The Santiaguito dome is a compound dome comprising four major vents. Dome growth has consisted of alternating exogenous (effusive outpouring of dacite lava flows), and endogenous (dome inflation) periods (Rose 1972). At present, activity is dominated by effusion of blocky lava flow down the east and southeast slopes of Caliente and by quasi-periodic phreatic or phreatomagmatic explosions. Powerful Vulcanian eruptions are sparse, but at least 15 such eruptions occurred from 19 July 1989 to November 1990. On 19 July 1990, four hikers from Quezaltenango were killed by a lateral blast from a powerful pyroclastic eruption.
Methodology
Before discussing techniques for monitoring activity at the dome it is necessary to briefly describe the volcanic hazards associated with dome growth.
The principal hazards directly associated with dome growth include:
Dome collapse -- Collapse of domes typically results in catastrophic debris avalanche events (e.g. 1150 BP collapse at nearby Cerro Quemado). The resulting avalanche can flow tens of kilometers downslope burying or sweeping away everything in its path. Edifice collapse at domes is commonly triggered by seismic events or intrusion of fresh magma into the edifice. Although large scale edifice collapse has never occurred at Santiaguito, the combination of steep slopes, a brecciated infrastructure, hydrothermal alteration, and occasional seismicity make dome collapse potentially likely at the dome.
Pyroclastic flows -- Historically pyroclastic flows from Santiaguito have traveled several km to a maximum of 12 km. Hot Pyroclastic flows (> 500 C) originate in several different ways including: Vulcanian eruption, plug collapse, collapse of blocky lava flows descending steep slopes, and collapse at the toe of lava flows. The 1929 pyroclastic flow killed hundreds of people, and destroyed property along the RíomNimá I & II drainages as far south as El Palmar.
Lava flows -- Lava flows move to slowly to threaten human, but they can destroy property. Viscous dacite lava flows, such as those that effuse from Santiaguito, rarely travel more than several kilometers and thus lava flows from the dome are unlikely to threaten present agricultural lands in the Río Nimá drainage. However, powerful explosions occurring at the distal toe of dacite lava flows are capable of producing hot pyroclastic flows capable of traveling many kilometers downvalley.
Mass wasting -- In the last several years photographic evidence reveals alarming erosional ,degradation of the north slope of Santiaguito. Wide scale erosion was first noted in February 1990 on the El Monje, La Mitad and El Caliente vents. On the basis of photographic evidence, the area on the north slope affected by mass wasting had increased from about 24,000 m[2] to nearly 90,000 m2. Since February 1990 erosion of the north slope has migrated laterally to include more of El Caliente and now includes El Brujo vent. Erosion on the south slope is not as extensive and fresh, erosional scarps are not as common. At present, erosion on the northwest flank of El Caliente vent appears to be undermining large Peléan spines at the summit.
The causes of this sudden increase in mass wasting are poorly known. One hypothesis is that magma intrusion into La Mitad and El Monje inflated the central part of the dome producing radial fractures along which sluffing of unconsolidated volcanic breccia occurred. A second hypothesis is that the combination of climate, dome age and the nature of dome deposits, and the probable presence of a strong hydrothermal system with subsequent through-going hydrothermal alteration results in dome instability and erosional breakdown. As large volumes of material are removed by erosion it becomes increasingly more likely that the infrastructure of the dome could collapse.
Collapse at Santa María -- The highly oversteepened south face of Santa María could potentially collapse. Such an event would send a debris avalanche sweeping over Santiaguito, potentially destroying the dome and depressurizing the feeding magma chamber. This could result in a catastrophic lateral blast event similar to the one that occurred on 18 May 1980 at Mt. Saint Helens.
Monitoring Techniques
In this section we describe methods to provide fundamental data on past and present dome growth at Santiaguito. The majority of the methods are low cost, require little training, and could be accomplished by employees of INSIVUMEH stationed at Guatemala City and the Santiaguito Volcano Observatory.
1) Meteorological station
At least 15 powerful pyroclastic eruptions occurred at Santiaguito from August through November 1990. The eruptions were coincident with the rainy season. A correlation exists between climate and volcanic eruptions at other domes (e.g. Mt. Saint Helens). Measurements of meteorological data -- amount and velocity of rainfall, barometric pressure, temperature, humidity, wind direction and velocity, cloud cover -- at the Santiaguito Volcano Observatory is necessary to determine dome response to climate.
2) Photographic history of the dome.
Information from dated photographs and videotapes should yield dome volumes and profiles and locations and style of eruptive activity through time, and provide a better understanding of the role of exogenous versus endogenous growth. This in turn could provide data on the nature and probability of hazards associated with both types of growth.
A systematic photographic program using three dedicated sites is essential to documenting continued evolution of Santiaguito. Periodic videotapes of the dome and the 1902 explosion crater would be an important complement to this program.
