 Geosphere, there are several conflicting definitions for geosphere. The geosphere may be taken as the collective name for the lithosphere, the hydrosphere, the cryosphere, and the atmosphere. In Aristotelian physics, the term was applied to four spherical natural places, concentrically nested around the center of the Earth. Geosphere is described in the lectures Physica and Meteorologica. They were believed to explain the motions of the four terrestrial elements, Earth, Water, Air and Fire. In modern texts and in Earth system science, geosphere refers to the solid parts of the Earth. It is used along with atmosphere, hydrosphere, and biosphere to describe the systems of the Earth. The interaction of these systems with the magnetosphere is sometimes listed. In that context, sometimes the term lithosphere is used instead of geosphere or solid Earth. The lithosphere, however, only refers to the uppermost layers of the solid Earth oceanic and continental crustal rocks and uppermost mantle. Since space exploration began, it has been observed that the extent of the ionosphere or plasma sphere is highly variable, and often much larger than previously appreciated, at times extending to the boundaries of the Earth's magnetosphere or geopagnetosphere. This highly variable outer boundary of geogenic matter has been referred to as the geopause to suggest the relative scarcity of such matter beyond it, where the solar wind dominates.