The lithosphere comprises all the hard, cold, and solid rock of the surface of the planet (crust), the hot liquid rock close to the center of the planet, the hot semi-solid rock that lies below the crust, and the solid iron core of the planet.
The hydrosphere comprises all the planet's liquid, solid, and gaseous water. The biosphere comprises all the living species of the planet, and the atmosphere comprises all the air of the planet.
All these spheres are closely associated. For example, various birds (biosphere) fly through the air (atmosphere), while water (hydrosphere) generally flows through the soil (lithosphere).
The spheres are so closely associate with each other that a modification in one sphere generally leads to a modification in one or more of the other spheres. Such modifications, which occurs within an ecosystem are considered as events.
