chemical weathering of rocks on the continents is one source of elements found in sea water.
Mass extinctions occur for many reasons. Changes in the environment might make it impossible for the local life to survive, diesease or drought might strike, or there might be a natural disaster. These extinctions make good divisions in geologic time because they are easy to locate over a wide area and mark a definitive change in the Earth's systems.
I’m sorry I don’t speak what ever launguage that is
