Mean sea level is an average level of the surface of one or more of Earth's oceans from which heights such as elevation may be measured. MSL is a type of vertical datum – a standardised geodetic datum –, used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and aircraft flight levels. A common and straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location. Sea levels can be affected by many factors and are known to have varied over geological time scales; however 20th century and current millennium sea level rise is caused by global warming, careful measurement of variations in MSL can offer insights into ongoing climate change. The term above sea level refers to above mean sea level. Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level. Instantaneous sea level varies quite a lot on several scales of space.
This is because the sea is in constant motion, affected by the tides, atmospheric pressure, local gravitational differences, salinity and so forth. The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum. For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point. Still-water level or still-water sea level is the level of the sea with motions such as wind waves averaged out. MSL implies the SWL further averaged over a period of time such that changes due to, e.g. the tides have zero mean. Global MSL refers to a spatial average over the entire ocean. One measures the values of MSL in respect to the land. In the UK, the Ordnance Datum is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921, the vertical datum was MSL at the Victoria Liverpool. Since the times of the Russian Empire, in Russia and other former its parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 1.230m below the average sea level. In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level, it is used for main part of Africa as official sea level. As for Spain, the reference to measure heights below or above sea level is placed in Alicante. Elsewhere in Europe vertical elevation references are made to the Amsterdam Peil elevation, which dates back to the 1690s. Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008. Height above mean sea level is the elevation or altitude of an object, relative to the average sea level datum, it is used in aviation, where some heights are recorded and reported with respect to mean sea level, in the atmospheric sciences, land surveying.
An alternative is to base height measurements on an ellipsoid of the entire Earth, what systems such as GPS do. In aviation, the ellipsoid known as World Geodetic System 84 is used to define heights; the alternative is to use a geoid-based vertical datum such as NAVD88. When referring to geographic features such as mountains on a topographic map, variations in elevation are shown by contour lines; the elevation of a mountain denotes the highest point or summit and is illustrated as a small circle on a topographic map with the AMSL height shown in metres, feet or both. In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case, see Amsterdam Airport Schiphol. To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or geodetic datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field.
In reality, due to currents, air pressure variations and salinity variations, etc. this does not occur, not as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as ocean surface topography, it varies globally in a range of ± 2 m. Adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month Metonic cycle and the 223-month eclipse cycle on the tides. Several terms are used to describe the changing relationships between sea level and dry land; when the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps; the term "steric" refers to global changes in sea level due to thermal expansion and salinity variations. The term "isostatic" refers to changes in
In geology, depositional environment or sedimentary environment describes the combination of physical and biological processes associated with the deposition of a particular type of sediment and, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more that direct modern analogues are not available. Continental Alluvial Aeolian – Processes due to wind activity Fluvial LacustrineTransitional Deltaic – Silt deposition landform at the mouth of a river Tidal Lagoonal – A shallow body of water separated from a larger body of water by barrier islands or reefs Beach – Area of loose particles at the edge of the sea or other body of water Lake – A body of still water, in a basin surrounded by landMarine Shallow water marine environment Upper shoreface – The portion of the seafloor, shallow enough to be agitated by everyday wave action Lower shoreface – The portion of the seafloor, the sedimentary depositional environment, that lies below the everyday wave base Deep water marine environment – Flat area on the deep ocean floor Reef – A bar of rock, coral or similar material, lying beneath the surface of waterOthers Evaporite – A water-soluble mineral sediment formed by evaporation from an aqueous solution Glacial Volcanic Tsunami – Sedimentary unit deposited by a tsunami Depositional environments in ancient sediments are recognised using a combination of sedimentary facies, facies associations, sedimentary structures and fossils trace fossil assemblages, as they indicate the environment in which they lived.
Harold G. Reading. 1996. Sedimentary Environments: Processes and Stratigraphy. Blackwell Publishing Limited. Sedimentary Environments Classification Charts Depositional environments on e-notes
Beds are the layers of sedimentary rocks that are distinctly different from overlying and underlying subsequent beds of different sedimentary rocks. Layers of beds are called strata, they are formed from sedimentary rocks being deposited on the Earth's solid surface over a long periods of time. The stratigraphy are layered in the same order that they were deposited, allowing a differentiation of which beds are younger and which ones are older; the structure of a bed is determined by its bedding plane. Beds can be differentiated including rock or mineral type and particle size; the term is applied to sedimentary strata, but may be used for volcanic flows or ash layers. In a quarry, bedding is a term used for a structure occurring in granite and similar massive rocks that allows them to split in well-defined planes horizontally or parallel to the land surface. Other kinds of beds are graded beds. Cross beds are not layered horizontally and are formed by a combination of local deposition on the inclined surfaces of ripples or dunes, local erosion.
