Category: Geography Notes PPT

Every rock is formed under various chemicals, physical, and biological conditions, resulting in the formation of a diverse group of minerals that can be used in a variety of applications. Different forms of rocks exit on earth such as strong, fragile, permeable, and impermeable.

Marling, a centuries-old method, and the more recent application of rock powder to soils are both interventions in the organic energy budget of soils. Both are long-acting, slow-release fertilisers, or better yet, soil conditioners. An analysis of beliefs and skills from the 1st century AD to the 1800s is presented for marls, while a review of the recent analytical and empirical literature is presented for rock powders.

      

             

Marl Mining

In the 1800s, marl was widely used as a soil conditioner in Central New Jersey. Blue marl was the most popular marl in 1863. Blue marl usually composed of 38.70 percent silicic acid and sand, 30.67 percent oxide of iron, 13.91 percent carbonate of lime, 11.22 percent water, 4.47 percent potash, 1.21 percent magnesia, 1.14 percent phosphoric acid, and 0.31 percent sulphuric acid, but exact composition and properties differed depending on which layer it was contained in. Marl was in demand for farm use and this continued for farming in the 21st century also.

Formation of Marl

Both clastic and chemical-biogenic origins are possible for the rocks.

Carbonate may be washed in as detritus, and it often returns to plankton carbonate skeletons or biochemically precipitated calcite.

It is critical to characterise the Marl in order to address all relevant geotechnical engineering issues. The typology, creation, physical, chemical properties, as well as feedback from some geotechnical field and laboratory tests are essential things to look for in marl mineral.

Finding Marl

Marl is common lacustrine sediment found in post-glacial lake-bed sediments, particularly beneath peat bogs. It’s been used as a soil conditioner and a neutralizer for acidic soil.

Marl is found in India in association with limestone. Marl resources can be found in Gujarat’s Amreli, Junagarh, Jamnagar, and Porbandar districts. The cement industry consumes the marl primarily for the manufacture of cement.

Types of Marl

Marl soils are fine, cohesive sedimentary rocks deposits and cover a large area of Algiers’ south-east, east, and west, where most of the city’s and region’s growth is still underway. The largest marl soil deposit was discovered to be more than 200 metres deep, forming a homogeneous and massive substratum in most cases. The geotechnical activity is determined by carbonate amount and clay mineral type. There are the following types of marls depending on the degree of weathering and they are Weathered, Intact and Intermediate.

Usage of Marl

Food, drugs, jewellery, pencils, makeup, paths, tools, floors, monuments, sculptures, and other items all contain rocks. Many different types of rocks have been used as building blocks for buildings since ancient times and are still in use today. Many sectors like medical, industrial and others use Marl.

• Soil Conditioner– Marl has traditionally been used as a soil conditioner and acid neutralizer

• Decoratives- Marl is also used to decorate and in floor tiles. Also, it is used as a building stone.

• Creative’s-With marl, small figures, artifacts, sculpture and jewellery can be made.

• Cement Manufacturing- Marl obtained is also used in the process of making cement. The material used for cement manufacture is called Marlbrook.

Roads and highways construction has a base of Marl as it consists of carbonate (CaCO3) and clays with varying percentages and occasional traces of organic matter, silt, or sand. The base and sub-base layers of highway pavements use Marl. This form of soil is weak and sensitive to water, and immersion will result in a significant loss of bearing ability.

The soil is recommended to use in construction projects because of its high water sensitivity and strength. Before recommendation, proper engineering treatment is required.

Glauconite

Glauconite is an iron potash rich mica of the mica group mineral that has green colour. This mica mineral is very powdery, chalky and has very low weathering resistance. It crystallizes together with monoclinic geometry.

Once mined, greensand (glauconite) is dried and utilized as a soil conditioner. Greensand is also quite commonly used in water softeners principally to remove iron from the water. Recent research has also revealed that greensand has the potential for execution as a filter of heavy metals from industrial waste water and landfill leachates.

Marl and Scientific Study

Various types of experimental research have been conducted on marl deposits. They are good study sites for recovering a variety of fossil plant and animal remains, which can be used to analyze evolving ecosystems in the postglacial period.

