Why do scientist use mass extinction to separate one era from another

One of the main reasons the united states occupied japan after world war ii was that

What id the primary human cause of the global warming a. storage of nuclear fuel rods b. burning of fossil fuels c. building of high-density housing d. violent conflicts over natural resources

Using plate tectonics and elastic rebound theory, describe why juan de fuca plate subduction produces so few earthquakes. what will happen in the future?