Erosion is the process of removing rock particles by water, wind, ice or gravity.
In ancient times, mountains were used as a natural barrier (used for protection) from the weather and neighboring countries. They're neighboring countries may go into a "great depression" and seek for new natural resources. They may acomplish this by attacking near by villages, but if you were on the other side of the mountain from them, you were safe since it is very long and dangerous expedition.
A dense oceanic plate plunges beneath the North American plate (subduction). As the oceanic plate sinks down beneath the Earth's interior, beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape. The water vapor rises to the pliable mantle above the subducting plate causing some of the mantle to melt.