Monazite is a rare-earth element-rich phosphate mineral that is predominantly reddish-brown in colour.

Monzite Formula: The Monazite formula is (Ce,La,Nd,Th)(PO4,SiO4), and in igneous and metamorphic rocks such as granite, pegmatite, schist, and gneiss, it is contained in small isolated grains as an accessory mineral. These grains withstand weathering and accumulate in soils and sediments downslope from the host rock. They are mined for their rare earth and thorium content when they reach high enough concentrations.

  

Monazite price per ton- Monazite costs currently range from＄1680 to＄1900 per ton-1.

 

Is Monazite Mineral or a Group of Minerals?

Monazite chemical formula is (Ce,La,Nd,Th)(PO4,SiO4), which indicates that cerium, lanthanum, neodymium, and thorium can provide all substitutes for one another in the mineral’s structure, as well as silica for phosphate. Monazite is a mineral that is found in a number of solid-solution series with other minerals.

The term “monazite” also refers to a group of monoclinic phosphate and arsenate minerals with similar compositions and crystal structures. Below is a list of minerals that belong to the monazite group. It’s worth noting that monazite comes in a variety of forms.

Monazite Mineral Groups

 

Physical Properties of Monazite

Monazite is a resinous to vitreous mineral with a yellowish-brown to reddish-brown or greenish-brown lustre. It’s transparent, and big grains or well-formed crystals are uncommon. Where monazite is abundant locally, granular masses can be seen. It has a cleavage that is pleasant and distinct. It has a hardness of 5 to 5.5. It has a high specific gravity that varies between 4.6 and 5.4 depending on its composition.

Properties of Monazite

 

Geological Occurrence of Monazite Mineral

Monazite is known for its accumulation rather than its formation. It forms when igneous rocks crystallise and clastic sedimentary rocks metamorphose. Monazite, one of the most resistant minerals, concentrates in the weathering debris as these rocks weather. Monazite concentrations in soils and sediments near weathering outcrops can be higher than in the source rock.

The liberated monazite grains then begin their descent. They are eventually taken to a stream or a dry wash. Gravity and running water aid in the separation of hard grains of monazite and other heavy minerals from lighter minerals. They collect behind boulders, on the inside bends of stream channels, and eventually work their way down into the sediment deposit’s lower levels. Any of them flow into the sea and settle in deltaic, beach, or shallow water sediments.

Monazite Mineral Mining

Monazite mining is concentrated in placer deposits since they are easier to mine and contain higher concentrations of monazite than hard rock deposits. Gold, silver, magnetite, ilmenite, rutile, zircon, and a number of gemstones are all heavy minerals that accumulate with monazite. The recovered heavy sands are extracted to extract the heavy minerals, with the light fraction being returned to the deposit. Heavy minerals have been dredged from stream sediments, alluvial terraces, coastal sediments, beach terraces, and shallow-water sediments.

The offshore waters of India, Malaysia, Vietnam, and Brazil now contain the majority of the world’s monazite. The most extensive offshore monazite deposits are found in southern India and Sri Lanka. Australia was once the world’s largest producer of monazite, and its monazite reserve is considered to be the world’s largest. It hasn’t been a major producer since the 1990s when public opposition forced the closure of mining on Frasier Island.

In the United States, monazite is not actually mined. It was previously mined in Idaho from stream placer deposits. The Idaho batholith weathered to form these deposits. Monazite was also extracted as a byproduct from offshore deposits along the United States’ southeast coast, from North Carolina to Florida. Many states have inland and offshore reserves, but they are small and low-grade as compared to what is currently extracted in other countries.

Monazite Mineralization and Extraction

When released by the weathering of pegmatites, monazite minerals accumulate in alluvial sands due to their high density. Other heavy minerals of commercial interest, such as zircon and ilmenite, can be found in these so-called placer deposits, which are mostly sandy or fossil beach sands. Using gravity, magnetic, and electrostatic separation, monazite can be isolated as a nearly pure concentrate.

