■ SEISMIC DISCONTINUITY
#Interiorofearth
◇ Seismic discontinuities are the regions in the earth where seismic waves behave a lot different compared to the surrounding regions due to a marked change in physical or chemical properties.
◇ Mohorovicic Discontinuity (Moho): separates the crust from the mantle.
◇ Asthenosphere: highly viscous, mechanically weak and ductile part of mantle.
◇ Gutenberg Discontinuity: lies between the mantle and the outer core.
■ SURGE TECTONICS
#Interiorofearth
◇ Surge tectonics refers to the genesis of global surge waves caused by changes (weakening in the regional gravity fields due to upward movement of deformable magma in the surge channels in the lithosphere (crust, 100-200 km thick upper layer of the earth) lying above mantle.
◇ Strictly speaking, surge tectonics means upward motion of fluid magma in surge channels magma conduits), rise in temperature of regional oceanic water and consequent decrease in pressure and shift in regional gravity field of oceanic crust.
◇ The motions in the surge channels are caused by earth’s rotation.
◇ Magma, while rising through the surge channels, undergoes its transformation (deformation; i.e. it becomes lighter (decrease in density) and less compact and hence expands.
◇ This consequent expansion in magma reduces gravitational attraction in the surge channels and weakens the regional gravity fields.
■ EARTH’S MAGNETIC FIELD
#Magneticfield
◇ A 'field' is a region in which a body experiences a force owing to the presence of other bodies.
◇ Gravitational fields determine how bodies with mass are attracted to each other.
◇ In electric fields, objects that have an electric charge are attracted or repelled from each other.
◇ Magnetic fields determine how electric currents that contain moving electric charges exert a force on other electric currents.
■ MAGNETIC POLES
#Magneticfield
◇ A magnet's North pole is thought as the pole that is attracted by the Earth's North Magnetic Pole when the magnet is suspended so it can turn freely.
◇ Since opposite poles attract, the North Magnetic Pole of the Earth is the south pole of its magnetic field.
◇ Magnetic dipole field (simple north-south field like that of a simple bar magnet) is usually aligned fairly closely with the Earth's rotation axis; in other words, the magnetic poles are usually fairly close to the geographic poles, which is why a compass works.
◇ However, the dipole part of the field reverses after a few thousand years causing the locations of the north and south magnetic poles to switch.
◇ The terms magnetic north and magnetic south are not to be confused with geographic north and geographic south, and geomagnetic north and geomagnetic south.
■ GEOMAGNETIC REVERSAL
#Magneticfield
◇ A geomagnetic reversal or a reversal in earth’s magnetic field is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged.
◇ Based on palaeomagnetism (is the study of the record of the Earth's magnetic field in rocks, sediment, or archaeological materials).
◇ It is observed that over the last 20 million years, magnetic north and south have flipped roughly every 200,000 to 300,000 years.
◇ The reversal is not literally 'periodic' as it is on the sun, whose magnetic field reverses every 11 years.
◇ The time between magnetic reversals on the Earth is sometimes as short as 10,000 years and sometimes as long as 25 million years.
◇ And the time it takes to reverse could be about a few hundred or a few thousand years.
◇ The magnetic poles emerge at odd latitudes throughout the process of the reversal.
■ NORMAL AND REVERSE FIELD
#Magneticfield
◇ The Earth's field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite.
◇ In Normal Polarity, Earth’s North Magnetic Pole is the South Pole of its Magnetic Field. In Reverse Polarity, Earth’s North Magnetic Pole is the North Pole of its Magnetic Field.
