Category: Geography Notes PPT

Ocean acidification means a significant reduction in the pH level of the ocean over an extended course of time, caused principally by the uptake of carbon dioxide (CO₂) from the atmosphere. The pH refers to the potential or power of hydrogen. 

For over 200 years, or since the industrial revolution, the consolidation of carbon dioxide (CO₂) in the atmosphere has risen due to the burning of fossil fuels and change in land usage. The ocean absorbs about 30% of the CO₂ which is released in the atmosphere, and as levels of atmospheric CO₂ rises, so do the levels in the ocean.

Ocean Acidification Causes

Carbon is the criminal. The main culprit behind the acidification in the ocean is the extra amount of carbon dioxide that humans have given rise to in the atmosphere by cutting down forests, burning fossil fuels, and other actions.

Impacts of Ocean Acidity on Ocean Life

The pH of the ocean varies within limits as an outcome of natural processes, and ocean organisms are well-adapted to sustain the alterations that they normally undergo. Some marine species could be able to adapt to more extreme changes—however many would suffer, and there will possibly be extinctions. A more acidic ocean won’t dismantle all marine life in the sea, but the increase in seawater acidity content by 30% that we have observed is already affecting some ocean organisms. Let’s check out how it impacts the atmosphere and habitat:

1. Coral Reefs: 

Acidification may constrain the growth of coral by corroding pre-existing coral skeletons while concurrently slowing the growth of new ones and the weaker reefs that out-turn will be more susceptible to erosion. This erosion will not only emerge from storm waves, but also from animals that pierce into or eat coral.

2. Mussels, Oysters, Urchins, And Starfish: 

Some of the serious effects on these organisms outstrip adult shell-building, but Mussels’ byssal threads, with which they hold on to rocks in the pounding surf, can’t cling well in acidic water. For time being, oyster larvae fail to even start growing their shells. In the initial 48 hours of life, oyster larvae experience a huge growth spurt, building their shells rapidly so they can begin feeding. But the more acidic seawater eats away at their shells before they can develop; this has already induced massive oyster die-offs in the U.S. Pacific Northwest.

3. Zooplankton (Tiny Drifting Animals): 

Oceans consist of the highest amount of actively cycled carbon in the world and are also quite significant in storing carbon. When shelled zooplanktons die and sink down to the seafloor, they carry their calcium carbonate shells along with them, which are accumulated as rock or sediment and stored for the future. This is a crucial way that carbon dioxide is eliminated from the atmosphere, decelerating the rise in temperature induced by the greenhouse effect. 

These little organisms reproduce rapidly so that they may be able to adapt to acidity comparatively better than massive, slow-reproducing animals. However, experiments in the carbon dioxide seeps (where pH scale is naturally low) have discovered that foraminifera are not capable of handling higher acidity very well, as their shells dissolve very quickly. One study even foresees that foraminifera from tropical areas will become extinct by the end of the century.

4. Plants and Algae: 

Plants and various algae may blossom under acidic conditions. These organisms form their energy by combining sunlight and carbon dioxide—thus extra carbon dioxide in the water doesn’t harm them, but helps.

How Carbon Dioxide Kills Ocean Life?

Acidification meaning must be clear to you by now. The oceans have always both absorbed and discharged carbon dioxide, commuting the carbon back and forth from the atmosphere to water. But the exchange happened gradually, usually over thousands or tens of thousands of years.

Humans have interrupted that slow exchange. In the middle of the 18th century, humans, since the start of the Industrial Revolution, humans have added some 400 billion tons of carbon to the environment. That’s a byproduct of the massive amounts of fossil fuels we burned for energy, the trees that have been cut down, the cement we’ve generated, and more.

The majority of carbon, in the gas, creates carbon dioxide (CO₂), which remains in the atmosphere, where it traps heat and bestows planetary warming. But every year, the ocean sucks up about 25% of all the extra CO₂.

In order to understand what the Paleocene Eocene Thermal Maximum is, it is necessary to know about the Paleocene Eocene Epochs. 

Paleocene Epoch: The Paleocene Epoch is a geological time-scale that lasted from 66 to 56 million years ago. This was the time of dinosaurs and is famously marked for the extinction event of non-avian dinosaurs because of an asteroid impact, along with 75% of living species. The end of the epoch is marked by Paleocene Eocene Thermal Maximum.

Eocene Epoch: The Eocene Epoch is the geological time-scale that lasted from 56 million years ago i.e. the end of Paleocene Epoch, to 33.9 million years ago i.e. the beginning of Oligocene Epoch. 

