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141-427: The petrifaction process occurs underground, when wood becomes buried in water or volcanic ash . The presence of water reduces the availability of oxygen which inhibits aerobic decomposition by bacteria and fungi. Mineral-laden water flowing through the sediments may lead to permineralization, which occurs when minerals precipitate out of solution filling the interiors of cells and other empty spaces. During replacement,

282-524: A British Airways Boeing 747-236B ( Flight 9 ) flew through the ash cloud from the eruption of Mount Galunggung , Indonesia resulting in the failure of all four engines. The plane descended 24,000 feet (7,300 m) in 16 minutes before the engines restarted, allowing the aircraft to make an emergency landing. On 15 December 1989, a KLM Boeing 747-400 ( Flight 867 ) also lost power to all four engines after flying into an ash cloud from Mount Redoubt, Alaska . After dropping 14,700 feet (4,500 m) in four minutes,

423-471: A jigsaw puzzle . Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age must lie between the two known ages. Because rock sequences are not continuous, but may be broken up by faults or periods of erosion , it is very difficult to match up rock beds that are not directly next to one another. However, fossils of species that survived for

564-595: A characteristically dark coloured ash containing ~45–55% silica that is generally rich in iron (Fe) and magnesium (Mg). The most explosive rhyolite eruptions produce a felsic ash that is high in silica (>69%) while other types of ash with an intermediate composition (e.g., andesite or dacite ) have a silica content between 55 and 69%. The principal gases released during volcanic activity are water , carbon dioxide , hydrogen , sulfur dioxide , hydrogen sulfide , carbon monoxide and hydrogen chloride . The sulfur and halogen gases and metals are removed from

705-448: A clay matrix. Particle surfaces are often coated with aggregates of zeolite crystals or clay and only relict textures remain to identify pyroclast types. The morphology (shape) of volcanic ash is controlled by a plethora of different eruption and kinematic processes. Eruptions of low-viscosity magmas (e.g., basalt) typically form droplet shaped particles. This droplet shape is, in part, controlled by surface tension , acceleration of

846-544: A collision that formed the Moon about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about 4,440  million years ago . There is evidence on the Moon of a Late Heavy Bombardment by asteroids from 4,000 to 3,800 million years ago . If, as seems likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away. Paleontology traces

987-602: A common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is too little information to achieve this, and paleontologists have to make do with junctions that have several branches. The cladistic technique is sometimes fallible, as some features, such as wings or camera eyes , evolved more than once, convergently  – this must be taken into account in analyses. Evolutionary developmental biology , commonly abbreviated to "Evo Devo", also helps paleontologists to produce "family trees", and understand fossils. For example,

1128-451: A constant rate. These " molecular clocks ", however, are fallible, and provide only a very approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two. Earth formed about 4,570  million years ago and, after

1269-403: A data source that is not limited to animals with easily fossilised hard parts, and they reflect organisms' behaviours. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them. Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of

1410-856: A few millimeters requires removal before airports can resume full operations. Ash does not disappear (unlike snowfalls) and must be disposed of in a manner that prevents it from being remobilised by wind and aircraft. Ash may disrupt transportation systems over large areas for hours to days, including roads and vehicles, railways and ports and shipping. Falling ash will reduce the visibility which can make driving difficult and dangerous. In addition, fast travelling cars will stir up ash, generating billowing clouds which perpetuate ongoing visibility hazards. Ash accumulations will decrease traction, especially when wet, and cover road markings. Fine-grained ash can infiltrate openings in cars and abrade most surfaces, especially between moving parts. Air and oil filters will become blocked requiring frequent replacement. Rail transport

1551-567: A fortunate accident during other research. For example, the 1980 discovery by Luis and Walter Alvarez of iridium , a mainly extraterrestrial metal, in the Cretaceous – Paleogene boundary layer made asteroid impact the most favored explanation for the Cretaceous–Paleogene extinction event – although debate continues about the contribution of volcanism. A complementary approach to developing scientific knowledge, experimental science ,

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1692-599: A forward-facing surface, that are tuned to detect volcanic ash. This system can detect ash concentrations of <1 mg/m to > 50 mg/m , giving pilots approximately 7–10 minutes warning. The camera was tested by the easyJet airline company, AIRBUS and Nicarnica Aviation (co-founded by Dr Fred Prata). The results showed the system could work to distances of ~60 km and up to 10,000 ft but not any higher without some significant modifications. In addition, ground and satellite based imagery, radar , and lidar can be used to detect ash clouds. This information

1833-413: A good level of removal of suspended particles. Chlorination may have to be increased to ensure adequate disinfection. Many households, and some small communities, rely on rainwater for their drinking water supplies. Roof-fed systems are highly vulnerable to contamination by ashfall, as they have a large surface area relative to the storage tank volume. In these cases, leaching of chemical contaminants from

1974-503: A lack of water for hygiene, sanitation and drinking. Municipal authorities need to monitor and manage this water demand carefully, and may need to advise the public to utilise cleanup methods that do not use water (e.g., cleaning with brooms rather than hoses). Wastewater networks may sustain damage similar to water supply networks. It is very difficult to exclude ash from the sewerage system. Systems with combined storm water/sewer lines are most at risk. Ash will enter sewer lines where there

2115-814: A minor group until the first jawed fish appeared in the Late Ordovician . The spread of animals and plants from water to land required organisms to solve several problems, including protection against drying out and supporting themselves against gravity . The earliest evidence of land plants and land invertebrates date back to about 476  million years ago and 490  million years ago respectively. Those invertebrates, as indicated by their trace and body fossils, were shown to be arthropods known as euthycarcinoids . The lineage that produced land vertebrates evolved later but very rapidly between 370  million years ago and 360  million years ago ; recent discoveries have overturned earlier ideas about

2256-483: A minor part of the geologic record, hot spring deposits are important to paleontologists because such deposits sometimes preserve more delicate plant parts in exquisite detail. These Lagerstätte deposits include the Paleozoic Rhynie Chert and East Kirkton Limestone beds, which record early stages in the evolution of land plants. Most of the color in petrified wood comes from trace metals. Of these, iron

