The pit-craters and pit-crater-filling lavas of Masaya volcano |
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| Authors: | Andrew J L Harris |
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| Institution: | (1) HIGP/SOEST, University of Hawaii, 1680 East-West Road, Honolulu, HI 96822, USA |
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| Abstract: | Lava flowing into a pit crater will become entrapped to form an inactive lava lake. At Masaya volcano (Nicaragua) pit filling
lavas are exposed in the walls of Nindiri, Santiago and San Pedro pits. Mapping of these lavas shows that fill can involve
emplacement of both ’a’a and pahoehoe, with single fill units ranging in thickness from 2 to 22 m. Thick units with columnar
joints were emplaced as simple inactive lava lakes during high effusion rate episodes. Sequences of thinner units, which can
form pit floor shields or compound lakes, were emplaced at lower effusion rates. Lava withdrawal caused unsupported sections
of three 20-m-thick units to subside, resulting in unit flexure and faulting, and viscous peeling features reveal that subsidence
occurred while at least one unit was still partially molten. Where withdrawal has not occurred, fill sequences are flat lying
and symmetrically distributed around the feeder structures (cinder cones and dykes). The filled Nindiri pit holds 5 × 107 m3 of lava in a 215-m-thick sequence. Partial fill of Santiago pit with 1 × 107 m3 of lava has filled the pit with a 110-m-thick lava sequence, of which ∼50% has been consumed by formation of a secondary
pit. Altogether, 6.4 × 107 m3 of lava was erupted into Nindiri and Santiago during 1525–1965, with 94% of this volume remaining pit-contained; the remainder
forms a north flank lava flow field. Pit development and filling is a dynamic and ephemeral process, having short-lived effects
on volcano morphology, where pits develop and fill over hours-to-centuries. However, pits play an important role in shaping
an edifice, representing lava sinks and controlling whether lavas are trapped or able to spread onto the flanks. |
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| Keywords: | Pit craters Lava lakes Lava units Masaya |
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