Downloaded from gsabulletin.gsapubs.org on 17 July 2009 Geological Society of America Bulletin Santiaguito Volcanic Dome, Guatemala WILLIAM I ROSE, JR. Geological Society of America Bulletin 1972;83;1413-1434 doi: 10.1130/0016-7606(1972)83[1413:SVDG]2.0.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. 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Department of Geology and Geological Engineering, Michigan Technological University, Houghton, Michigan 49931 Santiaguito Volcanic Dome, Guatemala ABSTRACT INTRODUCTION AND GEOLOGIC SETTING The Santiaguito volcanic dome in southwest- The Santiaguito Dome is located in south- ern Guatemala is a multiple extrusive dome western Guatemala (14°44'30" N. lat, 91°34' that has shown constant volcanic activity since W. long), on the active volcanic chain of Cen- its birth in 1922. Fourteen extrusive units are tral America. Williams (I960) and Bonis (1965) mapped. Five of these are volcanic domes; the have discussed the regional setting; the local remaining nine are lava flows which generally setting is described by Stoiber and Rose (1969). cling to the sides of the domes. The volume of Santiaguito is a multiple extrusive dome which material extruded at Santiaguito since 1922 began forming in 1922 inside the large ex- (0.7 km3) is a small fraction of the volume of plosion crater created in 1902 on the south- pyroclastic debris from the 1902 eruption of western slope of Santa Maria Volcano (Fig. 1). Santa Maria (5.5 km3), Santiaguito's parent A comparison of the historic activities of composite cone. Extrusion of the dome began Central American volcanoes (Table 1) shows in the center of the explosion crater created by that Santa Maria-Santiaguito is one of five Santa Maria's 1902 activity. This crater was most active volcanoes in the last 150 yrs, and if volumetrically much smaller (0.5 km3) than the only the last 50 yrs are considered, it is the amount of material erupted during the 1902 most active. event, and local slumping near the crater has Many active volcanoes in Guatemala lie on occurred and is continuing along a series of linear fractures transverse to the dominant west- east-trending faults. The general westward northwest volcanic trend. Santiaguito, Santa growth of the dome complex and many of the Maria, and Cerro Quemado lie along such a structural features on Santiaguito are controlled northeast-trending structure. This trend is par- by these near-vertical faults. The domes are allel to fault scarps on the southeast side of studded with Pelean spines. Twenty-five new Cerro Quemado and along the bottom and chemical analyses are presented, showing San- sides of the Rio Samala Canyon (Williams, tiaguito's eruptive products to be soda-rich 1960, p. 39). The largest fault in the canyon, dacite of the calc-alkaline suite. The dome lava the Zunil fault, has been traced inland 25 km has differentiated quite significantly from the to Totonicapan. Alignment of Chicaval and overwhelmingly abundant pyroxene andesite Siete Orejas volcanoes, northwest of Santia- magma which makes up Santa Maria and the guito, is also northeast. From Lake Atitlan older volcanic rocks in the area. Trace element eastward, fractures transverse to the volcanic analyses of the lavas, along with major element belt are more nearly north trending. Dollfus data suggest a differentiation by fractional and de Montserrat (1868) first called attention crystallization under constant or increasing to these transverse trends, observing that the PC>2. Sr isotope determinations could not de- most seaward volcanic vent showed the most tect contamination of the lavas by radiogenic prolific recent activity. Table 1 shows that this crustal material; the Sr87/Sr86 ratio in San- generality is still true. Other volcano pairs that tiaguito rock averages .7043. The Santiaguito lie along apparent transverse trends are Atitlan lavas are tridymite-bearing hypersthene dacites, and Tollman, Fuego and Acatenango, and characterized by strongly zoned plagioclase Izalco and Santa Ana. Central American north- phenocrysts and oxyhornblende. Compositional trending transverse structures are discussed in and petrographic comparisons are made with more detail by Dengo and