Integrated geophysical and hydrothermal models of flank degassing and fluid flow at Masaya volcano, Nicaragua

Abstract

[1] We investigate geologic controls on circulation in the shallow hydrothermal system of Masaya volcano, Nicaragua, and their relationship to surface diffuse degassing. On a local scale ($250 m), relatively impermeable normal faults dipping at $60 control the flowpath of water vapor and other gases in the vadose zone. These shallow normal faults are identified by modeling of a NE-SW trending magnetic anomaly of up to 2300 nT that corresponds to a topographic offset. Elevated SP and CO 2 to the NW of the faults and an absence of CO 2 to the SE suggest that these faults are barriers to flow. TOUGH2 numerical models of fluid circulation show enhanced flow through the footwalls of the faults, and corresponding increased mass flow and temperature at the surface (diffuse degassing zones). On a larger scale, TOUGH2 modeling suggests that groundwater convection may be occurring in a 3-4 km radial fracture zone transecting the entire flank of the volcano. Hot water rising uniformly into the base of the model at 1 Â 10 À5 kg/m 2 s results in convection that focuses heat and fluid and can explain the three distinct diffuse degassing zones distributed along the fracture. Our data and models suggest that the unusually active surface degassing zones at Masaya volcano can result purely from uniform heat and fluid flux at depth that is complicated by groundwater convection and permeability variations in the upper few km. Therefore isolating the effects of subsurface geology is vital when trying to interpret diffuse degassing in light of volcanic activity.

Distribution of Vapor and Condensate in a Hydrothermal System: Insights From Self‐Potential Inversion at Mount Tongariro, New Zealand

“…A broad negative anomaly (N3, −600 mV) separates the Ketetahi and Te Maari thermal areas. Anomalies around the thermal surface features are typically subdued (±100 mV), similar to those observed around fumarolic areas at Masaya volcano, Nicaragua, (Pearson et al, ). A local negative‐to‐positive pair (N2 and P3, −950 and 700 mV) is mapped southwest of North Crater to the east of the Waihi fault zone.…”

“…Self‐potential (SP) is a simple geophysical technique to map subsurface fluid flow and is frequently applied at volcano hydrothermal systems (e.g., Brothelande et al, ; Legaz et al, ; Pearson et al, ). Interpretation however is usually qualitative inhibiting deeper understanding of the system dynamics.…”

Tracking Formation of a Lava Lake From Ground and Space: Masaya Volcano (Nicaragua), 2014–2017

“…Each “degassing crisis” has typically lasted years to decades (the most recent started in 1993) and has typically fluctuated between vent‐opening phases (mostly due to unroofing of small chambers beneath the crater floor) and vent‐closing phases (due to rockfalls inside the crater; Rymer et al, ). Occasionally, the latter crater wall collapse events have been followed by vent‐clearing ash explosions, such as in 2001 (Duffell et al, ) and in April–May 2012 (Global Volcanism Program, ; Pearson et al, ). After a period of reduced volcanic activity between late 2012 and late 2015, incandescence was again reported on the crater floor by INETER on 11 December 2015 (Global Volcanism Program, ).…”

Structures controlling volcanic activity within Masaya caldera, Nicaragua

“…In this particular geographic location, buried bodies with higher remnant magnetization values tend to generate dipolar anomalies with a positive response south and a negative response north of the object. A previous study by Pearson et al [2012] in a small area within Masaya caldera (around San Fernando fissure, LF2, Figure 2) showed a NE-SW trending magnetic anomaly of 2300 nT corresponding with the outline of a previously known fault.…”

“…The high concentration of ferromagnetic minerals in basalts makes mafic volcanoes a good setting for magnetic studies, since the presence of basaltic dikes (evidence of deep fracturing) can gen-erate important magnetic anomalies, and therefore become good indicators of structural features [e.g. Stamatakos et al 1997;Connor et al 1997;La Femina et al 2002;Lopez-Loera et al 2010;Pearson et al 2012]. A disadvantage of this method is the numerous short wavelength features that can be found in the data due to the inhomogeneous superficial distribution of volcanic rocks/sediments.…”