Paleocene Eocene Thermal Maximum (PETM): It is a time period in the geological history of Earth when there was globally more than 5℃ – 8℃ temperature rise across the Earth. It is also known as initial Eocene Thermal Maximum 1 (ETM).

Characterising Paleocene Eocene Thermal Maximum

Towards the end of the Paleocene Epoch and the beginning of Eocene Epoch there was a temperature rise of 5℃ – 8℃ globally. The temperature rise was observed across the event which lasted from 20,000 to 50,000 years. The entire period was warm and has been known for increased temperature for approximately 2,00,000 years. The exact age and the duration of the event is arguable but has been estimated to have begun and occurred around 55 million years ago, the boundary of Paleocene Eocene Epochs. 

The starting of the Eocene Thermal Maximum has been said to have occurred because of the increased volcanic activity or volcanism and the uplift that is associated with the North Atlantic Igneous Province (a large area in north atlantic region centered on iceland) that lead to significant changes in the carbon cycle of Earth which in turn lead to the global temperature rise of 5℃ – 8℃. The period is marked by the high decrease observed in the amount of 13C stable isotope of carbon all over the world. This led to the decrease of the 13C/12C ratio of marine and terrestrial carbonates and organic carbon. From the combined data obtained of isotopes, 𝛿13C, 𝛿11B, 𝛿18O it is observed that approximately 12,000 Gigatonnes of carbon was released into the atmosphere for a period of 50,000 years, with 0.24 Gigatonnes per year average. This means that at least 44,000 Gigatonnes of carbon dioxide responsible for global warming was released into the atmosphere due to extreme geological events. 

Changes During Paleocene Eocene Thermal Maximum

Apart from the rise in the carbon dioxide levels due to volcanic activity throughout the globe, there were several other changes that led to drastic changes in the animal and plant life both on land and in the ocean. These other changes are observed by the stratigraphic sections of the rock from this period that reveal numerous changes. 

Many of the fossil records show major and significant changes and turnovers in the profile of organisms over the Earth. Examples include the changes in the marine realm, there was a mass extinction of the benthic foraminifera and a global expansion of the subtropical dinoflagellates. There was also an appearance of increase in the population of the planktic foraminifera and calcareous nannofossils. All these were observed during the beginning stages of PETM. The mammalian sea-animals also sprung and got the advantage of the wide resources available to them. There is trace evidence of the impacts of the rising temperatures and increase in the carbon dioxide content of the atmosphere leading to decrease in the amount of dissolved oxygen in the oceanic water, in turn affecting the life of deep-sea species. But one specific development is known about the increase in the population of heavily calcified algae and weakly calcified forams which occurred due to acidification of the water bodies. 

Similar things were observed on land as well. The modern mammalian order including the primates suddenly appeared in the continents of Europe and North America. There is a widespread migration known for Asian mammals towards the north, because of the prevailing humid conditions. Also, the population of the mammalian species increased many fold during this time. Increase in the levels of CO2 may have facilitated the physical trait of dwarfing in turn encouraging speciation. Some 13,000 to 22,000 years after the initiation of the PETM, many animals belonging to the mammalian orders such as Artioctyla, horses and primates spread around the globe. The deposition of the sediments also changed the outcrops and many of the drill cores that spanned the time interval. 

Recovery and Comparison With Current Global Warming

In order to understand the climatic conditions of olden times such as the Paleocene Epoch about 56 million years ago, climate proxies are used. Climate proxies are the preserved physical characteristics of the past, that show us visible proof and help us understand and reconstruct the possible behaviour of the climate during that time. From such climate proxies it is stated that the recovery might have occurred with an increase in the biological productivity. Increased biological productivity might have helped to form an appropriate carbon cycle and would have transported carbon to the deep oceans. This would have occurred with higher global temperatures, higher CO2 levels, and increased supply of nutrients from continental weathering (because of higher temperatures and higher rainfall) and depositions from volcanic eruptions. 

The evidence of such an increased biological productivity leading to recovery is given by the bio-concentrated barium. But there is a possibility that the bio-concentrated barium may have been because of barium dissolved with methane which is a later event and a possibly recent event. The diversification of the plant species, especially the ones near the shores indicate that productivity increase in such areas as the weather there would have been warm and fertilized run-off which had outweighed the reduction of productivity in the deep ocean areas. 