The monazite-(Ce) composition is invariably found in monazite sand deposits. In such monazites, the lanthanides usually contain 45–48 percent cerium, 24 percent lanthanum, 17 percent neodymium, 5 percent praseodymium, and trace amounts of samarium, gadolinium, and yttrium. Europium concentrations are typically low, averaging about 0.05 percent. The Lindsay Chemical Division of American Potash and Chemical Corporation, at the time the world’s largest producer of lanthanides, extracted South African “rock” monazite from Steenkampskraal in the 1950s and early 1960s.

The full collection of lanthanides was available from the Steenkampskraal monazite. Monazite’s extremely low concentrations of the heaviest lanthanides justified the term “rare” earth for these elements, which came with correspondingly high costs. Monazite’s thorium content varies, but it can reach 20–30% in some cases. Monazite derived from some carbonatites or tin ore veins in Bolivia is practically thorium-free. Commercial monazite sands, on the other hand, usually contain 6 to 12 percent thorium oxide.

1. Acid Cracking –

The initial method for extracting thorium and lanthanide from monazite was to heat it with concentrated sulfuric acid for several hours at temperatures between 120 and 150 °C. Several different methods to isolate thorium from the lanthanides were developed as a result of variations in the acid-to-ore ratio, the degree of heating, and the amount of water applied afterwards. One of the processes caused the thorium to precipitate out in the crude form as phosphate or pyrophosphate, leaving a solution of lanthanide sulphates from which the lanthanides could be quickly precipitated double sodium sulphate. The acid methods resulted in a significant amount of acid waste and a lack of phosphate content in the ore.

2. Alkaline Cracking- 

A more recent method employs a hot sodium hydroxide solution (73 percent) at a temperature of about 140 °C. The valuable phosphate content of the ore can be recovered as crystalline trisodium phosphate using this method. After treating the lanthanide/thorium hydroxide mixture with hydrochloric acid, a solution of lanthanide chlorides and an insoluble sludge of the less basic thorium hydroxide is obtained.

 

Monazite Uses

• Monazite is an important source of thorium, cerium, and other rare elements.

• It is often mined as a byproduct of heavy mineral deposits.

• Monazite sand is used in construction and casting. 

 

Rare Earth Metal Extraction From Monazite

The rare earth elements are an essential part of modern life products and green technologies. Now let’s understand how rare earth metals are extracted from monazite. The following steps below detail the extraction of rare earth metals from the monazite ore. Neutralizations and filtrations are the major requirements in this process.

1. Monazite ore should be ground to a fineness of 150 micrometres in a grinder. Monazite ore is composed of 55–60 per cent rare-earth metal oxides, 24–29 per cent P₂O₅, 5–10 per cent ThO2, and 0.2–0.4 per cent U₃O₈.

2. At input temperatures of 150 to 180 °C, pulverised monazite is blended with strongly concentrated sulfuric acid (93 per cent acid). The acid-to-ore proportion differs based on the ore’s density. The digester is stirred continuously with a powerful impeller and continues to perform at temperatures ranging from 200 to 300 °C. Before the ore, acid is compensated into the reactor and warmed. The insoluble item coats the pulverised ore grains. The heat generated by the exothermic responses raises the temperature in the reactor. The viscosity of the remedy has risen to the point where it resembles dough after 15 minutes. The product takes 3 to 4 hours to respond. Before the solution stiffens, it is removed from the digester. 

3. The components of the reactor are cooled to 70 °C and leached with 30 °C water, with a proportion of sulfuric acid to sand excluded of 1.6 to 2.5. The water-to-ore mass proportion is ten parts water to one part ore. This leaching process takes 12 to 15 hours to complete.

4. At 30°C, dilute monazite sulfate with 6–7 parts water. To establish a preferential crystallisation of thorium-phosphate cake, add NH₃(aq) to neutralise to a pH of 1.1.

5. Obtain thorium phosphate precipitate from nullified monazite solution after filtration.

6. To make condensed thorium phosphate, nourish thorium-phosphate cake through a dryer set to 120°C.

7. To generate a rare-earth-metal precipitate at a pH of 2.3, incorporate NH₃ (aq) to the remainder monazite solution.

8. To achieve a pH of 6, add NH₃ (aq) to the leftover filtrate. This results in a uranium-rich precipitate.

9. To obtain uranium concentrate, filter the leftover solution. Thorium-phosphate concentrate, RE hydroxides, and uranium concentrate are the finished products of this procedure.

