Magma Magma is the molten rock of highly gaseous and mobile nature. It is generated under great pressure within the earth. On solidification and crystallisation, it forms igneous rock.
Magnetic Equator Also termed aclinic line, magnetic equator is a line joining points on the earth's surface where the magnetic needle exhibits no magnetic inclination. The magnetic inclination is the acute angle between the vertical and the direction of the earth's magnetic field in the plane of the magnetic meridian.
Magnetic Poles Magnetic poles refer to the two specific points on the earth's magnetic field at which the lines of magnetic force are vertical. Their positions undergo a slow change and they are not synonymous with the geographical poles. Currenfly the magnetic north pole is located on Prince of Wales Island in Canada, while the magnetic south pole is located on South Victoria Land in Antarctica.
Magnetic Storm The temporary disturbance of the earth's magnetic field due to solar flares and sun spots is called magnetic storm. Magnetic storms result in disruption in radio and telegraphic communications and magnetic surveys.
Mango Showers Heavy showers that occur in India between March and May, i.e., before the breaking of the south-west monsoon, are called mango showers. They are termed mango showers due to the fact the mangoes begin to ripen with these rains.
Mangrove Mangrove refers to a vegetation type consisting of dense swamp forest. It consists of a number of broadleaved shrubs and trees that are adapted to the intertidal environment. Mangrove requires muddy conditions and shallow water.
Mantle Mantle is that portiop. of the interior of the earth which lies beneath the crust and above the core. It consists of ultrabasic rocks. The upper limit of the mantle is Mohorovicic discontinuity.
Maquis (Macchia) Maquis refers to a low evergreen shrub formation on acidic soils in the mediterranean region, e.g., France. It is considered a secondary vegetation on the lands which have been cleared of their natural oak forests by human activity.
Mariculture Mariculture is the farming of the sea. It includes fishing.
Marsh The term marsh refers to a poorly drained tract of land with patches of open water. Because of their high organic content, marsh so~ls are often very fertile when properly drained. Rushes, reeds and certain mosses are characteristics of marshland.
Massif A French term, massif reefers to an upland region that is formed by a moutainous plateau.
Meander Meander is a pronounced curve or loop in the course of a river channel. The flow of water in a meander is not even, the Current being fastest on the outside of the meander bend. The outer side of a meander is characterised by river-cliff or bluff, while its inner side is marked by gently shelving slip-off slope.
Meridian A line of longitude passing from the North to South Pole forming a semi-great circle (half of a great circle) is called a meridian. From the Greenwich meridian, all other meridians are measured. The' term meridian day is used for the day on which the international date line is crossed by a traveller.
Mesosphere Mesosphere is the layer of atmosphere which occurs between the stratopause and the mesopause (or between the stratosphere and the ionosphere).
Metamorphic Rocks Rocks which have been altered from their original state by the process of metamorphism are known as metamorphic rocks. Metamorphism is the process by which rocks-are altered in their mineralogy, texture and internal structure owing to external sources of heat and pressure leading to formation of new rocks.
Meteor Meteor refers to. a bod y of rock and metal travelling at a great speed through space. When drawn into the earth's gravitational field, a meteor becomes luminous due to friction as it falls through the atmosphere, and is seen as a shooting star. If it-reaches the earth's surface, it is called a meteorite.
Meteorology Meteorology is the scientific study of atmosphere. It is usually confined to the study of the tropospheric and stratospheric processes only as these are the layers in which surface weather is generated.
Midnight Sun Midnight sun refers to a phenomenon in which the sun remains in the horizon when viewed between latitudes 631,2°-90° north and south of the equator. The duration of the phenomenon increases towards the poles where it may be observed for six months of each year. The regions of northern Scandinavia (Lapland) are referred to as the Lands of the Midnight Sun.
Mid~Oceanic Ridge (Median Ridge) A large-scale linear elevation rising from the ocean floor is called ffiidoceanic ridge or median ridge. Mid-oceanic ridge marks the boundary of adjoining constructive plate margins. Such ridges occur where the plates of the earth's surface are moving apart, allowing material to well up from the interior. Mid-Atlantic ridges and Chagos-St. Paul Ridges in the Indian Ocean are the examples of the largest median ridges.
Mineral A mineral is a natural inorganic substance possessing a definite chemical composition. It is almost always in crystalline form.
Mist Mist refers to the degree of atmospheric obscurity midway between haze and fog. It results from condensation within the lower layers of the atmosphere.
Mistral A French term, mistral refers to a cold dry and usually strong northerly or northwesterly wind blowing in South France, especially in Rhone delta area. It is most common in winter when pressure is low over the' Mediterranian and high over Central Europe.
Mohs' Scale Named after its inventor, Moh's scale is the ~ scale that is used to determine the hardness of minerals. , ,In this scale, teQ, selected minerals are arranged so that each one can scratch all of those which precede it on the scale.
Monadnock Monadnock refers to an isolated hill that rises above the surrounding flat or gently undulating plain or peneplain. Formed by denudation, a monadnock is an outcrop of more resistant rocks.
Monsoon Monsoon refers to seasonal reversal of winds, pressure and rainfall in the tropical regions. South East Asia provides the best example of the monsoon area in the world. Monsoon wind includes any wind which has periodic alternations of direction and velocity related to the seasonal reversal of the subtropical jet stream and pressure over continents and neighbouring oceans.
Moon Moon is the only satellite of the earth. It has approximately one~fourth of the earth's diameter and has about one-sixth of the earth's surface gravitation. Its mass is 1/81 of the earth.
Moraine Moraine refers to an accumulation of rocks, boulders, pebbles and clay by a glacier or ice-sheet.
Mountain A portion of the land surface rising considerably above the surrounding country either as a single eminence or in a range or chain is called mountain. Mountains are formed by the process of orogenesis,
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