Earth has a gravitational field and a magnetic field. Both are closely related to Earth's shape and internal structure, and they form the basis of modern science and technology.
Basic Knowledge
Gravity: The resultant force of Earth's gravitational attraction and the centrifugal force caused by its rotation. At the equator, it is weaker than at the poles because of the effect of centrifugal force. The standard gravitational acceleration is 9.80665 m/s². Also, when high-density rocks exist underground, gravity becomes slightly stronger (a gravity anomaly). By measuring this, gravity surveys are conducted to investigate underground structures.
Geomagnetism (Earth's magnetic field): Earth behaves like a large magnet and has a magnetic field in the north-south direction. The geomagnetic north pole(magnetic north) is slightly different from the geographic North Pole (true north), and this angular difference is called magnetic declination. The angle between the magnetic field and the horizontal plane is called the magnetic inclination. At the equator, magnetic inclination≈0°, while at the poles it is≈90°.
The source of geomagnetism is thought to be the dynamo effect caused by convection of liquid iron in the outer core. Geomagnetism deflects charged particles (cosmic rays and solar wind) from space so that they do not directly strike Earth, serving as a shield that protects life.
Gravity（gravitational attraction+centrifugal force. Equator < poles）
Gravity anomaly（A deviation from the theoretical value. Reflects underground density structures）
Geomagnetism (Earth's magnetic field)（Generated by convection of liquid iron in the outer core）
Magnetic declination（The angular difference between magnetic north and true north）
Magnetic inclination（The angle between the magnetic field direction and the horizontal plane. Equator≈0°・poles≈90°）
Geomagnetic reversal（The phenomenon in which Earth's magnetic field has reversed direction many times in the past. Recorded in the remanent magnetism of rocks）
In Depth
Geomagnetism has reversed many times in the past, and records of these reversals remain as remanent magnetism in seafloor basalt. When lava erupted from a mid-ocean ridge cools, it becomes magnetized according to the direction of geomagnetism at that time. Therefore, striped magnetic anomaly zones are distributed symmetrically on both sides of the ridge. This is one piece of evidence for the seafloor spreading hypothesis.
The strength of the present geomagnetic field has gradually decreased over the past several thousand years, and scientists are paying attention to whether this is a sign of an approaching geomagnetic reversal. If a reversal occurs, geomagnetism temporarily weakens, possibly increasing exposure to cosmic rays.
auroras are a phenomenon in which charged particles from the solar wind are trapped by geomagnetism and emit light when they collide with the atmosphere near the polar regions.
- Gravity=gravitational attraction+centrifugal force. Smallest at the equator and largest at the poles
- Source of geomagnetism=dynamo effect of liquid iron in the outer core
- Magnetic declination=the difference between magnetic north and true north
- Magnetic inclination=small at the equator and large at the poles
- Evidence of geomagnetic reversals=remanent magnetic stripes on the seafloor
- Geomagnetism protects life from cosmic rays
Points to Note
① Do not confuse magnetic declination(the difference between magnetic north and true north) with magnetic inclination(the tilt of the magnetic field). ② The geomagnetic north pole (magnetic north pole) is in a different location from the geographic North Pole and moves every year. ③ Gravity is weaker at the equator and stronger at the poles for two reasons: “the equator is farther from the center (greater centrifugal force)+centrifugal force reduces gravitational attraction.”
Practice
- Explain the leading theory about the source of geomagnetism.
- What is the difference between the direction indicated by the north end of a compass needle (magnetic north) and the geographic North Pole (true north) called?
- Give two reasons why gravity is stronger at the poles than at the equator.