> the global Earth system is enormously complex.
> physical conditions at the surface change incredibly over space and in time

> there are about 90 chemical elements on Earth (excluding man-made elements) combining in various ways to form millions of compounds, both organic and inorganic

> you can think of the Earth as being made of concentric spheres of matter in different physical states.
> the four great spheres of Earth:

1. Lithosphere - uppermost layer of solid Earth (from surface to up to 150 km deep).
2. Hydrosphere - water on Earth, including oceans, lakes and rivers, glaciers, etc.
   >  think of hydrosphere as lying on top of solid foundation, the lithosphere.
3. Atmosphere - gaseous layer around the Earth. Most of atmosphere is between the surface and 30 km above it. Atmosphere is the outermost Earth sphere enclosing the other ones.
4. Biosphere - all living organisms and their habitat. The core of biosphere (most biomass) is located right at the surface, where all Earth spheres intersect. Lifeforms utilize gases from the atmosphere, water from the hydrosphere, nutrients from the lithosphere. In other words, biosphere is dependent on other three realms.

> the four realms are not independent, but rather intrinsically interconnected
> soil contains mineral particles (lithosphere), water (hydrosphere), air (atmosphere) and organic matter, alive and dead (biosphere).

> weather systems are generated in the atmosphere, but depend on factors such as evaporation of moisture from the hydrosphere and from plants in the biosphere.
> moist air is  transported from one place to another by air currents that depend on the nature of the surface - the lithosphere.
> volcanic eruptions occur in the lithosphere, but affect the other realms substantially.

• Life layer - intersection of all spheres at the surface and near it. This is a very thin layer, it lies within 100 m or so from the surface. This is the place where life on Earth is most protected and enjoys most stability.

• Geosphere - all 4 spheres taken together. It therefore extends from the bottom of lithosphere to the top of atmosphere.