 

Mineral Processing

Mineral processing is the method of separating highly valued minerals from mining waste, or sludge, in crude ores and mineral goods. It is the initial procedure that most ores go through after they are mined in order to offer a more intensive material for resource extraction metallurgy processes. Comminution and concentration are the most important functions in a modern mineral processing plant, but sampling and analysis and dewatering are also important.

In an attempt to obtain data needed for the monetary valuation of ores and concentrates, routine sampling and analysis of the raw material being handled are carried out. Modern plants also have fully automated control systems that analyse the material as it is filtered in real-time and make certain changes that are needed to produce the pure or richest possible concentrate at the verge of the lowest possible cost of operation.

The Oligocene period is the geologic epoch related to the Paleogene period this extends from about 33.9 million to 23 million years. Like other older geologic periods, the rocks which define the epoch are truly identified but these exact dates of the start and end of the epoch are a little uncertain. 

The name Oligocene originated in the year 1854 by a palaeontology must have named Heinrich Ernst Beyrich. His name comes from the Ancient which refers to the extant forms of mollusks. This period is preceded by Eocene Epoch which is then followed by the Miocene Epoch. While Oligocene is the third and also final epoch of the Paleogene Period.

Oligocene Period 

Oligocene Period is the third and last major worldwide division of the Paleogene Period which is 65.5 million to about 23 million years ago. This is the interval between 33.9 million to 23 million years ago. The Oligocene Epoch is here subdivided into two ages, we present their corresponding rock stages as well:

The Rupelian Age

The Chattian Age. 

This is being followed by the Eocene Epoch and was succeeded by the Miocene Epoch, while the first epoch was the Neogene period. The term used ‘Oligocene’ is being derived from Greek which actually means the “epoch of the few recent forms,” this is being referred to as the sparseness of the number of these modern animals which originated during that time.

In the western part of Europe, the beginning of the Oligocene period was marked by the invasion of the sea which was brought with it by the new mollusks which is a prominent characteristic of the epoch. The marine conditions did not exist in this period for long, however, the brackish and freshwater conditions soon prevailed during this time.

Oligocene Animals 

The earliest forms of amphicyonids, canids, tayassuidae, camels, protoceratops, and the anthracotheres showed in this time. Birds like Caprimulgiformes who have gaping mouths to catch the insects also appeared in this period. Diurnal raptors like the falcons, hawks, and eagles with seven to ten families of rodents also originated during the Oligocene Period.

Aluvarus praeimperialis is a type of bony fish, which is extinct, they are known from headless fossil specimens that are found in the Elam Formation.  

Oligocene Climate

Oligocene cherished a climate that appeared to have been of a temperate type, there were many regions that enjoyed the subtropical climatic conditions in this region. The grasslands also expanded and the forested regions dwindled in this time. Here the tropical vegetation flourished along the borders of the Sea named Tethyan. Then existed warm, swampy conditions. This specific type of condition prevailed over much of Germany and had extensive deposits of lignite coal.

Oligocene Epoch Animals 

Another prominent group of Oligocene was the marine organisms – foraminiferans, protists. This group was quite similar to amoebas but they bore a complex, and a general calcareous test, or the shell. The prominent foraminiferans were known as the nummulites which were large, lens-shaped foraminiferans. The other marine form is essentially quite modern in approach.  

The terrestrial consists of the invertebrate life where abundant and diverse vegetation grew. The deposits of the lake and stream on the Isle of Wight in England also contain the remains, often well preserved. The Baltic and many other forms of Oligocene insects that includes butterflies, bees, ants, and spiders live here in amber.  

Oligocene Fossils 

The climatic change occurred approximately 34 million years ago in Africa and not in Asia. More specifically said, the climate marking of the Eocene-Oligocene Transition (this occurred about 34 million years ago) which could have acted as an evolution filter, this allows different types of primates to evolve in the land of Africa while compared to Asia. The ensuing dominance of the subset of primates was known as anthropoids which were located in the African regions, this must-have led to the evolution of humans in this part according to a paper in Science. Even though the early fossils were found in Asia, the actual evolutions of human beings occurred in Africa. 

