The earth behaves like a magnet. The magnetic field at a point on the surface of the earth is such as would be caused by a dipole magnet placed near its centre. About 97% of the earth's main magnetic field is of internal origin in the inner molten core of the earth and the rest is produced by the atmospheric electric current system attributed to extra-terrestrial causes like the sun and etc. Thus "Geomagnetism is that branch of science which deals with the study and characteristics of geomagnetic field".

The earth's magnetic field or, in modern usage, the geomagnetic field resembles, at a first approximation, the field produced by a magnetic dipole situated at the earth’s centre. The axis of the magnetic dipole makes and angle of 1.5° with the earth’s axis of rotation and cuts the earth’s surface at the Geomagnetic poles.

The actual magnetic field of the earth departs from the above model. The actual magnetic poles are situated where the lines of force are perpendicular to the Earth’s surface as observed by the Dip needle. Therefore, the magnetic poles are also called dip poles. The Dip equator or magnetic equator is situated where the lines of force are horizontal as observed with the dip needle. The magnetic meridian is the projection of the actual line of force on the horizontal plane. The two magnetic poles are some 500 to 1000 miles distant from the respective geomagnetic poles. The field intensity is about 0.65< near the poles and 0.25< near the equator. Decrease of intensity with height above ground proceeds approximately as the cube of the distance from the earth’s centre.

Magnetic Parameters:
 
1.Inclination I

horizontal plane is the inclination I, also called dip. I is positive when the geomagnetic field vector is directed down ward (i.e. in the northern hemisphere) and Z varies +90° at the north magnetic pole and –90° at the south magnetic pole.

The scalar magnitude of the field vector is called the total intensity F, which is always positive and varies between 0.25 gauss (<)in South America and 0.80 gauss near the north magnetic pole.

The projection of the geomagnetic field vector on the horizontal plane is the horizontal intensity H which is always positive and can assume values between zero at the magnetic poles and 0.40 gauss at the magnetic equator.

The projection of the geomagnetic field vector on the vertical is the vertical intensity Z which is zero at the magnetic equator, +0.60 gauss at the north magnetic pole and –0.70 gauss at the south magnetic pole.

The angle made by the magnetic meridian and the true north meridian is the magnetic declination D, which is positive or east when the magnetic meridian is east of the true north meridian. Near the magnetic poles, D may vary from 180° to –180°. A t lower latitudes D is confined to ± 30°.

The projection of H on the true meridian is the geomagnetic north component X, which is always positive except for small areas near the magnetic poles.

The projection of H on the true east direction is the geomagnetic east component Y. The geomagnetic east component is positive when directed eastward and is of the same algebraic sign as the magnetic declination.

Most materials are composed of molecules made of atoms, each of which have electrons orbiting nuclear protons, but where the motion of one electron, essentially a tiny electric current, generates a magnetic field that is cancelled by the magnetic field generated by the motion of another electron. In magnetic materials this cancellation is incomplete, and so the atoms of the material have small net electric currents and they thus generate small magnetic fields. As a result, the material exhibits a magnetic field. It is a ‘magnet’.(FIG-2)

The needle of a compass is a small magnet, one that is allowed to pivot in the horizontal plane. The needle experiences a torque from the ambient magnetic field of the Earth. The reaction to this torque is the needle’s preferred alignment with the horizontal component of the geomagnetic field. The ‘north’ end of the compass needle is simply the north end of the magnet, and it is the end of the compass needle that points in the general direction of the geographic north pole; naturally, the ‘south’ end of the compass needle is the south end of the magnet and it points in the opposite direction, towards the general direction of the geographic south pole.

At most places on the Earth's surface, the compass doesn’t point exactly toward geographic north. The deviation of the compass from true north is an angle called 'declination'

The crust of the Earth has some permanent magnetization, and the core of the Earth, the outer part of which is liquid iron and the inner of which is solid iron, generates its own magnetic field, sustaining the main part of the field we measure at the surface.

            The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The "skin" of Earth, called the crust, is very thin like the skin of a peach. There are two types of crust, oceanic and continental. As you might suspect from the names, oceanic crust is usually found under the oceans, and continental crust is found on the continents. Oceanic crust is made primarily of a rock called basalt. It contains almost as much iron and magnesium as the mantle rock. Oceanic crust also contains thin layers of limestone, serpentine, and clay. Limestone is made of calcium and carbon dioxide. Serpentine and clay are rocks like basalt that contain parts of water molecules. An important fact about these rocks is that when they are heated, the carbon dioxide and water "boil" out as gases.

Continental crust is made mostly of granite, a rock that contains smaller amounts of iron and larger amounts of quartz. As you may know from just looking around, continental crust also contains some limestone, sandstone, and basalt--a little bit of just about everything. An important fact about continental crust is that it is less dense than basalt or mantle rocks. Thus it "floats" on the mantle rocks like a piece of wood in water. In fact, you and I live on dry land because the continents "float" much higher in the mantle "ocean" than oceanic crust does (just as balsa floats higher than pine).

The equator is the intersection of the Earths surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In more simple language, it is an imaginary line on the Earth's surface approximately equidistant from the North Pole and South Pole that divides the Earth into a Northern Hemisphere and a Southern Hemisphere.

The outer core is in a state of turbulent convection. This sets up a process that is a bit like a naturally occurring electrical generator, where the convective kinetic energy is converted to electrical and magnetic energy. Basically, the motion of the electrically conducting iron in the presence of the Earth's magnetic field induces electric currents. Those electric currents generate their own magnetic field, and, as the result of this internal feedback, the process is self-sustaining, so long as there is an energy source sufficient to maintain convection.(FIG-6)

Reversing of the polarity of the magnetic field is called paleo-magnetisam. When lavas are deposited on the Earth’s surface, and subsequently freeze, and when sediments are deposited on ocean and lake bottoms, and subsequently solidify, they often preserve a signature of the ambient magnetic field at the time of deposition