Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Minerals

What is a mineral?


To a geologist, a mineral is a naturally occurring solid, formed by geologic processes, that has a crystalline structure and a definable chemical composition. Almost all minerals are inorganic. Let’s pull apart this mouthful of a definition and examine its meaning in detail.
  • Naturally occurring: True minerals are formed in nature, not in factories. We need to emphasize this point because in recent decades, industrial chemists have learned how to synthesize materials that have characteristics virtually identical to those of real minerals. These materials are not minerals in a geologic sense, though they are referred to in the  commercial world as synthetic minerals.  
  • Formed by geologic processes: Traditionally, this phrase implied processes, such as solidification of molten rock or direct precipitation from a water solution, that did not involve living organisms. Increasingly, however, geologists recognize that life is an integral part of the Earth System. So, some  geologists consider solid, crystalline materials produced by organisms to be minerals too. To avoid confusion, the term “biogenic mineral” may be used when discussing such  materials. 
  • Solid: A solid is a state of matter that can maintain its shape indefinitely, and thus will not conform to the shape of its container. Liquids (such as oil or water) and gases (such as air) are not minerals.  
  • Crystalline structure: The atoms that make up a mineral are not distributed randomly and cannot move around easily. Rather, they are fixed in a specific, orderly pattern. A material in which atoms are fixed in an orderly pattern is called a crystalline solid. 
  • Definable chemical composition: This simply means that it is possible to write a chemical formula for a mineral. Some minerals contain only one element, but most are compounds of two or more elements. For example, diamond and graphite have the formula C, because they consist entirely of carbon. Quartz has the formula SiO2 it contains the elements silicon and oxygen in the proportion of one silicon atom for every two oxygen atoms. Calcite has the formula CaCO3, meaning it consists of a calcium (Ca ) ion and a carbonate (CO3 ) ion. Some formulas are more complicated: for example, the formula for biotite is K(Mg,Fe)3(AlSi3O10)(OH)2. 
  • Inorganic: Organic chemicals are molecules containing some carbon-hydrogen bonds. Sugar (C12H22O11), for example, is an organic chemical. Almost all minerals are inorganic. Thus, sugar and protein are not minerals. But, we have to add the qualifier “almost all” because mineralogists do consider about 30 organic substances formed by “the action of geologic processes on organic materials” to be minerals. Examples include the crystals that grow in ancient deposits of bat guano.
With these definitions in mind, we can make an important distinction between minerals and glass. Both minerals and glass are solids, in that they can retain their shape indefinitely. But a mineral is crystalline, and glass is not. Whereas atoms, ions, or molecules in a mineral are ordered into a crystal lattice, like soldiers standing in formation, those in a glass are arranged in a semi-chaotic way, like people at a party, in small clusters or chains that are neither oriented in the same way nor spaced at regular intervals. If you ever need to figure out whether a substance is a mineral or not, just check it against the criteria listed above. Is motor oil a mineral? No it’s an organic liquid. Is table salt a mineral? Yes it’s a solid crystalline compound with the formula NaCl. Is the hard material making up the shell of an oyster considered to be a mineral? Microscopic examination of  an oyster shell reveals that  it consists of calcite, so it can be called a biogenic mineral. Is rock candy a mineral? No. Even though it is solid and crystalline, it’s made by people and it consists of sugar (an organic chemical).
Credits: Stephen Marshak (Essentials of Geology)

Geology Degree

Geology Degree

As a geology graduate, your mastery in undertaking field and lab examinations joined with your team-working, correspondence and systematic abilities make you an alluring prospect for some businesses.

Definition of Geology

Geology is a science that studies the Earth and the materials that it’s made of. It looks at the rocks that the Earth is composed of, the structure of the earth’s materials, and the processes acting upon those materials that cause the Earth to evolve. Through the study of geology we can understand the history of the Earth. Geologists decipher evidence for plate tectonics, the evolutionary history of life, and the past climates the Earth has been through. Geology also includes the study of organisms that have inhabited the planet, and how they’ve changed over time.

Currently we use geology for mineral and hydrocarbon exploration, evaluating water resources, predicting natural hazards, finding remedies for environmental problems, providing insights into past climate change, and geotechnical engineering. Through geology degrees people can study geology, become a geologist, and use their knowledge to improve our Earth.

