Bull Volcanol (1998) 59:345–355 Q Springer-Verlag 1998 ORIGINAL PAPER H. Rymer 7 B. van Wyk de Vries 7 J. Stix G. Williams-Jones Pit crater structure and processes governing persistent activity at Masaya Volcano, Nicaragua Received: 16 May 1997 / Accepted: 29 October 1997 Abstract Persistent activity at Masaya Volcano, Nica- eruptions, lava flows and pit crater formation. In histor- ragua, is characterised by cycles of intense degassing, ic times lava lakes have been common, and two lava lava lake development and pit crater formation. It pro- flows have been erupted (1670 and 1772). In the past vides a useful site to study the processes which govern 150 years the volcano has formed only two pit craters, such activity, because of its easy accessibility and rela- with episodic lava lake development and feeble strom- tively short cycles (years to decades). An understanding bolian eruptions, but it has maintained a dynamic mag- of the present activity is important because Masaya is matic system manifested by voluminous gas emission. visited by large numbers of tourists, is located close to Five locally catastrophic degassing crises since 1852 major cities and has produced voluminous lavas, plin- have degassed approximately 10 km3 of basaltic magma ian eruptions and ignimbrites in the recent past. We (Stoiber et al. 1986), contributing significantly to global 3 provide structural and geophysical data that character- SO2 and CO2 output. At least four large (1–10 km ) ise the “normal” present state of activity. These indi- caldera-forming eruptions have also taken place be- cate that the ongoing degassing phase (1993 to present) tween 2700 and 30000 BP (Williams 1983; van Wyk de was not caused by fresh magma intrusion. It was asso- Vries 1991). ciated with shallow density changes within the active As with other active volcanoes in populated areas, Santiago pit crater. The activity appears to be asso- Masaya is a major tourist attraction. As such, even ciated predominantly with shallow changes in the pit small increases in activity pose a serious risk and are crater structure. More hazardous activity will occur potentially damaging to the local economy. For this only if there are significant departures from the present reason it is desirable to understand the processes that gravity, deformation and seismic signatures. control variations in activity, with an aim to predicting changes in activity, including any return to the more de- Key words Masaya 7 Santiago 7 Pit 7 Structure 7 structive phenomena. On a broader scale, it is neces- Microgravity 7 Magma 7 Gas sary to understand the mechanisms of long-term persis- tent activity in order to develop models for the rates of magmatic processes at these sites. Introduction We present structural and geophysical (gravity, ground deformation and SO2 flux) data from the most Masaya Volcano is persistently active, with a vigorously recent degassing crisis at Masaya (1993 to present). The degassing lava lake periodically visible. Geological evi- aim is to constrain the shallow structure of the magma dence shows several cycles of pyroclastic cone-building plumbing system, determine the geophysical signatures of the present activity and predict future changes. An initial gravity decrease between February 1993 and April 1994 of up to 90 mGal close to Santiago pit crater Editorial responsibility: D. Swanson indicates that a shallow density reduction occurred just Hazel Rymer (Y) 7 Benjamin van Wyk de Vries below the crater bottom. This gravity decrease corre- Department of Earth Sciences, The Open University, lates with increasing gas flux, indicating a change in the Milton Keynes, MK7 6AA, UK Fax: c44 1908 652558 shallow plumbing system. A subsequent gradual in- e-mail: h.rymer6open.ac.uk crease in gravity between April 1994 and March 1997 of up to 56 mGal has occurred at the same time as minor John Stix 7 Glyn Williams-Jones Department of Geology, UniversitÉ de Montreal, CP. pit crater collapse and a decline of degassing. The grav- 6100, Montreal, Canada ity and deformation data indicate that changes oc- 346 curred at shallow depths, and that the crisis was not ac- companied by major intrusion. General geology Masaya Volcano is a large basaltic shield volcano, lo- cated 20 km south of Managua, Nicaragua (Fig. 1a). It is composed of a nested set of calderas and craters, the largest of which is Las Sierras shield and caldera (van Wyk de Vries 1993). Within this caldera lies Masaya Volcano sensu stricto, a shallow shield composed of basaltic lavas and tephras (Fig. 1b). This hosts Masaya caldera, formed 2.5 ka ago by an 8-km3 basaltic ignim- brite eruption (Williams 1983). Inside this caldera a new basaltic complex has grown from eruptions mainly on a semi-circular