Third question: How much salt is there in the sea water?
In the most ancient pre-historic period erupting volcanoes emitted large quantities of gases and steam which in turn became massive clouds filling up the sky. Rain fell incessantly for hundreds of years. Rainwater collected in low-lying areas and formed seas for the first time.
The earliest rain water was somewhat salty or saline because gases like hydrogen chloride emitted by volcanoes had dissolved in it. Rainwater also dissolved salts on the ground. Although rocks and stones remained almost unaffected, their soluble salt (e.g. sodium and magnesium) continued to dissolve in rain and went on increasing the salinity of sea water year after year. Today the salt content in sea water is 1 kilogram for each cubic foot of water on an average. If all the salt is extracted from sea water its quantity will be sufficient to cover all the land of earth with a layer of salt about 500 feet thick!