Pit crater structure at Volcan Masaya

Introduction

Masaya Volcano is persistently active, with a vigorously degassing lava lake periodically visible. Geological evidence shows several cycles of pyroclastic cone-building eruptions, lava flows and pit crater formation. In historic times lava lakes have been common, and two lava flows have been erupted (1670 and 1772). In the past 150 years the volcano has formed only two pit craters, with episodic lava lake development and feeble strombolian eruptions, but it has maintained a dynamic magmatic system manifested by voluminous gas emission.

General Geology

Masaya volcano is a large basaltic shield volcano, located 20km 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). 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.

Historic activity has been confined to the Masaya and Nindiri cones. They are steep sided on their western ad southern sides, where exposures in the pit craters indicate a predominance of pyroclastic material, either coarse scoria and spatter layers, or fine, laminated ash beds. The northern and eastern sides are less steep and surfaces mostly by lavas. Exposures in the pit craters record a higher proportion of lava flows on these sides.

Pit Crater Structure and History

Masaya Pit Crater

Masaya pit crater, located centrally within the Masaya cone, is now vegetated but probably formed after the late sixteenth century, as it was not described in its present state by Oviedo (1855) writing in 1525. It has nearly vertical upper walls of scoria and lava above steep scree slopes that lead to a flat base and a 10-m-high spatter cone (Fig. 1c). This cone is fresher than the rest of the crater and was probably erupted concurrently with the 1772 lava flow. East of Masaya pit crater is a lava-filled crater, which is probably the “funnel shaped” crater that Oviedo (1855) mentions, and which was probably filled sometime between 1529 and 1670 (McBirney 1956). The “funnel shaped” nature of this crater, and the local abundance of scoria, suggest that it originated from pyroclastic cone-building activity rather than pit collapse.

Nindiri Pit Crater

At the time of Oviedo’s observations (1525), Nindiri crater was approximately 210m deep, with a vent at the bottom hosting a lava lake (McBirney 1956). Nindiri is now filled by frozen lava lakes erupted between 1570 and 1670 (Fig. 1c). A lava lake overflowed Nindiri to the north in 1670, forming a thick 3-km long aa flow (Fig. 1c). The lava surface in the crater subsequently sagged downward on circular faults. These faults, now cut by the San Pedro and Santiago crater walls, dip outward at approximately 80 degrees, and the lava flows are tilted inward. 

A final lava lake was erupted onto the sagged crater floor (Fig. 3). Spatter on the edge of San Pedro and within Nindiri indicates that this may be the lava flow described by Montessus de Ballore (McBirney 1956). He reported that the flow was extruded in July 1852, a few years before the formation of Santiago and San Pedro pit craters in 1858-1859 (McBirney 1956).

San Pedro Pit Crater

San Pedro pit crater is a vertically sided cylinder, 400 meters wide and approximately 200 meters deep. The base is mostly scree, descending to a small flat area under a slight overhang directly below eastern Nindiri side. This flat area may be the remains of a lava lake. 

Santiago Pit Crater

Since its formation in 1858-1859, Santiago has been the main site of activity at Masaya, hosting the vent for all major degassing episodes except that of 1906 from fissures on the north flank of Masaya cone (McBirney 1956). After its initial collapse, Santiago pit crater was a vertical cylinder approximately 150 meters deep and 600 meters wide. The original scree-covered floor was resurfaced in 1948 and 1965 with lava lakes, which are broken by concentric faults and are similar to those seen in Nindiri (McBirney 1956). One of these faults is still visible on the north side of the crater (Fig. 2b). From 1948 until 1986 a small circular vent was the locus of activity (Fig. 2b, i). Santiago now consists of the main crater 150 meters deep, and an inner crater 150 meters deep, in which the active vent is located (Fig. 2b, iv). The walls of the east and north side expose flat layers of lava, either thin lava flows or lakes (Fig. 2a).

Santiago Pit Crater Development 1986-1997

Progressive deepening of the Santiago crater began in November 1986 and continues through 1997. Observations in this period illustrate how pit crater formation proceeds.

In 1986 there was an 80-meters-wide vent at the bottom of the Santiago crater (Fig. 2b, i). In 1979-1980 magma had been occasionally visible near the surface, but in 1986 only a red glow was visible. During early to mid 1986 the vent was degassing vigorously, although less so than in the crisis of 1980-1983. By late 1986 the vent had enlarged toward the southwest, and a large portion of the southwest crater wall had collapsed (Fig. 2b, ii). The collapses blocked the vent and the degassing stopped. Two new vents had opened by 1987, and observations by van Wyk de Vries (1993) during this period indicated that the vents widened with depth.

By January 1988 these two vents were both degassing at varying rates (Fig. 2b, iii). Gas then obscured the new inner crater until late 1988, by which time it had widened 50 meters and deepened by approximately the same amount to an uneven base. In February 1989 several new vents opened in the new crater bottom, one was lava filled, one produced small strombolian eruptions and one glowed red. A fourth vent in the northeast corner of the crater opened into a subcrater cavern. It first glowed red, but as the vent enlarged lava fountaining became visible below an overhanging wall. This lasted for approximately 1 day, after which the vent glowed red (Fig. 2v, iv). Each of these vents formed by the opening of cracks and then the collapse of material into the vent. 

Eruptive activity stopped in April 1989, and in May the roof of the fourth cavern collapsed farther to reveal a deeper vent. This became the focus of weak strombolian activity during May and June 1989 (Fig. 2b, v). Over the latter half of 1989 eruption noises could be heard, but the source was too deep within the vent to be seen. During this period rockfalls became frequent in Santiago, and fissures opened up above the southern wall. In November 1989 a large collapse occurred from this wall and filled the crater bottom.

A vent became established again in June 1993, hosting a new lava pool and building a small cone of ejecta (Fig. 2b, vi). This vent plunged approximately 80 degrees under the northeast wall of the inner crater to approximately 40 meters depth, from where it appeared to extend vertically. The vent was partially blocked by a rockfall in April 1997, but gas still escaped through the boulders. 

Characteristics of the pit craters at Masaya Volcano

Each of the pit craters is a nearly vertical cylinder whose walls may be slightly overhanging. The bottoms of the craters are funnel shaped unless they have been refilled by lava lakes. Sections though Nindiri indicate that collapse occurred mainly along outward-dipping faults. Such fault scarps are also present in Santiago. Vertical faults and fractures around Santiago indicate that the steep walls of the pit craters are probably formed by collapse above the outward-dipping fault set.

Much of the collapse of the inner crater of Santiago since 1986 has occurred by a process distinct from collapse along faults: that of unroofing small chambers, as observed during the progressive collapse of Santiago inner crater between 1988 and 1991 and again during 1997. These collapses have happened episodically and rapidly; usually a small hole appears first, which then collapses to reveal a cavern. The caverns have varied in size from 5 to 30 meters diameter. During the 1989 activity four cavernous vents were present. The vents themselves covered approximately 15% of the inner crater floor area. Subsequent collapse revealed the caverns below, which took up approximately 25% of the area of the inner crater, although not all of these were open at the same time.

Rymer et al., Pit crater structure and processes governing persistent activity at Masaya Volcano, Bulletin of Volcanology, Springer-Verlag, October 29th, 1997