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. After its initial collapse, Santiago pit crater was a vertical cylinder approximately 150 m deep and 600 m 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. One of these faults is still visible on the north side of the crater. From 1948 until 1986 a small circular vent was the locus of activity. Santiago now consists of the main crater 150 m deep, and an inner crater 150 m deep, in which the active vent is located.
The walls of the east and north side expose flat layers of lava, either thin lava flows or lakes. These lavas lap onto a cinder cone to the southeast and abut old pit crater walls to the southwest and northeast. These old crater walls are composed mainly of fine ash and scoria layers. The bedding be-comes indistinct approximately 150 m down the south-western side, and below this a massive breccia forms the wall of the inner crater. On the north side the layering is distinct to 30 m above the 1965 lava layer, below which there is a massive breccia. The western side of the crater wall is composed of the lava infill of Nindiri crater. The upper crater walls of Nindiri dip steeply inward, gradually becoming less steep 100-200 m down, where there are large scree fans. The base of the walls, now exposed in the inner crater, consist of a massive homogeneous breccia. The concentric downthrown faults in Nindiri are well exposed in the walls. They dip approximately 70-80 degrees outward and extend into highly fractured areas, where no discrete fault planes can be seen. In addition to the fractures and faults, several areas of massive rock locate solidified magma-filled cavities in the lower part of the crater during the formation of the inner crater of Santiago.
Much of the collapse of the inner crater at 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 m 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. There have been many small collapses of this type since 1986, and they have been more common in the waning period of the degassing cycle. Some of these events have exposed lava-filled vents, such as in February 1989 and May 1989. The appearance of these lakes may be caused by the level of the crater floor reaching down to the magma level, not the usual opposite case where magma rises to the surface. (Excert from Rymer et al., 1998).