My Science School

The plate Tectonics theory was formulated in the 1960s to explain the phenomena of continental drift and seafloor spreading, and the formation of the major physical features of the Earth’s surface. The Earth’s outermost layer is regarded as a jigsaw of rigid major and minor plates up to 100 kilometers thick, which move relative to each other, probably under the influence of convection currents in the mantle beneath. Major land forms occur at the margins of the plates, where plates are colliding or moving apart – for example, volcanoes, fold mountains, ocean trenches, and ocean ridges.

            At times, the crust crumples gradually to form ranges of Fold Mountains such as the Himalayas. Andes (South America) and the Rockies (North America). Sometimes two plates will slide past each other – as in the San Andreas Fault, California, where the movement of the plates sometimes led to sudden jerks, causing the earthquakes common in the San Francisco-Los Angeles area. Most of the earthquake and volcano zones of the world are, in fact, found in regions where two plates meet or are moving apart.

            According to the theory of continental drift in geology, about 250 million years ago, the Earth consisted of a single large continent (Pangaea), which subsequently broke apart to form the continents known today. During that time, the rest of the Earth was covered by the Panthalassa Ocean. Later on Pangaea split into two land masses – Laurasia in the north and Gondwanaland in the south – which subsequently broke up into several continents. These then drifted slowly to their present positions.

            The existence of a single “supercontinent” was proposed by German meteorologist Alfred Wegener 1912. (There are reports that well known scientists made similar observations centuries ago.) Plate Tectonics was formulated by Canadian geophysicist John Tuzo Wilson and has gained widespread acceptance among earth scientists.

Waves are formed everywhere on the sea or for that matter on any large water body. There are two physical mechanisms that control and maintain waves. For most waves, gravity is the restoring force that displaces the surface to be accelerated back towards the mean surface level. The kinetic energy gained by the fluid returning to its rest position causes it to overshoot, resulting in the oscillating wave motion.

            In the case of ripples, the restoring force is surface tension, wherein the surface acts like a stretched membrane. Waves on sea surface are generated by the action of the wind.

            The height of simple waves is the elevation difference between the top of a crest and the bottom of a trough. The height of wind waves increases with increasing wind speed and with increasing duration and fetch of the wind. Together with height, the dominant wavelength also increases. Finally, however, the waves reach a state of saturation, because they attain the maximum significant height to which the wind can raise them, even if duration and fetch are unlimited.

            After becoming swell, the waves may travel thousands of kilometers, particularly if the swell is from the great storms. In travelling, the swell waves gradually become lower; energy is lost by internal friction, air resistance and by energy dissipation because of divergence of the directions of propagation.

            When waves run into shallow water, their speed of propagation, height and wavelength decrease. In the final stage, the shape of the wave’s changes, and the crests become narrower and steeper until, finally, the waves become breakers (surf). Generally, this occurs where the depth is 1.3 times the wave height.

            Particles of sand, sometimes sorted by water transport into deposits of remarkably uniform size, are continuously being firmed, often from the bedrock of earth, by weathering and erosion by chemical and physical forces.

            The physical forces include water, wind and ice, in the form of the plant grindstones of glaciers.

            Sand particles may be glued together along with other minerals to form new sedimentary rocks, like the familiar sandstone, and sedimentary rocks may in turn by weathered into new sand.

            Quartz (a compound of the elements silicon and oxygen) is a very common component of primary rocks and is resistant to destruction by either mechanical or chemical means.

            It is not surprising that it makes up the biggest share of both sand and the larger than the very fine particles of clay and silt and smaller than gravel and pebbles.

            One widely used scale, the Wentworth Udden scale, puts the size of sand particles at approximately 0.0025 inches to 0.08 inches.

            The machinery earth provides for turning out such small hard particles if far cheaper to operate than any commercial equipment that could do the same job.

            Though there is a dwindling supply of mined sand on the continents because of restrictions on land use, there is a very large potential supply of offshore sand in the shallow shelf seas, presumably the results of cons of both the ocean’s undertow, chewing on the continents, and water and wind transport, carrying sand from inland.