2397-901: A minor role in the determination of grain shape in phreatomagmatic eruptions. In this sort of eruption, the rising magma is quickly cooled on contact with ground or surface water. Stresses within the "quenched" magma cause fragmentation into five dominant pyroclast shape-types: (1) blocky and equant; (2) vesicular and irregular with smooth surfaces; (3) moss-like and convoluted; (4) spherical or drop-like; and (5) plate-like. The density of individual particles varies with different eruptions. The density of volcanic ash varies between 700 and 1200 kg/m for pumice, 2350–2450 kg/m for glass shards, 2700–3300 kg/m for crystals, and 2600–3200 kg/m for lithic particles. Since coarser and denser particles are deposited close to source, fine glass and pumice shards are relatively enriched in ash fall deposits at distal locations. The high density and hardness (~5 on

2538-838: A range of different pyroclasts dependent on the eruptive process. For example, ash collected from Hawaiian lava fountains consists of sideromelane (light brown basaltic glass) pyroclasts which contain microlites (small quench crystals, not to be confused with the rare mineral microlite ) and phenocrysts . Slightly more viscous eruptions of basalt (e.g., Strombolian) form a variety of pyroclasts from irregular sideromelane droplets to blocky tachylite (black to dark brown microcrystalline pyroclasts). In contrast, most high-silica ash (e.g. rhyolite) consists of pulverised products of pumice (vitric shards), individual phenocrysts (crystal fraction) and some lithic fragments ( xenoliths ). Ash generated during phreatic eruptions primarily consists of hydrothermally altered lithic and mineral fragments, commonly in

2679-555: A rapid increase in knowledge about the history of life on Earth and to progress in the definition of the geologic time scale , largely based on fossil evidence. Although she was rarely recognised by the scientific community, Mary Anning was a significant contributor to the field of palaeontology during this period; she uncovered multiple novel Mesozoic reptile fossils and deducted that what were then known as bezoar stones are in fact fossilised faeces . In 1822 Henri Marie Ducrotay de Blainville , editor of Journal de Physique , coined

2820-535: A relatively short time can be used to link up isolated rocks: this technique is called biostratigraphy . For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period. If rocks of unknown age are found to have traces of E. pseudoplanus , they must have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and have a short time range to be useful. However, misleading results are produced if

2961-417: A sequential leaching experiment on ash from the 1980 eruption of Mount St. Helens , chloride salts were found to be the most readily soluble, followed by sulfate salts Fluoride compounds are in general only sparingly soluble (e.g., CaF 2 , MgF 2 ), with the exception of fluoride salts of alkali metals and compounds such as calcium hexafluorosilicate (CaSiF 6 ). The pH of fresh ash leachates

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3102-403: A significant health risk to those without pre-existing respiratory conditions . The health effects of volcanic ash depend on the grain size, mineralogical composition and chemical coatings on the surface of the ash particles. Additional factors related to potential respiratory symptoms are the frequency and duration of exposure, the concentration of ash in the air and the respirable ash fraction;

3243-465: A source of silica, but tetraethyl orthosilicate has proven more promising. Petrified wood has limited use in jewelry, but is mostly used for decorative pieces such as book ends, table tops, clock faces, or other ornamental objects. A number of Ancestral Puebloan structures near Petrified Forest National Park were constructed of petrified wood, including the Agate House Pueblo. Petrified wood

3384-604: A steady increase in brain size after about 3  million years ago . There is a long-running debate about whether modern humans are descendants of a single small population in Africa , which then migrated all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide at the same time as a result of interbreeding . Life on earth has suffered occasional mass extinctions at least since 542  million years ago . Despite their disastrous effects, mass extinctions have sometimes accelerated

3525-485: Is a particularly fine example of fluvial accumulations of driftwood. Volcanic ash is particularly suitable for preservation of wood, because large quantities of silica are released as the ash weathers. The presence of petrified wood in a sedimentary bed is often an indication of the presence of weathered volcanic ash. Petrified wood can also form in arkosic sediments, rich in feldspar and other minerals that release silica as they break down. The warm supermonsoon climates of

3666-426: Is affected by temperature and moisture content, but exclusion of oxygen is the most important factor preserving wood tissue: Organisms that decompose lignin must have oxygen for their life processes. As a result, fossil wood older than Eocene (about 56 million years old or older) has lost almost all its holocellulose, and only lignin remains. In addition to microbial decomposition, wood buried in an alkaline environment

3807-438: Is also often loosely used to refer to all explosive eruption products (correctly referred to as tephra ), including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gases shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash

3948-641: Is also produced when magma comes into contact with water during phreatomagmatic eruptions , causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometres away. Due to its wide dispersal, ash can have a number of impacts on society, including animal and human health problems, disruption to aviation, disruption to critical infrastructure (e.g., electric power supply systems, telecommunications, water and waste-water networks, transportation), primary industries (e.g., agriculture), and damage to buildings and other structures. Volcanic ash

4089-664: Is also used in New Age healing . Petrified wood is found worldwide in sedimentary beds ranging in age from the Devonian (about 390 million years ago), when woody plants first appeared on dry land, to nearly the present. Petrified "forests" tend to be either entire ecosystems buried by volcanic eruptions, in which trunks often remain in their growth positions, or accumulations of drift wood in fluvial environments. Amethyst Ridge at Yellowstone National Park shows 27 successive forest ecosystems buried by eruptions, while Petrified Forest National Park

4230-502: Is classified as a human carcinogen by the International Agency for Research on Cancer . Guideline values have been created for exposure, but with unclear rationale; UK guidelines for particulates in air (PM10) are 50 μg/m and USA guidelines for exposure to crystalline silica are 50 μg/m . It is thought that the guidelines on exposure levels could be exceeded for short periods of time without significant health effects on

4371-584: Is composed only of eukaryotic cells, and the earliest evidence for it is the Francevillian Group Fossils from 2,100  million years ago , although specialisation of cells for different functions first appears between 1,430  million years ago (a possible fungus) and 1,200  million years ago (a probable red alga ). Sexual reproduction may be a prerequisite for specialisation of cells, as an asexual multicellular organism might be at risk of being taken over by rogue cells that retain

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4512-401: Is formed during explosive volcanic eruptions and phreatomagmatic eruptions, and may also be formed during transport in pyroclastic density currents . Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles (dominantly water and carbon dioxide ) to exsolve into gas bubbles. As more bubbles nucleate a foam is produced, which decreases the density of

4653-416: Is generally controlled by the mechanical properties of the wall rock broken up by spalling or explosive expansion of gases in the magma as it reaches the surface. The morphology of ash particles from phreatomagmatic eruptions is controlled by stresses within the chilled magma which result in fragmentation of the glass to form small blocky or pyramidal glass ash particles. Vesicle shape and density play only