The initial Eocene Thermal Maximum 1 has been the centre point of investigation as an analog to understand the effects of global warming. Although, the adverse effects of global warming and the reason are much worse today as compared to the period of PETM. Humans emit 10 Gigatonnes of carbon dioxide into the atmosphere as compared to the 0.24 Gigatonnes produced per year during PETM. Another difference is that during the PETM the planet was ice-free and the effects of global warming on such areas isn’t clear but the effects will be known by today as the water level rises all over the world due to melting glaciers and melting ice at the poles. Also, it is said that the cause of the PETM were the widespread volcanic eruptions and the K-T extinction event but they are still debatable and the cause, details and the overall significance of the event remain uncertain. It has already been established that the peak carbon addition to the ocean-atmosphere system during the PETM is much slower and lower than the carbon addition by human activities. Also, it is said that the current methane emission regime is similar to the one that occurred during the PETM. Thus, it is understandable what such emissions can lead to and the changes and impact that it can have on the species life and nature. 

On Earth, there are several types of geographical features of landmasses. Examples include various kinds of landforms mountains, plains, plateaus that are created over eras of geological history. A plateau landform is a landform made up of an extensive area of flat land which is usually bounded by a steep slope on all sides. Sometimes it can be enclosed by elevated portions of land such as hills and mountains. 

The criteria which determine the physical features of a plateau are relatively low relief structures and some altitude. Even though both the mountains and plateaus are higher than their surrounding areas, the significant difference between them is that the mountains keep on elevating continuously forming an inverted cone-like structure whereas the plateaus are flat-lands with some altitude.

Definition of Plateau Landform

Mentioned above was a brief description of the plateau landform. Based on the given information the plateau landform definition can be stated as follows:

Plateau: In geology and physical geography, a plateau, also known as a high plain or a flatland, is an area of elevated or raised terrains as compared to all the surrounding sides or at least one side, which are completely almost flat surfaces at the top. 

A plateau can be surrounded by deep hills on all or at least one side. This is because they are formed because of a number of processes such as volcanic magma, lava extrusions, or erosions due to the water and the glaciers. Usually, the top of the plateaus is a wide flat area of landmass but some of them can be short flatlands as well. Also, depending on their surrounding or neighbouring environment plateau landforms can be classified into intermontane, piedmont or continental. Given below is the picture of plateau landform:

Geological and Geographical Features of Plateau Landform

As the plateau landforms are formed like the mountain landforms owing to the volcanic activity, lava extrusions, or erosions due to water and glaciers, these landforms are differentiated and classified into various types depending on the geological and geographical features, surrounding or creating them.

Classification of Plateau Landform Based on the Formation

Depending on the basis of the geologic activity, the plateau landform is distributed into the following types:

Volcanic plateau is also a lava plateau landform. Hence, the lava plateau definition is any flat-land landform produced due to any volcanic activity. The lava plateau formation is also caused by the volcanic upwelling of magma or lava extrusion. The magma rising from the mantle causes the ground to swell upward. Because of this large, flat areas of rock get uplifted to form a plateau. In the extrusion phenomenon, the lava spreads outward from cracks and weak areas in the crust, resulting in the formation of a lava plateau.

When the glaciers from the mountain tops erode away, they leave huge flat areas between the mountain ranges which forms the plateau. Rivers are also responsible for creating large flat-lands by eroding huge surfaces and broken by deep narrow valleys giving dissected plateau landform. Computer modelling studies provide some information that plateau landform can also be formed as a result of the feedback between tectonic plates. The Appalachian plateau landforms are an example of the dissected plateau landforms. 

Classification of Plateau Landform Based on Surrounding Environment

Given below are the classifications of the plateaus based upon the surrounding environment:

• Intermontane plateaus are the ones that are classified based on the surrounding mountains. The plateaus are the flat-lands with bordering mountains. The Tibetan plateau is one such example.

• Lava plateau or volcanic plateau is the one surrounded by volcanic activity centres as clear from the definition of lava plateau. The magma comes out of the fixtures and cracks in the crust and leads to lava plateau formation. The Deccan Plateau in India is one such lava plateau. Other such examples, including Antrim plateau in Netherland, Columbia plateau landforms in the United States are examples of the lava plateau.

• Piedmont plateau landforms are the ones that are surrounded by the mountains on one side and by a plain or a sea on the other. The Piedmont plateau in the Eastern United States falling between the Appalachian Mountains and the Atlantic coastal plain is one such example. 

• The last one is the continental plateau landforms. These are the ones surrounded by oceans or plains, forming away from the mountains. One such example is the Antarctic Plateau in East Antarctica.

