Gum disease generally develops because of improper brushing and flossing habits, although underlying medical conditions may have a correlation to greater incidence. The bacteria that are found in the mouth as a result of these poor habits begin to affect the surrounding bone and tissues below the gumline.
Most earthquakes occur at the boundaries where the plates meet. In fact, the locations of earthquakes and the kinds of ruptures they produce help scientists define the plate boundaries. There are three types of plate boundaries: spreading zones, transform faults, and subduction zones.
Earthquakes and volcanic eruptions occur due to plate movements. Plates are found under the crust of the Earth. These phenomena occur near plate boundaries. Countries that are on plate boundaries include Japan, Indonesia, Peru, Chile and the western edge of the United States.