A Geology Degree

If you’re interested in studying geology, there are a few different degree options open to you in both undergrad and graduate education. The following are a few options:
  • Bachelor of Arts in Geology: The BA in geology degree is intended for students who plan to pursue teacher certification, natural resource management, scientific or technical writing, and other fields that combine a strong liberal arts background with science training. BA classes may include earth materials, minerals, igneous and metamorphic rocks, oceanography, principles of astronomy, deformation of the Earth, sedimentary processes, earth surface processes, and field methods.
  • Bachelor of Science in Geology: The BS in geology degree differs from the BA in that it has a strong mathematical component. It’s typically designed for students planning to pursue graduate study in geology, or work as a professional geologist. Courses may include: History of the Earth, Earth materials, deformation of the Earth, sedimentary processes, Earth surface processes, field methods, chemistry, physics, physics in electricity and magnetism, and calculus classes.
  • Master of Science in Geology: This is a graduate degree in geology. Master programs are advanced geology degrees with a focus on geology classes. They typically come in both thesis and non-thesis options. Those who want to get a master’s in geology degree must have an undergraduate degree in geology or a closely related science field. Sometimes they’ll let applicants without a bachelor’s degree in geology to take pre-requisite classes before beginning a master’s program. Pre-requisite classes include: physical geology, mineralogy, paleobiology, petrography, geologic field methods, stratigraphy, igneous/metamorphic petrogenesis, structural geology, sedimentary petrogenesis, and introduction to geophysics.
  • Doctorate in Geology: A PhD is the highest level of degree a person can get in geology. These programs are designed to develop creative scholarship and to prepare the student for a professional career in the geological sciences. Typically a person chooses a specialisation or focus such as geochemistry, geology, geophysics, planetary geology, minerals, or more. Students can be admitted into PhD programs with either a bachelor’s or master’s degree in geology. Depending on the previous degree earned, a PhD may take one to two years of study.
In all degree levels of geology, the goal is for students to master basic concepts and vocabulary in geology. Through these programs you’ll learn the following materials:
  • Plate tectonics
  • Origin and classification of rocks and minerals
  • Geological time scale and how this relates to major events in the history of Earth and its life
  • Geophysical properties of the Earth and crustal deformation
  • Processes that shape the surface of the Earth
  • Environmental hazards and issues
You’ll also be expected to:
  • Develop skills in observing and recording geologic features and processes
  • Develop competency in the interpretation of earth science data, including both qualitative and quantitative analyses
  • Achieve competence in: locating and interpreting scientific literature,
  • Giving oral presentations,
  • Using computers at a level consistent with current professional practice
  • Be able to express earth science concepts in writing

Specializations Within Geology

Not all geologists study the same thing. Since the Earth is so large there are many areas that a geologist can focus on. The following are the most common types of geology specialisations within geology degrees:
  • Economic Geology: These geologists help locate and manage the Earth’s natural resources. These resources may include petroleum, coal, and minerals such as iron, copper, and uranium. Typically these resources are used for profit companies.
  • Mining Geology: This is a common form of geology that focuses on extracting mineral resources from the Earth. Typically these resources are of economic interest. They may include gemstones, metals, and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, silica, sulphur, chlorine, and helium.
  • Petroleum Geology: These geologists study locations below the Earth’s surface where extractable hydrocarbons may be. They especially focus on petroleum and natural gas. Petroleum geologists also study the formation of sedimentary basins, where gas reservoirs are typically found. They look into the sedimentary and tectonic evolution, and the present-day positions of rocks within the basins.
  • Engineering Geology: This is the application of geologic principles to engineering practice. The main purpose of engineering geology is to assure that the geologic factors affecting the location, design, construction, operation, and maintenance of engineering works are properly addressed in engineering and building projects. In civil engineering geological principles are used in order to analyse the mechanical principles of the material on which structures are built, or will be built. This allows tunnels to be built without collapsing, and bridges and skyscrapers to be built with sturdy foundations.
  • Environmental Geology: Geology can be applied to various environmental problems. It can be applied in steam restoration, the restoration of brownfield, the understanding of the interactions between natural habitat and the geologic environment, and more.
  • Hydrology Geology: Groundwater hydrology, or hydrogeology, is used to locate groundwater. This can often provide a ready supply of uncontaminated water and is especially important in arid regions. It’s also used to monitor the spread of contaminants in groundwater wells.
  • Natural Hazards Geology: Geologists study natural hazards in order to enact safe building codes. They also make warning systems that are used to prevent loss of property and life amidst natural hazards. The kind of natural hazards that geologists’ study include: avalanches, earthquakes, floods, landslides, debris flows, river channel migration, avulsion, liquefaction, sinkholes, subsidence, tsunamis, and volcanoes.

Day in a Life of a Geologist

In general, geologists work to understand the history of our planet. As noted above, there are many different types of geology that geologists can focus on. The better understanding geologists have of the Earth’s history, the better they can foresee how events and processes of the past and present may influence the future. We rely on geologists to find a good supply of Earth’s products, such as natural gases and minerals. Every structure that we build we must know about the ground it sits on. Our food and fibre comes from soil, which we must understand to produce food. Protection against geologic hazards also depends on a geologists’ understanding of them.
Geologists are responsible for many important contributions to society. In fact, we rely on them in everyday life without even realising it. Geologists use a number of field, laboratory, and numerical modelling methods to decipher Earth history. Through geological investigations, geologists use primary information related to the study of rocks (petrology), sedimentary layers (stratigraphy), and positions of rocks and their deformation (structural geology). In many cases, geologists also study modern soils, rivers, landscapes, and glaciers. They investigate past and current life and biogeochemical pathways, and use geophysical methods to investigate the subsurface of the Earth. The following are the typical categories that geologist’s work fall under:
  • Earth processes: The Earth is constantly evolving and changing due to natural disasters and climate change. These include landslides, earthquakes, floods and volcanic eruptions, all of which can be hazardous to people. Some geologists’ jobs are to understand these processes. Through understanding them they help build important structures where they won’t be damaged, or create maps of areas that have flooded in the past and that could be flooded in the future. These kinds of maps can be used in developing communities and housing areas. They can determine if certain houses need flood insurance or protection.
  • Earth materials: Humans use the Earth’s materials on a daily basis. We use oil that’s produced from wells, metals produced from mines, water drawn from underground sources, and more. A geologist’s work could involve studying rocks that contain important metals, finding and planning mines that could produce metals, finding oil reservoirs, and more.
  • Earth History: A current issue for our world is climate change. Many geologists are currently working to learn more about past climates of the Earth and how they’ve changed over time. Through understanding these patterns, geologists can understand our current climate change and what the future results of that change may be.