Geographic Distribution of the Plateau Landforms

There are many different types of plateaus found all over the world across all the continents. The plateaus formed owing to the thermal expansion of the lithosphere are associated with the hotspots. Examples include Yellowstone plateau in the USA, the Massif Central in France, Ethiopian Plateau in Africa. 

The plateaus formed due to the crustal shortening and internal natural drainage lie within major mountain ranges and generally in dry climates. They are usually found in North Africa, Turkey, Iran and Tibet at the points of collision of the African, Arabian, and Indian continental landmasses with the Eurasian continent. The Altiplano is lying between the Cordillera Occidental which is composed of volcanoes and the Cordillera Oriental beneath which the Brazilian shield is being thrust. These areas correspond to the period of Cenozoic time when they underwent crustal shortening. In each of the cases, the surface of the plateau flat-lands includes strongly deformed pre-Cenozoic rocks and the flat-land sediment. 

The Serra Geral is mostly formed of basalt rocks, cap a plateau on the Atlantic coast of Brazil. This plateau landform erupted about 135 million years ago before the separation of the African and South American tectonic plates. In North America, the Columbia plateau landforms, along the Columbia river which is composed of basalts, ejected over the same hotspot which underlies the Yellowstone landform today. These are some of the most unique forms of lava plateau landmasses.

Fun Fact

One of the interesting facts about plateau landform is that the origin of some of them is unknown. Examples include the Iberian Peninsula and north-central Mexico which show a topography that is largely high and flat. The Mexican crustal shortening occurred during the Late Cretaceous and Early Cenozoic period. Another example of crustal shortening is some parts of Spain formed during the Cenozoic period. Even though there are these high elevations at both the plateau landforms they are not supported by a thick crust. Instead, they are probably underlined by a hot mantle, until conclusively proved otherwise.  

Red soil is a type of soil that is characterized by its reddish color. It is also known as Terra Rossa, which is Italian for “red earth.” Red soil is found in areas where the climate is warm and humid, such as in tropical and subtropical regions. It is usually fertile and good for agriculture. Red soil is formed from the weathering of rocks that contain iron oxides. The most common type of rock that contributes to red soil is called basalt. The process of weathering breaks down the basalt into smaller pieces, and then the smaller pieces are broken down further into silt and clay. 

The iron oxides in the rocks are what give the soil its reddish colour. The fertility of red soil is due to its high levels of organic matter and nutrients, such as nitrogen, phosphorus, and potassium. These nutrients are essential for plant growth. Red soil is also well-drained, which is important for agriculture. Red soil helps plants to grow better. The red colour of the soil comes from the iron oxide in the rocks. The fertility of red soil is due to its high levels of organic matter and nutrients. Red soil is well-drained, which is important for agriculture.

Soil Types

Characteristics of Red Soil

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There are many different types of soil, each with its unique characteristics. One way of classifying soils is by their colour. There are three main categories based on their colours: red soil, black soil, and alluvial soil. These three colours reflect the types of rocks from which they formed. All soils, no matter what color, contain water and air as well as organic materials such as plants and animal remains. Red soil is a type of soil that is characterized by its reddish colour. It is also known as Terra Rossa, which is Italian for “red earth.” Red soil is found in areas where the climate is warm and humid, such as in tropical and subtropical regions. It is usually fertile and good for agriculture. Red soil is formed from the weathering of rocks that contain iron oxides. 

The most common type of rock that contributes to red soil is called basalt. The process of weathering breaks down the basalt into smaller pieces, and then the smaller pieces are broken down further into silt and clay. The iron oxides in the rocks are what give the soil its reddish colour. The fertility of red soil is due to its high levels of organic matter and nutrients, such as nitrogen, phosphorus, and potassium. These nutrients are essential for plant growth. Red soil is also well-drained, which is important for agriculture.

What is Red Soil?

Red soil is considered to be soil that generally develops in warm temperatures and moist climates. They are developed under deciduous conditions and are generally found in mixed forests. They have thin organic and mineral layers overlying a yellowish-brown leached layer which can be seen resting on an illuvial red layer. Red soils are generally formed from sedimentary rocks which are rich in iron. These soils are not suitable for cultivating because they are low in nutrients, making them poor growing soil. We learn more about red soil on this page. 