3) Visual Observation of the dome
Since November 1990 trained INSIVUMEH personal employed have observed the dome daily and recorded the time and nature of vertical eruptions, rock avalanches, etc. This monitoring is essential for recognizing a change in dome behavior.
4) Deformation
Dome deformation is a principal indicator of change in dome growth pattern. A geographical position system (GPS) or electronic distance meter (EDM) would provide essential data on subtle changes in dome morphology and provide an early-warning system of potentially hazardous dome behavior.
5) Topographic base maps
It is possible, and inexpensive, to produce topographic base maps from aerial photographs. The data resolved in large scale topographic maps provides a baseline with which to monitor the effects of mass wasting as well as dome growth.
6) Satellite imagery
AVHRR satellites carrying thermal detectors fly over the volcanic highlands of Guatemala twice daily. Digital thermal imagery of Santa María-Santiaguito is probably available t hrough the National Oceanographic and Atmospheric Administration (NOAA) and can be acqquired through NOAA NESDIS. Image analysis would provide quantitative data on radiances that can be used to infer emitting areas temperatures of the El Caliente vent.
7) Analysis of ash
The size, shape, and composition of ash reflects the role of magmatic versus phreatomagmatic gases in pyroclastic eruptions. This in turn provides information on the state of the volcano.
8) Comparative study of Cerro Quemado
The Cerro Quemado dome complex is located 5 km north of Santiaguito. Despite possessing similar geochemical compositions and being of similar age, the eruptive style of the two domes is quite different. Petrologic and geochemical studies of the domes may provide clues to explain this difference.
Recommendations
We recommend the following actions should be taken immediately to effectively monitor growth at Santiaguito, to reconstruct the historical evolution of the dome, and to mitigate volcanic hazards at the dome. Most of our recommendations can be met by the trained staff of the Volcanological, Seismological, and Meteorological sections of INSIVUMEH, provided that basic equipment (vehicles, 35 mm camera and tripod, meteorological equipment, etc.) and supplies (gasoline, film, sampling bags, etc.) are available.
1) We recommend the installation of a meteorological station at the Santiaguito Volcano Observatory. This station should include basic meteorological equipment: rain gauge, wind gauge, barometer, thermometer, and an instrument that measures humidity. Data should be gathered 3 or 4 times daily.
2) We highly recommend daily or weekly photographs and stereo pairs of the dome and the 1902 explosion crater be taken from several sites. Suitable sites include: Santiaguito Volcano Observatory, Hotel Magermann, and the summit of Santa María. The latter site provides excellent coverage of the active vent of El Caliente.
The cost of several cameras, tripods, lenses, and film is probably well under $700. Moreover, on occasion some film makers donate equipment in order to reap promotional benefits. We suggest that the head of the volcanology section of INSIVUMEH solicit camera or film companies for donated materials.
3) We strongly recommend deploying geographical position system equipment , tiltmeters, and electronic distance meter on Santiaguito. We further recommend deploying deformation equipment near the scarp of the 1902 explosion crater in order to monitor slope stability. This equipment should be monitored by observatory personnel.
4) Local volunteers from the fincas and towns near Santa María-Santiaguito are important potential resources. In the past, villagers of Llano del Pinal provided Dr. Richard Stoiber with valuable data on changes in behavior of Santiaguito. We recommend recruiting villagers and farmers for gathering ash and making observations of the dome.
5) USGS facilities in Flagstaff are capable of rapid production of large scale topographic base maps from aerial photographs. We recommend aerial photographs of Santa María- Santiaguito from 1940 on, available from Michigan Technological University and the Smithsonian archives, be used to produce topographic base maps.
6) In 1929 the Santiaguito dome was considerably smaller than it is now, yet it produced t he largest pyroclastic flow at the dome. The origin of this pyroclastic flow is poorly known. We recommend geologic mapping should be combined with geochemical, petrographic, and stratigraphic studies to allow for inference of the origin and potential hazard from pyroclastic flows at Santiaguito.
7) NOAA distributes numerical data from AVHRR overflights. We strongly ecommend that the directors of INSIVUMEH contact NOAA NESDIS about obtaining images of Santiaguito.
8) It is essential to install and maintain a seismic net of 4-6 seismometers around Santiaguito. The information gathered by the net will provide important constraints on the magma system beneath the dome.
9) Facilities for scanning electron microscope (SEM) for the analysis of ash are available at the Geological Survey of Austria (Inst. Geowissenschaften/Prospektion). Dr. Obenholzner of the institute has offered free analysis of fresh ash samples. We recommend that INSIVUMEH personel make it a habit to collect ash whenever possible and mail the ash to Dr. Obenholzner for analysis.
10) We recommend that a study of the interaction between the groundwater system and dome growth mechanisms be conducted.