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Rock Microstructure
Rock microstructure includes the texture and small-scale structures of a rock. The words ''texture'' and ''microstructure'' are interchangeable, with the latter preferred in modern geological literature. However, ''texture'' is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance. Textures are ''penetrative fabrics'' of rocks; they occur throughout the entirety of the rock mass on a microscopic, hand-specimen, and often outcrop scale. This is similar in many ways to foliations, except a texture does not necessarily carry structural information in terms of deformation events and orientation information. Structures occur on a hand-specimen scale and above. Microstructure analysis describes the textural features of the rock, and can provide information on the conditions of formation, petrogenesis, and subsequent deformation, folding, or alteration events. Sedimentary microstructures Description of sedimentary ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Texture (geology)
In geology, texture or rock microstructure refers to the relationship between the materials of which a rock is composed. The broadest textural classes are crystalline (in which the components are intergrown and interlocking crystals), fragmental (in which there is an accumulation of fragments by some physical process), aphanitic (in which crystals are not visible to the unaided eye), and glassy (in which the particles are too small to be seen and amorphously arranged).Texture & Genesis of Rocks, Introductory Geology Laboratory, Christopher DiLeonardo, Ph.D., Marek Cichanski, Ph.D., Earth & Space Sciences, De Anza College The geometric aspects and relations amongst the component particles or crystals are referred to as the '' crystallographic texture'' or ''preferred orientation''. Textures can be quantified in many ways. The most common parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pressure Solution
In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress within the same rock or their complete removal from the rock within the fluid. It is an example of diffusive mass transfer. The detailed kinetics of the process was reviewed by Rutter (1976), and since then such kinetics has been used in many applications in earth sciences. Occurrence Evidence for pressure solution has been described from sedimentary rocks that have only been affected by compaction. The most common example of this is bedding plane parallel stylolites developed in carbonates. In a tectonic manner, deformed rocks also show evidence of pressure solution including stylolites at a high angle to bedding. The process is also thought to be an important part of the developmen ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Igneous Rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust. Geological significance Igneous and metamorphic rocks make up 90–95% of th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is common in igneous and metamorphic rock and is occasionally found as small flakes in sedimentary rock. It is particularly prominent in many granites, pegmatites, and schists, and "books" (large individual crystals) of mica several feet across have been found in some pegmatites. Micas are used in products such as drywalls, paints, fillers, especially in parts for automobiles, roofing and shingles, as well as in electronics. The mineral is used in cosmetics and food to add "shimmer" or "frost." Properties and structure The mica group is composed of 37 phyllosilicate minerals. All crystallize in the monoclinic system, with a tendency towards pseudohexagonal crystals, and are similar in structure but vary in chemical composition. Micas ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cleavage (geology)
Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the kind of cleavage feature that develops. Generally, these structures are formed in fine grained rocks composed of minerals affected by pressure solution. Cleavage is a type of rock foliation, a fabric element that describes the way planar features develop in a rock. Foliation is separated into two groups: primary and secondary. Primary deals with igneous and sedimentary rocks, while secondary deals with rocks that undergo metamorphism as a result of deformation. Cleavage is a type of secondary foliation associated with fine grained rocks. For coarser grained rocks, schistosity is used to describe secondary foliation. There are a variety of definitions for cleavage, which may cause confusion and debate. The terminology used in this article is based largely ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mylonite
Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock. Formation Mylonites are ductilely deformed rocks formed by the accumulation of large shear strain, in ductile fault zones. There are many different views on the formation of mylonites, but it is generally agreed that crystal-plastic deformation must have occurred, and that fracturing and cataclastic flow are secondary processes in the formation of mylonites. Mechanical abrasion of grains by milling does not occur, although this was originally thought to be the process that formed mylonites, which were named from the Greek μύλος ''mylos'', meaning mill. Mylonites form at depths of no less than 4 km. There are many different mechanisms that accommodate crystal-plastic ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shear (geology)
Boudinaged quartz vein (with strain fringe) showing ''Fault (geology)">sinistral shear sense'', Starlight Pit, Fortnum Gold Mine, Western Australia In geology, shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics. The process of shearing occurs within brittle, brittle-ductile, and ductile rocks. Within purely brittle rocks, compressive stress results in fracturing and simple faulting. Rocks Rocks typical of shear zones include mylonite, cataclasite, S-tectonite and L-tectonite, pseudotachylite, certain breccias and highly foliated versions of the wall rocks. Shear zone A shear zone is a tabular to sheetlike, planar or curviplanar zone composed of rocks that are more highly strained than rocks adjace ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Facies
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formation), and the changes that may occur in those attributes over a geographic area. A facies encompasses all of the characteristics of a rock including its chemical, physical, and biological features that distinguish it from adjacent rock. The term facies was introduced by the Swiss geologist Amanz Gressly in 1838 and was part of his significant contribution to the foundations of modern stratigraphy, which replaced the earlier notions of Neptunism. Types of facies Sedimentary facies Ideally, a sedimentary facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment. Sedimentary facies are either descriptive or interpretative. Sedimentary facies are bodies of sediment ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstones) are garnets and staurolites, which stand out in well- foliated metapelites (such as schists) against the platy mica matrix. A similar type of crystal is a '' phenocryst'', a large crystal in an igneous rock. Porphyroblasts are often confused with ''porphyroclasts'', which can also be large outstanding crystals, but which are ''older'' than the matrix of the rock. If a porphyroblastic mineral has small inclusions of minerals within it, the mineral is described as poikiloblast A poikiloblast is a porphyroblast mineral with small inclusions of the previous rock in it. From the texture (if any) shown in the inclusion, the deformation history of the rock can be interpreted, as ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Crenulation
In a geological context, crenulation or crenulation cleavage is a fabric formed in metamorphic rocks such as phyllite, schist and some gneiss by two or more stress directions causing the formation of the superimposed foliations. Formation Crenulations form when an early planar fabric is overprinted by a later planar fabric. Crenulations form by recrystallisation of mica minerals during metamorphism. Micaceous minerals form planar surfaces known as foliations perpendicular to the principal stress fields. If a rock is subjected to two separate deformations and the second deformation is at some other angle to the original, growth of new micas on the foliation planes will create a new foliation plane perpendicular to the plane of principal stress. The angular intersection of the two foliations causes a diagnostic texture called a ''crenulation'', which may involve folding of the earlier mica foliations by the later foliation. Recognition Recognising a crenulation in a rock may requi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Metamorphic Rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by their protolith, their chemical and mineral makeup, and their texture. They may be formed simply by being deeply buried beneath the Earth's surface, where they are subject to high temperatures and the great pressure of the rock layers above. They can also form from tectonic processes such as continental collisions, which cause horizontal pressure, friction, and dis ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