4794-416: Is good evidence that pyroclastic flows produce high proportions of fine ash by communition and it is likely that this process also occurs inside volcanic conduits and would be most efficient when the magma fragmentation surface is well below the summit crater. Ash particles are incorporated into eruption columns as they are ejected from the vent at high velocity. The initial momentum from the eruption propels

4935-477: Is hard to decide at what level to place a new higher-level grouping, e.g. genus or family or order ; this is important since the Linnaean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it must be renamed. Paleontologists generally use approaches based on cladistics , a technique for working out the evolutionary "family tree" of a set of organisms. It works by

5076-486: Is highly variable, depending on the presence of an acidic gas condensate (primarily as a consequence of the gases SO 2 , HCl and HF in the eruption plume) on the ash surface. The crystalline-solid structure of the salts act more as an insulator than a conductor . However, once the salts are dissolved into a solution by a source of moisture (e.g., fog, mist, light rain, etc.), the ash may become corrosive and electrically conductive. A recent study has shown that

5217-399: Is inflow/infiltration by stormwater through illegal connections (e.g., from roof downpipes), cross connections, around manhole covers or through holes and cracks in sewer pipes. Ash-laden sewage entering a treatment plant is likely to cause failure of mechanical prescreening equipment such as step screens or rotating screens. Ash that penetrates further into the system will settle and reduce

5358-701: Is less vulnerable, with disruptions mainly caused by reduction in visibility. Marine transport can also be impacted by volcanic ash. Ash fall will block air and oil filters and abrade any moving parts if ingested into engines. Navigation will be impacted by a reduction in visibility during ash fall. Vesiculated ash ( pumice and scoria ) will float on the water surface in ‘pumice rafts’ which can clog water intakes quickly, leading to over heating of machinery. Paleontologist Paleontology ( / ˌ p eɪ l i ɒ n ˈ t ɒ l ə dʒ i , ˌ p æ l i -, - ən -/ PAY -lee-on- TOL -ə-jee, PAL -ee-, -⁠ən- ), also spelled palaeontology or palæontology ,

5499-450: Is most commonly associated with trees that were buried in fine grained sediments of deltas and floodplains or volcanic lahars and ash beds. A forest where such material has petrified becomes known as a petrified forest . Petrified wood forms when woody stems of plants are buried in wet sediments saturated with dissolved minerals. The lack of oxygen slows decay of the wood, allowing minerals to replace cell walls and to fill void spaces in

5640-443: Is often said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena. This approach cannot prove a hypothesis, since some later experiment may disprove it, but the accumulation of failures to disprove is often compelling evidence in favor. However, when confronted with totally unexpected phenomena, such as the first evidence for invisible radiation , experimental scientists often use

5781-594: Is one that contained an extinct "crocodile-like" marine reptile, which eventually came to be known as the mosasaurid Mosasaurus of the Cretaceous period. The first half of the 19th century saw geological and paleontological activity become increasingly well organised with the growth of geologic societies and museums and an increasing number of professional geologists and fossil specialists. Interest increased for reasons that were not purely scientific, as geology and paleontology helped industrialists to find and exploit natural resources such as coal. This contributed to

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5922-401: Is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better. Although radiometric dating requires very careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to the element into which it decays shows how long ago

6063-626: Is passed between meteorological agencies, volcanic observatories and airline companies through Volcanic Ash Advisory Centers (VAAC) . There is one VAAC for each of the nine regions of the world. VAACs can issue advisories describing the current and future extent of the ash cloud. Volcanic ash not only affects in-flight operations but can affect ground-based airport operations as well. Small accumulations of ash can reduce visibility, produce slippery runways and taxiways, infiltrate communication and electrical systems, interrupt ground services, damage buildings and parked aircraft. Ash accumulation of more than

6204-499: Is physically, socially, and economically disruptive. Volcanic ash can affect both proximal areas and areas many hundreds of kilometres from the source, and causes disruptions and losses in a wide variety of different infrastructure sectors. Impacts are dependent on: ash fall thickness; the grain size and chemistry of the ash; whether the ash is wet or dry; the duration of the ash fall; and any preparedness , management and prevention (mitigation) measures employed to reduce effects from

6345-475: Is rapidly broken down by inorganic reactions with the alkali. Wood is preserved from decomposition by rapid entombment in mud, particularly mud formed from volcanic ash. The wood is then mineralized to transform it to stone. Non-mineralized wood has been recovered from Paleozoic formations, particularly Callixylon from Berea Sandstone , but this is very unusual. The petrified wood is later exposed by erosion of surrounding sediments. Non-mineralized fossil wood

6486-515: Is rapidly destroyed when exposed by erosion, but petrified wood is quite durable. Some 40 minerals have been identified in petrified wood, but silica minerals are by far the most important. Calcite and pyrite are much less common, and others are quite rare. Silica binds to the cellulose in cell walls via hydrogen bonding and forms a kind of template. Additional silica then replaces the cellulose as it decomposes, so that cell walls are often preserved in great detail. Thus silicification begins within

6627-642: Is removed from pyroclastic density currents in co-ignimbrite ash plumes. Physical and chemical characteristics of volcanic ash are primarily controlled by the style of volcanic eruption. Volcanoes display a range of eruption styles which are controlled by magma chemistry, crystal content, temperature and dissolved gases of the erupting magma and can be classified using the volcanic explosivity index (VEI) . Effusive eruptions (VEI 1) of basaltic composition produce <10 m of ejecta, whereas extremely explosive eruptions (VEI 5+) of rhyolitic and dacitic composition can inject large quantities (>10 m ) of ejecta into

6768-521: Is the most important, and it can produce a range of hues depending on its oxidation state . Chromium produces bright green petrified wood. Variations in color likely reflect different episodes of mineralization. In some cases, variations may come from chromatographic separation of trace metals. Wood can also be petrified by calcite , as occurs in concretions in coal beds. Wood petrified by calcite tends to retain more of its original organic material. Petrification begins with deposition of goethite in

6909-451: Is the scientific study of life that existed prior to the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossils to classify organisms and study their interactions with each other and their environments (their paleoecology ). Paleontological observations have been documented as far back as the 5th century BC. The science became established in

7050-503: Is thought to have been propelled by coevolution with pollinating insects. Social insects appeared around the same time and, although they account for only small parts of the insect "family tree", now form over 50% of the total mass of all insects. Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over 6  million years ago . Although early members of this lineage had chimp -sized brains, about 25% as big as modern humans', there are signs of

