Category: Geography Notes PPT

The permanent magnetic properties left in rocks, sediments, and other natural elements of Earth during the time of its formation ages ago is known as remanent magnetism of natural remanent magnetization (NRM) or Palaeomagnetism.

Remanence comes from the word ‘remanent’, which means “that which remains”. It refers to the magnetization left in a substance, even when the induced magnetic field, that is, the external magnetic field, is removed. Due to remanent magnetization, information can be derived on the orientation of the Earth’s magnetic field through the magnetic memory in such natural elements. The Earth’s magnetic field plays a massive role in remanent magnetization.

Types of Natural Remanent Magnetization

Depending Upon the Natural Processes and Various Geomagnetic Signals, NRM Can be Classified Into Three Types –

1. Thermo-Remanent Magnetization (TRM): In this magnetization, the minerals in the rocks cool through their Curie temperature (Tc) in the presence of a magnetic field and acquire magnetic properties. 350°C to 700°C is the Curie temperature of most of the magnetic minerals. Above this temperature, the magnetic domain in the minerals aligns themselves by the Earth’s magnetic field. While cooling, these magnetic domains tend to freeze and acquire a remanence which is seen to be stable over time. Any further change in the Earth’s magnetic field cannot affect its remanence. It is generally observed in igneous rocks. For example, the iron-titanium oxides present in the igneous rocks are due to TRM.  

2. Chemical Remanent Magnetization (CRM): In this magnetization, the magnetic mineral surpasses a critical grain size, the blocking diameter, and grows through it. Until the required diameter is reached, the magnetic domain aligns itself with the Earth’s magnetic field. Once this diameter is crossed, the mineral grain locks the area and retains the remanence with stability for years to come. Ferromagnetic minerals undergo chemical reactions like,

1. The ferromagnetic minerals are precipitated from solution or,

2. The pre-existing minerals are altered to ferromagnetic minerals.

Chemical remanent magnetization occurs due to fluid migration, metamorphosis, or while intrusive rocks cool down slowly. When new magnetic minerals are formed, weathering processes also lead to CRM. For example, the formation of hematite from magnetite is due to CRM.

3. Detrital Remanent Magnetization (DRM): In this magnetization, the mineral grains having detrimental origins carry either thermo-remanent magnetization or chemical remanent magnetization, get deposited. As long as the grains are in contact with water or are in the topmost layer of the water-saturated sediment, the mineral grains keep on aligning themselves corresponding to the Earth’s magnetic field. Due to dewatering and compaction, the grains get locked mechanically, thereby preserving the area’s direction. DRM occurs when the sedimentary rocks are deposited and undergo lithification.

Conclusion

Remanent magnetism leads to various paleomagnetic studies like continental drift, that is, the moving away of continents and polar wandering, which means changing magnetic poles from time to time. It forms the basis of magnetostratigraphy and Palaeomagnetism. By determining the type of remanent magnetization, the history of the substance and its magnetization properties are figured out. 

The characteristic remanent magnetization, or the primary component, is the most critical part of remanence. It is acquired during the formation of the rock or substance. Any additional element formed accounts henceforth for the secondary component. To separate these two components, alternating field or thermal demagnetization is carried out, which strips away the natural remanent magnetization stepwise, and the characteristic magnetic element can be inferred.

Did You Know?

1. Magnets have their saturation levels. Once a magnet is saturated, it cannot be magnetized further. Hence, it cannot be made a stronger magnet. 

2. Most de-magnetized magnets can be re-magnetized to their original strength provided it has not been damaged under extreme heat. 

3. A magnetic field can be blocked by using a material that attracts a magnet. Depending upon the thickness of the material, magnetic fields can be either partially blocked or permanently barred.

4. It is possible to machine magnets. Rubber-type or flexible magnet materials are generally used for machining. Magnets are first unmagnetized using soft grinding wheels and diamond tools and then machined. Hard magnet materials require specialized techniques for machining and, so, are generally not preferred for machining.

