Category Weather, Climate, Ecology

WHAT IS THE DIFFERENCE BETWEEN PHYSICAL AND CHEMICAL WEATHERING?

Physical weathering is also known as mechanical weathering. It is a process, initiated by humans, plants or animals, which breaks down rocks and minerals on the surface of Earth. It changes just the shape or size of the rocks and minerals. Chemical weathering, on the other hand, happens when the chemical composition of the rock and soil changes, forming new chemical combinations and a different internal structure.

Physical weathering is also called as mechanical weathering. This is the process where rocks breakdown without altering their chemical composition. Physical weathering can occur due to temperature, pressure or snow. There are two main types of physical weathering. They are freeze thaw and exfoliation.

Freeze-thaw is the process where water goes into the cracks of the rock, then freezes and expands. This expansion causes rock to break apart. Changing temperature also causes rocks to expand and contract. When this happens over a period of time, rock parts starts to break down. Due to the pressure, cracks can be developed parallel to the land surface which leads to exfoliation.

Physical weathering is prominent in the places where there is little soil and few plants. For example, in desserts surface rocks are subjected to regular expansion and contraction due to temperature changes. Also, in mountain tops, snow keeps melting and freezing which causes physical weathering there.

Chemical weathering is the decomposition of rocks due to chemical reactions. This changes the composition of the rock. This often takes place when rain water reacts with minerals and rocks. Rain water is slightly acidic (due to dissolution of atmospheric carbon dioxide, carbonic acid is produced), and when the acidity increases chemical weathering also increases. With the global pollution, acid rains occur now, and this increases chemical weathering more than the natural rate.

Other than water, temperature is also important for chemical weathering. When the temperature is high, the weathering process is also high. This releases minerals and ions in rocks into surface waters. There are three main types as to how the chemical weathering occurs. They are solution, hydrolysis and oxidation. Solution is the removal of rock in solution due to acidic rain water. This is sometimes called carbonation process, since the rain water acidity is due to carbon dioxide. Hydrolysis is the breakdown of rock to produce clay and soluble salts by acidic water. Oxidation is the breakdown of rock due to oxygen and water.

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WHAT IS CARBONATION?

Decaying leaves and plant matter give out carbon dioxide, which is also present in the air around us. Carbon dioxide dissolves in water to create carbonic acid through a process called carbonation. This acid can, over time, dissolve rocks, especially limestone. Limestone is a soft rock that consists mainly of calcium carbonate, which reacts with rainwater, dissolving away to create huge caves and cave complexes.

Carbonation is the chemical reaction between carbon dioxide present in the air, and the hydration compounds of the cement in concrete structures. The rate of carbonation depends on the physical characteristics such as the design, on-site preparation, production and protection, as well as external factors, such as the location and degree of exposure to contaminants and other environmental factors. Carbonation may lead to the corrosion of the reinforcement steel and deterioration of concrete structures.

The carbonation process starts immediately when concrete is exposed to air. Carbon dioxide (CO2) penetrates the concrete through the pores where it reacts with the calcium hydroxide and the moisture in the pores to form calcium carbonate. The carbon dioxide combines with the pore water to form a dilute carbolic acid which acts to reduce the concrete’s alkalinity.

Carbonation reduces the concrete’s natural alkalinity from pH13 to about pH8. Whereas a high pH provides a passivation layer around the steel, at pH below 9.5, the passivation layer breaks down and exposes the reinforcement steel to the corrosive effects of water and air.

When steel rusts, it expands in volume and exerts force on the surrounding concrete, causing the concrete to crack and spall at a rate that increases exponentially if the corrosion is not prevented.

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WHAT IS HONEYCOMB WEATHERING?

When salt water that collects on the rough surface of rocks, or seeps into cracks, evaporates, it leaves behind salt crystals. Over time, these crystals alter the rock, forming hundreds and thousands of tightly joined pits called honeycombs that are a classic example of both physical and chemical weathering.

Honeycomb weathering occurs throughout the world, but the origin remains a matter of controversy. Wind erosion, exfoliation, frost shattering, and salt weathering have been proposed as explanations, although few attempts have been made to substantiate these hypotheses with chemical or mineralogical studies.

Chemical analyses and field observations indicate that honeycomb weathering in coastal exposures of arkosic sandstone near Bellingham, Washington, results from evaporation of salt water deposited by wave splash. Microscopic examination of weathered surfaces show that erosion results from disaggregation of mineral grains rather than from chemical decomposition. Thin walls separating adjacent cavities seem to be due to protective effects of organic coatings produced by microscopic algae inhabiting the rock surface. Cavity walls are not reinforced by precipitation of elements released by weathering, as has often been suggested at other locations. Honeycomb weathering develops rapidly and can be observed on surfaces that were planar less than a century ago.

