Structural geology is the study of the three dimensional distribution of rock bodies and their planar or folded surfaces, and their internal fabrics. Geology the term fabric describes the spatial and geometric configuration of all the elements that make up a particular rock.
Structural geology includes features of and overlaps with facets of geomorphology, metamorphism and geotechnical studies. Geomorphology (from Greek: γη ge, "earth" μορφή morfé, "form" and λόγος Logos, "knowledge" Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i Geotechnical engineering is the branch of Civil engineering concerned with the engineering behavior of earth materials By studying the three dimensional structure of rocks and regions, inferences on tectonic history, past geological environments and deformation events can be made. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere These can be fixed in time using stratigraphical controls as well as geochronology, to determine when the structural features formed. In the Natural sciences under the umbrella of Natural history, Geochronology is the Science of determining the absolute age of rocks, Fossils
More formally stated it is the branch of geology that deals with the geological processes through which the application of a force results in the transformation of a shape, arrangement or internal fabric of the rock into another shape, arrangement or internal fabric. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere Petroleum structural geologists can interpret prospect or basin scale geology using several techniques. These techniques include the interpretation of surface data, well data, remote sensing data and seismic data. Many structural geologists now use 2D/3D geological modelling software in order to integrate these varied datasets. Geologic modelling is the applied science of creating computerized representations of portions of the Earth's crust, especially oil
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Use and importance
The study of geologic structures has been of prime importance in economic geology, both petroleum geology and mining geology. Economic geology is concerned with earth materials that can be utilized for economic and/or industrial purposes Petroleum geology refers to the specific set of geological disciplines that are applied to the search for Hydrocarbons ( Oil exploration) Mining is the extraction of valuable Minerals or other geological materials from the earth usually (but not always from an Ore body Folded and faulted rock strata commonly form traps for the accumulation and concentration of fluids such as petroleum and natural gas. In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes Petroleum ( L petroleum, from Greek πετρέλαιον, lit Natural gas is a Gaseous Fossil fuel consisting primarily of Methane but including significant quantities of Ethane, Propane, Faulted and structurally complex areas are notable as permeable zones for hydrothermal fluids and the resulting concentration areas for base and precious metal ore deposits. Hydrothermal circulation in its most general sense is the circulation of hot water 'hydros' in the Greek meaning water and 'thermos' meaning heat An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining Veins of minerals containing various metals commonly occupy faults and fractures in structurally complex areas. These structurally fractured and faulted zones often occur in association with intrusive igneous rocks. In Geology, an intrusion is a body of Igneous rock that has Crystallized from molten Magma below the surface of the Earth. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock They often also occur around geologic reef complexes and collapse features such as ancient sinkholes. In nautical terminology a reef is a rock, sandbar, or other feature lying beneath the surface of the water (six fathoms or less at low water A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or Cenote, is a natural depression Deposits of gold, silver, copper, lead, zinc, and other metals, are commonly located in structurally complex areas. Gold (ˈɡoʊld is a Chemical element with the symbol Au (from its Latin name aurum) and Atomic number 79 Silver (ˈsɪlvɚ is a Chemical element with the symbol " Ag " (argentum from the Ancient Greek: ἀργήντος - argēntos gen Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 Characteristics Lead has a dull luster and is a dense, Ductile, very soft highly Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30
Structural geology is a critical part of engineering geology, which is concerned with the physical and mechanical properties of natural rocks. Engineering Geology is the application of the geologic sciences to engineering practice for the purpose of assuring that the geologic factors affecting the location design construction Structural fabrics and defects such as faults, folds, foliations and joints are internal weaknesses of rocks which may affect the stability of human engineered structures such as dams, road cuts, open pit mines and underground mines or road tunnels. In geology the term joint refers to a fracture in rock where there has been no lateral movement in the plane of the fracture (up down or sideways of one side A dam is a barrier that divides waters. Dams generally serve the primary purpose of retaining water while other structures such as Floodgates, Levees Open-pit mining, also known as opencast mining and open-cut mining and strip mining, refers to a method of extracting rock or Minerals Underground hard rock Mining refers to various underground mining techniques used to excavate hard minerals such as those containing metals like Gold, A tunnel is an underground passageway The definition of what constitutes a tunnel is not universally agreed upon
Geotechnical risk, including earthquake risk can only be investigated by inspecting a combination of structural geology and geomorphology. Geotechnical engineering is the branch of Civil engineering concerned with the engineering behavior of earth materials An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer Geomorphology (from Greek: γη ge, "earth" μορφή morfé, "form" and λόγος Logos, "knowledge" In addition areas of karst landscapes which are underlain by underground caverns and potential sinkholes or collapse features are of importance for these scientists. Karst topography is a landscape shaped by the dissolution of a layer or layers of soluble Bedrock, usually Carbonate rock such as Limestone A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or Cenote, is a natural depression In addition, areas of steep slopes are potential collapse or landslide hazards.
