Category: Geography Notes PPT

Mohorovicic discontinuity, also known as moho discontinuity is the boundary between Earth’s Crust and its Mantle. The moho lies at the depth of 30-50 km below the continents and 5-10 km below the sea level in the ocean. It is defined by the discontinuous changes in the velocity of seismological waves as they pass through the densities of rock. The moho lies almost entirely within the lithosphere.

Named after the pioneering Croatian Seismologist Andrija Mohorovicic, the moho divides both the oceanic crust and continental crust from the underlying mantle. The Mohorovicic discontinuity was first introduced in 1990 by Mohorovicic, when he observed that seismograph from shallow focus earthquakes has 2 sets of P waves and S- waves, one that followed a direct path near the Earth’s surface and another refracted by the high velocity medium. 

What is Moho Discontinuity?

The ‘Moho Discontinuity’ is the boundary between the crust and mantle. It has been named so as it was discovered by Croatian Seismologist Andrija Mohorovicic. This boundary marks a change in the seismic waves velocity from the crust to the uppermost mantle within the lithospheric plate. 

Who Discovered Moho Discontinuity?

Croatian Seismologist Andrija Mohorovicic is first credited with discovering and defining the Moho.  In 1990, the Mohorovicic observed the data from a local earthquake in Zagreb where he observed two different sets of P waves and Q waves propagating out from the focus of the earthquake. Mohorovivis knew that the waves caused by earthquakes travelling at velocities are proportional to the density of the material carrying them. On account of this information, he theorized that the second set of waves could only be caused by a sharp transition in density in the Earth crust, which could account for such a drastic change in the eave velocity. Using the velocity data from the earthquake, Mohorovicic was able to calculate the depth of the Moho to be approximately 54 km, which was backed up by subsequent seismological studies.      

             

How Deep is the Moho Discontinuity?

The Moho discontinuity marks the lower limit of the Earth crust. The Moho discontinuity is 5-10 kilometres (3-6) mi below the ocean base and 20 – 90 kilometres (10 – 60) mi below the continental surface with an average of 35 kilometres (22 mi). Mohorovicis further used his discovery to study thickness variations of the crust which has a relatively uniform thickness, while continental thickness is thickest under the mountain ranges and thinner under the plains. 

Moho Role

Moho has played a large role in the field of geology and Earth Science well for a century.  By observing the Moho reflective nature, and how it affects the speed of W waves, scientists were able to hypothesize about Earth’s composition. This study gave rise to Earth seismology.

The Project Mohale in the 1960s attempted to drill to the Moho from deep – ocean regions.  After an initial success in establishing deep ocean drilling, the project underwent political and scientific opposition, mismanagement, cost overturn, and it was cancelled in 1966.

Did You Know?

• Moho discontinuity is defined as the boundary between Earth’s crust and the mantle, observed by a sharp increase in the velocity of the seismic waves passing through the Earth.

• The name Mohorovicic discontinuity was received from Croatian geophysicist Andrija Mohorovičič, in 1909.

• As per the recent studies, the moho is now realised as a transition zone rather than a sharp discontinuity.

Oceanic ridge is a continuous submarine chain of mountains that extends roughly 80,000 km (that is 50,000 miles) among all the oceans present in the world. In the ocean basin, these ocean ridges are the largest features. Geographically speaking, these oceanic ridges are the most distinct and prominent feature on the surface of the earth after the continents and oceans being the prominent ones. 

Previously these features were also referred to as the mid-ocean ridges. While the largest ocean ridge, the East Pacific Rise is quite a distance from a mid-ocean location, hence, we can say that the generalization is not exactly correct. Further, students must not confuse these oceanic ridges with aseismic ridges, which originated apart from these.  

Mid Ocean Range

A mid-ocean ridge also known as the mid-oceanic ridge is an underwater mountain range, which is formed by the action of plate tectonics. There is uplifting of the ocean floor, which is caused due to the convection currents as it rises in the mantle which is beneath the oceanic crust and here it creates magma. The creation of magma is done where the two tectonic plates meet at a divergent boundary.

These mid-ocean ridges of the world are all connected and they form a single global mid-oceanic ridge system which is a part of every ocean that makes up the mid-oceanic ridge system. Here the longest mountain range in the world is about 60,000 km in length.

There are two processes involved in this, one is ridge-push and the other is slab-pull, they are responsible for the spreading seen at the mid-ocean ridges, while there is some uncertainty about the fact which is dominant.