 

 Magma is predominantly a molten silicate saturated with gases that are dissolved in it. It has a marked quantity of easily voltiling compounds ( O vapour, C , HF, HCI, etc).

            Owing to the high pressure existing in deeper part of the earth where volatile compounds are in a dissolved state within magma, diminishing its viscosity and increasing the degree of its mobility and chemical activity.

            Formation of magmatic sources under the earth is in general a continuous process. They are accumulated in the upper part of the asthenosphere (33-140 km deep from the surface of the earth) which then ascends into the upper levels of the earth’s crust.

            The movement of the magma towards the earth surface is conditioned by hydrostatic pressure along with considerable increase in the volume, which accompanies the transition of solid rocks into the molten state.

            Some magma melts penetrate and break through the horizons of earth surface and some invasive magma on its way to earth surface and solidifies at certain depth within the earth.

            Volcanism unites all the processes connected with the outflow on the earth’s surface. The volatile components, which, in the deeper regions owing to high pressure and temperature remains in the magma in dissolved state, are released it on the way to the earth’s surface. The products of volcanic eruptions include liquid, solid and gaseous materials (varitia).

            Liquid products of volcanic eruptions are represented by lava. And are classified as acid, medium, basic (or) ultra basic depending upon its chemical composition especially  (silica) content.

            Lavas of ultra basic or basic are poor in silica and rich in Ferro-magnesium compounds with temperature existing at the surface at the time of outflow being C and characterized by low viscosity and high mobility. So they easily move and spread themselves over a considerable distances and form sheets and streams of undulating surface. Thus the lava flowing out of a volcano is hot.

            Acid and medium lavas rich in silica with surface temperature of C - C possess high degree of viscosity and little mobility and more for short distance and quickly solidify forming small streams and blocks.

            Solid particles also emerge during volcanic explosion as a result of an ejection into the atmosphere and dispersion of huge masses of lava as well as fragments of rocks.

            Depending on size they are classified as volcanic bombs, Lapilli, volcanic sand and volcanic ash. Volcanic ash is the main product of eruption. Gaseous products released are made up of water vapour (60-90 percent followed by S, , Co, , HCI,HF, etc.)

            One of the most rare and mysterious forms of lightning is ball lightning. It is a ball of luminosity that usually occurs near the impact point of a flash and moves horizontally at a speed of a few centimeters per second. It can penetrate closed windows, is usually accompanied by a hissing sound and has a life time of several seconds. The colour is quiet variable and the ball often ends with an explosion-however, it is not usually destructive. Also called as globe lightning, it occurs at times of intense electrical activity in the atmosphere. These balls are said to be plasmas. (Plasma is a completely ionized state of mater, at high temperature, in which positive and negative Ions freely move about.). However, no theory has so far satisfactorily explained the behavior of a ball as scientists have not been able to reproduce it in the laboratory. Lightning ball is comparatively rare sight and so next time you see it, take a picture.

            Clouds are masses of tiny water droplets and ice crystals that float in the air. As such they do not have any colour. But some look white and some grey.

            Some change shape continuously as parts of the cloud evaporate when they come into contact with the warmer air.

            Clouds are classified mainly by their appearance dimension, shape, structure and texture. While stratus clouds are sheet-like, fair weather cumulus clouds consists of piled-up masses of dazzlingly white clouds. They are made of water droplets. Cirrus clouds are curly white made of ice crystals at higher altitudes. In these clouds, water droplets or ice crystals are loosely packed and so light can pass through them without much loss in intensity.

            Some of the clouds which cause rain are the stratus and stratocumulus clouds which are near-earth clouds. Stratocumulus clouds are not as thick as stratus clouds and so they have light and dark areas.

            Altrostratus clouds form smooth white or grey sheets across the sky. Sometimes these clouds are so thick that the Sun or the Moon cannot be seen through it. At times the difference in thickness may cause relatively light patches between dark parts but the surface of the clouds do not show any relief.

            Nimbostratus is low, amorphous and occurs at higher altitudes. These are dark, grey and uniform. Cumulonimbus is the main rain clouds which are black. This is because light cannot penetrate through them as they are deep and densely packed with water droplets, ice and snow particles.