Job Options


Occupations specifically identified with your degree include:
  • Building geologist 
  • Geochemist 
  • Geophysicist/field seismologist 
  • Geoscientist 
  • Hydrogeologist 
  • Seismic mediator 
  • Mudlogger 
  • Wellsite geologist 
Occupations where your degree would be helpful include:
  • Drilling Engineer
  • Ecological specialist 
  • Geophysical information processor 
  • Minerals surveyor 
Keep in mind that numerous businesses acknowledge applications from graduates with any degree subject, so don't confine you're supposing to the employments recorded here.

Work Experience

Hands on work in both the UK and abroad is a key a portion of topography courses as it gives down to earth experience. A few courses offer a year out, either abroad or in industry, an awesome chance to expand your ability set and build up a system of contacts.
A few graduates decide to improve their capabilities and abilities by doing paid or intentional deal with fleeting natural tasks in the UK or abroad. Time getting work experience or shadowing can assist you with settling on choices about your future vocation and you'll see it inspiring when you apply your ability to tackle issues in an alternate connection.

Typical Employers

Numerous geography graduates enter callings specifically identified with their degree. Prominent parts incorporate investigation and creation, water supply, natural building and topographical looking over. Different regions incorporate ecological arranging, hydrogeology and contamination control. Average managers of geography graduates include: 
  • the oil, gas and petroleum area; 
  • the groundwater business; 
  • ecological consultancies; 
  • structural building and development organisations.

Skills for your CV

Considering geography you'll increase particular learning identified with your project of study and module decisions. The down to earth field work you do as a major aspect of your degree outfits you with ability in field and research facility examinations. 

Transferable aptitudes from your course include: 
  • aptitudes in perception, information gathering, examination and elucidation; 
  • the capacity to get ready, process and present information; 
  • the capacity to handle data in a scope of distinctive mediums, e.g. literary, numerical, oral, graphical; 
  • composed and verbal relational abilities; 
  • report composing abilities; 
  • critical thinking aptitudes and horizontal considering; 
  • self-inspiration and flexibility; 
  • team-working aptitudes and the capacity to deal with your own particular activity.

Further Study

Further study is a prevalent alternative for geography graduates. In case you're keen on getting into a specific field of geography, for example, mining building, designing topography or the minerals business, what about taking an applicable M.Sc course? 
For instance, taking a M.Sc in petroleum geoscience is a possibility for those needing to get into the petroleum business. Different cases of postgraduate courses include:
  • petroleum engineering;
  • petroleum geophysics;
  • earth sciences;
  • hydrogeology;
  • waste management;
  • nuclear decommissioning.
A little number of understudies proceed onto PhD. By learning at postgraduate level, you'll build up your authority information, research aptitudes and relational abilities.

Rock layers

Rock layers


In geology and related fields, a stratum (plural: strata) is a sedimentary rock layer or soil with inside reliable qualities that recognize it from different rock layers. The "stratum" is the crucial unit in a stratigraphic section and structures the study's premise of stratigraphy.

Characteristics of rock layers

Every rock layer is for the most part one of various parallel rock layers that lie one upon another, set around characteristic procedures. They may stretch out over a huge number of square kilometres of the Earth's surface. Strata are normally seen as groups of diverse shaded or contrastingly organized material uncovered in bluffs, street cuts, quarries, and waterway banks. Individual groups may fluctuate in thickness from a couple of millimetres to a kilometre or more. Every band speaks to a particular method of affidavit: stream residue, shoreline sand, coal bog, sand ridge, magma bed, and so on.

Naming of rock layers

Geologists study rock strata and sort them by the material of beds. Each particular layer is normally doled out to the name of sheet, generally in view of a town, stream, mountain, or locale where the arrangement is uncovered and accessible for study. For instance, the Burgess Shale is a thick introduction of dim, once in a while fossiliferous, shale uncovered high in the Canadian Rockies close Burgess Pass. Slight refinements in material in an arrangement may be portrayed as "individuals" (or now and again "beds"). Arrangements are gathered into "gatherings" while gatherings may be gathered into "supergroups".

Formation

An formation or geological formation is the basic unit of lithostratigraphy. An arrangement comprises of a sure number of rock strata that have an equivalent lithology, facies or other comparable properties. Developments are not characterized on the stone's thickness strata they comprise of and the thickness of distinctive formation can thus change broadly. 
The idea of formally characterized layers or strata is key to the geologic control of stratigraphy. Arrangements can be separated into individuals and are themselves regularly divided in gatherings.