Mother nature is lovable and commendable and everyone loves visiting different natural landforms and spending quality time. A sea arch is one among them formed due to erosion of rocks and sand. Due to variable rates of erosion, rocks collide and break to form minute particles that further dissolve in the seawater. These deposits take millions of years to deposit and form rock in different shapes. 

The best part of such disposition is the formation of new shapes and wave-cut platforms that cause hollow arches. Arches are formed by only roof cave remains within a rock. This naturally formed arch has a spectacular shape when ocean waves cause a rate in rock movement. Let us understand different ocean arch famous around the world with the major specification for each. 

What are Two Coastal Landforms Formed by the Sea Arches?

Usually, when the ocean waves erode a weaker section of rocks, it forms different shapes. Arches are the hollow caves formed due to continuous strike of wave and wind on the weaker section of rocks. The ocean waves hit soft rocks and form arches generally by the beachside. However, the large and massive arches are visited with a boat such that people can quickly drive watercraft through it. It is a coastal arch that naturally either forms a bridge shape and a door style to move from one section of the ocean to another. 

When huge ocean waves cut through rocks, it forms an upper portion like a bridge causing a hollow shape at the bottom. Thus the ocean arch forms a bridge where people usually love visiting being a tourist spot. These take millions of years to develop.

An arch forms causing hollow inside a rock forms a door style. People visit this spectacular arch ocean through boats that pass by from one side of the sea to another. 

Major Coastal Arch in the World

Huge ocean waves throughout the world erode weaker sections from the rocks which form arches. These natural processes take thousands of years but present beautiful scenery before us to spend some good time. There is a list below stating some major arches formed with ocean waves that are highly explored. 

It is one song gem arch that pinpoints on Donegal Coastline on Mullaghmullan Peninsula. Crohy Head Sea Arch is also famous as “Breeches” that defines the beautiful arch of Ireland. It is a perfect tourist spot which is 8 km west of Dungloe. It is around 25 meters high above the seashore.

Thousands of years took to form the greatest arch ocean of Ireland- The great Pollet Arch. The arch is situated at the north of Doagh Beg, the small village east of Fanad Peninsula. Thousands of tourists visit this arch to admire its beauty, moving 8 km west to Dungloe. 

Hōlei Sea Arch is a stark lava rock formation formed in the Pacific Ocean. It is about 27.4 meters high above seashores but supposed to have a limited life span. It is formed through differential erosion which is a cut from the cliff of ancient lava.

Some formations of the arch ocean take place due to the collision of lava with ocean water. One such example is the Pokowai Sea Arch which is formed with the collision of volcanic lava with pacific water. It is visible from Highway 31 around #29 Mile Marker. It is the safest spot to view and photograph. 

Kingsgate Bay Sea Arch is famous for its chalky rock structure, which is very pleasing. It is a sandy cave that seems to be life-threatening due to regular chalk falls, which have no protection from sea walls. However, people usually make family visits to Kingsgate Bay beach.  

Sea Arch Efficiency Units

Sea Arch Efficiency Units is an accommodation area to plan your stay near the seashore. You enjoy a standard quality stay with tons of services. People visit this place because of the comprehensive and pleasant ocean view for different small ocean arches. 

Any rock belonging to the group of sedimentary rocks that largely or entirely consists of silicon dioxide (SiO₂) in either of the two forms of quartz or amorphous silica and cristobalite is widely known as siliceous rock. In the class of the sedimentary rocks, the rocks that are formed as a result of the chemical precipitation are included in the category and examples of siliceous rock or silica rock, as it contains silicon dioxide as the main constituent. This siliceous sedimentary rock does not include any rock that is formed due to detrition or fragmentation.  