Mid Atlantic Ridge

The Mid-Atlantic Ridge abbreviated as MAR is the mid-oceanic ridge which is divergent or is constructive with a plate boundary that is located along the Atlantic Ocean floor. Also, this serves to be the longest mountain range in the world. The Mid-Atlantic Ridge separates North America from the Eurasian and African Plate; this happens in the North Atlantic. Also, this ridge separates North America from Azores Triple Junction. While, in the South of Atlantic, the ridge separates the African and the South American plates. The ridge further extends from a place in the Gakkel Ridge which is called the Mid-Arctic Ridge to the northeast in Greenland, proceeding southward to Bouvet Triple Junction in the Southern part of the Atlantic. 

We know that the Mid-Atlantic Ridge is majorly an underwater feature, while portions of it have elevation to the extent above the sea level, for example in Iceland. 

Submarine Ridge

Submarine Ridge is quite elongated and steep-sided elevation of the ocean and the sea floors. The Submarine ridges extend up to hundreds and thousands of kilometers, lengthwise and the width is about several hundred km. Here the peaks often rise above sea level to form many such islands. 

In the submarine continental margins, the submarine ridges are very rare, while their structure being analogous to the structure of those of the mountains which are located on the adjacent parts of these continents. In the transition zone, the submarine ridges appear mainly like the island arcs.

On the ocean floor, the submarine ridges may be block, or block-folding, or like the volcanic ridges.  The mid-ocean ridges are the largest submarine ridge. 

Indian Ocean Ridge

Talking about the Indian Ocean Ridge, the Carlsberg Ridge which is also a submarine ridge of the Arabian Sea and the Indian Ocean is a prominent feature. This ridge is a portion of the Mid-Indian Ridge and this extends from the vicinity of Rodrigues Island to the Gulf of Aden, basically, this means northwest to southeast. This ridge separates the Arabian Sea located in the northeast from the Somali Basin in the southwest.

East Pacific Rise

The East Pacific Rise is also a mid-oceanic ridge which has a divergent tectonic plate with a boundary that is located along the Pacific Ocean floor. This separates the Pacific Plate in the west from the North American Plate, from the Rivera Plate, from the Cocos Plate, the Nazca Plate and also from the Antarctic Plate. This runs southward from the Gulf of California which is in the Salton Sea basin, South California to a point near 55° S, 130° W, then it joins the Pacific-Antarctic ridge.

The pegmatite minerals, pink, grey-white in colour, is an intrusive igneous body of substantially variable grain size. Having an exceptionally large crystal composition may also contain minerals that are not found in other types of rocks. The pegmatite often includes coarse crystal growth and is widely used as a principal ingredient for making glass and ceramics. A pegmatite rock can be segregated within a related plutonic rock or a dike that violates the surrounding country rock. A pegmatite is at least one centimetre in diameter.

            

   

Pegmatite Formation 

Formed during the final stage of magma’s crystallization is pegmatit. It is clearly a pegmatite if it is wholly made of crystals. With pegmatite’s large crystals are accredited to low-viscosity fluids which permit ions to be very mobile. During the stage of crystallization, the melt generally consists of a substantial amount of dissolved water and various other volatiles such as fluorine, chlorine, and carbon dioxide. During the early crystallization process, water is not eliminated, thus its concentration in the melt mushrooms as the crystallization progresses. 

Finally, there is a profusion of water, and pockets of water isolated from the melt. These pockets of highly heated water are superbly rich in dissolved ions. The ions in the water are relatively more mobile than ions in the melt. This enables them to move freely and create crystals quickly. This is why crystals of a pegmatite blossom so large.

The severe conditions of crystallization sometimes form crystals that are some meters in length and weigh over a ton. Pegmatites produce from waters that isolate from magma in the late stages of crystallization; this activity commonly takes place in small pockets beside the margins of a batholith. Pegmatite can also produce fractures that develop along the margins of the batholith. This is also the way the pegmatite dikes are produced.

Pegmatite Composition

The pegmatite mineralogy exercises control from some form of feldspar, frequently with mica and usually with quartz. Pegmatite includes most minerals linked with granite and granite-related hydrothermal systems. However, It is not feasible to measure the mineralogy. This is subjected to the difficulty in counting and sampling mineral grains in a rock that may consist of crystals from centimetres to meters.

Pegmatites involve mafic and carbonate-containing sequences that include garnet, often almandine or spessartine

Syenite pegmatites are depleted of quartz and have huge feldspathoid crystals instead.

Where is Pegmatite Found?

Pegmatite is found all across the world. They are the most copious antiquated rocks, particularly of the great geologic age. Some are found in huge intrusive igneous rocks, while others are spread over rocks enclosing intrusive magmatic rocks.

