The volcanic arcs and oceanic trenches that partly encircle the Pacific Basin form the so-called “Ring of Fire.” This zone, which is notorious for frequent earthquakes and volcanic eruptions, coincides with the edges of one of the world's major tectonic plates. More than half of the world’s active volcanoes above sea level are part of this ring.

In the figure above, from the U.S. Geological Survey, the oceanic trenches are shown in blue-green. The volcanic island arcs, although not labelled, are parallel to, and always landward of, the trenches. For example, the island arc associated with the Aleutian Trench is represented by the long chain of volcanoes that make up the Aleutian Islands.

All the Earth’s tectonic plates move. And as they move, intense geologic activity occurs at the plate edges, where one of three things may occur. When the plates move away from each other, magma from within the Earth percolates up to form new seafloor. When the plates move toward each other, they will either slide past one another, or one plate will become submerged under the other.

When parts of the plate boundary slide past one another in opposite directions — such as the San Andreas Fault — minor earthquakes occur. The faults also may create cliffs thousands of feet high on the ocean bed. When two plates collide and one is forced under the other, deep into the Earth’s interior, the subsumed plate encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises to the Earth’s surface and erupts, forming chains of violent volcanoes like the Ring of Fire. These narrow plate-boundary sites, known as subduction zones, are also associated with the formation of deep ocean trenches and big earthquakes. When there is an earthquake under the sea, one side of the ocean floor suddenly drops downward, beneath the top edge of the subducting plate. The resulting vertical fault will generate a tsunami, or harbor wave, with often devastating consequences.

On August 26,1883, the island volcano of Krakatau (“Krakatoa”) in Indonesia erupted, setting into motion a series of huge tsunamis, some reaching a height of nearly 40 meters (more than 120 feet) above sea level. The tsunamis killed more than 36,000 people on the islands of Java and Sumatra and were recorded or observed throughout the Indian Ocean, Pacific Ocean, U.S. West Coast, South America, and even as far away as the English Channel.

Sources: U.S. Geological Survey and the British Broadcasting Company.