NGDIR News Section-- Santiaguito Lava Dome Complex.
Santa Maria Volcano: Introduction
Santa Maria, a stratovolcano in the southwestern Guatemalan volcanic highlands, is the site of one of the twentieth century's largest eruptions. It is also the home of Santiaguito, one of the most active lava dome complexes in the world. The group of four lava domes formed at the foot of Santa Maria twenty years after the volcano's devastating 1902 eruption, and the domes have been growing ever since. The currently active dome, El Caliente, is the site of regular ash-and-gas explosions, and this minor but persistent activity has drawn many tourists to catch a glimpse of explosive silicic eruptions.
Santa Maria Volcano: Plate Tectonic Setting
Santa Maria is located in the volcanic highlands of Guatemala, which parallel the Pacific coast of the country. The highlands were formed by the subduction of the Cocos plate under the Caribbean plate, which resulted in the formation of a line of stratovolcanoes that stretches along much of the Pacific coast of Central America. In Guatemala, these volcanoes overlie a basement of carbonate as well as igneous and metamorphic rocks; many xenoliths ("foreign" rock fragments) found in lavas erupted from the stratovolcanoes are composed of limestone, granite, and gneiss.
Santa Maria Volcano Geology and Hazards
Santa Maria is a 30,000 year old andesitic stratovolcano built on a basement of older rocks formed by ancient volcanic eruptions. The 0.5 km3 (0.1 mi3) crater in the south flank of the volcano exposes a spectacular sequence of alternating pyroclastic and lava flows and lahar deposits. The crater was formed by a massive Plinian eruption in 1902.
Following the 1902 eruption, the dacitic lava domes of Santiaguito began forming in the crater. The dome complex has since grown to include four domes totaling more than 1 km3 (0.25 mi3) of material. The domes rise more than 500 meters (1,600 feet) above the base of the stratovolcano.
While the main cone of Santa Maria is no longer active, the domes of Santiaguito have created a number of volcanic hazards since their growth began. The land around the volcano has been used for agriculture for centuries, especially coffee plantations, which puts the people living and working there in continual danger. The towns of El Palmar and San Felipe - which are located directly south of the domes - and the city of Quetzaltenango to the north of Santa Maria, are several places that often have to deal with hazards from the volcano.
The bulk of the domes were built by the extrusion of lava flows and spines, but the dacite lava is so viscous that it poses no immediate hazard on eruption. Collapses of the spines, the tips of the lava flows, or larger portions of the domes themselves can, however, create dangerous pyroclastic flows; the collapse of material in eruption columns formed by ash-and-gas explosions can also create pyroclastic flows.
Drifting ash from eruptions often lands on the towns and cities near the volcano, and can cause hazardous breathing conditions as well as damaging crops. Finally, lahars (volcanic mudflows) are an especially common hazard in the streams and rivers below the domes, since this area of Guatemala experiences an intense summer rainy season. Water falling on the slopes of Santa Maria and on the domes mixes easily with loose ash and rock and washes quickly downhill, choking the rivers below with mud and boulders. The original town of El Palmar was destroyed by lahars in the 1980s, and the new town could still be threatened by future mudflows.
Santa Maria: Eruption History
There is no historical record of eruption at Santa Maria. The oldest lava flows that make up the volcano are 30,000 years old, but there are few dates for younger deposits. Magnetic data suggests that most of the growth occurred during a 1000 to 3000 year period prior to 25,000 years ago, although more accurate dates are not yet available. The cone-building period was followed by a long period of quiescence interrupted by occasional small-volume lava flows from flank vents. (Conway et al, 1993)
In November 1902, following several large earthquakes that caused significant damage in Guatemala and neighboring countries, Santa Maria experienced one of the biggest eruptions in the twentieth century. It lasted for several weeks, created an 0.5 km3 (0.1 mi3) crater in the volcano's south flank, and spread more than 5 km3 (1.2 mi3) of tephra as far away as Mexico. The eruption crater continued to be active for some months afterward, with several geysers erupting from a short-lived crater lake.
In 1922, new seismic activity heralded the eruption of a single dacitic lava dome in the 1902 crater. The dome, initially named Santiaguito, grew quickly, reaching 0.2 km3 (0.05 mi3) in only three years. A devastating dome collapse occurred in 1929, sending pyroclastic density currents down the river valleys below the domes; more than 3,000 people were killed and the plantations in the path of the pyroclastic flows were destroyed.
Following this collapse, activity at Santiaguito began to move westward from the original vent (now called Caliente), eventually forming three more lava domes (La Mitad, El Monje, and El Brujo) by the 1960s. From 1972-1975, both Caliente and El Brujo (the domes on either end of the complex) were active at the same time, producing lava flows, pyroclastic flows, and ash-and-gas eruptions. Activity has been restricted to the Caliente dome since 1975, and has included regular ash-and-gas eruptions from the dome summit as well as lava flows that travel down its flanks. Caliente has experienced several significant events since the 1929 dome collapse, including large eruptions and pyroclastic flows in 1973, 1989, and most recently in April of 2010.
Did You Know?
Many of the plantations near Santa Maria and Santiaguito grow coffee beans, which thrive in the rich volcanic soil (and seem to be relatively unaffected by the occasional ashfall). One of the major purchasers of these beans is Starbucks.
Halite - otherwise known as rock salt - can be found in eruptive deposits around Santa Maria, and is thought to come from beds of evaporites deep beneath the volcano.