The sun drives the water cycle. The sun evaporates water from the oceans, seas, lakes and other bodies of water and also from plants through the process of transpiration. The evaporated water then forms into clouds and when it gains enough mass it falls back to Earth. This is a closed system so that means that this is the same amount of water that has always been present on Earth.
The youngest age of the shale is 910 ka. According to the principle of inclusions, the shale formed before the tephra, so the tephra must be younger than 910 ka.
