Scientists view Earth as a system of interconnected components that interact
and affect each other. The principal subsystems of Earth are the atmosphere,
hydrosphere, biosphere, lithosphere, mantle, and core. Earth is considered a
dynamic planet that continually changes because of the interaction among its
various subsystems and cycles.
Geology, the study of Earth, is divided into two broad areas: Physical geology
is the study of Earth materials, as well as the processes that operate within
Earth and on its surface; historical geology examines the origin and evolution
of Earth, its continents, oceans, atmosphere, and life.
The scientific method is an orderly, logical approach that involves gathering
and analyzing facts about a particular phenomenon, formulating hypotheses
to explain the phenomenon, testing the hypotheses, and finally proposing a
theory. A theory is a testable explanation for some natural phenomenon that
has a large body of supporting evidence. Both the theory of organic evolution
and plate tectonic theory are theories that revolutionized biology and geology,
The Universe began with the Big Bang approximately 14 billion years ago.
Astronomers have deduced this age by observing that celestial objects are
moving away from each other in an ever-expanding universe. Furthermore,
the universe has a pervasive background radiation of
2.7 K above absolute zero (2.7 K = -270.3°C), which is thought to be the faint
afterglow of the Big Bang.
About 4.6 billion years ago, our solar system formed from a rotating cloud of
interstellar matter. As this cloud condensed, it eventually collapsed under the
influence of gravity and flattened into a counterclockwise rotating disk.
Within this rotating disk, the Sun, planets, and moons formed from the
turbulent eddies of nebular gases and solids.