Geophysics

I

Introduction

Geophysics, branch of science that applies physical principles to the study of the Earth. Geophysicists examine physical phenomena and their relationships within the Earth; such phenomena include the Earth's magnetic field, heat flow, the propagation of seismic (earthquake) waves, and the force of gravity. The scope of geophysics also broadly includes outer-space phenomena that influence the Earth, even in subtle ways; the effects of the Sun on the Earth's magnetic field; and manifestations of cosmic radiation and the solar wind.

II

Areas of Study

Subdivision of the wide-ranging subject matter of geophysics into various branches involves categorizing specific endeavours. Strictly speaking, however, the discipline embraces all fields devoted to researching the Earth's interior, atmosphere, hydrosphere (waters), and ionosphere (ionized upper atmosphere). Related fields are included in the following descriptions.

A

Solid Earth Physics

Embracing all fields devoted to the Earth's interior, solid Earth physics involves studying the behaviour of Earth materials from the crust down to the core, particularly as they relate to the Earth's size and shape, gravity, magnetism, and seismicity. The specialized field of geodesy is concerned with determining the Earth's size and shape and locating precise points on its surface. Involved in this study are the determination of the Earth's gravitational field and observation of variations in the Earth's rotation, the location of the poles, and tides. Two new techniques for making geodetic measurements, Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR), have been used to determine, within a fraction of a centimetre, the rates at which the continents are moving towards or away from each other. See Plate Tectonics.

B

Terrestrial Magnetism

Geomagnetism refers to the study of magnetic phenomena exhibited by the Earth and its atmosphere. Generation of the magnetic field seems to be related to the motion of fluid, electrically conducting material within the Earth, so that the planet acts as a self-exciting dynamo. The conducting material and the geomagnetic field may mutually control each other. Study of this problem is known as magnetohydrodynamics or hydromagnetics. The study of how the magnetic field has changed throughout the Earth's history, called palaeomagnetism, provided the first strong evidence for the theory of plate tectonics.