Usefulness of formation

The definition and acknowledgement of formations permit geologists to correspond geologic strata crosswise over wide separations in the middle of outcrops and exposures of rock strata. 

Developments were at first depicted to be the crucial geologic time markers in view of relative ages and the law of superposition. The divisions of the land time scale were the formations depicted and put in sequential request by the geologists and stratigraphers of the eighteenth and nineteenth hundreds of years. 

Current modification of the geologic sciences has limited formations to lithologies, in light of the fact that lithological units are shaped by depositional situations, some of which may continue for a huge number of years and will transgress chronostratigraphic interims or fossil-based routines for relating rocks. For instance, the Hamersley Basin of Western Australia is a Proterozoic sedimentary bowl where up to 1200 million years of sedimentation is saved inside of the in place sedimentary stratigraphy, with up to 300 million years spoke to by a solitary lithological unit of grouped iron arrangement and shale. 

Geologic developments are typically sedimentary rock layers, yet might likewise be transformative rocks and volcanic streams. Molten nosy rocks are for the most part not separated into formations.

Defining lithostratigraphic formations

Formations are the main formal lithostratigraphic units into which the stratigraphic section all over ought to be partitioned totally on the premise of lithology. 

The difference in lithology between arrangements required to legitimize their foundation shifts with the multifaceted nature of the geography of an area and the point of interest required for geologic mapping and to work out its geologic history. 

Formations must have the capacity to be depicted at the size of geologic mapping honed in the area. The thickness of developments may run from not as much as a meter to a few thousand meters. 

Geologic arrangements are normally named for the geographic territory in which they were initially portrayed. 

Entirely, developments can't be characterized on whatever other criteria aside from essential lithology. Nonetheless, it is frequently helpful to characterize biostratigraphic units in light of paleontological criteria, chronostratigraphic units taking into account the stones' age, and chemostratigraphic units in view of geochemical criteria. 

Succession stratigraphy is an idea which challenges the thought of strict lithostratigraphic units by characterizing units in light of occasions in sedimentary bowls, for example, maritime relapses and transgressions. These groupings are a mix of chronostratigraphic units, connected by time, and depositional environment connected by the geologic occasions which happened around then, paying little respect to the grain size of the silt. 

The expression "formation" is regularly utilized casually to allude to a particular gathering of rocks, for example, those experienced inside of a sure profundity range in an oil well.

What is Earth made of?

What is Earth made of?

The proportions of major elements making up the mass of the whole Earth.
At this point, we leave our fantasy space voyage and turn our attention inward to the materials that make up the solid Earth, because we need to be aware of these before we can discuss the architecture of the Earth’s interior. Let’s begin by reiterating that the Earth consists mostly of elements produced by fusion reactions in stars and by supernova explosions. Only four elements (iron, oxygen, silicon, and magnesium) make up 91.2% of the Earth’s mass; the remaining 8.8% consists of the other 88 elements (figure above). The elements of the Earth comprise a great variety of materials.
  • Organic chemicals. Carbon-containing compounds that either occur in living organisms or have characteristics that resemble compounds in living organisms are called organic chemicals. 
  • Minerals. A solid, natural substance in which atoms are arranged in an orderly pattern is a mineral. A single coherent sample of a mineral that grew to its present shape is a crystal, whereas an irregularly shaped sample, or a fragment derived from a once-larger crystal or cluster of crystals, is a grain. 
  • Glasses. A solid in which atoms are not arranged in an orderly pattern is called glass. 
  • Rocks. Aggregates of mineral crystals or grains, or masses of natural glass, are called rocks. Geologists recognize three main groups of rocks. (1) Igneous rocks develop when hot molten (liquid) rock cools and freezes solid. (2) Sedimentary rocks form from grains that break off pre-existing rock and become cemented together, or from minerals that precipitate out of a water solution. (3) Metamorphic rocks form when pre-existing rocks change in response to heat and pressure. 
  • Sediment. An accumulation of loose mineral grains (grains that have not stuck together) is called sediment. 
  • Metals. A solid composed of metal atoms (such as iron, aluminium, copper, and tin) is called a metal. An alloy is a mixture containing more than one type of metal atom. 
  • Melts. A melt forms when solid materials become hot and transform into liquid. Molten rock is a type of melt geologists distinguish between magma, which is molten rock beneath the Earth’s surface, and lava, molten rock that has flowed out onto the Earth’s surface. 
  • Volatiles. Materials that easily transform into gas at the relatively low temperatures found at the Earth’s surface are called volatiles. 
The most common minerals in the Earth contain silica (a compound of silicon and oxygen) mixed in varying proportions with other elements. These minerals are called silicate minerals. Not surprisingly, rocks composed of silicate minerals are silicate rocks. Geologists distinguish four classes of igneous silicate rocks based, in essence, on the proportion of silica to iron and magnesium. In order, from greatest to least proportion of silica to iron and magnesium, these classes are felsic (or silicic), intermediate, mafic, and ultramafic. As the proportion of silica in a rock increases, the density (mass per unit volume) decreases. Thus, felsic rocks are less dense than mafic rocks. Many different rock types occur in each class, as will be discussed in detail in Chapters 4 through 7. For now, we introduce the four rock types whose names we need to know for our discussion of the Earth’s layers that follows. These are (1) granite, a felsic rock with large grains; (2) gabbro, a mafic rock with large grains; (3) basalt, a mafic rock with small grains; and (4) peridotite, an ultramafic rock with large grains.

