Why is the Pacific realm so geologically active?
The coastal areas that surround the Pacific Ocean along North America, South America, and Asia and encompass tens of thousands of miles or coastline are commonly referred to as the "Ring of Fire". This is because there is so much geological activity associated with this area, including earthquakes and volcanoes. In fact, the majority of the world's active seismic activity occurs in this area. This is because the entire Pacific coastal rim is a series of tectonic plate boundaries.
The entire lithosphere of the Earth is made up of a series of tectonic plates that literally float on the asthenosphere and move very slowly over time. Along North and South America, the Pacific, Nazca, and some smaller plates border the North and South American plates. Along Asia and the island systems of the South Pacific like Micronesia and Polynesia, the Pacific plate makes contact with the Filipino and Australian Plates. These plates subduct (move underneath) each other and slide along each other, this causing seismic activity and earthquakes. In addition, diverging plates and moving hotspots cause volcanic activity.
The island of Australia is in the center of a plate, thus it is not associated with a lot of geological activity. The same can be said of the Atlantic coastal areas. The plate boundaries are in the middle of the Atlantic Ocean, thus the coastal areas are not associated with heavy geological activity.