Graded beds shows a gradual change in clast sizes from one side of the bed to the other. A normal grading is when there are bigger grain sizes on the older side while an inverse grading is when there are smaller grain sizes on the older side. By knowing the type of beds, geologists can determine the relative ages of the rocks. A bed is the smallest lithostratigraphic unit ranging in thickness from a centimeter to several meters and distinguishable from beds above and below it; the thickness of the bed is determined by the time period involving the deposition of the rocks. Thick Bed - 100cm Thick Bed - 30cm Medium Bed - 10cm Thin Bed - 3cm Very Thin Bed - 1cm Thinner than 1cm is called a Lamina In geotechnical engineering a bedding plane forms a discontinuity that may have a large influence on the mechanical behaviour of soil and rock masses in, for example, foundation, or slope construction. There are geologic principles that the beds follow. Though there can be cases where the principles do not apply due to faults, they are true for most cases.
Law of Superposition is the law that states that the oldest rocks are deposited first and has the younger layers deposited last, as long as the beds have not been overturned through tectonic activities. This is used to date their relative ages. Law of Original Horizontality states that if the beds are not horizontal the layers were caused to either fold or tilt through tectonic activities, they were all deposited horizontally due to gravity. Law of Lateral Continuity states; this means that if two places separated by erosional features have similar rocks, it could mean that they were continuous. Cross-Cutting Relationship states, it helps with dating the rocks. Geological unit Lamination Stratigraphy Stratum Lamina, Laminaset and Bedset. Campbell, Charles V. Sedimentology, vol. 8, issue 1, pp. 7-26
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particules and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes; the particles that form a sedimentary rock are called sediment, may be composed of geological detritus or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, transported to the place of deposition by water, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and piling up on the floor of water bodies. Sedimentation may occur as dissolved minerals precipitate from water solution; the sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.
Sedimentary rocks are only a thin veneer over a crust consisting of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata; the study of sedimentary rocks and rock strata provides information about the subsurface, useful for civil engineering, for example in the construction of roads, tunnels, canals or other structures. Sedimentary rocks are important sources of natural resources like coal, fossil fuels, drinking water or ores; the study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps with other disciplines in the Earth sciences, such as pedology, geomorphology and structural geology. Sedimentary rocks have been found on Mars. Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, a fourth category for "other" sedimentary rocks formed by impacts and other minor processes.
Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic rocks are composed of quartz, rock fragments, clay minerals, mica. Clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel and mud; the classification of clastic sedimentary rocks parallels this scheme. This tripartite subdivision is mirrored by the broad categories of rudites and lutites in older literature; the subdivision of these three broad categories is based on differences in clast shape, grain size or texture. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz and lithic framework grains and the abundance of a muddy matrix between the larger grains.
Composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Quartz sandstones have >90% quartz grains Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grainsAbundance of muddy matrix material between sand grains When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud. "Clean" sandstones with open pore space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes. Six sandstone names are possible using the descriptors for grain composition and the amount of matrix. For example, a quartz arenite would be composed of quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme is used by sedimentologists, common names like greywacke and quartz sandstone are still used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles; these fine-grained particles are transported by turbulent flow in water or air, deposited as the flow calms and the particles settle out of suspension. Most authors presently
Petrology is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous and sedimentary petrology. Igneous and metamorphic petrology are taught together because they both contain heavy use of chemistry, chemical methods, phase diagrams. Sedimentary petrology is, on the other hand taught together with stratigraphy because it deals with the processes that form sedimentary rock. Lithology was once synonymous with petrography, but in current usage, lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks while petrography is the speciality that deals with microscopic details. In the petroleum industry, lithology, or more mud logging, is the graphic representation of geological formations being drilled through, drawn on a log called a mud log; as the cuttings are circulated out of the borehole they are sampled and tested chemically when needed. Petrology utilizes the fields of mineralogy, optical mineralogy, chemical analysis to describe the composition and texture of rocks.