Environmental geologists and hydrogeologists or hydrologists need to understand structural geology because structures are sites of groundwater flow and penetration, which may affect, for instance, seepage of toxic substances from waste dumps, or seepage of salty water into aquifers. Environmental geology, like Hydrogeology, is a multidisciplinary field of applied science and is closely related to Engineering geology and somewhat related to Hydrogeology ( hydro- meaning water and -geology meaning the study of the Earth) is the area of Geology that deals with the distribution and Hydrology (from Greek Yδωρ hudōr, "water" and λόγος logos, "study" is the study of the movement distribution and quality of Groundwater is Water located beneath the Ground surface in Soil pore spaces and in the Fractures of lithologic formations An aquifer is an underground layer of Water -bearing Permeable rock or unconsolidated materials ( Gravel, Sand, Silt, or Clay
Plate tectonics is structural geology on a large scale, usually referring to the structural effects of plate collisions and other plate tectonic features. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere
Rock macro-structures
On a large scale, structural geology is the study of the three dimensional relationships of stratigraphic units to one another within terranes of rock or within geological regions. A terrane in Geology is a fragment of crustal material formed on or broken off from one Tectonic plate and accreted — " sutured "
This branch of structural geology deals mainly with the orientation, deformation and relationships of stratigraphy (bedding), which may have been faulted, folded or given a foliation by some tectonic event. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic This is mainly a geometric science, from which cross sections and three dimensional block models of rocks, regions, terranes and parts of the Earth's crust can be generated.
Study of regional structure is important in understanding orogeny, plate tectonics and more specifically in the oil, gas and mineral exploration industries as structures such as faults, folds and unconformities are primary controls on ore mineralisation and oil traps. Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere Petroleum ( L petroleum, from Greek πετρέλαιον, lit This page is about the physical properties of gas as a state of matter A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific An unconformity is a buried Erosion surface separating two rock masses or strata of different ages indicating that Sediment deposition was not An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining
Modern regional structure is being investigated using seismic tomography and seismic reflection in three dimensions, providing unrivaled images of the Earth's interior, its faults and the deep crust. Seismic Tomography is a methodology for estimating the earth's properties Seismology (from Greek grc σεισμός seismos, "earthquake" and grc -λογία -logia) is the scientific study of Earthquakes Further information from geophysics such as gravity and airborne magnetics can provide information on the nature of rocks imaged in the deep crust. Geophysics, a major discipline of Earth sciences, is the study of the Earth by quantitative physical methods especially by seismic, electromagnetic Gravitation is a natural Phenomenon by which objects with Mass attract one another
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Rock microstructures
Rock microstructure or texture of rocks is studied by structural geologists on a small scale to provide detailed information mainly about metamorphic rocks and some features of sedimentary rocks, most often if they have been folded. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary In geology the term joint refers to a fracture in rock where there has been no lateral movement in the plane of the fracture (up down or sideways of one side Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics. Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock)
Textural study involves measurement and characterisation of foliations, crenulations, metamorphic minerals, and timing relationships between these structural features and mineralogical features. Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic Crenulation is a texture formed in Metamorphic rocks such as Phyllite, Schist and some Gneiss by two or more stress directions resulting
Usually this involves collection of hand specimens, which may be cut to provide petrographic thin sections which are analysed under a petrographic microscope. Petrography is that branch of Petrology which focuses on detailed descriptions of rocks. A petrographic microscope is a Microscope used in Petrology and Optical mineralogy to identify rocks and Minerals in Thin sections
Principles of structural geology
Measurement conventions
The inclination of a planar structure in geology is measured by strike and dip. Rock microstructure includes the texture of a rock and the small scale rock structures Strike and dip refer to the orientation or attitude of a Geologic feature The strike is the line of intersection between the planar feature and a horizontal plane, taken according to the right hand convention, and the dip is the magnitude of the inclination, below horizontal, at right angles to strike. For example; striking 25 degrees East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.
Alternatively, dip and dip direction may be used as this is absolute. Dip direction is measured in 360 degrees, generally clockwise from North. For example, a dip of 45 degrees towards 115 degrees azimuth, recorded as 45/115. Note that this is the same as above.