Discovering the Earth’s Internal Layers


People have speculated about what’s inside our planet since ancient times. What is the source of incandescent lavas that spew from volcanoes, of precious gems and metals found in mines, of sparkling mineral waters that bubble from springs, and of the mysterious forces that shake the ground and topple buildings? In ancient Greece and Rome, the subsurface was the underworld, Hades, home of the dead, a region of fire and sulphurous fumes. Perhaps this image was inspired by the molten rock and smoke emitted by the volcanoes of the Mediterranean region. In the 18th and 19th centuries, European writers thought the Earth’s interior resembled a sponge, containing open caverns variously filled with molten rock, water, or air. In fact, in the popular 1864 novel Journey to the Centre of the Earth, by the French author Jules Verne, three explorers hike through interconnected caverns down to the Earth’s centre.
How can we explore the interior for real? We can’t dig or drill down very far. Indeed, the deepest mine penetrates only about 3.5 km beneath the surface of South Africa. And the deepest drill hole probes only 12 km below the surface of northern Russia compared with the 6,371 km radius of the Earth, this hole makes it less than 0.2% of the way to the centre and is nothing more than a pinprick. Our modern image of the Earth’s interior, one made up of distinct layers, is the end product of many discoveries made during the past 200 years.
The first clue that led away from Jules Verne’s sponge image came when researchers successfully measured the mass of the whole Earth, and from this information derived its average density. They found that the average density of our planet far exceeds the density of common rocks found on the surface. Thus, the interior of the Earth must contain denser material than its outermost layer and can’t possibly be full of holes. In fact, the mass of the Earth overall is so great that the planet must contain a large amount of metal. Since the Earth is close to being a sphere, the metal must be concentrated near the centre. Otherwise, centrifugal force due to the spin of the Earth on its axis would pull the equator out, and the planet would become a disk. (To picture why, consider that when you swing a hammer, your hand feels more force if you hold the end of the light wooden shaft, rather than the heavy metal head.) Finally, researchers realized that, though molten rock occasionally oozes out of the interior at volcanoes, the interior must be mostly solid, because if it weren't, the land surface would rise and fall due to tidal forces much more than it does.
An early image of Earth’s internal layers.
Eventually, researchers concluded that the Earth resembled a hard-boiled egg, in that it had three principal layers: a not-so-dense crust (like an eggshell, composed of rocks such as granite, basalt, and gabbro), a denser solid mantle in the middle (the “white,” composed of a then-unknown material), and a very dense core (the “yolk,” composed of an unknown metal) (figure above). Clearly, many questions remained. How thick are the layers? Are the boundaries between layers sharp or gradational? And what exactly are the layers composed of?

Clues from the Study of Earthquakes: Refining the Image

Faulting and earthquakes.
When rock within the outer portion of the Earth suddenly breaks and slips along a fracture called a fault, it generates shock waves (abrupt vibrations), called seismic waves, that travel through the surrounding rock outward from the break. Where these waves cause the surface of the Earth to vibrate, people feel an earthquake, an episode of ground shaking. You can simulate this process, at a small scale, when you break a stick between your hands and feel the snap with your hands (figure above).
In the late 19th century, geologists learned that earthquake energy could travel, in the form of waves, all the way through the Earth’s interior from one side to the other. Geologists immediately realized that the study of earthquake waves travelling through the Earth might provide a tool for exploring the Earth’s insides, much as ultrasound today helps doctors study a patient’s insides. Specifically, laboratory measurements demonstrated that earthquake waves travel at different velocities (speeds) through different materials. Thus, by detecting depths at which velocities suddenly change, geoscientists pinpointed the boundaries between layers and even recognized subtler boundaries within layers. For example, such studies led geoscientists to subdivide the mantle into the upper mantle and lower mantle, and subdivide the core into the inner core and outer core.

Pressure and Temperature Inside the Earth


In order to keep underground tunnels from collapsing under the pressure created by the weight of overlying rock, mining engineers must design sturdy support structures. It is no surprise that deeper tunnels require stronger supports: the downward push from the weight of overlying rock increases with depth, simply because the mass of the overlying rock layer increases with depth. In solid rock, the pressure at a depth of 1 km is about 300 atm. At the Earth’s centre, pressure probably reaches about 3,600,000 atm. Temperature also increases with depth in the Earth. Even on a cool winter’s day, miners who chisel away at gold veins exposed in tunnels 3.5 km below the surface swelter in temperatures of about 53°C (127°F). We refer to the rate of change in temperature with depth as the geothermal gradient. In the upper part of the crust, the geothermal gradient averages between 20°C and 30°C per km. At greater depths, the rate decreases to 10°C per km or less. Thus, 35 km below the surface of a continent, the temperature reaches 400°C to 700°C, and the mantle-core boundary is about 3,500°C. No one has ever directly measured the temperature at the Earth’s centre, but calculations suggest it may exceed 4,700°C, close to the Sun’s surface temperature of 5,500°C.