7191-457: Is thought to supply the cations involved in the deposition of sulfate and halide salts . While some 55 ionic species have been reported in fresh ash leachates , the most abundant species usually found are the cations Na , K , Ca and Mg and the anions Cl , F and SO 4 . Molar ratios between ions present in leachates suggest that in many cases these elements are present as simple salts such as NaCl and CaSO 4 . In

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7332-567: The Carboniferous through Permian periods seem to have favored this process. Preservation of petrified forests in volcanic ash beds is less affected by climate and preserves a greater diversity of species. Areas with a large number of petrified trees include: Volcanic ash Volcanic ash consists of fragments of rock, mineral crystals , and volcanic glass , produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash

7473-567: The Middle Ages the Persian naturalist Ibn Sina , known as Avicenna in Europe, discussed fossils and proposed a theory of petrifying fluids on which Albert of Saxony elaborated in the 14th century. The Chinese naturalist Shen Kuo (1031–1095) proposed a theory of climate change based on the presence of petrified bamboo in regions that in his time were too dry for bamboo. In early modern Europe ,

7614-539: The Mohs Hardness Scale ) together with a high degree of angularity, make some types of volcanic ash (particularly those with a high silica content) very abrasive. Volcanic ash consists of particles (pyroclasts) with diameters less than 2 mm (particles larger than 2 mm are classified as lapilli ), and can be as fine as 1 μm. The overall grain size distribution of ash can vary greatly with different magma compositions. Few attempts have been made to correlate

7755-528: The Neogene - Quaternary . In deeper-level deposits in western Europe are early-aged mammals such as the palaeothere perissodactyl Palaeotherium and the anoplotheriid artiodactyl Anoplotherium , both of which were described earliest after the former two genera, which today are known to date to the Paleogene period. Cuvier figured out that even older than the two levels of deposits with extinct large mammals

7896-637: The Permian–Triassic extinction event . Amphibians Extinct Synapsids Mammals Extinct reptiles Lizards and snakes Extinct Archosaurs Crocodilians Extinct Dinosaurs Birds Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in paleontology have been based just on misunderstandings over names. Linnaean taxonomy is commonly used for classifying living organisms, but runs into difficulties when dealing with newly discovered organisms that are significantly different from known ones. For example: it

8037-513: The Permian–Triassic extinction event . A relatively recent discipline, molecular phylogenetics , compares the DNA and RNA of modern organisms to re-construct the "family trees" of their evolutionary ancestors. It has also been used to estimate the dates of important evolutionary developments, although this approach is controversial because of doubts about the reliability of the " molecular clock ". Techniques from engineering have been used to analyse how

8178-454: The embryological development of some modern brachiopods suggests that brachiopods may be descendants of the halkieriids , which became extinct in the Cambrian period. Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating . This technique

8319-410: The eruption column . Within pyroclastic density currents particle abrasion occurs as particles violently collide, resulting in a reduction in grain size and production of fine grained ash particles. In addition, ash can be produced during secondary fragmentation of pumice fragments, due to the conservation of heat within the flow. These processes produce large quantities of very fine grained ash which

8460-699: The infrastructure critical to supporting modern societies, particularly in urban areas, where high population densities create high demand for services. Several recent eruptions have illustrated the vulnerability of urban areas that received only a few millimetres or centimetres of volcanic ash. This has been sufficient to cause disruption of transportation, electricity , water , sewage and storm water systems. Costs have been incurred from business disruption, replacement of damaged parts and insured losses. Ash fall impacts on critical infrastructure can also cause multiple knock-on effects, which may disrupt many different sectors and services. Volcanic ash fall

8601-521: The " jigsaw puzzles " of biostratigraphy (arrangement of rock layers from youngest to oldest). Classifying ancient organisms is also difficult, as many do not fit well into the Linnaean taxonomy classifying living organisms, and paleontologists more often use cladistics to draw up evolutionary "family trees". The final quarter of the 20th century saw the development of molecular phylogenetics , which investigates how closely organisms are related by measuring

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8742-463: The 18th century as a result of Georges Cuvier 's work on comparative anatomy , and developed rapidly in the 19th century. The term has been used since 1822 formed from Greek παλαιός ( 'palaios' , "old, ancient"), ὄν ( 'on' , ( gen. 'ontos' ), "being, creature"), and λόγος ( 'logos' , "speech, thought, study"). Paleontology lies on the border between biology and geology , but it differs from archaeology in that it excludes

8883-548: The Early Cambrian , along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether this Cambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern-looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the "weird wonders" are evolutionary "aunts" and "cousins" of modern groups. Vertebrates remained

9024-459: The Earth's organic and inorganic past". William Whewell (1794–1866) classified paleontology as one of the historical sciences, along with archaeology , geology, astronomy , cosmology , philology and history itself: paleontology aims to describe phenomena of the past and to reconstruct their causes. Hence it has three main elements: description of past phenomena; developing a general theory about

9165-446: The SiO 2 is not attached to another element to create a new mineral. However, magmas containing less than 58% SiO 2 are thought to be unlikely to contain crystalline silica. The exposure levels to free crystalline silica in the ash are commonly used to characterise the risk of silicosis in occupational studies (for people who work in mining, construction and other industries,) because it

9306-412: The ability to reproduce. The earliest known animals are cnidarians from about 580  million years ago , but these are so modern-looking that they must be descendants of earlier animals. Early fossils of animals are rare because they had not developed mineralised , easily fossilized hard parts until about 548  million years ago . The earliest modern-looking bilaterian animals appear in

9447-481: The addition of water. Volcanic ash is also produced during phreatomagmatic eruptions. During these eruptions fragmentation occurs when magma comes into contact with bodies of water (such as the sea, lakes and marshes) groundwater, snow or ice. As the magma, which is significantly hotter than the boiling point of water, comes into contact with water an insulating vapor film forms ( Leidenfrost effect ). Eventually this vapor film will collapse leading to direct coupling of

9588-515: The appearance of moderately complex animals (comparable to earthworms ). Geochemical observations may help to deduce the global level of biological activity at a certain period, or the affinity of certain fossils. For example, geochemical features of rocks may reveal when life first arose on Earth, and may provide evidence of the presence of eukaryotic cells, the type from which all multicellular organisms are built. Analyses of carbon isotope ratios may help to explain major transitions such as

