Abstract A geothermal exploration project was conducted in 1969–70 over an area of about 38,000 km 2 in the Cenozoic volcanic zone of western Nicaragua (Fig. 1). The project included a regional geological and geochemical evaluation of the geothermal potential of the area and more detailed geological, geophysical, and borehole studies at two geothermal prospects—San Jacinto–Tisate and Momotombo. Western Nicaragua is a region of widespread late Cenozoic volcanism, with activity locally continuing to the present (McBirney and Williams, 1965; McBirney, 1958). Tertiary acidic ash-flow tuffs predominate east of the Nicaraguan depression (Fig. 1), whereas Quaternary andesites and basalts are present to the west. An impressive chain of Quaternary volcanic centers is localized along the southwestern margin of the Nicaraguan depression, which is a regional structure of probable volcano-tectonic origin (McBirney and Williams, 1964). This geologic setting is very favorable for development of hyperthermal conditions and, thus, geothermal potential. Relatively little was known about Nicaraguan thermal activity until recently, when brief descriptions were given by McBirney (1958) and Del Giudice (1959). An inventory, with temperature and some chemical data, of most of the water wells and many of the springs in the project area, by the Catastro i Inventario de Recursos Naturales de Nicaragua project was helpful as a preliminary indication of several thermal manifestations.

Boron contents were measured in predominantly mafic Quaternary lavas from the Central American Volcanic Arc to evaluate along-strike variations in subduction processes. Despite the significant range in B concentrations (∼2 to 46 ppm), B/La ratios vary in a systematic fashion along the arc: Higher values (>1) are typical between Guatemala and northern Costa Rica, whereas low values (most <0.5) typify central Costa Rica and western Panama. Because B/La is highly correlated with 10 Be/Be (r 2 = 0.94, excluding one sample), B/La may be a useful indicator of subduction contributions to the magma sources. If enrichments of both B and 10 Be are proportional to the flux of subducted sediment, along-strike variations in B/La suggest at least a twofold variation in this flux with maximum values below western Nicaragua and minimum values below Costa Rica and western Panama where the Cocos Ridge is being subducted. These data may also reflect significant differences in thermal state of the downgoing slab, which in turn differentially affect release patterns of fluids and fluid-mobile trace elements and possibly melting processes beneath different parts of the arc. The following scenario is suggested to explain the geochemical results. Steep subduction of cold slab beneath the northwestern arc favors more efficient subduction of fluid components to depths beneath the volcanic front. Their release provides fluid-mobile elements to the overlying mantle, which upon melting produces calcalkalic magmas. Shallow subduction of warmer slab beneath the southeastern arc favors shallow release of fluids and limits fluid-related metasomatism of sub-arc mantle beneath the volcanic front. Under such conditions, B/La and Ba/La ratios in the sub-arc mantle vary little from values seen in oceanic island basalts. Magmas in this part of the arc nevertheless display the highest La/Yb and lowest Ba/La and B/La ratios, which are consistent with prior light rare-earth-element enrichment in the source, significantly lower degrees of melting, or a combination thereof. Because some of the largest volcanoes occur in Costa Rica, and magma flux there is nearly an order of magnitude higher than elsewhere in the arc, source enrichment is considered to be the more plausible explanation. It is proposed that Quaternary magma production below Costa Rica involved lithospheric sources containing trapped or “stored” melt components, but this enrichment process is unlikely to have involved arc magmas or subduction fluids because we see no B-enrichment.

The youngest highly explosive basaltic eruptions from Masaya Caldera in central western Nicaragua produced five main pyroclastic deposits: the San Antonio Tephra, La Concepción Tephra, the Masaya Triple Layer, and the Masaya Tuff with the Ti cuan-te pe Lapilli. This tephra sequence was deposited over the past ∼6000 yr. The distribution and physical characteristics of these deposits suggest they originated from the Masaya Caldera. They have volumes ranging from 0.2 km 3 for La Concepción Tephra to 3.9 km 3 for the Masaya Tuff and cover minimum areas of 600–1600 km 2 . All deposits formed by violent eruptions discharging 10 11 to 10 12 kg of magma, thus reaching eruption magnitudes between 4.3 and 5.9 and volcanic explosivity indices of 3–4. An analysis of hazards for the main population centers around the Masaya Caldera shows that, if there were a similar eruption today, the most vulnerable communities would be Ticuantepe, Nindirí, and Masaya. In addition, La Concepción, southwest of the caldera, and the capital Managua, more than 15 km to the northwest, could be affected.