Non metal mineral resources

Non metal mineral resources

Society uses many non-metallic mineral resources, also known as industrial minerals, as well. From the ground, we get the stone used to make roadbeds and buildings, the chemicals for fertilizers, the gypsum in drywall, the salt filling salt shakers, and the sand used to make glass the list is endless. This section looks at a few of these materials and explains where they come from.

Dimension Stone 

Stone production in quarries.
The Parthenon, a colossal stone temple rimmed by 46 carved columns, has stood atop a hill overlooking the city of Athens for almost 2,500 years. No wonder“stone,” an architect’s word for rock, outlasts nearly all other construction materials. We use stone to make facades, roofs, curbs, steps, counter tops, and floors. We value stone for its visual appeal as well as its durability. The names that architects give to various types of stone may differ from the formal rock names that geologists use. For example, architects refer to any polished carbonate rock as “marble,” whether or not it has been metamorphosed. Likewise, they refer to any crystalline rocks containing feldspar and/or quartz as “granite,” regardless of whether the rock has an igneous or a metamorphic texture, or a felsic or mafic composition.
To obtain intact slabs and blocks of rock known as dimension stone in the trade for architectural purposes, workers must carefully cut rock out of the walls of quarries (a in figure above). (Note that a quarry provides stone, whereas a mine supplies ore.) To cut stone slabs, quarry operators split rock blocks from bedrock by hammering a series of wedges into the rock, or slice it off bedrock by using a wireline saw, a thermal lance, or a water jet. A wireline saw consists of a loop of braided wire moving between two pulleys. In some cases, as the wire moves along the rock surface, the quarry operator spills abrasive (sand or garnet grains) and water onto the wire. The movement of the wire drags the abrasive along the rock and grinds into it. Alternatively, the quarry operator may use a diamond-coated wire, cooled with pure water. A thermal lance looks like a long blowtorch: a flame of burning diesel fuel, stoked by high-pressure air, pulverizes rock and thereby cuts a slot. More recently, quarry operators have begun to use an abrasive water jet, which squirts out water and abrasives at very high pressure, to cut rock.

Crushed Stone and Concrete 

Crushed stone forms the substrate of highways and rail roads and serves as the raw material for manufacturing cement, concrete, and asphalt. In crushed-stone quarries (b in figure above), operators use high explosives to break up bedrock into rubble that they then transport by truck to a jaw crusher. This reduces the rubble into usable chunks.
Most of the buildings and highways constructed in the past two centuries consist of bricks attached to each other by mortar, or of walls, floors, columns, and roads made of concrete that has been spread into a layer or poured into a form. Both mortar and concrete start out as a slurry, but when allowed to set, they harden into a hard, rock-like substance. The slurry from which mortar and concrete form consists of “aggregate” (sand and/or gravel) mixed with water and cement. Before mixing, the cement in mortar and concrete is a powder that consists of lime (CaO), quartz (SiO2), aluminium oxide (Al2O3), and iron oxide (Fe2O3); typically, lime accounts for 66% of cement, silica for 25%, and the remaining chemicals for about 9%. When cement is mixed with water, the chemicals comprising it dissolve. Mortar and concrete set when these chemicals react and a complex assemblage of new mineral crystals grows and binds together pre-existing solid grains in effect, the “cement” in mortar or concrete serves the same purpose as the “cement” in a sandstone or conglomerate.
It appears that the ancient Romans were the first to use cement they made it from a mixture of volcanic ash and limestone. In the 18th and early 19th centuries, cement was produced by heating specific types of limestone (which happened to contain calcite, clay, and quartz in the correct proportions) in a kiln up to a temperature of about 1450nC; the heating releases CO2 gas and produces “clinker,” chunks consisting of lime and other oxide compounds. Manufacturers crushed the clinker into cement powder and packed it in bags for transport. But limestone with the exact composition necessary to make such “cement” is fairly rare, so most cement used today is Portland cement, made by mechanically mixing limestone, sandstone, and shale in just the right proportions, before heating in a kiln to provide the correct chemical make-up. Isaac Johnson, an English engineer, came up with the recipe for Portland cement in 1844; he named it after the town of Portland, England, because he thought that concrete made from it resembled rock exposed there. 