The term hade is occasionally used and is the deviation of a plane from vertical i. e. (90°-dip).
Fold axis plunge is measured in dip and dip direction (strictly, plunge and azimuth of plunge). The orientation of a fold axial plane is measured in strike and dip or dip and dip direction.
Lineations are measured in terms of dip and dip direction, if possible. Lineations in Structural geology are linear structural features within rocks Often lineations occur expressed on a planar surface and can be difficult to measure directly. In this case, the lineation may be measured from the horizontal as a rake or pitch upon the surface.
Rake is measured by placing a protractor flat on the planar surface, with the flat edge horizontal and measuring the angle of the lineation clockwise from horizontal. The orientation of the lineation can then be calculated from the rake and strike-dip information of the plane it was measured from, using a stereographic projection. In Geometry, the stereographic projection is a particular mapping ( function) that projects a Sphere onto a plane
If a fault has lineations formed by movement on the plane, eg; slickensides, this is recorded as a lineation, with a rake, and annotated as to the indication of throw on the fault. pedology, the study of soils in their natural environments a slickenside is a surface of the cracks produced in soils containing a high proportion of swelling Clays
Generally it is easier to record strike and dip information of planar structures in dip/dip direction format as this will match all the other structural information you may be recording about folds, lineations, etc. , although there is an advantage to using different formats that discriminate between planar and linear data.
Plane, fabric, fold and deformation conventions
The convention for analysing structural geology is to identify the planar structures, often called planar fabrics because this implies a textural formation, the linear structures and, from analysis of these, unravel deformations. Rock microstructure includes the texture of a rock and the small scale rock structures
Planar structures are named according to their order of formation, with original sedimentary layering the lowest at S0. Often it is impossible to identify S0 in highly deformed rocks, so numbering may be started at an arbitrary number or given a letter (SA, for instance). In cases where there is a bedding-plane foliation caused by burial metamorphism or diagenesis this may be enumerated as S0a. In Geology and Oceanography, diagenesis is any chemical physical or biological change undergone by a Sediment after its initial deposition and during
If there are folds, these are numbered as F1, F2, etc. Generally the axial plane foliation or cleavage of a fold is created during folding, and the number convention should match. For example, an F2 fold should have an S2 axial foliation.
Deformations are numbered according to their order of formation with the letter D denoting a deformation event. For example D1, D2, D3. Folds and foliations, because they are formed by deformation events, should correlate with these events. For example an F2 fold, with an S2 axial plane foliation would be the result of a D2 deformation.
Metamorphic events may span multiple deformations. Sometimes it is useful to identify them similarly to the structural features for which they are responsible, eg; M2. This may be possible by observing porphyroblast formation in cleavages of known deformation age, by identifying metamorphic mineral assemblages created by different events, or via geochronology. A porphyroblast is a large Mineral Crystal in a Metamorphic rock which has grown within the finer grained groundmass. In the Natural sciences under the umbrella of Natural history, Geochronology is the Science of determining the absolute age of rocks, Fossils
Intersection lineations in rocks, as they are the product of the intersection of two planar structures, are named according to the two planar structures from which they are formed. For instance, the intersection lineation of a S1 cleavage and bedding is the L1-0 intersection lineation (also known as the cleavage-bedding lineation).
Stretching lineations may be difficult to quantify, especially in highly stretched ductile rocks where minimal foliation information is preserved. Where possible, when correlated with deformations (as few are formed in folds, and many are not strictly associated with planar foliations), they may be identified similar to planar surfaces and folds, eg; L1, L2. For convenience some geologists prefer to annotate them with a subscript S, for example Ls1 to differentiate them from intersection lineations, though this is generally redundant.
Stereographic projections
Stereographic projection of structural strike and dip measurements is a powerful method for analyzing the nature and orientation of deformation stresses, lithological units and penetrative fabrics. In Geometry, the stereographic projection is a particular mapping ( function) that projects a Sphere onto a plane
References
- G. H. Davis and S. J. Reynolds (1996). The structural geology of rocks and regions. 2nd Edition. Wiley. ISBN 047 1526215.
- C. W. Passchier & R. A. J. Trouw (1998). Microtectonics. Berlin: Springer. ISBN 3-540-58713-6.
See also
- Crenulation
- Engineering geology
- Fault (geology)
- Fold (geology)
- Foliation (geology)
- Geomorphology
- Geotechnical
- Lineation (geology)
- List of publications in geology
- List of rock textures
- Metamorphism
- Rock microstructure
- Shear (geology)
- Stereographic projection
- Vergence (geology)
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