Classification of Rocks

Rocks are an integral part of the Earth’s crust. They are found in various shapes and sizes and are formed because of extreme geological processes going on in the crust of the Earth. They are majorly classified into three distinct types based on their formation processes and because of which they have their own unique characteristics. The classification of the rocks is as given below:

• Igneous Rocks: These are the types of rocks that are formed because of the solidification of the lava and the magma. This solidification can occur either below the surface or above the surface of the Earth. When it occurs below the surface, they are known as intrusive rocks and they are highly crystalline in feature whereas, when the solidification takes place above the surface, they are known as extrusive rocks and they are highly glassy in feature. Granite is one of the most common examples of igneous rocks in the world and it is an intrusive igneous rock. 

• Sedimentary Rocks: These are the rocks that are formed by the deposition of the fragments of rocks that weathered away due to exogenous processes. Mostly these rocks are formed at the river beds and the fragments are converted into rocks by compaction through a process called lithification. Limestone and silica rock is the most common form of sedimentary rocks found across the world. 

• Metamorphic Rocks: These are the kind of rocks that are formed because of the recrystallisation of the already existing rocks or fragments of rocks. There are various types of metamorphic rocks depending on the layer patterns. Gness is the best example that is found for metamorphic rocks. 

Sedimentary Rocks

As mentioned before the sedimentary rocks are the ones that are formed by the settling of the fragments of rocks that are subjected to different exogenous processes. These fragments are transported because of different processes such as the river flow and then over time are settled whenever suitable conditions are present. These deposits are converted into rocks through compaction in a process called lithification. The sedimentary rocks are classified into the following three major categories depending on their mode of formation:

I. Mechanically Formed: Examples include sandstone, limestone, shale, loess, etc.

II. Organically Formed: Examples include geyserite, chalk, limestone, coal, etc.  

III. Chemically Formed: Chert, limestone, halite, potash, etc. are examples of such types. 

As can be seen, most of the types of rock examples that are formed due to chemical precipitation are the ones that are mostly included in the category of siliceous sedimentary rocks. The siliceous chert is one of the most common forms of such rocks followed by the siliceous limestone which is also one of the pretty common examples of siliceous rocks. Other examples of siliceous sedimentary rocks, also known as silica rocks as these are rocks high in silica, include siliceous dolomite and calc gneiss. Some of the less common forms also include siliceous sandstones in places where they are formed because of certain chemical processes. Of the different types of available silicate mineral rocks, pyroxene rock is also one of them which is nothing but a type of inosilicate i.e. the chain silicates of silicate tetrahedra. 

These siliceous rocks are also not devoid of life. They are known to commonly form the silica-secreting organisms such as the radiolarians, diatoms or other types of sponges. These are one of the salient features of siliceous rocks.

Spinel is a gemstone which is available in a variety of colours. Pure Spinel stone is colourless, but impurities owe to the wide range of colours. The most valuable and preferable Spinel colour is a deep blood red, though blue spinel, black spinel, orange, yellow, pink, and purple colours also make fine gemstones. Spinel is commonly fluorescent in ultraviolet light, and this can be quite useful in its identification.

Spinel gem has a very limited supply, and thus gem and jewellery corporations don’t really market this gemstone to the consumer.

Factual Information About Spinel Stones

Black Spinel

Black spinel is actually the black variety, opaque known as Pleonast, contains a good luster and is used as a black gemstone.

Blue Spinel Meaning

Blue Spinel may have a resemblance to Zircon, sapphire and Topaz; while pink Spinel may imitate Morganite, pink Topaz, and pink Tourmaline. Purple Spinel stone may seem similar to Amethyst, though Spinel is generally bluer in tone than Amethyst.

Uses of Spinel Stone   

The deep-red variety, often popularized as a Ruby Spinel, is the most prized form and is actually an excellent alternative for Ruby. Spinel is also a significant pink gemstone.

Spinel naturally takes place in octahedral crystals. In well-carved red crystals, gem cutters may even out a crystal into an ideal octahedron. These “crystals” can occasionally be worn as a jewellery piece without any further faceting.

Though spinel has been used in jewellery since primitive times, this gemstone has gained the attention it deserves. Until the onset of modern gemology in the early 20th centuries, spinel was often determined as corundum, since they are found in the same mines. Nonetheless, these minerals are chemically isolated.