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Types of Red Soil

Red soil for plants is available in various types. Let’s discuss some of these soils:-

Red Clay Soil

Red clay soil is commonly known as Ultisols. They come under the twelve soil orders, which were introduced by the United States Department of Agriculture Soil Taxonomy. These are considered to be mineral soils that do not have any calcareous material present in them. These soils have 10% less weatherable minerals in the extremely top layer and also have a saturation base of less than 35% throughout the soil. You can find ultisols in tropical regions that experience humid temperatures. They are found in regions such as Africa, Asia, and South America.

Red Loam Soil

According to researchers, this red loam soil is formed by the decomposition of granite, gneiss charnockite, and diorite rocks. These soils are found to be cloddy, porous, and deficient in concretionary materials. These soils don’t have enough nitrogen, phosphorus, and various organic materials. These soils contain a sufficient amount of potash. Red sandy loam soil is not good for agriculture because they are not that fertile. These soils are mainly found in Andhra Pradesh, Telangana, Karnataka, Eastern Tamil Nadu, Orissa, Jharkhand, Madhya Pradesh, and many other states.

Red Laterite Soil

Red laterite soil is a type of soil that is considered as a brick also. This type of soil is rich in iron and aluminium. They are usually formed in hot and wet tropical areas. These soils are red because of the iron oxide content; this mineral gave the soil a red colouration. These soils are developed when the underlying parent rock starts intensive and prolonged weathering. The areas where you can find the majority of laterites are situated between the tropics of cancer and the tropics of Capricorn. Laterites are considered to be a source of aluminium ores.

Red Yellow Soil

Red, yellow soil is a type of soil that is formed under broad-leaved forests. Generally, these types of forests are found in humid subtropical regions. These soils are believed to have an acid reaction within them. The humus content in these soils is significantly less as compared to other soils. The red-yellow colour of the soil is due to the presence of ferric hydroxide in the soil. As per many researchers, the thickness of these soil ranges between 30-70 cm. The thickness of these soils varies in different places. One can find these types of soil in China, the Southern United States, southeastern Australia, and New Zealand.

Red Sandy Soil

This type of soil is light, warm, and dry. They tend to have acidic content in them and are low in nutrients. Red sandy soil is generally known as light soil because of a high proportion of sand and less clay (clay always weighs more than sand). These soils are easy to work with because they have water drainage that is quick most of the time. As these soils don’t have a sufficient amount of nutrients and organic matters; thus, these soils are not preferable for cultivation. In simple words, these soils are poor fertility soils. These soils are found in regions where the rainfall is arid, semi-arid, and humid.

Red Gravel Soil

This type of soil is found with 20% gravel in the topmost layer; sometimes, it is more than 20% also. These gravels are distributed and scattered throughout the soil. This type of soil affects the growth of plants because of the gravel content. These gravels act as a barrier to plant growth. Plants also can’t develop in these types of soils because they can’t get enough nutrients from red gravel soil. This soil lacks fertilizers. The gravels present in the soil penetrate the roots causing damage to the plants. So it is clear that agriculture is not a good choice on these soils.

Gravels present in the soil help to break up the clay soil and create air pockets where oxygen is stored. Plants that can grow in this type of soil are perennials, ornamental grasses, and herbs.

Use of Red Soil in Agriculture

Red soil is often used for agriculture because it is fertile and well-drained. It is especially beneficial for crops that require a lot of nutrients, such as bananas and sugarcane. Red soil can also be used to grow other types of crops, such as vegetables and fruits.

Following are the types of crops that can be grown in red soil:

• Bananas

• Sugarcane

• Fruits like oranges, pineapples, and avocados

• Vegetables like carrots, potatoes, celery, beets, spinach, beans, peas, and corn. 

Red soil is used to grow these crops because it has lots of nutrients needed for plant growth, such as nitrogen, phosphorus, and potassium. The soil is also well-drained, which is important for agriculture.

From our context, we understand that red soil is mostly loamy and hence cannot retain water like the black soil. However, with the proper use of fertilizers and irrigation techniques, red soil can give good yield of cotton, wheat, rice, pulses, millets, tobacco, oil seeds, potatoes and fruits.

Sandstone is a type of rocks which are neither too rough nor too fine and whose origin can be any type of rocks namely Igneous or metamorphic but someone asked you if sandstone is an example of which type of rock then the answer is sedimentary rocks. The classification of the sandstone can be done basically on two important factors i.e. texture as well as mineral composition. The two famous schemes of classifications are given by Robert H. Dott (1964) who was an American Petrologist and whose scheme was based on the concepts of P.D. Krynine & F.J. Pettijohn whereas another scheme was given by R.L. Folk (1974). The external structure because of the depositional activities as well as the composition of the sandstones is a very important factor to understand these rocks. In this article, we will learn what sandstones are and the types of sandstone in detail which will be helpful to understand this topic in Geography, Geology, Petrology, etc.

