Geologia, do grego γη- (ge-, "a terra") e λογος (logos, "palavra", "razão"),

Geologia, do grego γη- (ge-, "a terra") e λογος (logos, "palavra", "razão"), é a ciência que estuda a Terra, sua composição, estrutura, propriedades físicas, história e os processos que lhe dão forma. É uma das Ciências da Terra. A Geologia foi essencial para determinar a idade da Terra, que se calculou ter cerca de 4.6 mil milhões (br. bilhões) de anos e a desenvolver a teoria que afirma que a litosfera terrestre se encontra fragmentada em várias placas tectónicas e que se deslocam sobre o manto superior fluido e viscoso (astenosfera) de acordo com um conjunto de processos denominado tectónica de placas. O geólogo ajuda a localizar e a gerir os recursos naturais, como o petróleo e o carvão, assim como metais como o ferro, cobre e urânio, por exemplo. Muitos outros materiais possuem interesse económico: as gemas, bem como muitos minerais com aplicação industrial, como asbesto, pedra pomes, perlita, mica, zeólitos, argilas, quartzo ou elementos como o enxofre e cloro.

A Astrogeologia é o termo usado para designar estudos similares de outros corpos do sistema celestes.

A palavra "geologia" foi usada pela primeira vez por Jean-André Deluc em 1778, sendo introduzida de forma definitiva por Horace-Bénédict de Saussure em 1779.
A Geologia relaciona-se directamente com muitas outras ciências, em especial com a Geografia, e Astronomia. Por outro lado a Geologia serve-se de ferramentas fornecidas pela Química, Física e Matemática, entre outras, enquanto que a Biologia e a Antropologia servem-se da Geologia para dar suporte a muitos dos seus estudos.

1 História
2 Campos da geologia e disciplinas relacionadas
3 Importantes princípios da geologia
4 Ver também
5 Ligações externas

Na China, Shen Kua (1031 - 1095) formulou uma hipótese de explicação da formação de novas terras, baseando-se na observação de conchas fósseis de um estrato numa montanha localizada a centenas de quilómetros do oceano. O sábio chinês defendia que a terra formava-se a partir da erosão das montanhas e pela deposição de silte.

A obra, Peri lithon, de Teofrasto (372-287), estudante de Aristóteles permaneceu por milénios como obra de referência na ciência. A sua interpretação dos fósseis apenas foi revogada após a Revolução científica. A sua obra foi traduzida para latim, bem como para outras línguas europeias.

O médico Georg Agricola (1494-1555) escreveu o primeiro tratado sobre mineração e metalurgia, De re metallica libri XII 1556 no qual se podia encontrar um anexo sobre as criaturas que habitavam o interior da Terra (Buch von den Lebewesen unter Tage). A sua obra cobria temas como a energia eólica, hidrodinâmica, transporte e extracção de minerais, como o alumínio e enxofre.

Nicolaus Steno (1638-1686) foi o autor de vários princípios da geologia como o princípio da sobreposição das camadas, o princípio da horizontalidade original e o princípio da continuidade lateral, três princípios definidores da Estratigrafia. James Hutton é visto frequentemente como o primeiro geólogo moderno. Em 1785 apresentou uma teoria intitulada Teoria da Terra (Theory of the Earth) à Sociedade Real de Edimburgo. Na sua teoria, explicou que a Terra será muito mais antiga do que tinha sido suposto previamente, a fim de permitir "que houvesse tempo para para ocorrer erosão das montanhas de forma a que os sedimentos originassem novas rochas no fundo do mar, que ulteriormente foram levantadas e constituíram os continentes." Hutton publicou uma obra com dois volumes acerca desta teorias em 1795.
Em 1811 George Cuvier e Alenxandre Brongniart publicaram a sua teoria sobre a idade da Terra, baseada na descoberta, por Cuvier, de ossos de elefante em Paris. Para suportar a sua teoria os autores formularam o princípio da sucessão estratigráfica.
Em 1830 Sir Charles Lyell publicou pela primeira vez a sua famosa obra Princípios da Geologia, publicando contínuas revisões até à sua morte em 1875. Lyell promoveu com sucesso durante a sua vida a doutrina do uniformitarismo, que defende que os processos geológicos são lentos e ainda ocorrem nos dias hoje. No sentido oposto, a teoria do catastrofismo defendia que as estruturas da Terra formavam-se em eventos catastróficos únicos, permanecendo inalteráveis após esses acontecimentos.

