Room 2 at Woodlands School asks :-
How did the sea form?
James White, a geologist at Otago University, responded.
The sea contains most of the earth's water. Water originally started to form on earth when the young, hot planet's surface cooled below the boiling point of water. Once the surface was cool enough for water to stand, volcanoes over millions and millions of years continued to add water to the surface. It arrived in volcanic gases, which are mostly steam, and then condensed to help form the sea.
After a long time, the amount of water in the sea stopped increasing because less 'new' water was coming up out of the volcanoes. There are still a lot of volcanoes producing steam, but most of this water is recycled. This happens because some water from the seas gets trapped in sediment on the seafloor where it is carried down into the earth at 'subduction zones'. Most of the volcanoes we see today, like Ruapehu or White Island, are found with these subduction zones. The subduction zone for Ruapehu and White Island is under the Hikurangi trench, along the east coast of the North Island.
The 'saltiness' of seawater isn't from volcanoes directly. Salts in rocks and soils on land are slowly dissolved (chemically broken down) by water (rainwater, river water etc). Rivers then carry these salts to the oceans. There is little salt dissolved in river water so they don't seem salty to us. The ocean is salty mostly because, over millions and millions of years, rivers have been carrying into it dissolved salts from the chemical breakdown of rocks and soils on land.
Once the water is in the ocean, some of it evaporates to form clouds, leaving the salt behind in the water. The clouds drift over land, making rain that feeds the rivers, carrying yet more salt to the ocean. The amount of salt in the ocean stays fairly constant even though the rivers continue to bring in salt because it is in chemical balance. If salt levels get too high, salt crystals form and separate from the water, taking away the excess salt.