The theory of plate tectonics proposes that the earth crust is broken up in large plates. The plate boundaries do not necessarily match the coastlines of continents. A plate can consist of continental crust, oceanic crust, or both. In most cases, continents are part of larger plates that extend for hundreds of miles offshore. Many plate boundaries are far out in the middle of the ocean. There are three types of plate boundaries: divergent, convergent, and transform.

• Divergent boundaries exist where plates move away from each other, pushed apart by heated, material moving upwards from the asthenosphere. Divergent boundries undersea are known as Seafloor Spreading Zones. An additional force involved in divergence may be the subduction of the heavier, older, and thicker crust at the opposite ends of each diverging plate. As the heavy edge sinks, it pulls the rest of the plate with it, away from the divergent boundary. Magma at the divergent boundary hardens, adding new crust to the edges of the separating plates. Scientists often refer to these as constructive boundaries, due to the construction of new material. Mid-ocean ridges are examples of this type of boundary. These ridges frequently resemble submarine mountain ranges, portions of which are high enough to break the ocean’s surface (such as the Galapagos Islands).
• Convergent boundaries boundaries where two plates collide. When an oceanic plate, such as the Nazca Plate which moves eastwards under the southeastern Pacific Ocean, meets a continental edge such as South America, the denser and heavier oceanic crust is normally subducted and partially melted beneath the continental plate. Ocean trenches at the boundary of the plate and mountain chains on the continental plate often result. Earthquakes can occur at these plate margins, shifting plates by up to 5 metres (about 15 feet) at once. Such faults exist in Chile, Japan, Taiwan, the Philippines, New Zealand, and Sumatra. At subduction zones, where one plate moves beneath the other, the subducted plate is dragged downwards into the earth's mantle until it reaches a depth where high temperatures partially melt the rock. The resulting magma then rises along vertical fissures and reaches the surface through a volcanic vent. Volcanoes along the Andes in South America and the Cascade Range in North America are examples of volcanoes that formed on continental crust overlying subduction zones. When fissures open up on the seafloor, volcanic islands form as a result, such as Japan and the Philippines.
• At a transform boundary, plates move past each other in opposite directions. Little volcanic activity accompanies transform boundaries, but large, shallow earthquakes can occur. The San Andreas Fault in California, United States, is the most famous example of this type of boundary. The interaction of plates at a transform boundary does not normally lead to volcanic activity.

At divergent plate boundaries, where two plates move away from each other, magma wells up along the linear boundary. Iceland is a volcanic land mass on top the Mid-Atlantic Ridge, a divergent plate boundary. New additions along this ridge, such as the island of Surtsey, still continue to be created. A third type, known as transform boundaries, exists when two plates slide alongside each other. 

The Galapagos Rift

Where the Galapagos Rift intersects the East Pacific Rise, there is a region of intense underwater volcanic activity. This active region is being extensively studied by scientists in deep-diving research submarines. It was here that the first sightings of the deep-ocean chemosynthetic marine ecosystem were made. This system supports a multitude of deep-sea animals without using sunlight as the basis for the food chain. It is supported by bacteria that consume chemicals emitted by underwater volcanoes along the rift zone.

The East Pacific Rise

Like other mid-oceanic ridges, the East Pacific Rise is a region of seafloor spreading. It is considered a fast-spreading centre, with the crustal plates diverging at a rate of up to 18 centimetres (7 inches) each year. Constant volcanic activity pushes the ocean floor apart, and hot, molten lava bubbles up through the earth’s crust to form new crustal material. This rapid spreading rate means that the new crust is thin, and the slopes of the ridge are low and gentle. In contrast, along a slow-spreading centre such as the Mid-Atlantic Ridge, the new oceanic crust builds up into high, rugged, volcanic peaks.

Another unique feature of the East Pacific Rise is the presence of hydrothermal vents, where extremely hot, sulphurous water shoots up from sculpted undersea chimneys, which are called black smokers. These vents support ecosystems of unusual creatures, such as clusters of giant tube worms and delicate siphonophores—tentacled creatures that look like underwater dandelions. Sunlight cannot penetrate to the ocean depths here, but bacteria feed on the sulphur in the water through a process called chemosynthesis to support the life forms of the hydrothermal vents.

Subduction

An important effect of the melting of subducted ocean crust is the production of new magma. When subducted ocean crust melts, the magma that forms may rise from the plane of subduction deep within the mantle, erupting on the earth’s surface. Eruption of magma melted by subduction has created long, arc-shaped chains of volcanic islands, such as Japan, the Philippines, and the Aleutians. Where an oceanic plate is subducted beneath continental crust, the magma produced by subductive melting erupts from volcanoes situated among long, linear mountain chains, such as the Andes in South America.