Durante o século XIX a geologia debateu-se com a questão da idade da Terra. As estimativas variavam entre alguns milhões e os 100.000 mil milhões de anos. No século XX o maior avanço da geologia foi o desenvolvimento da teoria da tectónica de placas nos anos 60. A teoria da deriva dos continentes foi inicialmente proposta por Alfred Wegener e Arthur Holmes em 1912, mas não foi totalmente aceite até a teoria da tectónica de placas ser desenvolvida.

Uma descrição ilustrada de um sinclinal e anticlinal frequentemente estudados na Geologia estrutural e Geomorfologia. Há muitos campos diferentes dentro da disciplina geologia, e seria difícil listá-los a todos. De qualquer forma entre eles incluem-se:
Cartografia geológica
Geologia de engenharia
Estratigrafia
Geodesia
Geofísica
Geologia ambiental
Geologia económica
Geologia estrutural
Geologia do petróleo
Geologia médica
Gemologia
Geomorfologia
Geoquímica
Geotectónica
Hidrogeologia
Mineralogia
Paleontologia
Pedologia
Petrologia
Sedimentologia
Sismologia
Vulcanologia

A geologia rege-se por princípios que permitem, por exemplo, ao observar a disposição actual de formações estabelecer a sua idade relativa e a forma como foram criadas.

Princípio da Sobreposição das Camadas
Segundo este princípio, em qualquer sequência a camada mais jovem é aquela que se encontra no topo da sequência. As camadas inferiores são progressivamente mais antigas. Este princípio pode ser aplicado em depósitos sedimentares formados por acresção vertical, mas não naqueles em que a acresção é lateral (por exemplo em terraços fluviais). O princípio da sobreposição das camadas é válido para as rochas sedimentares e vulcânicas que se formam por acumulação vertical de material, mas não pode ser aplicado a rochas intrusivas e deve ser aplicado com cautela às rochas metamórficas.

Princípio da Horizontalidade Original
O princípio da horizontalidade original afirma que a deposição de sedimentos ocorre em leitos hotizontais. A observação de sedimentos marinhos e não marinhos numa grande variedade de ambientes suporta a generalização do princípio.

Princípio das Relações de Corte
Este princípio, introduzido por James Hutton, afirma que uma rocha ígnea intrusiva ou falha que corte uma sequência de rochas, é mais jovem que as rochas por ela cortadas. Esse princípio permite a datação relativa de eventos em rochas metamórficas, ígneas e sedimentares, sendo fundamental para o trabalho em terrenos orogênicos jovens e antigos. Este princípio é válido para qualquer tipo de rocha cortada por umas das estruturas acima relacionadas.

Princípio dos Fragmentos Inclusos
Este princípio de datação relativa diz que os fragmentos de rochas inclusas em corpos ígneos (intrusivos ou não) são mais antigos que as rochas ígneas nas quais estão inclusos. Este princípio, juntamente com o princípio das relações de corte, é fundamental em áreas formadas por grandes corpos intrusivos permitindo a datação relativa não só de rochas estratificadas, mas também de rochas ígneas e metamórficas.

Princípio da Sucessão Faunística
O Princípio da Sucessão Faunística ou Príncipio da Identidade Paleontológica, diz que os grupos de fósseis (animal ou vegetal) ocorrem no registro geológico segundo uma ordem determinada e invariável, de modo que, se esta ordem é conhecida, é possível determinar a idade relativa entre camadas a partir de seu conteúdo fossilífero. Esse princípio, inicialmente utilizado como um instrumento prático, foi posteriormente explicado pela Teoria da Evolução de Charles Darwin. Diversos períodos marcados por extinção de grande parte do conteúdo fossilífero são conhecidos na história da Terra e levaram ao desenvolvimento da Teoria do Catastrofismo.

Lista de rochas
Lista de minerais
Rocha
Mineral
História da geologia
Escala de tempo geológico

Instituto de Geociências da Universidade de São Paulo
Serviço Geológico do Brasil
Sociedade Brasileira de Geologia
Teoria da terra de James Hutton
Sociedade Brasileira de Geologia
Departamento de Geociências da Universidade de Aveiro
U.S.G.S. - Organismo governamental Americano com informação sobre as ciências da terra e da vida
Material didáctico sobre geociências
Glossário geológico ilustrado

Mineral exploration

Mineral exploration is the process undertaken by companies, partnerships or corporations in the endeavour of finding ore (commercially viable concentrations of minerals) to mine.
Mineral exploration is a much more intensive, organised and professional form of mineral prospecting and, though it may at times use the methods of hobby prospecting, the process of mineral exploration is much more involved.