others (1970). other modern volcanic domes and other Cen- Central American volcanoes lie north and tral American lavas. inland of an offshore trench, and the volcanic Geological Society of America Bulletin, v. 83, p. 1413-1434, 18 figs., May 1972 1413 Downloaded from gsabulletin.gsapubs.org on 17 July 2009 1414 WILLIAM I. ROSE, JR. Figure 1. Santa Maria (right) and Santiaguito from the south: January 1970. chain has long been recognized as an island arc. chemical analyses of these units appear in Recent compilation of earthquake data and Table 2. earthquake mechanism studies (Molnar and Sykes, 1969) indicate a Benioff zone dipping Tertiary (?) Andesite Flows from the trench under the volcanic chain. The The oldest rocks of the Santiaguito area are depth from the surface to the seismic zone flat-lying basaltic andesite flows lying to the under Santiaguito is about 100 to 120 km, a north and northwest of the dome. These rocks depth typical for volcanoes producing high form the basement for the Santa Maria cone alumina suites (Kuno, 1966). The Benioff zone but their age is not established. They are under Guatemala and eastern Mexico has a deeply denuded and are interpreted as being of much greater seismicity than the same zone Tertiary age, based on proximity to rocks farther east or west. This fact helped Molnar mapped as Tertiary in the valley of the Rio and Sykes (1969, p. 1666-1667) to conclude Ocosito (Bonis, 1965), and along Zufiil ridge that underthrusting rates were greater in this (Williams, 1960, p. 26), and based upon the area. convention (Bonis, 1965, p. 62) of assigning Tertiary age when erosional and destructional PREVIOUS WORK forms are dominant (in contrast to units show- A summary of previous work at Santiaguito ing original depositional and constructional has already been published (Stoiber and Rose, forms, which are interpreted as Quaternary). 1969, p. 479). Recently works dealing with the The flows are rarely interbedded with laharic chemistry of fumarolic activity at Santiaguito and ash-flow materials and have a thickness of have appeared (Stoiber and Eberl, 1969; more than 500 m. They are undeformed, except Stoiber and Rose, 1970). The 1902 eruption of by faults. Directly north of Santiaguito the Santa Maria is discussed in a separate article volcanic rocks of Santa Maria's cone cover the (Rose, 1972). The present paper formed part of Tertiary rocks; to the south the rocks are the author's doctoral thesis (Rose, 1970). covered by laharic and fluvial deposits of the coastal slope. VOLCANIC UNITS The Tertiary flows near Santiaguito are Descriptions of volcanic units of the San- olivine-bearing pyroxene andesites (no. 1118 in tiaguito area (Fig. 2) are below. In most cases Table 2). Phenocrysts of twinned labradorite Downloaded from gsabulletin.gsapubs.org on 17 July 2009 Downloaded from gsabulletin.gsapubs.org on 17 July 2009 SANTIAGUITO VOLCANIC DOME, GUATEMALA 1415 TABLE 1. RELATIVE "ACTIVITY" OF CENTRAL AMERICAN VOLCANOES, BASED ON DATA FROM CATALOGUE Interbedded Flows, Pyroclastic Deposits OF ACTIVE VOLCANOES (MOOSER AND OTHERS, 1958) and Laharic Materials of Santa Maria AND SUPPLEMENTED BY RECENT OBSERVATIONS Flows, pyroclastic debris, and laharic mate- Volcano No. of decades since No. of semi -decades 1820 in which eruptions since 1920 in which rials directly associated with the Santa Maria were reported (Max = 15) eruptions were re- composite cone occur on three sides of the ported (Max =10) dome. The surface of the cone north and south Tacana" 2 0 of the 1902 crater is thickly mantled with the Tajumulco 1 0 1902 eruption debris, but in places erosion has Santa Marfa- Santiaguito 6 10 exposed andesite flows and laharic units. A AtitlSn 3 0 laharic unit fills most of the valley floor north Acatenango 1 2 of the isla and casita (Fig. 2); it is up to 50 m Fuego 8 9 thick and has a well developed soil layer on Pacaya 2 2 top, in which the fossil 1902 forest was rooted. Santa Ana 2 1 East of the casita, andesite flows underlie the Izalco 15 9 1902 debris. San Salvador 1 0 Islas Quemadas 1 0 The interior of the composite cone of Santa San Miguel 9 7 Maria is irregularly exposed in the steep walls Conchaguita 1 0 of the 1902 explosion crater (Fig.