetrative, which means that the texture occurs virtually all the rock bodies at microscopic scale. Texture of metamorphic rocks depends on the shape of the minerals, their modes of growth and mutual arrangement. Structure is a term used for large- scale features that occur on the hand sample, outcrop or even regional scale. Structure of a metamorphic rock depends on the interrelations of various Textures within the same rock unit and are frequently dominated by the directive forces due to the unequal pressure.

FOLIATION and LINEATION Any penetrative set of more or less parallel surfaces or we can say any planar fabric element can be termed as FOLIATION. Any penetrative set of more or less parallel lines or we can say a linear fabric element can be termed as LINEATION.

TYPES of FOLIATION According to Turner and Weiss(1963), Compositional layering Preferred orientation of platy minerals. Shape of deformed grains. Grain size variation. Preferred orientation of platy minerals in a matrix without preferred orientation. Preferred orientation of lenticular mineral aggregates. Preferred orientation of fractures. Combination of the above

Types of lineation Preferred orientation of elongated mineral aggregates  Preferred orientation of elongate minerals  Lineation defined by platy minerals Fold axes (especially of crenulations) Intersecting planar elements

CATACLASTIC STRUCTURE Its produced under stress and in absence of high temperature, whereby rocks are subjected to shearing and fragmentation. Only the durable minerals partly survive the crushing force and the less durable ones are powdered. Thus, when the resistant minerals and rock fragments stand out in pseudoporphyritic manner in the finer materials, its known as ‘porphyroclastic structure’. When the rocks are highly crushed into fine grained rocks, they are called MYLONITES. Since all these structures are formed due to cataclasis, they are, as a whole, known as CATACLASTIC STRUCTURE.

SCHISTOSE STRUCTURE One of the most obvious structural features of most of the metamorphic rocks is the alignment of platy or flaky minerals in parallel layers. The rock with schistose structure is made up of broadly parallel or sub-parallel layers of flaky, platy or rod-like minerals making it very weak in the direction of parallelism.

GNEISSOSE STRUCTURE Biotite Gneiss from Norway In this structure, bands or folia of platy or flaky minerals alternate with those of equidimensional and granular minerals. These bands generally are of contrasting colors, composition and texture. Presence of gneissose structure is an indication of advance degree of metamorphism under combined action of high temperature, pressure and active participation of fluids .

Garnet Gneiss Gneissic structure in a rock from Karelia

MACULOSE STRUCTURE This is a kind of structure that is characterised by a spotted appearance of the rock that may be caused due to the formation of large sized crystals (porphyroblasts) within an otherwise fine grained rock. This structure is a typical product of thermal (contact) metamorphism and results generally due to incomplete recrystallisation of the constituent minerals.

An Augen structure in a rock found in Lake Alamo, Arizona Another very similar kind of structure shown by the metamorphic rocks is Augen Structure. This structure is typical of metamorphic rocks in which the effect of crushing under pressure is clearly associated with recrystallisation. In this structure some of the resistant minerals get deformed into thin , lenticular, lens like shapes giving a characteristic appearance to the rock.

SLATY STRUCTURE Its also termed as slaty cleavage . This structure commonly develops in shale that undergone slight metamorphism. Rocks showing a slaty structure are very fine grained composed mainly of microscopic flakes of mica.

conclusion The study of structure and texture of the metamorphic rocks is a vast topic. Small scale structures lead to the formation of large sized structure, or we can say texture lead to the formation of the overall structure of the rock, hence study of texture is very important to understand the structure of the rock.

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