Oligocene Mammals

The Oligocene period marked the advent of large gigantic mammals. Rhinoceroses also reached great heights similar to the animal called Paraceratherium. Dogs and cats were land carnivores animals while dogs and cats were the primary land carnivores and the whales were carnivores at sea. The mammals during the Oligocene period were “Micro-mammals” who experienced a period of diversification. This period was also characterized by free change of animals among the northern continents, as this was seen by the similarity of invertebrate faunas.

Oligocene Plants

Grasses were the plants that first grew near the water margins in the Eocene after which they became more common in the open habitats. In Northern America the flora also consisted of a mixture of subtropical elements, among others popular were – cashews and lychee trees, also there thrived temperate trees as well such as roses, beech, and pine.Oligocene Epoch Animals

Geology is defined as the study of the Earth, studying the materials by which it is made. Geology also studies the structure of those materials and the processes which act upon them. This also includes the study of other organisms that thrive on our planet.

The Geologists for this matter study the materials, its process-related, products, the physical nature, and the history of the Earth. The Geomorphologists study the Earth’s landforms and its definite landscapes which is in relation to the geologic and the climatic processes and they also study human activities. 

Petrology is another branch of geology that studies the rocks and the conditions under which they structure out. Petrology has three other subdivisions, they are – igneous, metamorphic, and sedimentary petrology.

Petrology Meaning 

Petrology is defined as the study of rocks – namely igneous, sedimentary and metamorphic rocks. The process and forms and thus get transformed. While mineralogy is the study of chemistry, it studies the crystal structure and physical properties of the mineral constituents of rocks. Both these processes – petrological and mineralogical are very much sensitive to the environmental conditions. Hence, the compositions of rocks, and the minerals they consist of, are quite interesting to answer the most basic questions which also satisfy a wide range of geological disciplines.

We use petrology to study how the volcanoes were formed and their magmatic sources, via this study we also get to know the evolution of continental crust during the growth and destruction of the mountain belts, the genesis of the accessory minerals like the REE phosphates in all rock types, the origins of economic concentrations of minerals and the petroleum, the make-up of the atmosphere, ocean and life on Earth through the passage of time, and the geological processes which occur on other planets.  

Igneous Petrology

Igneous petrology studies the identification, classification, origin, evolution, and also processes of formation of the crystallization of these igneous rocks. Most of the rocks which are available for study thus come from the Earth’s crust, while a few like eclogitic are derived from the mantle. 

While studying igneous petrology, the researcher generally employs a phase of equilibrium approach that is very much comparable to the mineral assemblages which are found in the naturally occurring and synthetic rocks. 

From this, we can learn quite a good deal about the melting of an igneous rock. We also can study the reverse process of the crystallization of these minerals that are from a melt (or from the liquid phase). 

Mineralogy and Petrology 

Mineralogy and Petrology embrace and include manuscripts from the classical fields of crystallography, mineralogy, petrology, geochemistry, along with their applications in academic experimentation and also in research. Here the materials science and engineering, for technology, industry, environment, or society. The journal also strongly promotes the cross-fertilization which occurs among the Earth-scientific and they are applied materials-oriented for the disciplines. These are purely descriptive manuscripts based on regional topics which will not be considered.

Sedimentary Petrology 

In the field, where sedimentary petrology is studied, the main concern is with the description and with the classification of the sedimentary rocks, it studies the interpretation of the processes of the transportation and also the deposition of these sedimentary materials which forms the rocks. The environment which prevailed during the sediments was responsible for this. 

Metamorphic Petrology

Metamorphic petrology majorly covers the chemical and also the physical work which is done in the natural systems quite in response to the changing physical conditions. The petrogenetic processes like recrystallization, continuous and discontinuous reactions, mixed volatile reactions, and also deformation is being addressed here. The principles which are related to metamorphic petrology are applied to a number of orogenic events through geologic time, and modern advances in research in metamorphic petrology are then explored.