            If we look from an aircraft, flying at high altitudes, these clouds will look dazzlingly white, as they reflect all the light falling on them. But for an observer on the ground the clouds may look black and be raining. Clouds are thus white and black at the same time!

   Many windblown drops can be forced together to form what Weather reporters call ‘sheeting rain’, but rain is always born as minuscule drops of condensed  water vapour explains ‘Clouds and Weather’ by John A. Day and Vincent J. Schaefer (Houghton Miffin Company), U.S. The formation of these droplets depends on the right amount of water vapour at the right pressure and temperature, but it also requires the presence of tiny solid particles of matter in the air on which the water vapour can gather and condense.

            These bits of dust and salt are called cloud condensation nuclei. Salt starts collecting vapour at about 80 percent relative humidity, while bits of clay begin to take on water molecules at 100 percent relative humidity.

            

            Rainfall is measured in terms of the level or height to which water is collected or accumulated on a flat surface through rain. It is usually expressed in millimeters to the nearest whole number. Rather than measuring all the rainfall falling over a large areas, which is impractical, rainfall is measured at a number of points over the area. There are many instruments for measuring rainfall; the most commonly used is the rain gauge.

            Rain gauge consists of a funnel (5”-6”in diameter), a measuring tube (usually one tenth of the funnel in diameter to measure accurately even the trace amounts of rainfall) and the outer cylindrical cover with a base. The rainfall falling into the funnel is directed into the measuring tube which is calibrated accordingly.

            The excess water, if any, overflows the tube and is collected within the outer cover. This is measured subsequently. Based on the material by which the parts of a rain gauge are made, it may be fibre glass type or metal type. For continuous recording of rainfall, recording rain gauge is used where the rise of water level is automatically monitored continuously. Recording rain gauge may be with float type recorder or weighing type recorder.

            In float type recorder the vertical movement of the float (with the rise of water level) is recorded by a pen on a chart fixed on a rotating drum; whereas in the weighing type, the weight    of the receiver is recorded by an weight balance. The rain gauge must be placed at horizontally (at about 12” height from the ground) at a distance of twice the height of the nearest objects like trees, buildings etc.

            Rainfall occurring in any place is simply measured as the height of the rainwater on the land in that place provided it is not lost due to run-off, evaporation etc. and the land is flat. Measuring rain this way is however impossible. A rain gauge must be used.

            A simple rain gauge which any one can use to measure rain at his place consists of a funnel (3”to 4” in diameter) fitted into a bottle (about 1 litre capacity) to collect the rain water and a measuring cylinder. (An air-vent is to be provided to prevent accumulation of water in the funnel in case of heavy down pours.)

            The rain gauge is kept on the ground in the open without obstructions from buildings, trees, etc. if the rainfall, over a period of time is 1cm at a place where the rain gauge is kept, then the height of the rainwater collected would also be 1 cm only if the bottle is flat at the bottom and its area cross section is the same as the area of the opening of the funnel.

            Since this specification cannot be followed the volume of water so collected has to be measured (this will be constant for a given size of funnel irrespective of the size or shape of the bottle) to know the amount of rainfall. Suppose the area of the opening of the funnel is 80 cm2then for 1 cm of rainfall the volume of water would be 80 cm2 x 1 cm that is 80 cm3. This amounts to 8 cm3 of water for every mm of rain.

            Thus if the total volume of rain water (in cm3) collected, over a specified period, is divided by 8, we get rainfall in mm in that place over the given period. To get accurate rainfall data quickly by directly observing the water level, a modified form of the above described rain gauge is used in all meteorological observatories. In this, rain water is collected in a narrow graduated tube so that the height of rain water is increased several times for the same amount of rain.

            This facilitates accurate measurement of even low rainfall like 1 mm or less. If the area of fifth of the area of the opening of the funnel then for 1 mm rainfall the height of rain water in the tube would be 5mm. if the graduations and made accordingly, the water level in the narrow tube directly gives the rainfall.

            For measuring continuous rain (which lasts several days on many occasions) automatic rain gauges are in use. In one type, called weighing type, the rainwater as it falls is weighed and translated into a continuous record on a clock-chart. Thus gives not only the total rainfall but also the time of its occurrence and its intensity.