Following which, as jewellery cartels have had an understanding that some of their treasured rubies and sapphires were actually spinels, the stone’s reputation suffered significantly. Synthetic spinel is common and inexpensive. It has commonly been used as a stimulant for other gems in birthstones and class rings, which has impacted its public perception.

Natural spinel, however, is a rare and beautiful gem and is deeply appreciated on its natural beauty by educated consumers.

Varieties of Spinel

Spinel is Synthesized in Different Varieties and is Thus Known in Different Names as Below:-

Sources of Spinel Stones

Found mainly and traditionally in Afghanistan, Myanmar, and Sri Lanka, spinels have been unearthed recently in different sites in Australia, Africa, Russia, and Vietnam. These gems are generally not mined from the hard rock fundamental deposits in which they form but rather from alluvial or placer deposits where weathered material has been washed downstream.

Care of Spinel Stones

Spinels are relatively hard and durable gemstones and thus need no special cleaning or care instructions. One can refer to the Gemstone Jewellery Care and Cleaning Guide for various cleaning systems.

Trade Names of Spinel

In prehistoric times, red spinels were referred to as “balas rubies,” possibly after Balascia, the primitive name of a region now partitioned between Afghanistan and Tajikistan. This was a significant source of these gems.

Although spinels were determined as sapphires and rubies centuries ago, they are now recognized as distinct gemstones. Nowadays, Any references made to “spinel ruby” or “spinel sapphire” are erroneous. Red or green garnets are sometimes called “Arizona spinels.” Almandines are wrongly called “candy spinels.”

Fun Facts

• Spinel is most popular for its deep red variety that nearly simulates Ruby.

• Spinel stone and ruby can be very difficult to distinguish as they not only look identical but are found in the same localities too.

• Fine red Spinel is rarer than Ruby of equal colour.

• Spinel stone cost is still undervalued, with its cost about 30% lower than comparable Ruby.

• The scarcity of good Spinel stone is a major factor for its lack of popularity and demand. This further hampers Spinel from being a mainstream gemstone.

• Spinel is easily synthesized, thus can be created in any colour by adding impurities.

• Synthetic Spinel often imitates many other gemstones such as Diamond, Zircon and Sapphire.

• The enormous “Ruby” that creates the centerpiece of the royal crown of England (the Black Prince’s Ruby) was actually identified to be a Spinel.

Note: Because of the plenty of synthetic Spinels, one must always be careful to only buy this gemstone from trusted sellers/jewellers. Remember that, almost all colourless Spinel gemstones are synthetic, as they rarely materialize colourless in nature.

Soil is the top and thin layer present on the surface of the Earth and it consists of various types such as silt, sand, clay, peat, loam, chalk, and various other types. We generally study the soil profile before jumping into various types of soil. The soil profile is nothing but a vertical representation of the various layers of the soil where every layer has different characteristics such as texture, colour, depth, composition, benefits, etc. Here, in this article, we will be discussing one of these layers comprehensively and will come to know what role this particular layer plays in the soil profile. 

In this, we will learn about subsoil, subsoil definition, subsoil meaning, its various types, characteristics, merits and other related aspects. This topic will help you in Geography and Geology or Geomorphology.

Introduction

Soil is formed by the actions of water, wind or climate and breaking down of rocks, etc. and it is the thin as well as the topmost layer of the crust of the Earth which consists of rock particles as well as organic matter. Its formation is basically based on the parent rock, climate, surface relief and natural vegetation. It is the platform where plant kingdom establish themselves and it is one of the most important and recognised functions of the soil is that it supports food production and plant growth by providing food and water to them. The study of this is known as pedology and V.V. Dokuchaev who was a Russian and was considered as the one who laid down the foundation of modern pedology.

There are different layers of soil under the crust of the Earth and also different types of soil are also found on the Earth. The layers of the soil can be divided into different horizons such as O Horizon, A Horizon, E Horizon, B Horizon, C Horizon and R Horizon. In this article, we are discussing the B Horizon or we can say the subsoil.