Igneous and Metamorphic Petrology 

Igneous Petrology

In this section we are aware of the principles of Igneous and Metamorphic Petrology which features over 250 contributions that are from more than 100 earth scientists based from 18 countries, In the Encyclopedia of Igneous and Metamorphic Petrology studies the nature and genesis of these igneous rocks which have crystallized into the form of molten magma. They are the metamorphic rocks which are residues of re-crystallization that are associated with the increases in temperature and pressure, this is mainly at considerable depths hidden in the Earth’s crust. 

The entries which range from the alkaline rocks to the rocks which are known as zeolite facies – provide other information on the concept of mineralogical, chemical and textural characters of these rock types.

Metamorphic petrology also covers the chemical and also the physical work which is done in natural systems, which is in response to changing the physical conditions. Petrogenetic is the processes like the recrystallization, continuous and the discontinuous reactions, which are mixed volatile reactions and the deformation is addressed thereby.

India is one of the most diverse countries in the world. Along with that, it’s the second most populated country in the world, with a population of close to 1.5 billion. With a population so massive, India contributes nearly 17% of the global population. In other words, every 6th person in the world is an Indian. The people are distributed randomly and unevenly in an area of 3.28 million square kilometres, with densely-populated cities to sparsely populated small villages and hilly terrain areas.

Distribution of Population in India

Among the 28 States and 8 Union Territories of India, Uttar Pradesh is the most populated state with a staggering population of 166 million people. That is a population more than most countries in the world. Meanwhile, Sikkim and Lakshadeep have the lowest population of 0.5 million and 60000, respectively. The top five most populated States in India are Uttar Pradesh, West Bengal, Bihar, Maharashtra, and Andhra Pradesh. About half of India’s population is concentrated in these states only.

Population Density

Population Density of a place is defined as the number of people living in that particular place divided by the geographical area. In other words, it’s the number of people living per unit area. It depends on several factors. The most prominent characteristic is the geographic location. People generally avoid hilly terrains due to harsh climatic conditions and their inability to sustain agriculture. Hence states like Himachal Pradesh and Assam are sparsely populated. On the other hand, river valleys, coastal areas, and the Northern plains are highly fertile and densely populated regions.

Population Growth and Decline: Population Change

The annual Growth rate is defined as the increase in people per every hundred people per year. For example, if a city’s population is 100 and 5 more people are added in a year, taking the population to 105, the Annual Growth rate would be 5%. The population of an area is not always destined to increase, and it may decrease as well. The population change, which includes increase and decreases, can be due to various factors like Birth, Death, and Migration. In India, the Birth rate has always been more than the death rate despite the major natural calamities. This is a significant reason behind the ever-increasing Indian population. The third reason for the population after birth and death is Migration. Migration includes two things, immigration, and emigration. Immigration is the number of people coming to an area, while emigration is the number of people leaving an area. Understandably, the population increases when immigration is more than emigration and decreases when the opposite happens.

Characteristics of Indian Population

After we have understood the various factors influencing the population, let us understand the characteristics of the Indian population.

Age Composition

The age composition of a country determines the social and economic structure. Age composition is nothing but the population of people of different age groups living in a country. The entire population is divided into three categories, children(below 15years), working-class(15-59 years), aged(59+ years). The more the working-class population in a country, the more economically stable and developing the country is. Additionally, a high child population implies a strong future ahead for the country.

Sex Ratio

The sex ratio is nothing but the number of females in the country for every 1000 males. This gives an idea of the proportion of females and males in the country and its culture and gender diversity. Unfortunately, India is one of the countries with the lowest sex ratio. But some of the states in India are those having more females than males. As per the 2011 census report in Kerala, the sex ratio is 964, whereas in union territory Puducherry, the ratio is 967 and regarded as the highest in India. In Haryana, the ratio is 877, and this is the lowest figure in India. Considering the entire population, the sex ratio of the country is 943.

Literacy Rate

The literacy rate is a vital part of the Population. It mainly determines the total development or the economic structure of the country. If the country has more literacy rate, it is more developed as the people do something innovative and contribute more towards GDP. As per the 2011 census, a standard was set for the literacy rate. According to this, if a person is more than seven years old and can read and write thoroughly, that particular person is called literate and counted in total literacy percentage. As per the 2011 census report, the literacy rate of India was 74.04%. Well, this number varies in different states. In Kerala, the literacy rate is around 96.2% and is regarded as the highest literary state in India, while Bihar accounts for the lowest literacy rate.

