<P> Earth has evolved through geological and biological processes that have left traces of the original conditions . The outer surface is divided into several gradually migrating tectonic plates . The interior remains active, with a thick layer of plastic mantle and an iron - filled core that generates a magnetic field . This iron core is composed of a solid inner phase, and a fluid outer phase . Convective motion in the core generates electric currents through dynamo action, and these, in turn, generate the geomagnetic field . </P> <P> The atmospheric conditions have been significantly altered from the original conditions by the presence of life - forms, which create an ecological balance that stabilizes the surface conditions . Despite the wide regional variations in climate by latitude and other geographic factors, the long - term average global climate is quite stable during interglacial periods, and variations of a degree or two of average global temperature have historically had major effects on the ecological balance, and on the actual geography of the Earth . </P> <P> Geology is the science and study of the solid and liquid matter that constitutes the Earth . The field of geology encompasses the study of the composition, structure, physical properties, dynamics, and history of Earth materials, and the processes by which they are formed, moved, and changed . The field is a major academic discipline, and is also important for mineral and hydrocarbon extraction, knowledge about and mitigation of natural hazards, some Geotechnical engineering fields, and understanding past climates and environments . </P> <P> The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations . </P>

Earth is considered a system because all its parts