Plate Boundaries

It is apparent that most of what we observe on the surface of the earth and at shallow depth is related in some way to plate tectonics. The beaches along the eastern margin of North America are the result of the weathering, erosion, and breakdown of a huge mountain range (the Appalachian Mountains) over the last 300 million years. The Appalachians were formed during the collision of continents to form a mega continent called Pangaea. The volcanic mountains that form the western margin of North America are the result of subduction of oceanic crust under continental crust. The subduction results in frictional heating and partial melting of material being dragged down into the mantle.

Practically all of the major processes that occur on the surface of the earth are related to plate tectonics. Earthquakes, volcanism, and mountain building are three of the most spectacular earth processes related to tectonic activity, however many other processes like metamorphism, sedimentary rock formation, among others are related directly or indirectly to plate tectonics.

There are three types of plate boundaries. They are divergent, convergent and oblique slip (transform). Each boundary type has its own characteristic geologic features and processes, by which it can be identified even millions of years after it has been active. It is clear that the area around the Appalachian Mountains was once a subduction zone even though it has not been active for the last 250 million years.

Pacific Ring of Fire

The Pacific Ring of Fire (or just The Ring of Fire) is an area where large numbers of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean. In a 40,000 km (25,000 mi) horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. The Ring of Fire has 452 volcanoes and is home to over 75% of the world's active and dormant volcanoes. It is sometimes called the circum-Pacific belt or the circum-Pacific seismic belt.

About 90%] of the world's earthquakes and 81% of the world's largest earthquakes occur along the Ring of Fire. The next most seismic region (5-6% of earthquakes and 17% of the world's largest earthquakes) is the Alpide belt, which extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. The Mid-Atlantic Ridge is the third most prominent earthquake belt.

The Ring of Fire is a direct result of plate tectonics and the movement and collisions of lithospheric plates. The eastern section of the ring is the result of the Nazca Plate and the Cocos Plate being subducted beneath the westward moving South American Plate.

The Cocos Plate is being subducted beneath the Caribbean Plate, in Central America. A portion of the Pacific Plate along with the small Juan de Fuca Plate are being subducted beneath the North American Plate. Along the northern portion the northwestward moving Pacific plate is being subducted beneath the Aleutian Islands arc. Further west the Pacific plate is being subducted along the Kamchatka Peninsula arcs on south past Japan. The southern portion is more complex with a number of smaller tectonic plates in collision with the Pacific plate from the Mariana Islands, the Philippines, Bougainville, Tonga, and New Zealand; this portion excludes Australia, since it lies in the center of its tectonic plate. Indonesia lies between the Ring of Fire along the northeastern islands adjacent to and including New Guinea and the Alpide belt along the south and west from Sumatra, Java, Bali, Flores, and Timor.

The famous and very active San Andreas Fault zone of California is a transform fault which offsets a portion of the East Pacific Rise under southwestern United States and Mexico. The motion of the fault generates numerous small earthquakes, at multiple times a day, most of which are too small to be felt. The active Queen Charlotte Fault on the west coast of the Queen Charlotte Islands, British Columbia, Canada, has generated three large earthquakes during the 20th century: a magnitude 7 event in 1929, a magnitude 8.1 occurred in 1949 (Canada's largest recorded earthquake) and a magnitude 7.4 in 1970.

Major Volcanic Areas in the Pacific Ring of Fire

In South America the Nazca plate is colliding with the South American plate. This has created the Andes and volcanoes such as Cotopaxi and Azul.

In Central America, the tiny Cocos plate is crashing into the North American plate and is therefore responsible for the Mexican volcanoes of Popocatepetl and Paricutun (which rose up from a cornfield in 1943 and became a instant mountains).

Between Northern California and British Columbia, the Pacific, Juan de Fuca, and Gorda plates have built the Cascades and the infamous Mount Saint Helens, which erupted in 1980.

Alaska's Aleutian Islands are growing as the Pacific plate hits the North American plate. The deep Aleutian Trench has been created at the subduction zone with a maximum depth of 25,194 feet (7679 meters).

From Russia's Kamchatka Peninsula to Japan, the subduction of the Pacific plate under the Eurasian plate is responsible for Japanese islands and volcanoes (such as Mt. Fuji).

The final section of the Ring of Fire exists where the Indo-Australian plate subducts under the Pacific plate and has created volcanoes in the New Guinea and Micronesian areas. Near New Zealand, the Pacific Plate slides under the Indo-Australian plate.