The Earth’s surface is made up of several major slabs of crust which move extremely slowly relative to one another and, over millions of years, have been responsible for the changing shape of the continents. These plates move in different directions at different rates, creating complex and differing boundaries. Where plates move apart, new crust is created at constructive (or divergent) margins; where plates move together, they create convergent (or destructive) margins; and where plates move laterally past one another, the margins are termed conservative. All of these boundaries are seismically active to a greater or lesser extent and usually display volcanic activity of some kind.
The Pacific Plate and Plate Tectonics
The Pacific Ring of Fire is the name commonly given to the belt of volcanic (and earthquake) activity around the Pacific Ocean. The ring traces the Pacific margin from New Zealand via Tonga and New Guinea, extending eastwards to encompass Java and Sumatra; the Philippines; the island arcs of the Pacific and Japan, and eastern Russia, Alaska and the western margins of the USA before heading seawards south of California. From here, the Ring of Fire continues south along the coast of Mexico and South America.
As the Pacific Plate is moving roughly north-west, its constructive margins are found to the south and the east (offset by lengths of conservative boundary), where it is moving away from the eastward-moving Nazca Plate adjacent to South America and the Antarctic Plate, which moves approximately south-eastward.
At the northern and western margins, the Pacific Plate comes into direct contact with several different plates: The Eurasian, Australian, Philippine and North American plates and an associated complex array of smaller ‘microplates’. Here, the relatively-dense oceanic crust sinks below the continental plates at destructive margins.
Conservative margins occur at various stages, linking different sections of constructive and destructive margins. The best-known conservative margin section is that associated with the San Andreas Fault zone, which runs along the western coast of the USA.