set of vents that include the Masaya and Nindiri cones. The latter host the pit craters of Masaya, Santiago, Nindiri and San Pedro. Observations in the walls of the pit craters indicate that there have been several episodes of cone and pit crater formation (Fig. 2). The floor of Masaya caldera is covered by poorly vegetated lavas, indicating resurfacing within the past 1000 or so years, but only two lava flows have erupted since the sixteenth century. The first, in 1670, was an overflow from the Nindiri pit, which at that time hosted a 1-km-wide lava lake. The other, in 1772, issued from a fissure on the flank of the Masaya cone. Since 1772 lava has appeared at the surface only in the Santiago pit crater (presently active) and possibly within Nindiri crater in 1852. Cone construction Fig. 1a–d Masaya Volcano. a Regional location of Masaya. b Historic activity has been confined to the Masaya and Map showing the Las Sierras Caldera enclosing the Masaya cal- dera, which in turn encloses the recent vents (black dots). Map Nindiri cones. They are steep sided on their western shows proximity of major towns (circles) and cities to Masaya cal- and southern sides, where exposures in the pit craters dera: approximately 1.5 million people live within 25 km of the indicate a predominance of pyroclastic material, either volcano. Heavy dashed line indicates location of COSPEC trav- coarse scoria and spatter layers, or fine, laminated ash erses. c Map of active crater area showing structural features, beds. The northern and eastern sides are less steep and such as eruptive fissures (dashed lines), pit crater edges (filled triangles) and explosion crater edges (open triangles). Lava flows surfaced mostly by lavas. Exposures in the pit craters and lakes (shaded) and tephra cover (blank) are also shown. Lav- record a higher proportion of lava flows on these sides. as and fissures are given dates of eruption (e.g. L1772 is the 1772 This asymmetrical construction is due to the prevailing flow and F1906 is the fissure that erupted gas in 1906). d Aerial easterly wind. The deposits are consistent with the photo of the same area as the map in c cones being built by lava fountain and strombolian ac- tivity accompanied by lava flows. steep scree slopes that lead to a flat base and a 10-m- high spatter cone (Fig. 1c). This cone is fresher than the Pit Crater structure and history rest of the crater and was probably erupted concurrent- ly with the 1772 lava flow. East of Masaya pit crater is a Masaya pit crater lava-filled crater, which is probably the “funnel shaped” crater that Oviedo (1855) mentions, and which Masaya pit crater, located centrally within the Masaya was probably filled sometime between 1529 and 1670 cone, is now vegetated but probably formed after the (McBirney 1956). The “funnel shaped” nature of this late sixteenth century, as it was not described in its crater, and the local abundance of scoria, suggest that it present state by Oviedo (1855) writing in 1525. It has originated from pyroclastic cone-building activity rath- nearly vertical upper walls of scoria and lava above er than pit collapse. 347 Fig. 2a, b Structure of pit craters. a Panoramic sketch of the walls of Santiago main a Panorama of Santiago main crater and inner craters, showing W NESW crater walls and major struc- Pc Pc Pc Plaza Sapper Plaza Oviedo tural features. Lava-dominated Nindiri Pc walls are indicated by L, teph- ra layers by dotted lines, and L crater breccias by speckled L L pattern. Pit crater walls indi- L L cated by Pc and closed trian- gles. The sketch of the inner 1965 crater floor pit crater shows the location of caverns and vents seen 100 m Inner crater Inner crater opening up from 1986 to 1989. b Maps of crater evolution at Santiago from 1986 to 1997. WWNES Collapsed 1965-86 vent These show locations of vents, caverns the enlarging of the inner Panorama of Santiago crater and fissures and col- inner crater 1948 and lapses in the main crater. A 1965 lavas fault in 1965 lava lake; B col- 100 m lapse fault on edge of Nindiri wall, which produced a major rockfall in November 1986; C 'El Cañon' lava lake and Strombolian two caverns open and degass- Active vent vent of 1989, ing in 1987–1988; D faults opening at the surface from b 1988 to present; E lava-filled vent of February 1989; F i. 4/86 ii. 11/86 iii. 1/88 iv. 2/89 chamber with lava fountain C within, during February 1989; A B G G the “Can˜ on” vent, briefly a F broad glowing hole in 1989, subsequently blocked, which still has fumarolic activity; H small strombolian vent active N in February 1989; I collapse E along faults on south side of 500 m D J Santiago; intermittent vent H from 1990 to 1993; K vent opened in June 1993; L the enlarged vent observed on 27 v.