The changes in structuring and gathering of minerals which occur during burial and heating are known as prograde metamorphism, whereas those that form during uplift and cooling of a rock indicate retrograde metamorphism.

The Metamorphic rocks are evolved due to the transformation of other rocks under high heat and high pressure. The process of physical and chemical change of rocks is known as metamorphism. The word metamorphism is taken from the Greek for “change of form.

Below is an overview of retrograde metamorphism of rocks

Formation of Metamorphic Rocks

Before metamorphism; the original rock or protolith may have been formed by solidification of a melt (and igneous protolith) or the lithification of discrete grains derived from weathering (a sedimentary protolith). Metamorphic rocks are formed in 3 ways. Three types of metamorphism are:

1. Contact Metamorphism – It takes place when magma intrudes the cooler upper body of the crust and makes a contact to create a metamorphic rock.

2. Regional Metamorphism – It occurs over broad areas of the crust, which have already undergone deformation over a period of time due to some event; resulting in mountain belts that have been exposed to extreme atmospheric conditions. Hydrostatic pressure, stress, temperature coupled with chemical activities.

3.Dynamic Metamorphism – Also known as cataclasis occurs from mechanical deformation with little long-term temperature change. Dynamic Metamorphism also occurs because of mountain-building.

Types of Metamorphic Rocks

Generally, the metamorphic rocks are considered to be the hardest as they undergo tremendous environmental changes. Metamorphic rocks can basically be found in two forms ie; foliated and nonfoliated. A few examples of foliated metamorphic rocks include Slate, schist, migmatite, phyllite, and gneiss. Marble and quartzite are nonfoliated metamorphic rocks.

The metamorphic grade is a general term we used to describe the temperature at which metamorphism occurs. Owing to the different temperature and pressure, different kinds of metamorphic rocks are formed with varied characteristics and would be significantly different from each other.

Sub-Division of Metamorphism

Metamorphism is divided into prograde and retrograde metamorphism. Prograde metamorphism is a change of mineral composition with increased heat and pressure. Whereas, Retrograde metamorphism is a change in mineral assemblage and its composition that occurs during uplift (releasing of pressure) and cooling (decreasing temperature) to reconstitute a rock, which is a rare process.

Factors That Prevent Retrograde Metamorphism

If retrograde metamorphism were a common process, then upon uplift and exposing metamorphic rocks would progressively return to mineral components stable at lower pressures and temperatures. Only 3 factors prevent retrograde metamorphism, two of which involve the fluid phase.

1. Faster chemical reactions due to high temperature

2. During the process of prograde metamorphism, a fluid phase vanishes as an outcome of the devolatilization reactions.

3. The fluid phase helps to catalyse chemical reactions.

Theory of Retrograde Metamorphism

The theory of Retrograde Metamorphism was first conceptualised by Becke in 1909 and later elaborated by Harker. It was Backe’s contention that these phyllites had formed from gneisses of the deep-seated zones of the earth’s crust as a result of a reversal of normal, progressive, regional metamorphism. To these rocks, he applied the term; diaphthorities.

As per Harker, “Retrograde Metamorphism”, “Retrogressive Metamorphism”, “Regressive Metamorphosis” and “Diaphthorities” are the terms which are not appropriately used as per their original connotations and confuse the readers as the precise definition of “Retrograde Metamorphism” has not been explained by most of the writers.

Process of Retrograde Metamorphism

Retrograde metamorphism in many ways is a reverse of prograde metamorphism. Retrograde reactions are usually very slow and may not impact only some parts of the rock and not the complete rock.

There are two factors that mitigate against complete retrogression of metamorphic rocks during their return.

a. Efficient removal of the water and carbon dioxide released.

b. Metamorphic reactions do not typically operate in reverse during cooling and reaction rates are increased by rising temperatures.

All the metamorphic rocks would eventually undergo a change in the mineral composition (Prograde metamorphism or retrograde) under the atmospheric conditions present near the earth. This process of change is called weathering.