Nonmetallic Minerals for Homes and Farms 

We use an astounding variety of non-metallic geologic resources without ever realizing where they come from. Consider the materials in a typical house or apartment. The foundation consists of concrete, made from limestone mixed with sand or gravel. The bricks in the exterior walls originated as clay, formed from the chemical weathering of silicate rocks and perhaps dug from the floodplain of a stream. To make bricks, workers mould wet clay into blocks and then bake it. Baking drives out water and causes metamorphic reactions that recrystallize the clay. The glass used to glaze windows consists largely of silica, formed by first melting and then freezing pure quartz sand from a beach deposit or a sandstone formation. Gypsum board (drywall), used to construct interior walls, comes from a slurry of water and the mineral gypsum sandwiched between sheets of paper. Gypsum (CaSO4s (2O) occurs in evaporite strata precipitated from seawater or saline lake water. Evaporites provide other useful minerals as well, such as halite, and serve as the source for lithium, a key element in computer and camera batteries. Modern technological innovations have also greatly increased the demand for the rare earth elements, a group of 17 elements including the lanthanides, scandium, and yttrium. While the names of rare earth elements are unfamiliar to most people, the elements themselves have become essential in the production of lasers, magnets, X-ray tubes, night vision goggles, camera lenses, and high-tech lamps. Rare earth elements aren't actually that rare, in terms of their abundance relative to other elements in the crust. But localities where ores have a high enough concentration to be mined are rare.
Credits: Stephen Marshak (Essentials of Geology)

Metals and Ores

Metal and Ores

Metal and Its Discovery 

Metals are opaque, shiny, smooth solids that can conduct electricity and can be bent, drawn into wire, or hammered into thin sheets. In this regard, they look and behave quite differently from wood, plastic, meat, or rock. This is because, unlike in other substances, the atoms that make up metals are held together by metallic bonds, so electrons can flow from atom to atom fairly easily and atoms can, in effect, slide past each other without breaking apart. The first metals that people used copper, silver, and gold can occur in rock as native metals. Native metals consist only of metal atoms, and thus look and behave like metal.
Gold occurs as native metal within quartz veins. The quartz breaks up to form sand, leaving nuggets of gold.
Gold nuggets, for example, are chunks of native metal that have eroded free of bedrock (figure above). Over the ages, people have collected nuggets of native metal from stream beds and pounded them together with stone hammers to make arrowheads, scrapers, and later, coins and jewellery. But if we had to rely solely on native metals as our source of metal, we would have access to only a tiny fraction of our current metal supply. Most of the metal atoms we use today originated as ions bonded to non-metallic elements in a great variety of minerals that themselves look nothing like metal. Only because of the chance discovery by some prehistoric genius that certain rocks, when heated to high temperatures in fire (a process called smelting), decompose to yield metal plus a non-metallic residue called slag, do we now have the ability to produce sufficient metal for the needs of industrialized society. 

What Is an Ore? 

Examples of ore minerals.
The minerals from which metals can be extracted are called ore minerals, or economic minerals. These minerals contain metal in high concentrations and in a form that can be easily extracted. Galena (PbS), for example, is about 50% lead, so we consider it to be an ore mineral of lead (a in figure above). We obtain most of our iron from haematite and magnetite. Copper comes from a variety of minerals, none of which look like copper (b in figure above). Geologists have identified a great variety of ore minerals. Many ore minerals are sulphides, in which the metal occurs in combination with sulphur (S), or oxides, in which the metal occurs in combination with oxygen (O).
To obtain the metals needed for industrialized society, we mine ore, rock containing native metals or a concentrated accumulation of ore minerals. To be an ore, rock must not only contain ore minerals, it must also contain a sufficient amount to make the rock worth mining. Iron constitutes only about 6.2% of the continental crust's weight but makes up about 30% to 60% of iron ore. The concentration of a useful metal in an ore determines the grade of the ore the higher the concentration, the higher the grade. Whether or not an ore of a given grade is worth mining depends on the price of metal in the market. 

How Do Ore Deposits Form? 

Ore minerals do not occur uniformly through rocks of the crust. If they did, we would not be able to extract them economically. Fortunately for humanity, geologic processes concentrate these minerals into accumulations called ore deposits. Simply put, an ore deposit is an economically significant occurrence of ore. The various kinds of ore deposits differ from each other in terms of which ore minerals they contain and which geologic conditions led to their formation. Below, we introduce a few examples.
Various processes that form ore deposits.

Magmatic deposits 

When a magma cools, sulphide ore minerals crystallize early, then, because sulphides tend to be dense, hey sink to the bottom of the magma chamber, where they accumulate; this accumulation is a magmatic deposit. When the magma freezes solid, the resulting igneous body may contain a concentration of sulphide minerals at its base. Because of their composition, such concentrations are known as “massive- sulphide deposits” (a in figure above).

Hydrothermal deposits 

Hydrothermal activity involves the circulation of hot-water solutions through a magma or through the rocks surrounding an igneous intrusion. These fluids dissolve metal ions. When a solution enters a region of lower pressure, lower temperature, different acidity, and/ or different availability of oxygen, the metals come out of solution and form ore minerals that precipitate in fractures and pores, creating a hydrothermal deposit (figure above b). Such deposits may form within an igneous intrusion or in surrounding country rock. If the resulting ore minerals disperse through the intrusion, we can also call the deposit a disseminated deposit, but if they precipitate to fill cracks in pre-existing rock, we can call the deposit a vein deposit; veins are mineral-filled cracks.
In recent decades, geologists have discovered that hydrothermal activity at the submarine volcanoes along mid-ocean ridges leads to the eruption of hot water, containing high concentrations of dissolved metal and sulphur, from a vent. When this hot water comes in contact with cold seawater, the dissolved components instantly precipitate as tiny crystals of metal-sulphide minerals (c in figure above). The erupting water, therefore, looks like a black cloud, so the vents are called “black smokers”. The minerals in the cloud eventually sink and form a pile of ore minerals around the vent. Since the ore minerals typically are sulphides, the resulting hydrothermal deposits constitute another type of massive-sulphide deposit.