1 Stages of mineral exploration
2 Area Selection
3 Target generation
3.1 Geophysical methods
3.2 Remote sensing
3.3 Geochemical methods
4 Resource evaluation
5 Reserve definition
6 Extraction
7 Greenfields vs Brownfields
8 See also

Area selection is a crucial step in professional mineral exploration. Selection of the best, most prospective, area in a mineral field, geological region or terrain will assist in making it not only possible to find ore deposits, but to find them easily, cheaply and quickly.

Area selection is based on applying the theories behind ore genesis, the knowledge of known ore occurrences and the method of their formation, to known geological regions via the study of geological maps, to determine potential areas where the particular class of ore deposit being sought may exist.

This process applies the disciplines of basin modelling, structural geology, geochronology, petrology and a host of geophysical and geochemical disciplines to make predictions and draw parallels between the known ore deposits and their physical form and the unknown potential of finding a 'lookalike' within the area selected.

Area selection is also influenced by the commodity being sought; exploring for gold occurs in a different manner and within different rocks and areas to exploration for oil or natural gas or iron ore. Areas which are prospective for gold may not be prospective for other metals and commodities.

Often a company or consortium wishing to enter mineral exploration may conduct market research to determine, if a resource in a particular commodity is found, whether or not the resource will be worth mining based on projected commodity prices and demand growth.
Area selection may also be influenced by previous finds, a process informed heavily by nearology, and may also be determined in part by financial and taxation incentives and tariff systems of individual nations.

The role of infrastructure may also be crucial in area selection, because the ore must be brought to market and infrastructure costs may render isolated ore uneconomic.
The ultimate result of an area selection process is the pegging or notification of exploration licenses, known as tenements.

The target generation phase involves investigations of the geology via mapping, geophysics and conducting geochemical or intensive geophysical testing of the surface and subsurface geology. In some cases, for instance in areas covered by soil, alluvium and platform cover, drilling may be performed directly as a mechanism for generating targets.

exploration geophysics
Geophysical instruments play a large role in gathering geological data which is used in mineral exploration. Instruments are used in geophysical surveys to check for variations in gravity, magnetism, electromagnetism (conductivity and resistivity of rocks) and a number of different other variables in a certain area. The most effective and widespread method of gathering geophysical data is via flying airborne geophysics.
Geiger counters and scintillometers are used to determine the amount of radioactivity. This is particularly applicable to searching for uranium ore deposits but can also be of use in detecting radiomatric anomalies associated with metasomatism. Airborne magnetometers are used to search for magnetic anomalies in the Earth's magnetic field. The anomalies are an indication of concentrations of magnetic minerals such as magnetite, pyrrhotite and ilmenite in the Earth's crust. It is often the case that such magnetic anomalies are caused by mineralization events and associated metals.

Ground-based geophysical prospecting in the target selection stage is more limited, due to the time and cost. The most widespread use of ground-based geophysics is electromagnetic geophysics which detects conductive minerals such as sulphide minerals within more resistive host rocks. Ultraviolet lamps may cause certain minerals to fluoresce, and is a key tool in prospecting for tungsten mineralisation.

Aerial photography is an important tool in assessing mineral exploration tenements, as it gives the explorer orientation information - location of tracks, roads, fences, habitation, as well as ability to at least qualitatively map outcrops and regolith systematics and vegetation cover across a region. Aerial photography was first used post world war two and was heavily adopted in the 1960's onwards. Since the advent of cheap and declassified Landsat images in the late 1970's an early 1980's, mineral exploration has begun to use satellite imagery to map not only the visual light spectrum over mineral exploration tenements but spectra which are beyond the visible. Satellite based spectroscopes allow the modern mineral explorationist, in regions devoid of cover and vegetation, to map minerals and alteration directly. Improvements in the resolution of modern commercially based satellites has also improved the utility of satellite imagery; for instance IKONIS satellite images can be generated with a 30cm pixel size.

