El Salvador, along with the rest of Central America, is one of the most seismologic-ally active regions on earth, situated atop three of the large tectonic plates that constitute the Earth's surface. The motion of these plates causes the area's earthquake and volcanic activity.
Most of Central America and the Caribbean Basin rests on the relatively motionless Caribbean Plate. The Pacific Ocean floor, however, is being carried northeast by the underlying motion of the Cocos Plate. Ocean floor material is largely composed of basalt, which is relatively dense; when it collides with the lighter granite rocks of Central America, the ocean floor is forced down under the land mass, creating the deep Middle America Trench that lies off the coast of El Salvador. The subduction of the Cocos Plate accounts for the frequency of earthquakes near the coast. As the rocks constituting the ocean floor are forced down, they melt, and the molten material pours up through weaknesses in the surface rock, producing volcanoes and geysers.
North of El Salvador, Mexico and most of Guatemala are riding on the westward-moving North American Plate that butts against the northern edge of the stationary Caribbean Plate in southern Guatemala. The grinding action of these two plates creates a fault (similar to the San Andreas fault in California) that runs the length of the valley of the Rio Motagua in Guatemala. Motion along this fault is the source of earthquakes in northernmost El Salvador.
El Salvador has a long history of destructive earthquakes and volcanic eruptions. San Salvador was destroyed in 1756 and 1854, and suffered heavy damage in the 1919, 1982, and 1986 tremors. The country has over twenty volcanoes, although only two, San Miguel and Izalco, have been active in recent years. From the early nineteenth century to the mid-1950s, Izalco erupted with a regularity that earned it the name "Lighthouse of the Pacific." Its brilliant flares were clearly visible for great distances at sea, and at night its glowing lava turned it into a brilliant luminous cone.
Two parallel mountain ranges cross El Salvador to the west with a central plateau between them and a narrow coastal plain hugging the Pacific. These physical features divide the country into two physiographic regions. The mountain ranges and central plateau, covering 85 percent of the land, comprise the interior highlands. The remaining coastal plains are referred to as the Pacific lowlands.The northern range of mountains, the Sierra Madre, form a continuous chain along the border with Honduras. Elevations in this region range from 1,600 to 2,700 meters. The area was once heavily forested, but overexploitation led to extensive erosion, and it has become semibarren. As a result, it is the country's most sparsely populated zone, with little farming or other development.
The southern range of mountains is actually a discontinuous chain of more than twenty volcanoes, clustered into five groups. The westernmost group, near the Guatemalan border, contains Izalco and Santa Ana, which at 2,365 meters is the highest volcano in El Salvador. Between the cones lie alluvial basins and rolling hills eroded from ash deposits. The volcanic soil is rich, and much of El Salvador's coffee is planted on these slopes.
The central plateau constitutes only 25 percent of the land area but contains the heaviest concentration of population and the country's largest cities. This plain is about 50 kilometers wide and has an average elevation of 600 meters. Terrain here is rolling, with occasional escarpments, lava fields, and geysers.
A narrow plain extends from the coastal volcanic range to the Pacific Ocean. This region has a width ranging from one to thirty-two kilometers with the widest section in the east, adjacent to the Golfo de Fonseca. Near La Libertad, however, the mass of the mountains push the lowlands out; the slopes of adjacent volcanoes come down directly to the ocean. Surfaces in the Pacific lowlands are generally flat or gently rolling and result from the alluvial deposits of nearby slopes.
El Salvador has over 300 rivers, the most important of which is the Rio Lempa. Originating in Guatemala, the Rio Lempa cuts across the northern range of mountains, flows along much of the central plateau, and finally cuts through the southern volcanic range to empty into the Pacific. It is El Salvador's only navigable river, it and its tributaries drain about half the country. Other rivers are generally short and drain the Pacific lowlands or flow from the central plateau through gaps in the southern mountain range to the Pacific.
Numerous lakes of volcanic origin are found in the interior highlands; many of these lakes are surrounded by mountains and have high, steep banks. The largest lake, the Lago de Ilopango, lies just to the east of the capital. Other large lakes include the Lago de Coatepeque in the west and the Lago de Güija on the Guatemalan border. The Cerron Grande Dam on the Rio Lempa has created a large reservoir, the Embalse Cerron Grande, in northern El Salvador.
Izalco has erupted at least 51 times since 1770. It earned the nickname "Lighthouse of the Pacific" because it was the most active volcano in Central America.