Secondary-enrichment deposits

Sometimes groundwater passes through ore-bearing rock long after the rock first formed. This groundwater dissolves some of the ore minerals and carries the dissolved ions away. When the water eventually flows into a different chemical environment (for instance, one with a different amount of oxygen or acid), it precipitates new ore minerals, commonly in concentrations exceeding that of the original deposit. A new ore deposit formed from metals that were dissolved and carried away from a pre-existing ore deposit is called a secondary-enrichment deposit. Some of these deposits contain spectacularly beautiful copper-bearing carbonate minerals, such as azurite and malachite. 

MVT ores 


Rain falling along one margin of a large sedimentary basin may sink into the subsurface and then flow as groundwater along a curving path that takes it first down to the bottom of the basin, and then eventually back up to the opposite margin of the basin, hundreds of kilometres away. At the bottom of the basin, temperatures become high enough that the water dissolves metals. As the water returns to the surface and enters cooler rock, these metals precipitate in ore minerals. Ore deposits formed in this way, containing lead- and zinc-bearing minerals, appear in dolomite beds of the Mississippi Valley region of the United States, and thus have come to be known as Mississippi Valley–type (MVT) ores. 

Sedimentary deposits of metals 

A Precambrian banded iron formation from northern Michigan.
Some ore minerals accumulate in sedimentary environments under special circumstances. For example, between 2.5 and 1.8 billion years ago, the atmosphere, which previously had contained very little oxygen, gained oxygen because of the evolution of abundant photo synthetic organisms. This change affected the chemistry of seawater so that large quantities of dissolved iron precipitated as iron oxide minerals that settled as sediment on the sea floor. The resulting iron-rich sedimentary deposits are known as a banded iron formation (BIF) (figure above), because after lithification they consist of alternating beds of Gray iron oxide (magnetite or haematite) and red beds of jasper (iron-rich chert).
The chemistry of seawater in some parts of the ocean today leads to the deposition of manganese-oxide minerals on the sea floor. These minerals grow into lumpy accumulations known as manganese nodules. Mining companies have begun to explore technologies for vacuuming up these nodules; geoscientists estimate that the worldwide supply of nodules contains 720 years’ worth of copper and 60,000 years’ worth of manganese, at current rates of consumption. 

Residual mineral deposits 

Recall from Interlude B that as rainwater sinks into the Earth, it leaches (dissolves) certain elements and leaves behind others, as part of the process of forming soil. In rainy, tropical environments, the residue left behind in soils after leaching includes concentrations of iron or aluminium. Locally, these metals become so concentrated that the soil itself becomes an ore deposit. We refer to such deposits as residual mineral deposits. Most of the aluminium ore mined today comes from bauxite, a residual mineral deposit created by the extreme leaching of rocks (such as granite) containing aluminium-bearing minerals.  The figure below shows the residual mineral deposits as well as placer deposits.

Placer deposits 

Placer deposits form where erosion produces clasts of native metals. Sorting by the stream concentrates the metals.
Ore deposits may develop when rocks containing native metals erode, producing a mixture of sand grains and metal flakes or nuggets (pebble-sized fragments). For example, gold accumulates in sand or gravel bars along the course of rivers, for the moving water carries away lighter mineral grains (quartz and feldspar) but can’t move the heavy metal grains (gold) so easily. Concentrations of metal grains in stream sediments are a type of placer deposit (figure above). Panning further concentrates gold flakes or nuggets swirling water in a pan causes the lighter sand grains to wash away, leaving the gold behind.

Where Are Ore Deposits Found? 

The Inca Empire of fifteenth-century Peru boasted elaborate cities and temples, decorated with fantastic masks, jewellery, and sculptures made of gold. Then, around 1532, Spanish ships arrived, led by conquistadors who quipped, “We Spaniards suffer from a disease that only gold can cure.” The Incas, already weakened by civil war, were no match for the armorclad Spaniards with their guns and horses. Within six years, the Inca Empire had vanished, and Spanish ships were transporting Inca treasure back to Spain. Why did the Incas possess so much gold? Or to ask the broader question, what geologic factors control the distribution of ore? Once again, we can find the answer by considering the consequences of plate tectonics. Several of the ore-deposit types mentioned above occur in association with igneous rocks. Igneous activity does not happen randomly around the Earth, but rather concentrates along convergent plate boundaries (specifically, in the overriding plate of a subduction zone), along divergent plate boundaries (along mid-ocean ridges), continental rifts, or hot spots. Thus, magmatic and hydrothermal deposits (and secondary-enrichment deposits derived from these) occur in these geologic settings. Placer deposits are typically found in the sediments eroded from such magmatic or hydrothermal deposits. The Inca gold formed in the Andes along a convergent plate boundary.

Credits: Stephen Marshak (Essentials of Geology)