geochemistry
The primary role of geochemistry, here used to describe assaying or geological media, in mineral exploration is to find an area anomalous in the commodity sought, or in elements known to be associated with the type of mineralisation sought. Regional geochemical exploration has traditionally involved use of stream sediments to target potentially mineralised catchments. Regional surveys may use low sampling densities such as one sample per 100 square kilometres. Follow-up geochemical surveys commonly use soils as the sampling media, possibly via the collection of a grid of samples over the tenement or areas which are amenable to soil geochemistry. Areas which are covered by transported soils, alluvium, colluvium or are disturbed too much by human activity (roads, rail, farmland), may need to be drilled to a shallow depth in order to sample undisturbed or unpolluted bedrock. Once the geochemical analyses are returned, the data is investigated for anomalies (single or multiple elements) that may be related to the presence of mineralisation. The geochemical anomaly is often field checked against the outcropping geology and, in modern geochemistry, normalised against the regolith type and landform, to reduce the effects of weathering, transported materials and landforms.

Geochemical anomalies may be spurious or related to low-grade or sub-grade mineralisation. In order to determine if this is the case, geochemical anomalies must be drilled in order to test them for the existence of economic concentrations of mineralisation, or even to determine why they exist in the place they exist. The presence of some chemical elements may indicate the presence of a certain mineral. Chemical analysis of rocks and plants may indicate the presence of an underground deposit. For instance elements like arsenic and antimony are associated with gold deposits and hence, are example pathfinder elements. Tree buds can be sampled for pathfinder elements in order to help locate deposits.

mineral resource classification
Resource evaluation is undertaken to quantify the grade and tonnage of a mineral occurrence. This is achieved primarily by drilling to sample the prospective horizon, lode or strata where the minerals of interest occur. The ultimate aim is to generate a density of drilling sufficient to satisfy the economic and statutory standards of a ore resource. Depending on the financial situation and size of the deposit and the structure of the company, the level of detail required to generate this resource and stage at which extraction can commence varies; for small partnerships and private non-corporate enterprises a very low level of detail is required whereas for corporations which require debt equity (loans) to build capital intensive extraction infrastructure, the rigor necessary in resource estimation is far greater. For large cash rich companies working on small ore bodies, they may work only to a level necessary to satisfy their internal risk assessments before extraction commences. Resource estimation may require pattern drilling on a set grid, and in the case of sulphide minerals, will usually require some form of geophysics such as down-hole probing of drillholes, to geophysically delineate ore body continuity within the ground. The aim of resource evaluation is to expand the known size of the deposit and mineralisation. A scoping study is often carried out on the ore deposit during this stage to determine if there may be enough ore at a sufficient grade to warrant extraction; if there is not further resource evaluation drilling may be necessary. In other cases, several smaller individually uneconomic deposits may be socialised into a 'mining camp' and extracted in tandem. Further exploration and testing of anomalies may be required to find or define these other satellite deposits.

Reserve definition
Reserve definition is undertaken to convert a mineral resource into an ore reserve, which is an economic asset. The process is similar to resource evaluation, except more intensive and technical, aimed at statistically quantifying the grade continuity and mass of ore.
Reserve definition also takes into account the milling and extractability characteristics of the ore, and generates bulk samples for metallurgical testwork, involving crushability, floatbility and other ore recovery parameters. Reserve definition includes geotechnical assessment and engineering studies of the rocks within and surrounding the deposit to determine the potential instabilities of proposed open pit or underground mining methods. This process may involve drilling diamond core samples to derive structural information on weaknesses within the rock mass such as faults, foliations, joints and shearing.

At the end of this process, a feasibility study is published, and the ore deposit may be either deemed uneconomic or economic.

Resource extraction
The ultimate goal of mineral exploration is the extraction, beneficiation and profitable and beneficial sale of mineral commodities.
Extraction methods may vary considerably and it is the discipline of engineers trained in mining engineering to determine the most safe, cost effective and efficient method of mining the ore body. Mineral exploration and development does not cease upon a decision to mine. Exploration of a brownfields nature is conducted to find near-mine repetitions, extensions and continuity of the existing ore body. In-mine exploration and grade control drilling is a major concern of operaing mines and can be an effective tool in adding value to existing mineral operations.
Often the lessons learned from studying an exposed ore body, both empirically and scientifically, are invaluable to the exploration geologist and geophysicist, for they get to see the proof of their concepts and the errors of the assumptions they used in the search for the ore body. It is always the case that the exact nature of the ore body does not exactly match the models used to find it.