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WHICH IS THE LARGEST CONSTELLATION?

Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees, and also the longest at over 100 degrees. Its southern end borders Libra and Centaurus and its northern end borders Cancer. It was included among the 48 constellations listed by the 2nd century astronomer Ptolemy. Commonly represented as a water snake, it straddles the celestial equator.

Despite its size, Hydra contains only one moderately bright star, Alphard, designated Alpha Hydrae. It is an orange giant of magnitude 2.0, 177 light-years from Earth. Its traditional name means “the solitary one”. Beta Hydrae is a blue-white star of magnitude 4.3, 365 light-years from Earth. Gamma Hydrae is a yellow giant of magnitude 3.0, 132 light-years from Earth.

Hydra has one bright binary star, Epsilon Hydrae, which is difficult to split in amateur telescopes; it has a period of 1000 years and is 135 light-years from Earth. The primary is a yellow star of magnitude 3.4 and the secondary is a blue star of magnitude 6.7. However, there are several dimmer double stars and binary stars in Hydra. 27 Hydrae is a triple star with two components visible in binoculars and three visible in small amateur telescopes. The primary is a white star of magnitude 4.8, 244 light-years from Earth. The secondary, a binary star, appears in binoculars at magnitude 7.0 but is composed of a magnitude 7 and a magnitude 11 star; it is 202 light-years from Earth. 54 Hydrae is a binary star 99 light-years from Earth, easily divisible in small amateur telescopes. The primary is a yellow star of magnitude 5.3 and the secondary is a purple star of magnitude 7.4. N Hydrae (N Hya) is a pair of stars of magnitudes 5.8 and 5.9. Struve 1270 (?1270) consists of a pair of stars, magnitudes 6.4 and 7.4.

The other main named star in Hydra is Sigma Hydrae (? Hydrae), which also has the name of Minchir, from the Arabic for snake’s nose. At magnitude 4.54, it is rather dim. The head of the snake corresponds to the ?shlesh? Nakshatra, the lunar zodiacal constellation in Indian astronomy. The name of Nakshatra (Ashlesha) became the proper name of Epsilon Hydrae since 1 June 2018 by IAU.

Hydra is also home to several variable stars. R Hydrae is a Mira variable star 2000 light-years from Earth; it is one of the brightest Mira variables at its maximum of magnitude 3.5. It has a minimum magnitude of 10 and a period of 390 days. V Hydrae is an unusually vivid red variable star 20,000 light-years from Earth. It varies in magnitude from a minimum of 9.0 to a maximum of 6.6. Along with its notable color, V Hydrae is also home to at least two exoplanets. U Hydrae is a semi-regular variable star with a deep red color, 528 light-years from Earth. It has a minimum magnitude of 6.6 and a maximum magnitude of 4.2; its period is 115 days.

Hydra includes GJ 357, an M-type main sequence star located only 31 light-years from the Solar System. This star has three confirmed exoplanets in its orbit, one of which, GJ 357 d, is considered to be a “Super-Earth” within the circumstellar habitable zone.

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Does deserts ‘breathe’ water vapor?

Deserts are arid ecosystems, receiving fewer than 25 cm of precipitation a year. They are hot dry and deserted. But the sand dunes aren’t just inert masses. They, in fact. “breathe” water vapor and are very much alive. Scientists have developed a super-sensitive probe that has recorded how water vapor from the surrounding air percolate between sand grains.

Researchers at Cornell University, New York, and University of Nantes, France, developed over a decade a new form of instrumentation called capacitance probes. to study the moisture content in sand dunes to better understand the process by which agricultural lands turn to desert. The probe uses multiple sensors to record everything from solid concentration to velocity to water content, all with unprecedented spatial resolution. It is so sensitive to moisture that it can pick up tiny films of water on a single grain of sand!

Conducting the research at Qatar, they combined data on wind speed and direction as well as ambient temperature and humidity. The study revealed just how porous sand is, with a tiny amount of air seeping through it.

When wind flows over the dune, it creates imbalances in the local pressure. This forces air to go into and out of the sand. “So, the sand is breathing, like an organism breathes,” the researchers note. This breathing could be the reason behind how microbes live deep in sand dunes, even when no liquid water is available. The researchers also found that at the surface of the dune, the probe measured less evaporation than scientists were predicting. This shows that the leaching of moisture from the sand dune to the atmosphere is a slow chemical process.

The team’s paper has been published in the Journal of Geophysical Research-Earth Surface. Probes that can sensitively measure moisture within sand could help experts find invisible signs of water, say, on Mars.

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