Introduction

If you are asked, sandstone is an example of which type of rock; then the answer will be those rocks whose large distribution is found on the Earth i.e. sedimentary rocks. The layers of sandstone layers can be found in each part and around 20 – 25% of these layers are found. It is composed of sediment materials that are formed from other processes such as weathering & erosion of other rocks. The rough materials will be smaller whereas smoother materials are those which depend on how far those materials are being transported. The size rate of sandstones lies between 0.0625mm ( 1/16 ) until 2 mm. The size of the material, which is called sand that can be very rough until the smallest material is very fine. These have a moderate grain size, neither too rough nor too fine. Therefore, it is easier to form the grain size of this sand in order to transport & to make them deposited. This is the reason sand deposits or sandstone can be found almost everywhere. We can know more about sandstone by its classification and when we know about quartz, feldspar, & rock fragments that are present in the sandstone, names of the sandstones can be known by their classification as well.  

Types of Sandstone

The sandstone classification scheme of Dott (1964) is one of the various classification schemes which is used by Geologists for the classification of sandstones. His scheme is considered as a modification of Gilbert’s classification of silicate sandstones whereas it also incorporates dual textural & compositional maturity concepts of R.L. Folk in only one classification system. This is based on the mineralogy of framework grains as well as on the type of matrix present in between them. In this, he has set the boundary at a 15% matrix between arenite & wackes. Besides these, he also breaks up various types of framework grains into 3 categories that can be found in the sandstone i.e quartz, feldspar as well as Lithic grains.  The types are mentioned below in detail:

Arenites

Arenites are a type of sedimentary rocks or basically, we can say a type of sandstone whose grain size lies between 0.0625 mm and 2 mm as well as it has also less than 15% clay matrix.

These arenites are those that contain more than 90% detrital quartz and these can also include small amounts of other grains such as feldspar, lithic fragments, etc. along with resistant grains like chert & minerals in the ZTR index. Quartz sandstone is simply rich in quartz content along with other material. Extensive weathering that occurs during or before the transport removed everything but quartz grains are more stable which results in this sandstone. Quartz Arenites are also known as ortho quartzites and sometimes also referred to as quartzose sandstone as well. These arenites are considered as the most possible & mature sedimentary rocks which are often known as ultra/super mature along with both textural as well as compositional maturity & are generally cemented by silica. The two primary environments ( usually sedimentary depositional areas ) that produce these types of sandstones are beaches or upper shoreface as well as aeolian processes. Mostly these sediments are being eroded and reworked over & over which leads to the formation of a new kind of sediments as well as rocks which are generally referred to as multicycle sand.

These arenites are those sandstones that contain less than 90% of quartz but more of feldspar. Here, feldspar is the main component & contains other unstable components as well such as lithic fragments & other minerals such as mica and heavy minerals, etc. These types of sandstones are having a pink/red colour basically due to the potassium feldspar or iron oxide as well as light grey to white and medium to coarse-grained minerals. If we talk about its texture maturity, it is usually immature whereas in terms of sorting of grains it is moderate to poorly sorted. These arenites can be judged from the sediments structure and generally occur on the craton or the continental shelf as well as sedimentary basins. The minerals contained in these type of sandstones are mostly associated with the form of plagioclase feldspar which is generally derived from quartz diorite as well as volcanic rock. The feldspar in this sandstone associated with arid to cold climates when the process of weathering is reduced.

These are the arenites that consist of unstable lithic fragments such as volcanic & metamorphic rocks, and also includes chert etc. These may consist of less than 90% quartz grain whereas rock fragments are considered to be more unstable than feldspar. If we talk about the colour of these sandstones they are usually light grey & dark grey whereas if we talk about sorting of grains, that are poorly sorted and textural maturity is immature to sub mature. They are generally formed under the environment where unstable material is found and may occur on marine turbidite sediments & fluvial and are generally found on the alluvial fan but can also be found in the Foreland basin in the fold-thrust belt.