Greenfields vs Brownfields
Exploration is termed either Greenfields or Brownfields depending on the extent to which previous exploration has been conducted on the tenements in question. Greenfields alludes to unspoilt grass, and brownfields to that which has been trodden on repeatedly. While loosely defined, the general meaning of brownfields exploration is that which is conducted within geological terranes within close proximity to known ore deposits. Greenfields are the remainder.
Greenfields exploration is highly conceptual, relying on the predictive power of ore genesis models to search for mineralisation in unexplored virgin ground. This may be territory which has been drilled for other commodities, but with a new exploration concept is considered prospective for commodities not sought there before. The success rate of exploration and the return on investment is low because exploration is an inherently risky business. Figures for success rates depend on the commodity in question but a good strike rate can be measured in the oil industry; the supergiant Prudhoe Bay oilfield was found on the 12th well drilled into the area. Within gold deposits a discovery hole may be one in one thousand and within some base metals commodities strike rates range from one in fifty to one in one hundred.
Greenfields exploration has a lower strike rate, because the geology is poorly understood at the conception of an exploration program but the rewards are greater because it is easier to find the biggest deposit in an area earlier, and it is only with more effort that the smaller satellite deposits are found. Brownfields exploration is less risky, as the geology is better understood and exploration methodology is well known, but since most large deposits are already found the rewards are incrementally less.


Mineral resource classification
Ore genesis
Exploration logging
Drilling rig
Prospecting

Geologic modelling is the applied science of creating computerized representations

Geologic modelling (or modeling) is the applied science of creating computerized representations of portions of the Earth's crust, especially oil and gas fields and groundwater aquifers.

In the oil and gas industry, realistic geologic models are required as input to reservoir simulator programs, which predict the behavior of the rocks under various hydrocarbon recovery scenarios. An actual reservoir can only be developed and produced once, and mistakes can be tragic and wasteful. Using reservoir simulation allows reservoir engineers to identify which recovery options offer the safest and most economic, efficient, and effective development plan for a particular reservoir. Geologic modelling is a relatively recent subdiscipline of geology which integrates structural geology, sedimentology, stratigraphy, paleoclimatology, and diagenesis. In 2 dimensions a geologic formation or unit is represented by a polygon, which can be bounded by faults, unconformities or by its lateral extent, or crop. In geological models a geological unit is bounded by 3-dimensional triangulated or gridded surfaces. The equivalent to the mapped polygon is the fully enclosed geological unit, using a triangulated mesh. For the purpose of property or fluid modelling these volumes can be separated further into an array of cells, often referred to as voxels combining the word volumetric and pixel. These 3D grids are the equivalent to 2D grids used to express properties of single surfaces.

1 Geologic modelling components
1.1 Structural framework
1.2 Rock type
1.3 Reservoir quality
1.4 Fluid saturation
2 Geologic modelling software
Structural framework
Incorporating the spatial positions of the major boundaries of the formations, including the effects of faulting, folding, and erosion (unconformities). The major stratigraphic divisions are further subdivided into layers of cells with differing geometries with relation to the bounding surfaces (parallel to top, parallel to base, proportional). Maximum cell dimensions are dictated by the minimum sizes of the features to be resolved (everyday example: On a digital map of a city, the location of a city park might be adequately resolved by one big green pixel, but to define the locations of the basketball court, the baseball field, and the pool, much smaller pixels - higher resolution - need to be used).
Rock type
Each cell in the model is assigned a rock type. In a coastal clastic environment, these might be beach sand, high water energy marine upper shoreface sand, intermediate water energy marine lower shoreface sand, and deeper low energy marine silt and shale. The distribution of these rock types within the model is controlled by several methods, including map boundary polygons, rock type probability maps, or statistically emplaced based on sufficiently closely spaced well data.
Reservoir quality
Reservoir quality parameters almost always include porosity and permeability, but may include measures of clay content, cementation factors, and other factors that affect the storage and deliverability of fluids contained in the pores of those rocks. Geostatistical techniques are most often used to populate the cells with porosity and permeability values that are appropriate for the rock type of each cell.
Fluid saturation
A 3D finite difference grid used in MODFLOW for simulating groundwater flow in an aquifer.
Most rock is completely saturated with groundwater. Sometimes, under the right conditions, some of the pore space in the rock is occupied by other liquids or gases. In the energy industry, oil and natural gas are the fluids most commonly being modelled. The preferred methods for calculating hydrocarbon saturations in a geologic model incorporate an estimate of pore throat size, the densities of the fluids, and the height of the cell above the water contact, since these factors exert the strongest influence on capillary action, which ultimately controls fluid saturations.
Geologic modelling software
Software packages have been designed for geologic modelling purposes to display, edit, digitise and automatically calculate parameters required by engineers, geologists and surveyors.
Three-dimensional stratigraphic modelling

