Why are active volcanoes, such as mt. st. helens, usually located in mountain ranges along the boundaries of tectonic plates?

Which of the following situations would most likely occur if a human-induced event caused the ocean water to become the same temperature around the world? deep ocean current circulation would increase salt distribution around the world's oceans. surface ocean current circulation would distribute heat around land masses at a faster rate. new, fresher water would float on top of dense salt water and limit thermohaline circulation. some land masses in the northern hemisphere would start to warm up over time, melting snow and ice. runoff of surface freshwaters and rainfall would no longer have an effect on the salinity of ocean waters.