Wackes Sandstone

Wackes are that sandstone that consists of more than 15% of the fine-grained clay matrix. It is also known as dirty sandstone which is a sedimentary rock with sand-sized grains of 0.063mm to 2 mm which is generally composed of rock fragments of a wide range of minerals such as pyroxenes, amphiboles, feldspars, and quartz, etc. and generally are poor sorted and angular grains. Here, the matrix has appreciable amounts of clay minerals which may constitute up to 50 %  of the volume and of the clay minerals, chlorite & biotite are more abundant as compared to muscovite & illite whereas kaolinite is absent. Thus the abundant matrix leads to binding the grains strongly which results in forming relatively hard rock. Here, quartz wackes are uncommon whereas feldspathic wackes are those type of sandstones that contain a greater than 15% matrix and Lithic wacke is a kind of sandstone which consists of greater than 15% the matrix.

Arkose Sandstone

Arkose sandstones are those sandstones that consists of at least or more than 25% of feldspar. It is rich in feldspar along with quartz as a dominant feature and also consists of mica or rock fragments as well. Sometimes it also consists of small amounts of calcite cement too. The grains are poorly rounded as well as less sorted as compared to pure quartz sandstone rock. If we talk about its colour, it is grey to reddish. The grains are usually fine to very coarse but end towards the coarser side in the scale. If we talk about fossils in these type of sandstones, they are considered rare. They are formed from the weathering of igneous or metamorphic rocks which are feldspar-rich as granitic rocks are most common, that is primarily composed of quartz & feldspar. These sediments need to be deposited rapidly in a cold/arid environment in such a way that feldspar does not undergo a significant process of chemical weathering as well as decomposition which is why this sandstone is considered texturally immature. 

Greywacke Sandstone

These types of sandstones are a heterogeneous mixture of lithic fragments as well as angular grains of quartz and feldspar & others. These are those sandstones that are characterized by quartz, feldspar, as well as small rock or lithic fragments which are generally poorly sorted angular grains & grey colour stone. It is a type of sedimentary rock that is usually found in Paleozoic strata and is texturally immature. They usually are grey, brown, yellow and black, dull-coloured rocks. These sandstones can contain a variety of minerals such as quartz, orthoclase & plagioclase feldspars, calcite, iron oxides & graphitic, carbonaceous matters, along with fragments of felsite, chert, slate, gneiss, various schists, & quartzite. These also can contain minerals such as biotite, chlorite, epidote, garnet, hornblende, tourmaline, augite, apatite,  sphene & pyrites. They are usually along with the edges of continental shelves or bottoms of trenches in the ocean as well as bases of mountain formational areas.  They can also be found along with shales as well as limestones and do not contain fossils but organic remains are common to be seen in the finer beds which are associated with them. 

Folk’s Classification

Robert L. Folk who was an American Petrologist as well as professor of the University of Texas gave a descriptive type of classification of sedimentary rocks. We will learn about Folk’s sandstone classification here. The basic philosophy of his says that the name of the rock must be able to convey as much information that is possible without knowing the complete description. With this respect, he proposed 5 vital sandstones properties use as defining characteristics and these properties are given below:

1. Grain size

2. Chemically Precipitated Cement, 

3. Textural Maturity

4. Miscellaneous transported constituents 

5. Clan designation

Folk said that in these properties, 2 and 4 are optional categories because they are not always observed in all type of sandstones whereas the other 3 are important ones and are always observed. The examples of rock names are mentioned below by using the Folk’s fivefold names:

He also used composition to classify the sandstones as similar to other schemes given before him on the basis of quartz (Q), feldspars (F), & rock fragments ( R) and the abundance of these components leads to help in knowing and classifying the sandstones and also on the basis of these, names are also being recognised. To understand and define the name of the clan, the sum of these components should be equal to 100% and those which do not fit are being disregarded. This QFR diagram helps a lot which is given below:

Another Classification

On the basis of hardness as well as colour, sandstones can be classified into four categories i.e grey sandstone, crystallized sandstone, hard sandstone & carbonate cemented sandstone.

• Gray Sandstones are those sandstones that consist of a mixture of the dark as well as light grains that lead to a rock that is grey in appearance and also which is generally not as hard as other types of sandstone. The hardness of this sandstone can be determined by breaking the core as well as by examining the broken core’s edges. The edge of this kind of sandstone core is relatively weak & crumbles with minor impact. Cores of other types of sandstone fracture with sharp edges and shatter into small as well as sharp fragments. 

• Crystallized Sandstones are those sandstones that are white as well as sugary and very hard & brittle in many cases. In some of the sandstones of this type, the grains can be seen as welded together as well as the cored rock has a smooth and polished appearance.