We review the most important types of volcanic hazards that have occurred in Nicaragua during the past ∼40,000 yr and that are expected to occur in the future. Population density within the potential hazard area is clearly essential in defining and understanding volcanic hazard and risk. There are three main groups of volcanic events that pose major hazards: Group 1 comprises several types of explosive volcanic eruptions that impact society (people and infrastructure) directly. The most hazardous types are pyroclastic surges, particularly those generated by water-magma interaction, pyroclastic fallout, and pyroclastic flows, as well as tsunamis generated by volcanic eruptions within and close to Nicaragua's large lakes. Group 2 includes nonexplosive volcanic activity such as lava flows and the permanent or episodic emission of volcanic gases from open vents. Group 3 comprises chiefly lahars generated by mixing of volcanic debris with water and volcano flank collapses (landslides) sometimes unrelated to synchronous volcanic eruptions but being conditioned chiefly by the stability of a volcanic edifice. We discuss the present database on the age and type of the most recent eruptions emphasizing those that potentially pose major hazards to the populated areas. These include volcanogenic tsunamis in Lake Managua and Lake Nicaragua, scoria cone and maar formation chiefly in the western part of Managua, and major explosive eruptions of Chiltepe and Masaya volcanoes, a large eruption from Masaya volcano having devastated the entire area of present Managua only ∼2000 yr ago. We discuss the most important techniques for monitoring volcanoes to detect unrest and predict the time and magnitude of upcoming eruptions, emphasizing techniques presently employed in Nicaragua. Finally, we address the subjects of risk assessment, including hazard and risk maps, and the importance of long-term development plans to reduce vulnerability.

Velocities from six continuous and 14 campaign sites within the boundaries of the Caribbean plate, including eight new sites from previously unsampled areas of Honduras and Nicaragua at the western edge of the Caribbean plate, are described and tested for their consistency with Caribbean–North America plate motion and a rigid Caribbean plate model. Sites in central Honduras and Guatemala move 3–8 mm yr −1 westward with respect to the Caribbean plate interior, consistent with distributed east-to-west extension in Guatemala and the western two-thirds of Honduras. A site in southern Jamaica moves 8 ± 1 mm yr −1 westward relative to the Caribbean plate interior, indicating that most or all of Jamaica is unsuitable for estimating Caribbean plate motion. Two sites in southern Hispaniola also exhibit anomalous motions relative to the plate interior, consistent with a tectonic bias at those sites. An inversion of the velocities for 15 sites nominally located in the plate interior yields a well-constrained Caribbean plate angular velocity vector that predicts motion similar to previously published models. Data bootstrapping indicates that the solution is robust to better than 1 mm yr −1 with respect to both the site velocities that are used to estimate the plate angular velocity and the site velocity uncertainties. That velocities at seven of eight GPS sites in eastern Honduras and Nicaragua are consistent with the motions of sites elsewhere in the plate interior indicates that much or all of eastern Honduras and Nicaragua move with the plate interior within the 1–2 mm yr −1 resolution of our data. It further suggests that the morphologically prominent, but aseismic Guayape fault of eastern Honduras is inactive. Tests for possible east-to-west deformation across the Beata Ridge and Lower Nicaraguan Rise in the plate interior establish a 95% upper bound of ∼2 mm yr −1 for any deformation across the two features, significantly slower than a published estimate of 9.0 ± 1.5 mm yr −1 during the past 23 Ma for deformation across the Beata Ridge.

Ignimbrites are deposits resulting from the eruption of volatile-rich, silicic magma. Few geochemical or petrological studies have been done concerning the Tertiary Ignimbrite Province of Central America in Honduras and Nicaragua. Previous work, using ages and geographical proximity, suggested that tephra layers recovered during Ocean Drilling Program (ODP) Leg 165 in the western Caribbean Sea were associated with deposits from explosive eruptions in Central America. A total of 112 marine tephra and 79 terrestrial samples from Nicaragua and Honduras were analyzed during this study. An electron microprobe was used for major oxides, and laser inductively coupled plasma–mass spectrometry (ICP-MS) was used for trace elements. The rare earth elements (REEs), due to their resistance to weathering, were used to correlate the marine tephra with the terrestrial samples. Cluster analysis resulted in the division of the samples into 6 geochemical groups. Visual inspection of these groups resulted in the reclassification of these 6 into 14 distinct geochemical groups. Each one of these geochemical groups displays unique REE trends relative to each other. The trends vary, relative to enriched mid-ocean-ridge basalt (EMORB), from strongly light (L) REE enriched with moderate to large, negative Eu anomalies, to nearly equal EMORB values without any Eu anomalies, to enriched REE with positive Eu anomalies. All but two groups consist of both marine and terrestrial samples. The geochemical correlation is strengthened using factor analysis, in which the REE values for each sample were reduced to two factors and replotted. Each group plots in a distinct field. The Lesser Antilles can be ruled out as contributing to the western Caribbean tephra due to the lack of any large-volume ignimbrite sheets within the arc, as well as the distinctly different REE trends of the magmas. The Sierra Madre Igneous Province in México is also ruled out as a potential source, due mainly to a significant age difference between Sierra Madre ignimbrites and the Caribbean Sea tephra layers. Besides the strong similarities in REE patterns, several of the groups and subgroups are geographically limited in extent, which might imply a specific group source location.