Petrologists include the principles of geochemistry and geophysics through the study of geochemical trends and cycles and the use of thermodynamic data and experiments in order to better understand the origins of rocks. There are three branches of petrology, corresponding to the three types of rocks: igneous and sedimentary, another dealing with experimental techniques: Igneous petrology focuses on the composition and texture of igneous rocks. Igneous rocks include plutonic rocks. Sedimentary petrology focuses on the texture of sedimentary rocks. Metamorphic petrology focuses on the composition and texture of metamorphic rocks Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are useful for investigating rocks of the lower crust and upper mantle that survive the journey to the surface in pristine condition, they are one of the prime sources of information about inaccessible rocks such as those in the Earth's lower mantle and in the mantles of the other terrestrial planets and the Moon.
The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built. Important publications in petrology Ore Pedology Atlas of Igneous and metamorphic rocks and textures – Geology Department, University of North Carolina Metamorphic Petrology Database – Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute Petrological Database of the Ocean Floor - Center for International Earth Science Information Network, Columbia University
Geology is an earth science concerned with the solid Earth, the rocks of which it is composed, the processes by which they change over time. Geology can include the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Modern geology overlaps all other earth sciences, including hydrology and the atmospheric sciences, so is treated as one major aspect of integrated earth system science and planetary science. Geology describes the structure of the Earth on and beneath its surface, the processes that have shaped that structure, it provides tools to determine the relative and absolute ages of rocks found in a given location, to describe the histories of those rocks. By combining these tools, geologists are able to chronicle the geological history of the Earth as a whole, to demonstrate the age of the Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, the Earth's past climates. Geologists use a wide variety of methods to understand the Earth's structure and evolution, including field work, rock description, geophysical techniques, chemical analysis, physical experiments, numerical modelling.
In practical terms, geology is important for mineral and hydrocarbon exploration and exploitation, evaluating water resources, understanding of natural hazards, the remediation of environmental problems, providing insights into past climate change. Geology is a major academic discipline, it plays an important role in geotechnical engineering; the majority of geological data comes from research on solid Earth materials. These fall into one of two categories: rock and unlithified material; the majority of research in geology is associated with the study of rock, as rock provides the primary record of the majority of the geologic history of the Earth. There are three major types of rock: igneous and metamorphic; the rock cycle illustrates the relationships among them. When a rock solidifies or crystallizes from melt, it is an igneous rock; this rock can be weathered and eroded redeposited and lithified into a sedimentary rock. It can be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric.
All three types may melt again, when this happens, new magma is formed, from which an igneous rock may once more solidify. To study all three types of rock, geologists evaluate the minerals; each mineral has distinct physical properties, there are many tests to determine each of them. The specimens can be tested for: Luster: Measurement of the amount of light reflected from the surface. Luster is broken into nonmetallic. Color: Minerals are grouped by their color. Diagnostic but impurities can change a mineral’s color. Streak: Performed by scratching the sample on a porcelain plate; the color of the streak can help name the mineral. Hardness: The resistance of a mineral to scratch. Breakage pattern: A mineral can either show fracture or cleavage, the former being breakage of uneven surfaces and the latter a breakage along spaced parallel planes. Specific gravity: the weight of a specific volume of a mineral. Effervescence: Involves dripping hydrochloric acid on the mineral to test for fizzing. Magnetism: Involves using a magnet to test for magnetism.
Taste: Minerals can have a distinctive taste, like halite. Smell: Minerals can have a distinctive odor. For example, sulfur smells like rotten eggs. Geologists study unlithified materials, which come from more recent deposits; these materials are superficial deposits. This study is known as Quaternary geology, after the Quaternary period of geologic history. However, unlithified material does not only include sediments. Magmas and lavas are the original unlithified source of all igneous rocks; the active flow of molten rock is studied in volcanology, igneous petrology aims to determine the history of igneous rocks from their final crystallization to their original molten source. In the 1960s, it was discovered that the Earth's lithosphere, which includes the crust and rigid uppermost portion of the upper mantle, is separated into tectonic plates that move across the plastically deforming, upper mantle, called the asthenosphere; this theory is supported by several types of observations, including seafloor spreading and the global distribution of mountain terrain and seismicity.
There is an intimate coupling between the movement of the plates on the surface and the convection of the mantle. Thus, oceanic plates and the adjoining mantle convection currents always move in the same direction – because the oceanic lithosphere is the rigid upper thermal boundary layer of the convecting mantle; this coupling between rigid plates moving on the surface of the Earth and the convecting mantle is called plate tectonics. The development of plate tectonics has provided a physical basis for many observations of the solid Earth. Long linear regions of geologic features are explained as plate boundaries. For example: Mid-ocean ridges, high regions on the seafloor where hydrothermal vents and volcanoes exist, are seen as divergent boundaries, where two plates move apart. Arcs of volcanoes and earthquakes are theorized as convergent boundaries, where one plate subducts, or moves, under another. Transform boundaries, such as the San Andreas Fault system, resulted in widespread powerful earthquakes.
Plate tectonics has provided a mechan
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, grain size, composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7; the naming of a lithology is based on the rock type. The three major rock types are sedimentary, metamorphic. Sedimentary rocks are further classified by whether they are carbonate. Siliciclastic sedimentary rocks are subcategorized based on their grain size distribution and the relative proportions of quartz and lithic fragments.
Carbonate rocks are classified with the Dunham or Folk classification schemes according to the constituents of the carbonate rock. The name of an igneous rock requires information on crystal mineralogy; this classification can be performed with a QAPF diagram. Metamorphic rock naming can be based on texture, metamorphic facies, and/or the locations in which they are found. Naming based on texture and a pelite protolith can be used to define slate and phyllite. Texture-based names are gneiss; these textures, from slate to gneiss, define a continually-increasing extent of metamorphism. Metamorphic facies are defined by the pressure-temperature fields. Additional metamorphic rock names exist: greenstone is a classification based on composition and being located Precambrian terranes, while quartzite is based only on composition, as quartz is too stable and homogeneous to change phase at typical metamorphic temperatures and pressures. In igneous and metamorphic rocks, grain size is a measure of the sizes of the crystals in the rock.
In igneous rock, this is used to determine the rate at which the material cooled: large crystals indicate intrusive igneous rock, while small crystals indicate that the rock was extrusive. As metamorphic reactions progress, the grains in metamorphic rocks can be broken down into smaller grains. In clastic sedimentary rocks, grain size is the diameter of the grains and/or clasts that constitute the rock; these are used to determine. In the case of sandstones and conglomerates, which cover a wide range of grain sizes, a word describing the grain size range is added to the rock name. Examples are "pebble conglomerate" and "fine quartz arenite". In rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description. In the case of sequences including carbonates, calcite-cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite using dilute hydrochloric acid and looking for effervescence.
The mineralogical composition of a rock is one of the major ways. In general, igneous rocks can be categorized by increasing silica content as ultramafic, intermediate, or felsic, though more mineral-specific classifications exist. Metamorphic facies, which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks, are determined by observing the mineral phases that are present in a sample; the colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded, sometimes against standard colour charts, such as that produced by the Rock-Color Chart Committee of the Geological Society of America based on the Munsell color system. The fabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks the main visible fabric is bedding and the scale and degree of development of the bedding is recorded as part of the description.
Metamorphic rocks, are characterised by linear fabrics. Igneous rocks may have fabrics due either to flow or to the settling out of particular mineral phases during crystallisation, forming cumulates; the texture of a rock describes the relationship between the individual grains or clasts that make up the rock. Sedimentary textures include the degree of sorting, grading and roundness of the clasts. Metamorphic textures include those referring to the timing of growth of large metamorphic minerals relative to a phase of deformation – before deformation porphyroclast – after deformation porphyroblast. Igneous textures include those. Rocks contain small-scale structures. In sedimentary rocks this may include sole markings, ripple marks and cross-bedding; these are recorded as they are characteristic of a particular depositional environment and may provide information on paleocurrent directions. In metamorphic rocks associated with the deeper levels of fault zones, small scale structures such as asymmetric boudins and microfolds are used to determine the sense of displacement across the zone.
In igneous rocks, sma