Properties and Characteristics of Pegmatite Rock

Pegmatites can be characterized in accordance to the elements or mineral of interest, for example, “lithian pegmatite” to define a Li-carrying or Li-mineral carrying pegmatite, or “boron pegmatite” for those having tourmaline.

There is often no rational way to differentiate pegmatites as per the chemistry simply because of the complexity of collecting a representative sample, but often groups of pegmatites can be differentiated on contact textures, accessory minerals, orientation and timing. These can be named either formally or informally as a category of intrusive rock or within a bigger igneous association.

While complex to be certain of extraction of pegmatite in the stringent sense, often pegmatites are termed as “granitic”, “metamorphic”, or “metasomatic”, depending upon the interpretations of the interrogating geologist.

Fun Facts

• Rocks with a texture similar to pegmatites are known as pegmatitic.

• Pegmatite is often traded commercially as a granite product.

• Facts About The Rock

• Crystals of pegmatite grow huge due to critical conditions of crystallization.

• Complex pegmatites come into existence because of the existence of exotic volatiles such as CO₂, chlorine, and fluorine.

• An example of an extremely huge crystal is at the Etta Mine in South Dakota. It was 5 feet in diameter, 42 feet long, and produced about 90 tons of spodumene.

• Pegmatites generally do not support extended mining operations.

• Rare components condensed in huge crystals may make pegmatite a competent source of valuable ore. Pegmatite deposits may also have industrial minerals, rare minerals and gemstones.

A polar vortex, also known as a circumpolar vortex is a large area of cold, rotating air that surrounds both of the Earth’s polar regions. It always exists in poles, but strengthens in winters and weakens in summers. 

The term ‘vortex’ in the circumpolar vortex refers to the counterclockwise flow of air that helps to keep the colder air near the poles. Most of the time during winters in the northern hemisphere, the polar vortex will expand, sending cold air southwards through the jet stream. This generally occurs during winters and is often associated with the large outbreaks of Arctic air in the United States. 

Polar Vortex Meaning

Polar vortex, or circumpolar vortex, the polar low, polar cyclone is a large area of persistent low pressure generally located above each of the Earth’s polar regions and containing a mass of extremely cold air. The altitude of this cyclone extends from the middle of the troposphere ( the lowest level of Earth’s atmosphere which spans the region from the surface up to 10 – 18 km [ 6 -11 miles high] km into the stratosphere (the atmospheric layer that extends from 10 -18 km to about 50 km [ 30 miles] high.

The cold air is contained within the polar vortex by the polar front Jet stream. The strength of the polar vortex changes with the season. It is strongest during the winter season in each hemisphere when the Equator is at greatest and temperature contrast between the poles. It may be weaker or entirely diminished during the warmer months of the year.

Polar Cyclone

Polar cyclones, also known as arctic cyclones or polar vortices are very large areas of low pressures. The term polar low should not be confused with the term polar cyclones. Polar cyclones are generally 1000 to 2000 kilometers wide in which the air is moving in the spiral counterclockwise in the northern hemisphere. The reason for this rotation is similar to tropical cyclones, the Coriolis effect. They also exist in the regions such as the Eurasian Arctic area, Greenland, and northern Canada with about 15 cyclones per winter.

Polar cyclones can occur at any time of the year, although summer polar cyclones are weaker than the ones that occur in the winters. 

Strength and Duration of Polar Vortex

The strength and duration of the polar vortex are as follows:

Strength

• Polar vortices or polar cyclones are weakest during summers and strongest during the winters. 

• During the volcanic eruptions in the tropics, a polar vortex will be strengthened and can stay that way for 2 years after the initial eruptions.

• Extratropical cyclones that emigrate into the higher latitudes when the polar vortex is weaker can disturb the single vortex creating small vortices within the polar air mass. Those individual vortices can continue for more than a month.

• The Antarctic polar vortex is more pronounced and persistent than the Arctic polar vortex.

• The climate anomalies La Nina significantly strengthen the polar vortices. 

• Strengthening the polar system within the troposphere that cools down the poles, intensifying the polar vortex.

Duration

• The Northern polar vortex comes about between Mid March Mid May. This event marks the transition from the winter season to the spring season. Hydrological cycles, growing seasons of vegetation, and overall ecosystem productivity are highly impacted by these events.

• The same transition also impacts sea ice, zone, air temperature, and cloudiness immensely.

• Early and late polar break events have occurred due to the variation in stratospheric flow structure and the upward spreading of planetary waves from the troposphere.