Subsoil Meaning

As per the soil profile, this is a kind of soil that lies below the surface soil but above the bedrocks. It is also called undersoil and B Horizon soil. It lies between E Horizon and C Horizon. It predominantly consists of leached materials as well as minerals also such as iron and aluminium compounds. The microscopic and macroscopic organisms help in making the surface soil rich but these organisms spend very little time in this subsoil soil. Below this layer, there is a layer of partially disintegrated rocks and below is a hard rock found. If we talk about the layers of other soil and subsoil, a Horizon is a topsoil layer, E Horizon is a leaching layer and subsoil is a layer present below them and contains minerals and organic matter from both above layers. Thus, mostly it contains a mixture and variables of the particles similar to the surface soil but it contains less humus or less percentage of organic matter.

Subsoil Definition 

Some of the standard definitions are given below:

• “Subsoil is the stratum of weathered material that underlies the surface soil” – Merriam-Webster

• “The subsoil is a layer of earth that is just below the surface soil but above the hard rock.” Or “the bed or stratum of earth or earthy material immediately under the surface soil.” – Collins dictionary

Features

The various characteristics of subsoil are mentioned below:

• It does not contain many plant nutrients but contains more minerals than the surface layer such as iron and aluminium compounds.

• It is sticky and light in colour.

• It is less fertile.

• It is also very challenging to manage this kind of soil.

• Water passes through the above soil to it and thus affects it as well.

• If we talk about subsoil compaction, It is more compact than the surface soil as it does not contain any air, and thus microorganisms and macroeconomics do not live here for long and no insects can survive here.

• It is also called undersoil.

• Clay, as well as human remains, are also accumulated here in this layer.

Benefits

Subsoil provides a number of benefits which are mentioned below:

• It helps and plays a vital role in the absorption of the water as well as drainage.

• It also supports hard features of the landscape like fencing, patios and paths, etc. Thus, it helps in giving strength to the plants whenever it rains.

• It is also rich in minerals and also helps in the growth of plants.

Types of Subsoil

It has a mixture of variable and small particles similar to the surface soil ie. sand, silt and clay but it has less percentage of humus as well as organic matter. The major types of this are mentioned below:

• Sandy: It is one of the foremost types of this layer which consists of small particles of disintegrated and weathered rocks particles. It is considered the poorest type of soil for growing plants. It is less fertile and has fewer nutrients as well as poor water holding capacity. Thus, it becomes very hard for the roots of the plants to hold water here but it is good for the drainage system. 

• Clay: This type of soil contains the most finely grained particles with the presence of clay minerals and it contains the smallest particles as compared to other sandy and silt types of soil. Here, these particles are tightly packed where there is no airspace. It has good water holding capacity which makes it hard for air and moisture to penetrate. When it is wet, it becomes very sticky but it becomes smooth when it is dried. It does not drain well and it is also considered the heaviest and the densest type of soil.

• Gravel: It is a type of granular soil that consists of coarse particles like sand and gravel. It is also called crushed stones. It is a combination and aggregation of fragmented rocks. It consists of loosely attached particles and it is not good for plant growth.

• Hard Rock: Usually, the soil is made up of breaking down rocks and it has different layers and at some point, hard rock soil can be seen as well which are formed by a number of various rocks that contain various minerals. This type of soil provides a good and strong foundation. This is the soil that helps in giving strength to the above soil and plants present on it and other landscape features whenever it rains or storms occur.

Did You Know?

• The word soil is known with different words in different regions or languages such as in Malay or Indonesian, it is known as Tanah and thus Tanah subsoil is also known in this way whereas in Danish soil is called as Jord or in Dutch as Grond.

