Non-testable hypotheses are also rejected. A theory is a grand or unifying hypothesis that has survived all tests devised to date. A theory is not just someone's opinion. Some examples of theories are Relativity in Physics, Evolution in Biology, and Plate Tectonics in Earth Sciences. Earth's Surficial Processes and External Heat Engine Surface processes are mostly driven by solar energy, the external heat engine.
Water is evaporated from the oceans and rains down on the continents.
Solidification of these sediments by burial to form sedimentary rocks is called lithification . This cycling of water is called the hydrologic cycle . Solar driven wind may also redistribute the sediments on the land surface. Understanding sedimentary rocks and processes and the record of life on the surface contained in these rocks led to the first great revolution in the Earth sciences, the recognition of the vast amounts of time required for the formation of the planet and the evolution of the landforms and life forms of the surface. Geologists of the early part of the last century recognized that Earth history must be many millions of years, not thousands as advocated by theologians.