The hydrosphere includes water that is on the surface of the planet, underground, and in the air. A planet's hydrosphere can be liquid, vapor, or ice. On Earth, liquid water exists on the surface in the form of oceans, lakes and rivers.
geologists often need to know the age of material that they find. they use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years. this is different to relative dating, which only puts geological events in time order.