Conclusion

Therefore, we can say at the end that subsoil acts as a base for the above soil and also provides strength to the surface soil. Though it does not contain so many nutrients which help in plant growth and it is not a fertile layer as compared to topsoil but it has its importance and contains the same variables as the surface soil such as sand, clay, silt etc. This subsoil lies below Horizon A and E and thus contains all the material passed by the above two layers. It helps in gardening activities and providing strength to the plants. In this article, we have covered the subsoil meaning along with various standard subsoil definition, types of subsoil, its features and benefits, etc. This article will help you whenever you study the soil profile and layers of the soil in Geography or Geology.

A thermocline popularly known as the thermal layer or even known as metalimnion in lakes is actually a thin but distinct layer in a large fluid body like in ocean or in the lake. In thermocline, the temperature varies drastically with the depth while it varies less in the layers that are above or below. In the ocean, the thermocline distinguishes the upper mixed layer from the deep water which is quite calmer deep below. 

 

Majorly depending on the season, turbulent and latitude extension, and also by mixing with the wind, the thermocline may be a semi-permanent feature present in the body of water where they basically occur. 

 

More about Thermocline

Thermoclines are visible in lakes as well. In cold weather, this layer leads to an occurrence called ‘stratification’. In the summertime, the warm water, which is less dense, will settle on the top of the colder, denser, and deep water, and in between them, a thermocline will be separating both temperature water. 

 

The warm is known as epilimnion, while the cold layer is called the hypolimnion. This happens as the warm water gets exposed to the direct sun in the daytime, there exists a stable system and little mixing of the warm water and the cold water which occurs particularly in calmer weather.

 

One result of this kind of stability is that when the summer wears on, there is much lesser oxygen that is below the thermocline. While the water below this thermocline never circulates on the surface and the organisms in the water deplete the availability of oxygen. 

 

When the winter approaches, the temperature of the surface water will drop, this happens because at nighttime the cooling dominates the heat transfer. Then a point is reached in where the density of the cooling surface water is much greater than the density of the deep water and then the overturning begins when the dense surface water moves down with the gravitational pull. This process is also backed by wind or by any other process like the currents for example which agitates the water.

 

Thermocline Depth

The thermocline is the oceanic water layer where the water temperature decreases quite rapidly with the increasing depth in them. There is a widespread and permanent thermocline that exists beneath this relatively warmer, well-mixed surface layer, from the depths of about 200 m (or 660 feet) to about around 1,000 m (that is 3,000 feet), where there is an interval of temperatures that diminishes steadily.

Seasonal Thermocline

A thermocline that is formed in the oceans in the summer days at relatively shallow depths, is due to surface heating and for the downward transport of heat which is caused by mixing with the water that is generated by the summer breeze.            

Permanent Thermocline

The permanent thermocline actually refers to the thermocline that is not affected by the seasonal or the diurnal changes in the surface which is by the forcing and is therefore located below the per annum maximum depth of the mixed layer and by the influential potentiality of the atmosphere.

Fishing Thermometer Thermocline

Put the thermocline in a band of water. This is done where the temperature changes rapidly from the hot on top to the cold below. This is often a number of feet thick. For the water properties varying at different temperatures, this forms a barrier that prevents the hot water on the top to mix with the cold water underneath.

This technique is quite important while fishing as the water which is above the thermocline is very hot for the fish to live there, they cannot survive there for long periods of time. This is so because the surface water is exposed to direct sunlight.

While the water below has low oxygen available for the fish. This happens as the oxygen gets into the water by the water plants and by the surface action. The water plants are usually shallow which places above the thermocline. The thermocline acts as a barrier for that oxygen to trickle the water down below.

Hence the fishes like to hang around the thermocline. This allows the fishes to find the balance for the want of oxygen and temperature. Thus, effortlessly the fishes can move to adjust the balance here.

Thermocline Temperature

The temperature measured in the deep ocean may gradually drop with the increasing depth. As the saline water will not freeze until this reaches −2.3 °C (27.9 °F) (which is colder as the depth and pressure increase gradually) the temperature that is below the surface is usually not far from getting zero degrees. Thus, the thermocline varies with increase and decrease in depth.