9729-516: The ash fall. Different sectors of infrastructure and society are affected in different ways and are vulnerable to a range of impacts or consequences. These are discussed in the following sections. Ash particles of less than 10 μm diameter suspended in the air are known to be inhalable, and people exposed to ash falls have experienced respiratory discomfort, breathing difficulty, eye and skin irritation, and nose and throat symptoms. Most of these effects are short-term and are not considered to pose

9870-457: The ashfall can become a health risk and drinking of water is not recommended. Prior to an ashfall, downpipes should be disconnected so that water in the tank is protected. A further problem is that the surface coating of fresh volcanic ash can be acidic. Unlike most surface waters, rainwater generally has a very low alkalinity (acid-neutralising capacity) and thus ashfall may acidify tank waters. This may lead to problems with plumbosolvency , whereby

10011-442: The atmosphere by processes of chemical reaction, dry and wet deposition, and by adsorption onto the surface of volcanic ash. It has long been recognised that a range of sulfate and halide (primarily chloride and fluoride ) compounds are readily mobilised from fresh volcanic ash. It is considered most likely that these salts are formed as a consequence of rapid acid dissolution of ash particles within eruption plumes , which

10152-625: The atmosphere increased their effectiveness as nurseries of evolution. While eukaryotes , cells with complex internal structures, may have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen from a poison to a powerful source of metabolic energy. This innovation may have come from primitive eukaryotes capturing oxygen-powered bacteria as endosymbionts and transforming them into organelles called mitochondria . The earliest evidence of complex eukaryotes with organelles (such as mitochondria) dates from 1,850  million years ago . Multicellular life

10293-414: The atmosphere. The types of minerals present in volcanic ash are dependent on the chemistry of the magma from which it erupted. Considering that the most abundant elements found in silicate magma are silicon and oxygen , the various types of magma (and therefore ash) produced during volcanic eruptions are most commonly explained in terms of their silica content. Low energy eruptions of basalt produce

10434-415: The bodies of ancient organisms might have worked, for example the running speed and bite strength of Tyrannosaurus , or the flight mechanics of Microraptor . It is relatively commonplace to study the internal details of fossils using X-ray microtomography . Paleontology, biology, archaeology, and paleoneurobiology combine to study endocranial casts (endocasts) of species related to humans to clarify

10575-689: The capacity of biological reactors as well as increasing the volume of sludge and changing its composition. The principal damage sustained by aircraft flying into a volcanic ash cloud is abrasion to forward-facing surfaces, such as the windshield and leading edges of the wings, and accumulation of ash into surface openings, including engines. Abrasion of windshields and landing lights will reduce visibility forcing pilots to rely on their instruments. However, some instruments may provide incorrect readings as sensors (e.g., pitot tubes ) can become blocked with ash. Ingestion of ash into engines causes abrasion damage to compressor fan blades. The ash erodes sharp blades in

10716-401: The causes of various types of change; and applying those theories to specific facts. When trying to explain the past, paleontologists and other historical scientists often construct a set of one or more hypotheses about the causes and then look for a " smoking gun ", a piece of evidence that strongly accords with one hypothesis over any others. Sometimes researchers discover a "smoking gun" by

10857-456: The cell walls, and the spaces within and between cells are filled with silica more gradually. Over time, almost all the original organic material is lost; only around 10% remains in the petrified wood. The remaining material is nearly pure silica, with only iron, aluminum, and alkali and alkaline earth elements present in more than trace amounts. Iron, calcium, aluminum are the most common, and one or more of these elements may make up more than 1% of

10998-554: The cell walls, followed by deposition of calcite in the void spaces. Carbonized wood is resistant to silicification and is usually petrified by other minerals. Wood petrified by minerals other than silica minerals tends to accumulate heavy metals, such as uranium , selenium , and germanium , with uranium most common in wood high in lignin and germanium most common in wood preserved in coal beds. Boron , zinc , and phosphorus are anomalously low in fossil wood, suggesting they are leached away or scavenged by microorganisms. Less commonly,

11139-763: The characteristics and evolution of humans as a species. When dealing with evidence about humans, archaeologists and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site , to discover the people who lived there, and what they ate; or they might analyze the climate at the time of habitation. In addition, paleontology often borrows techniques from other sciences, including biology, osteology , ecology, chemistry , physics and mathematics. For example, geochemical signatures from rocks may help to discover when life first arose on Earth, and analyses of carbon isotope ratios may help to identify climate changes and even to explain major transitions such as

11280-520: The chronological order in which rocks were formed, is useful to both paleontologists and geologists. Biogeography studies the spatial distribution of organisms, and is also linked to geology, which explains how Earth's geography has changed over time. Although paleontology became established around 1800, earlier thinkers had noticed aspects of the fossil record. The ancient Greek philosopher Xenophanes (570–480 BCE) concluded from fossil sea shells that some areas of land were once under water. During

11421-520: The cold water and hot magma. This increases the heat transfer which leads to the rapid expansion of water and fragmentation of the magma into small particles which are subsequently ejected from the volcanic vent. Fragmentation causes an increase in contact area between magma and water creating a feedback mechanism, leading to further fragmentation and production of fine ash particles. Pyroclastic density currents can also produce ash particles. These are typically produced by lava dome collapse or collapse of

11562-404: The column upwards. As air is drawn into the column, the bulk density decreases and it starts to rise buoyantly into the atmosphere. At a point where the bulk density of the column is the same as the surrounding atmosphere, the column will cease rising and start moving laterally. Lateral dispersion is controlled by prevailing winds and the ash may be deposited hundreds to thousands of kilometres from

11703-649: The column. Ash fallout is less concentrated during the final stages as the column moves downwind. This results in an ash fall deposit which generally decreases in thickness and grain size exponentially with increasing distance from the volcano. Fine ash particles may remain in the atmosphere for days to weeks and be dispersed by high-altitude winds. These particles can impact on the aviation industry (refer to impacts section) and, combined with gas particles, can affect global climate. Volcanic ash plumes can form above pyroclastic density currents. These are called co-ignimbrite plumes. As pyroclastic density currents travel away from

11844-403: The composition. Just what form the silica initially takes is still a topic of research. There is evidence of initial deposition as opal , which then crystallizes to quartz over long time periods. On the other hand, there is some evidence that silica is deposited directly as quartz. Wood can become silicified very rapidly in silica-rich hot springs. While wood petrified in this setting is only

11985-415: The compressor, reducing its efficiency. The ash melts in the combustion chamber to form molten glass. The ash then solidifies on turbine blades, blocking air flow and causing the engine to stall. The composition of most ash is such that its melting temperature is within the operating temperature (>1000 °C) of modern large jet engines . The degree of impact depends upon the concentration of ash in

12126-445: The date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago. It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common ancestor must have lived – by assuming that DNA mutations accumulate at

12267-586: The development of mammalian traits such as endothermy and hair. After the Cretaceous–Paleogene extinction event 66  million years ago killed off all the dinosaurs except the birds, mammals increased rapidly in size and diversity, and some took to the air and the sea. Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous between 130  million years ago and 90  million years ago . Their rapid rise to dominance of terrestrial ecosystems

12408-551: The development of the body plans of most animal phyla . The discovery of fossils of the Ediacaran biota and developments in paleobiology extended knowledge about the history of life back far before the Cambrian. Increasing awareness of Gregor Mendel 's pioneering work in genetics led first to the development of population genetics and then in the mid-20th century to the modern evolutionary synthesis , which explains evolution as

12549-477: The different levels of deposits represented different time periods in the early 19th century. The surface-level deposits in the Americas contained later mammals like the megatheriid ground sloth Megatherium and the mammutid proboscidean Mammut (later known informally as a "mastodon"), which were some of the earliest-named fossil mammal genera with official taxonomic authorities. They today are known to date to

12690-425: The droplets after they leave the vent, and air friction. Shapes range from perfect spheres to a variety of twisted, elongate droplets with smooth, fluidal surfaces. The morphology of ash from eruptions of high-viscosity magmas (e.g., rhyolite, dacite, and some andesites) is mostly dependent on the shape of vesicles in the rising magma before disintegration. Vesicles are formed by the expansion of magmatic gas before

12831-412: The effects of an ashfall, but there will not be service interruptions. The final step of drinking water treatment is disinfection to ensure that final drinking water is free from infectious microorganisms. As suspended particles (turbidity) can provide a growth substrate for microorganisms and can protect them from disinfection treatment, it is extremely important that the water treatment process achieves

12972-595: The electrical conductivity of volcanic ash increases with (1) increasing moisture content, (2) increasing soluble salt content, and (3) increasing compaction (bulk density). The ability of volcanic ash to conduct electric current has significant implications for electric power supply systems. Volcanic ash particles erupted during magmatic eruptions are made up of various fractions of vitric (glassy, non-crystalline), crystalline or lithic (non-magmatic) particles. Ash produced during low viscosity magmatic eruptions (e.g., Hawaiian and Strombolian basaltic eruptions) produce

13113-409: The end of the 20th century have been particularly important as they have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds. The last few decades of the 20th century saw a renewed interest in mass extinctions and their role in the evolution of life on Earth. There was also a renewed interest in the Cambrian explosion that apparently saw

13254-500: The engines were started just 1–2 minutes before impact. Total damage was US$ 80 million and it took 3 months' work to repair the plane. In the 1990s, a further US$ 100 million of damage was sustained by commercial aircraft (some in the air, others on the ground) as a consequence of the 1991 eruption of Mount Pinatubo in the Philippines . In April 2010, airspace all over Europe was affected, with many flights cancelled -which

13395-611: The eruption of Puyehue-Cordón Caulle , Chile. Volcanic ash clouds are very difficult to detect from aircraft as no onboard cockpit instruments exist to detect them. However, a new system called Airborne Volcanic Object Infrared Detector (AVOID) has recently been developed by Dr Fred Prata while working at CSIRO Australia and the Norwegian Institute for Air Research , which will allow pilots to detect ash plumes up to 60 km (37 mi) ahead and fly safely around them. The system uses two fast-sampling infrared cameras, mounted on

13536-410: The evolution of the human brain. Paleontology even contributes to astrobiology , the investigation of possible life on other planets , by developing models of how life may have arisen and by providing techniques for detecting evidence of life. As knowledge has increased, paleontology has developed specialised subdivisions. Vertebrate paleontology concentrates on fossils from the earliest fish to

13677-466: The evolutionary history of life back to over 3,000  million years ago , possibly as far as 3,800  million years ago . The oldest clear evidence of life on Earth dates to 3,000  million years ago , although there have been reports, often disputed, of fossil bacteria from 3,400  million years ago and of geochemical evidence for the presence of life 3,800  million years ago . Some scientists have proposed that life on Earth

13818-548: The exception of fluorine . The elements iron , manganese and aluminium are commonly enriched over background levels by volcanic ashfall. These elements may impart a metallic taste to water, and may produce red, brown or black staining of whiteware, but are not considered a health risk. Volcanic ashfalls are not known to have caused problems in water supplies for toxic trace elements such as mercury (Hg) and lead (Pb) which occur at very low levels in ash leachates. Ingesting ash may be harmful to livestock , causing abrasion of

13959-555: The exceptional events that cause quick burial make it difficult to study the normal environments of the animals. The sparseness of the fossil record means that organisms are expected to exist long before and after they are found in the fossil record – this is known as the Signor–Lipps effect . Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces ) and marks left by feeding. Trace fossils are particularly significant because they represent

14100-504: The focus of paleontology shifted to understanding evolutionary paths, including human evolution , and evolutionary theory. The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North America. The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection. Fossils found in China near

14241-449: The following: At the end of the 18th century Georges Cuvier 's work established comparative anatomy as a scientific discipline and, by proving that some fossil animals resembled no living ones, demonstrated that animals could become extinct , leading to the emergence of paleontology. The expanding knowledge of the fossil record also played an increasing role in the development of geology, particularly stratigraphy . Cuvier proved that

14382-580: The fossil record: different environments are more favorable to the preservation of different types of organism or parts of organisms. Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although there are 30-plus phyla of living animals, two-thirds have never been found as fossils. Occasionally, unusual environments may preserve soft tissues. These lagerstätten allow paleontologists to examine

14523-613: The general population. There have been no documented cases of silicosis developed from exposure to volcanic ash. However, long-term studies necessary to evaluate these effects are lacking. For surface water sources such as lakes and reservoirs, the volume available for dilution of ionic species leached from ash is generally large. The most abundant components of ash leachates (Ca, Na, Mg, K, Cl, F and SO 4 ) occur naturally at significant concentrations in most surface waters and therefore are not affected greatly by inputs from volcanic ashfall, and are also of low concern in drinking water, with

14664-439: The grain size characteristics of a deposit with those of the event which produced it, though some predictions can be made. Rhyolitic magmas generally produce finer grained material compared to basaltic magmas, due to the higher viscosity and therefore explosivity. The proportions of fine ash are higher for silicic explosive eruptions, probably because vesicle size in the pre-eruptive magma is smaller than those in mafic magmas. There

14805-675: The history and driving forces behind their evolution. Land plants were so successful that their detritus caused an ecological crisis in the Late Devonian , until the evolution of fungi that could digest dead wood. During the Permian period, synapsids , including the ancestors of mammals , may have dominated land environments, but this ended with the Permian–Triassic extinction event 251  million years ago , which came very close to wiping out all complex life. The extinctions were apparently fairly sudden, at least among vertebrates. During

14946-416: The history of Earth's climate and the mechanisms that have changed it  – which have sometimes included evolutionary developments, for example the rapid expansion of land plants in the Devonian period removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus helping to cause an ice age in the Carboniferous period. Biostratigraphy , the use of fossils to work out

15087-542: The immediate ancestors of modern mammals . Invertebrate paleontology deals with fossils such as molluscs , arthropods , annelid worms and echinoderms . Paleobotany studies fossil plants , algae , and fungi. Palynology , the study of pollen and spores produced by land plants and protists , straddles paleontology and botany , as it deals with both living and fossil organisms. Micropaleontology deals with microscopic fossil organisms of all kinds. Instead of focusing on individual organisms, paleoecology examines

15228-434: The index fossils turn out to have longer fossil ranges than first thought. Stratigraphy and biostratigraphy can in general provide only relative dating ( A was before B ), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the problems involved in matching up rocks of the same age across different continents . Family-tree relationships may also help to narrow down

15369-538: The interactions between different ancient organisms, such as their food chains , and the two-way interactions with their environments.   For example, the development of oxygenic photosynthesis by bacteria caused the oxygenation of the atmosphere and hugely increased the productivity and diversity of ecosystems . Together, these led to the evolution of complex eukaryotic cells, from which all multicellular organisms are built. Paleoclimatology , although sometimes treated as part of paleoecology, focuses more on

15510-463: The internal anatomy of animals that in other sediments are represented only by shells, spines, claws, etc. – if they are preserved at all. However, even lagerstätten present an incomplete picture of life at the time. The majority of organisms living at the time are probably not represented because lagerstätten are restricted to a narrow range of environments, e.g. where soft-bodied organisms can be preserved very quickly by events such as mudslides; and

15651-456: The investigation of evolutionary "family trees" by techniques derived from biochemistry , began to make an impact, particularly when it was proposed that the human lineage had diverged from apes much more recently than was generally thought at the time. Although this early study compared proteins from apes and humans, most molecular phylogenetics research is now based on comparisons of RNA and DNA . Fossils of organisms' bodies are usually

15792-409: The logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characters that are compared may be anatomical , such as the presence of a notochord , or molecular , by comparing sequences of DNA or proteins . The result of a successful analysis is a hierarchy of clades – groups that share

15933-513: The magma has solidified. Ash particles can have varying degrees of vesicularity and vesicular particles can have extremely high surface area to volume ratios. Concavities, troughs, and tubes observed on grain surfaces are the result of broken vesicle walls. Vitric ash particles from high-viscosity magma eruptions are typically angular, vesicular pumiceous fragments or thin vesicle-wall fragments while lithic fragments in volcanic ash are typically equant, or angular to subrounded. Lithic morphology in ash

16074-407: The magma, accelerating it up the conduit. Fragmentation occurs when bubbles occupy ~70–80 vol% of the erupting mixture. When fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a very efficient process of ash formation and is capable of generating very fine ash even without

16215-404: The majority of the remaining composition of hardwood while lignin, which is a polymer of phenylpropanes , is more abundant in softwood . The hemicellulose and lignin encrust and reinforce the cellulose microfibrils. Dead wood is normally rapidly decomposed by microorganisms, beginning with the holocellulose. The lignin is hydrophobic (water-repelling) and much slower to decay. The rate of decay

16356-409: The most informative type of evidence. The most common types are wood, bones, and shells. Fossilisation is a rare event, and most fossils are destroyed by erosion or metamorphism before they can be observed. Hence the fossil record is very incomplete, increasingly so further back in time. Despite this, it is often adequate to illustrate the broader patterns of life's history. There are also biases in

16497-414: The outcome of events such as mutations and horizontal gene transfer , which provide genetic variation , with genetic drift and natural selection driving changes in this variation over time. Within the next few years the role and operation of DNA in genetic inheritance were discovered, leading to what is now known as the "Central Dogma" of molecular biology . In the 1960s molecular phylogenetics ,

16638-427: The plant's cell walls act as a template for mineralization. There needs to be a balance between the decay of cellulose and lignin and mineral templating for cellular detail to be preserved with fidelity. Most of the organic matter often decomposes , however some of the lignin may remain. Silica in the form of opal-A, can encrust and permeate wood relatively quickly in hot spring environments. However, petrified wood

16779-436: The plume, the length of time the aircraft spends within the plume and the actions taken by the pilots. Critically, melting of ash, particularly volcanic glass, can result in accumulation of resolidified ash on turbine nozzle guide vanes, resulting in compressor stall and complete loss of engine thrust. The standard procedure of the engine control system when it detects a possible stall is to increase power which would exacerbate

16920-647: The power delivery process: Groundwater-fed systems are resilient to impacts from ashfall, although airborne ash can interfere with the operation of well-head pumps. Electricity outages caused by ashfall can also disrupt electrically powered pumps if there is no backup generation. The physical impacts of ashfall can affect the operation of water treatment plants. Ash can block intake structures, cause severe abrasion damage to pump impellers and overload pump motors. Ash can enter filtration systems such as open sand filters both by direct fallout and via intake waters. In most cases, increased maintenance will be required to manage

17061-452: The principal types of evidence about ancient life, and geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave body fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating , which provides absolute dates that are accurate to within 0.5%, but more often paleontologists have to rely on relative dating by solving

17202-443: The problem. It is recommended that pilots reduce engine power and quickly exit the cloud by performing a descending 180° turn. Volcanic gases, which are present within ash clouds, can also cause damage to engines and acrylic windshields, and can persist in the stratosphere as an almost invisible aerosol for prolonged periods of time. There are many instances of damage to jet aircraft as a result of an ash encounter. On 24 June 1982,

17343-419: The progressive encroachment of urban development into higher risk areas, closer to volcanic centres, increasing the human exposure to volcanic ash fall events. Direct health effects of volcanic ash on humans are usually short-term and mild for persons in normal health, though prolonged exposure potentially poses some risk of silicosis in unprotected workers. Of greater concern is the impact of volcanic ash on

17484-423: The proportion of ash with less than 10 μm diameter, known as PM 10 . The social context may also be important. Chronic health effects from volcanic ash fall are possible, as exposure to free crystalline silica is known to cause silicosis . Minerals associated with this include quartz , cristobalite and tridymite , which may all be present in volcanic ash. These minerals are described as ‘free’ silica as

17625-577: The quality of the fibre. As the usual pastures and plants become covered in volcanic ash during eruption some livestock may resort to eat whatever is available including toxic plants. There are reports of goats and sheep in Chile and Argentina having natural abortions in connection to volcanic eruptions. Volcanic ash can disrupt electric power supply systems at all levels of power generation, transformation, transmission, and distribution. There are four main impacts arising from ash-contamination of apparatus used in

17766-432: The radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers. Consequently, paleontologists must usually rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record, and has been compared to

17907-480: The replacement minerals in petrified wood are chalcocite or other sulfide minerals . These have been mined as copper ore at locations such as the Nacimiento Mine near Cuba, New Mexico . Scientists have attempted to duplicate the process of petrification of wood, both to better understand the natural petrification process and for its possible use as a ceramic material. Early attempts used sodium metasilicate as

18048-701: The same approach as historical scientists: construct a set of hypotheses about the causes and then look for a "smoking gun". Paleontology lies between biology and geology since it focuses on the record of past life, but its main source of evidence is fossils in rocks. For historical reasons, paleontology is part of the geology department at many universities: in the 19th and early 20th centuries, geology departments found fossil evidence important for dating rocks, while biology departments showed little interest. Paleontology also has some overlap with archaeology , which primarily works with objects made by humans and with human remains, while paleontologists are interested in

18189-478: The similarity of the DNA in their genomes . Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend. The simplest definition of "paleontology" is "the study of ancient life". The field seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, and what they can tell us about

18330-465: The slow recovery from this catastrophe a previously obscure group, archosaurs , became the most abundant and diverse terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates for the rest of the Mesozoic , and birds evolved from one group of dinosaurs. During this time mammals' ancestors survived only as small, mainly nocturnal insectivores , which may have accelerated

18471-631: The study of anatomically modern humans . It now uses techniques drawn from a wide range of sciences, including biochemistry , mathematics , and engineering. Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life , almost back to when Earth became capable of supporting life, nearly 4 billion years ago. As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates . Body fossils and trace fossils are

18612-617: The systematic study of fossils emerged as an integral part of the changes in natural philosophy that occurred during the Age of Reason . In the Italian Renaissance, Leonardo da Vinci made various significant contributions to the field as well as depicted numerous fossils. Leonardo's contributions are central to the history of paleontology because he established a line of continuity between the two main branches of paleontology – ichnology and body fossil paleontology. He identified

18753-580: The teeth, and hypersensibility to pressure in the legs and back. Ash ingestion may also cause gastrointestinal blockages. Sheep that ingested ash from the 1991 Mount Hudson volcanic eruption in Chile, suffered from diarrhoea and weakness. Ash accumulating in the back wool of sheep may add significant weight, leading to fatigue and sheep that can not stand up. Rainfall may result in a significant burden as it adds weight to ash. Pieces of wool may fall away and any remaining wool on sheep may be worthless as poor nutrition associated with volcanic eruptions impacts

18894-737: The teeth, and in cases of high fluorine content, fluorine poisoning (toxic at levels of >100 μg/g) for grazing animals. It is known from the 1783 eruption of Laki in Iceland that fluorine poisoning occurred in humans and livestock as a result of the chemistry of the ash and gas, which contained high levels of hydrogen fluoride . Following the 1995/96 Mount Ruapehu eruptions in New Zealand, two thousand ewes and lambs died after being affected by fluorosis while grazing on land with only 1–3 mm of ash fall. Symptoms of fluorosis among cattle exposed to ash include brown-yellow to green-black mottles in

19035-409: The volcano, depending on eruption column height, particle size of the ash and climatic conditions (especially wind direction and strength and humidity). Ash fallout occurs immediately after the eruption and is controlled by particle density. Initially, coarse particles fall out close to source. This is followed by fallout of accretionary lapilli , which is the result of particle agglomeration within

19176-413: The volcano, smaller particles are removed from the flow by elutriation and form a less dense zone overlying the main flow. This zone then entrains the surrounding air and a buoyant co-ignimbrite plume is formed. These plumes tend to have higher concentrations of fine ash particles compared to magmatic eruption plumes due to the abrasion within the pyroclastic density current. Population growth has caused

19317-423: The water is more aggressive towards materials that it comes into contact with. This can be a particular problem if there are lead-head nails or lead flashing used on the roof, and for copper pipes and other metallic plumbing fittings. During ashfall events, large demands are commonly placed on water resources for cleanup and shortages can result. Shortages compromise key services such as firefighting and can lead to

19458-447: The wood. Wood is composed mostly of holocellulose ( cellulose and hemicellulose ) and lignin . Together, these substances make up 95% of the dry composition of wood. Almost half of this is cellulose, which gives wood much of its strength. Cellulose is composed of long chains of polymerized glucose arranged into microfibrils that reinforce the cell walls in the wood. Hemicellulose, a branched polymer of various simple sugars , makes up

19599-406: The word "palaeontology" to refer to the study of ancient living organisms through fossils. As knowledge of life's history continued to improve, it became increasingly obvious that there had been some kind of successive order to the development of life. This encouraged early evolutionary theories on the transmutation of species . After Charles Darwin published Origin of Species in 1859, much of

19740-454: Was "seeded" from elsewhere , but most research concentrates on various explanations of how life could have arisen independently on Earth. For about 2,000 million years microbial mats , multi-layered colonies of different bacteria, were the dominant life on Earth. The evolution of oxygenic photosynthesis enabled them to play the major role in the oxygenation of the atmosphere from about 2,400  million years ago . This change in

19881-550: Was unprecedented-due to the presence of volcanic ash in the upper atmosphere from the eruption of the Icelandic volcano Eyjafjallajökull . On 15 April 2010, the Finnish Air Force halted training flights when damage was found from volcanic dust ingestion by the engines of one of its Boeing F-18 Hornet fighters. In June 2011, there were similar closures of airspace in Chile, Argentina, Brazil, Australia and New Zealand, following

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