Geomodeller3D
GSI3D
Gocad
Earth Vision
Irap RMS
Landmark
Petrel
PetroMod
Igeoss (Dynel2D/3D and Poly3D)
MineSight
Three-dimensional stratigraphic, faulting & bedrock modelling
Geomodeller3D
PetroMod
Modeling magma crystallizing processes
COMAGMAT-NET 3.3
COMAGMAT-Windows 3.58
MELTs
Groundwater modelling
FEFLOW
MODFLOW
GMS
Visual MODFLOW
ZOOMQ3D

BANDED IRON FORMATION

Banded iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite, alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed around three thousand million years before present (3 Ga), include banded iron layers, and the banded layers are a common feature in sediments for much of the Earth's early history.
Banded iron beds are less common after 1.8 Ga, although some are known that are much younger. The conventional concept is that the banded iron layers were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria, combining with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the substrate, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve. The banding is assumed to result from cyclic variations in available oxygen. It is unclear whether these banded ironstone formations were seasonal or followed some other cycle. It is assumed that initially the Earth started out with vast amounts of iron dissolved in the world's acidic seas. Eventually, as photosynthetic organisms generated oxygen, the available iron in the Earth's oceans was precipitated out as iron oxides. At the tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced pulses of free oxygen in the surface waters, alternating with pulses of iron oxide deposition

Water flowing over iron-rich beds
Later banded iron formations
Until fairly recently, it was assumed that the rare later banded iron deposits represent unusual conditions where oxygen was depleted locally and iron-rich waters could form then come into contact with oxygenated water. An alternate explanation of these later rare deposits is undergoing much research as part of the Snowball Earth hypothesis — wherein it is believed that an early equatorial supercontinent (Rodinia) was totally covered in an ice age (implying the whole planet was frozen at the surface to a depth of several kilometers) which corresponds to evidence that the earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 million years ago (mya) (See Cryogenian period, from 800 million years ago (mya, boundary defined chronometrically) to approximately 635 mya) prior to the Ediacaran wherein the earliest multicellular lifeforms appear. Alternatively, some geochemists suggest that BIFs could form by direct oxidation of iron by autotrophic (non-photosynthetic) microbes. The total amount of oxygen locked up in the banded iron beds is estimated to be perhaps twenty times the volume of oxygen present in the modern atmosphere.

Banded iron beds are an important commercial source of iron ore.
Banded iron formation
Archean eon -- time of formation of earth and first rocks
Chlorophyta -- a secondary source of oxygenation
Green algae -- see above
Iron (metallic element)
Ironstone (metallic mineral)
Metals -- group of similar atomic elements
Micropaleontology -- study of ancient microbes and microfossils
Mineralology -- study of minerals and their chemistry
Oxygen catastrophe -- the ancient oxygenation of the atmosphere
Prokaryota -- primitive, unicellular organisms without a nucleus
Archaea -- a newly-described domain of above microbes; includes extremophiles
Bacteria -- the more-familiar domain of above microbes
Cyanobacteria -- a bacterial kingdom previously called "blue-green algae"
Proterozoic eon -- second eon of geologic time (the time of oxygenation)
Paleoproterozoic era -- earliest/lower rock layer
Mesoproterozoic era -- middle geologic layer
Neoproterozoic era -- latest/upper rock layer
Taconite (mineral)
References
Jelte P. Harnmeijer, 2003, Banded Iron-Formation: A Continuing Enigma of Geology, University of Washington Doc format
Klein, Cornelis, 2005, Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins, American Mineralogist; October 2005; v. 90; no. 10; p. 1473-1499; DOI: 10.2138/am.2005.1871 http://ammin.geoscienceworld.org/cgi/content/short/90/10/1473 abstract.
Andreas Kappler, et al., 2005, Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria, Geology; November 2005; v. 33; no. 11; p. 865–868; doi: 10.1130/G21658.1 http://www.gps.caltech.edu/~claudia/papers/kappleretal_GEO2005.pdf