• When the polar break up is early, there is one warning period from late February to middle March whereas when the polar break up is late, there are two warning periods, one in January and the other in March.

• Sometimes, a mass polar vortex breaks off before the end of the final warning period. If large enough it can move into Canada and the Midwestern, Central, Southern, and Northeastern United States.

Polar Vortex UPSC Facts and Information

Following are some of the polar vortex UPSC facts and information:

• A polar vortex on Earth is usually in the middle and upper troposphere and stratosphere.

• The Arctic vortex has two main centers: one is over Baffin Island, the other is over Northeastern Siberia.

• Polar vortex and weather impacts due to stratospheric warnings.

Sea caves also known as littoral caves, is a type of cave which is mainly formed by the action of sea. The primary process involved is erosion. Sea caves are found throughout the world. Matainaka cave, on the Otoga coast of New Zealand’ South Island, has been verified as the world’s largest sea cave in length 1.54km or 5.051feet, by a survey in October 2012.

Who Lives in Sea Caves?

Sea caves are brimmed with life, both on their floors and walls. Such as anemones, starfish and sponges, living under these sea caves with dark zones may harbour organisms not commonly seen in shallow water.

Beach Caves in India

Sea caves are also identified by the ocean cave as well as beach caves.

• Borra caves, Andra Pradesh

• Bhimbetka rock shelters, Madhya Pradesh

• Amarnath cave, Jammu and Kashmir

• Undavalli caves, Andra Pradesh

• Vaishno devi, Jammu and Kashmir

• Udayagiri khandagiri caves, Orissa

• Elephanta cave, Maharashtra

• Badami caves, Karnataka

Avila Sea Caves

Cave Landing Beach is not easily accessible since it’s a hidden gem and thus makes it difficult for one to reach below cave landing in Avila beach CA. Most of the visitors to the Cave Landing parking will make an effort towards the downside of the main trail that leads to Pirates Cove Beach and the best known cave which is actually a tunnel. This tunnel has many nicknames including Smugglers Cave, leads to a viewpoint that looks west toward Port San Luis and the three piers of Avila Beach. This outlook also looks down on a small beach that appears nearly impossible to reach. Well it seems challenging but there is actually a way down.

Back at the parking lot there exists a trail just to the south of the main trail, in close proximity to the ocean, that heads in the same direction as the main trail but then drops down towards the ocean shore. It leads to a point at which a fixed rope is generally in place to provide the necessary support in descending a steep and the cliff. Remember that this descent is not possible without a rope particularly for those of us that are not technical climbers. If the rope is weakened or if one does feel convenient with rappelling, then they should not attempt the downward climb. While performing, wear shoes with good traction help.

How Long is Avila Beach?

Avila Beach consists of 3 piers i.e: Avila Beach Pier, 1,685 feet (514m) in length, and the California Polytechnic State University (Cal Poly SLO) Pier, part of the university’s marine research program, is not publicly accessible.

Not long ago, the pier was enhanced to be the site for whale watching as numbers of grays and humpback whales come into bays around the pier to feed and draw crowds during the seasons.

The Avila Beach Pier got a call to attention in a Super Bowl advertisement on February 7, 2010. Avila Beach was the primary shooting location for California films.

The beach measures less than 0.5 miles (0.8 km) long and is firmly based in San Luis Bay, which is formed by Point San Luis on the west and Fossil Point on the east. Avila Beach faces south and an elevation of 600 foot point at San Luis breaks the prevailing northwesterly winds. It is relatively warmer than the other beaches located on the central coast.

Meyers Beach Sea Caves

These are located in the North of Cornucopia, Wi. There are caves at the Northern end of Mawikwe bay and one caveat at the South end of Mawikwe bay. Meyers Beach Sea Cave trail is 4.6 mile. The trail offers a number of activity options and is rugged with stream crossings and steep slopes which is accessible year round. But in winter it may be snow packed or icy, where skiing is not recommended and use of snowshoes is difficult.

The mainland sea cave of the Apostle Island National Lakeshore, which are the similar caves that turned out to be the world’s most popular ice cave in winter, are easy to reach via boat and hiking trail from Meyers Beach. 

The Meyer’s Beach sea cave trail is extended through 2.3 miles of the Lakeshore Trail, which typically starts at the Meyer’s Beach parking lot near Cornucopia, Wisconsin. “The Bowl” is one of the best views of the hike.

Sea Cave Adventures

Exploring the Mysteries of Earth’s Underwater Caves

• Swimming

• Cave diving

• Scuba diving

• Snorkeling

• Waterrofting