• Hard Sandstones are those sandstones whose characteristics lie in-between grey sandstones as well as crystallized sandstones. These are the sandstones that are light to dark grey in colour similar to grey sandstones and also hard & brittle similar to crystallized sandstones. Some hard sandstones examples are specimens 24L, 24R, & upper 28R (this is the broken butt end of the core). This butt end can be compared with lower 28L which is considered as a mixture of light grey, dark grey as well as tan grains whereas should also be compared to the pure white grains in the specimen of crystallized sandstone such as upper 28L.

• Carbonate Cemented Sandstones are considered to be relatively rare sandstones & generally they are grey in colour and these types of sandstones grains are held together with calcium or iron carbonate cement. This kind of cement make the hardcore & often create a brown or yellow stain on the core sides such as 26R. When 10% HCL i.e hydrochloric acid is applied to the rock then calcium carbonate cement can be detected by effervescence whereas if we talk about Iron carbonates, they do not effervesce but they do have a deep brown as well as a sugary appearance on the broken butt end of the core such as e.g. lower 28R.

Additional Information 

Sandstones can be simply classified on the basis of their colours. The images of different types of sandstones are given below:

Conclusion

Thus, to sum up in the end we can say that on the basis of composition as well as structure of the rocks, we can learn about the sandstones and types of sandstone. The major components include quartz, feldspar as well as Lithic fragments. These sandstones are formed of sediment material that can originate from any kind of rocks i.e. igneous rocks or metamorphic rocks. In this article, we learn about sandstone classification given by Dott in which we learned Arenites sandstone, Wackes Sandstone, Arkose Sandstone and Greywacke Sandstone in which we learned different mineral composition and about their texture and other aspects. This topic will be helpful in Geography, Geology, Petrology, etc. to understand sandstones.

We have learned about various types of sandstone through different classifications. Let’s have a look at important questions related to it:

Earthquakes can take place anywhere between the Earth’s surface and about 700 kilometres beneath the surface. With respect to scientific purposes, this earthquake depth range of 0 – 700 km is classified into 3 zones: shallow, deep and intermediate.

Shallow quakes usually are disposed to be more damaging than deeper quakes. Seismic waves from deep quakes travel farther to the surface, depleting energy along the way.

Shallow Earthquake Geology

Shallow earthquakes are between the depth of 0 and 70 km; intermediate earthquakes, 70 – 300 km (43-186miles) deep; and deep earthquakes, 300 – 700 km (186-434 miles) deep. Usually, the word “deep-focus earthquakes” is used for earthquakes deeper than 70 km. All earthquakes deeper than 70 km are within high slabs of lithosphere which are sinking into the Earth’s mantle.

Deep Foci Earthquakes

The proof for deep-focus earthquakes first emerged in 1922 by H.H. Turner of Oxford, England. Earlier on, all earthquakes had been contemplated to have shallow focal depths. The presence of deep-focus earthquakes was validated in 1931 from researches of the seismograms of various earthquakes, which in turn results in the building of travel-time curves for deep and intermediate earthquakes.

How To Determine A Shallow Intermediate And Deep Foci Earthquake

1. A Seismograph: It is so far the most reliable method to determine the focal depth of an earthquake. That being said, the most obvious evidence on a seismogram that a great earthquake has a deep focus is the small height or amplitude of the recorded surface waves and an effortlessly simple attribute of the P and S waves. Although the surface-wave pattern does usually signal that an earthquake is either shallow or may contain some depth, the most appropriate technique method of identifying the focal depth of an earthquake is to read a depth phase recorded on the seismogram.

2. sP Phase: Another seismic wave used to identify the focal depth is the sP phase – an S wave considered as a P wave from the Earth’s surface at a point quite close to the epicenter. This wave is recorded after the pP by about ½ of the pP-P time duration. The depth of an earthquake is identified from the sP phase in a similar way as the pP phase by using the correct travel-time curves or depth tables for sP. If the pP and sP waves are able to be identified on the seismogram, an accurate focal depth can be established.

Difference Between Shallow and Deep Earthquakes

Remember that an earthquake’s destructive force is dependent not only on its strength but also on location, depth and distance from the epicenter.

Quakes can hit upon near the surface or deep within the Earth. Most earthquakes take place at shallow depths, as acclaimed by the U.S. Geological Survey.

That being said, Italy’s quake was recorded to be very shallow, originating between 4 km and 10 km underground. The magnitude measurements also differed a bit between magnitude 6 and 6.2.

In comparison, the 6.8 measurement of the quake in Myanmar was deeper at 84 km, which is regarded to be an intermediate depth. With that, let’s see how shallow differentiate from deep quakes. Refer to the table below: