<P> In 1902, the Santa Maria Volcano erupted violently in Guatemala, with the largest explosions occurring over two days, ejecting an estimated 5.5 km (1.3 mi) of magma . The eruption was one of the largest of the 20th century, only slightly less in magnitude to that of Mount Pinatubo in 1991 . The eruption had a volcanic explosivity index of 6 . Today Santiaguito is one of the world's most active volcanoes. (1) (2) </P> <P> Volcanoes of Mexico are related to subduction of the Cocos and Rivera plates to the east, which has produced large, explosive eruptions . Most active volcanoes in Mexico occur in the Trans - Mexican Volcanic Belt, which extends 900 kilometres (559 mi) from west to east across central - southern Mexico . A few other active volcanoes in northern Mexico are related to extensional tectonics of the Basin and Range Province, which splits the Baja California peninsula from the mainland . Popocatépetl, lying in the eastern half of the Trans - Mexican Volcanic Belt, is the second - highest peak in Mexico after the Pico de Orizaba . It is one of the most active volcanoes in Mexico, having had more than 20 major eruptions since the arrival of the Spanish in 1519 . The 1982 eruption of El Chichón, which killed about 2,000 people who lived near the volcano, created a 1 - km - wide caldera that filled with an acidic crater lake . Prior to 2000, this relatively unknown volcano was heavily forested and of no greater height than adjacent nonvolcanic peaks . </P> <P> The United States Geological Survey and the National Earthquake Information Center (NEIC) are located on the campus of the Colorado School of Mines in Golden, Colorado . Both monitor volcanos in the United States; in the western United States lies the Cascade Volcanic Arc . It includes nearly 20 major volcanoes, among a total of over 4,000 separate volcanic vents including numerous stratovolcanoes, shield volcanoes, lava domes, and cinder cones, along with a few isolated examples of rarer volcanic forms such as tuyas . Volcanism in the arc began about 37 million years ago, but most of the present - day Cascade volcanoes are less than 2,000,000 years old, and the highest peaks are less than 100,000 years old . The arc formed by the subduction of the Gorda and Juan de Fuca plates at the Cascadia subduction zone . This is a 680 - mi - long fault, running 50 mi (80 km) off the coast of the Pacific Northwest from northern California to Vancouver Island, British Columbia . The plates move at a relative rate over 0.4 in (10 mm) per year at a somewhat oblique angle to the subduction zone . </P> <P> Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area . When the "locked" zone stores energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture . Thermal and deformation studies indicate that the locked zone is fully locked for 60 km (about 40 mi) down - dip from the deformation front . Further down - dip, a transition from fully locked to aseismic sliding occurs . </P>

Type of volcano that is commonly found in the pacific ring of fire