Bulletin of Volcanology

, Volume 74, Issue 9, pp 2007–2021

An eruptive history of Maderas volcano using new 40Ar/39Ar ages and geochemical analyses

Authors

    • Department of Geological and Mining Engineering and SciencesMichigan Technological University
  • William I. Rose
    • Department of Geological and Mining Engineering and SciencesMichigan Technological University
  • Brian Jicha
    • Department of GeoscienceUniversity of Wisconsin—Madison
Research Article

DOI: 10.1007/s00445-012-0644-7

Cite this article as:
Kapelanczyk, L., Rose, W.I. & Jicha, B. Bull Volcanol (2012) 74: 2007. doi:10.1007/s00445-012-0644-7
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Abstract

Maderas volcano is a small, andesitic stratovolcano located on the island of Ometepe in Lake Nicaragua, Nicaragua, with no record of historic activity. Twenty-one samples were collected in 2010 from lava flows of Maderas. The selected samples were analyzed for whole-rock geochemistry using ICP-AES and/or were dated using the 40Ar/39Ar method. The results of these analyses were combined with previously collected data from Maderas as well as field observations to determine the eruptive history of the volcano and create a geologic map. The results of the geochemical analyses indicate that Maderas has higher concentrations of alkalies than most Nicaraguan and Costa Rican volcanoes including its nearest neighbor, Concepción volcano. It is also different from Concepción in that it displays higher incompatible elements. Determined age dates range from 179.2 ± 16.4 ka to 70.5 ± 6.1 ka. Based on these ages and the geomorphology of the volcano which is characterized by a bisecting graben, it is proposed that Maderas experienced two generations of development: initial build-up of the older cone including pre-graben lava flows, followed by post-graben lava flows. The ages also indicate that Maderas is markedly older than Concepción which is historically active. Volcanic hazards were also assessed. The 40Ar/39Ar ages indicate that Maderas has likely been inactive for tens of thousands of years and future volcanic eruptions are not considered an immediate hazard. However, earthquake and lahar hazards exist for the communities around the volcano. The steep slopes of the eroded older cone are the most likely sources of lahar hazards.

Keywords

40Ar/39Ar datingEruptive historyVolcanoMaderas

Introduction

Maderas volcano (11°26′44″ N, 85°30′54″ W) is a small, asymmetrical stratovolcano (Fig. 1) that forms half of the dumbbell-shaped Ometepe Island located in Lake Nicaragua, Nicaragua. The other half of Ometepe, which means “two mountains” in the Nahuatl language, is formed by the highly symmetrical stratovolcano Concepción. Due to its remote location and lack of historic activity, relatively little is known about Maderas compared to other Central American volcanoes whose eruptive histories are fairly well constrained (e.g., Irazu, Santa María) (Alvarado et al. 2006; Escobar-Wolf et al. 2010; Singer et al. 2011). However, its unique location and geomorphology make Maderas an atypical and important volcano to investigate.
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Fig. 1

Map of Central America with inset showing Lake Nicaragua and Ometepe. Dashed lines represent boundaries of the Nicaraguan depression (ND) and Median Trough (MT). Triangles represent volcanic centers

Field observations were combined with 24 new geochemical analyses of Maderas lavas, 88 previously collected geochemical analyses from both Maderas and Concepción volcanoes, and six new and one previously acquired 40Ar/39Ar age determination. These data were used to create a new geologic map and to assess the eruptive history and hazards posed by Maderas in order to reduce vulnerabilities for the small communities located around its flanks.

Tectonic setting

Maderas is one of 39 Quaternary volcanic centers that form the Central American volcanic front (CAVF), a 1,100-km chain of volcanoes that spans from the Guatemala–Mexico border to central Costa Rica, and it is the southernmost of 12 volcanic centers located in Nicaragua (Carr et al. 2003). The CAVF formed as a result of the subduction of the Cocos plate moving northeast beneath the Caribbean plate at a rate of 84 ± 5 mm year−1 near Nicaragua (DeMets 2001) (Fig. 1).

The volcanic centers along the CAVF form eight linear segments, each between 100 and 300 km in length, that form right steps along the volcanic front (Bolge et al. 2009) (Fig. 1). The largest of these right steps, ~40 km, occurs between Maderas and Orosí volcano in Costa Rica. The depth to the slab from Maderas is ~150 km (Syracuse and Abers 2006). Estimates of the dip angle of the slab below Maderas range from 65° (Syracuse and Abers 2006) to 80° (Funk et al. 2009). The crust beneath Maderas is estimated to be 24.6 ± 3.5 km in thickness (MacKenzie et al. 2008).

From east to west, Nicaragua is divided into four geologic regions: the Atlantic Coastal Plain, the interior highlands, the Nicaraguan depression, and the Pacific Coastal Plain (McBirney and Williams 1965). The Pacific Coastal Plain in southern Nicaragua is underlain by Cretaceous to Oligocene sedimentary rocks (Funk et al. 2009). To the east, the CAVF, including Maderas, lies within and nearly parallel to the axis of the roughly 600-km-long and 40–70-km-wide Nicaraguan depression (Fig. 1). Lake Nicaragua and Lake Managua are prominent features of this depression.

The stratigraphy and structure of the rocks of the Pacific lowlands and within the Nicaraguan depression and, therefore, underneath Maderas, have been described by McBirney and Williams (1965), Borgia and van Wyk de Vries (2003), Funk et al. (2009), and Saginor et al. (2011). The oldest known rocks in the area belong to the Nicoya Complex, a Jurassic-to-Cretaceous suite of igneous rocks (gabbros, plagiogranites, and basalts) and Mn radiolarites that are exposed on the Nicoya Peninsula in Costa Rica and extend into southern Nicaragua (de Boer 1979; Hoernle et al. 2004; Denyer and Baumgartner 2006). Above the Nicoya Complex lies a sequence of flysch deposits from the Rivas, Brito, and Masachapa formations deposited within the Nicaragua depression that range in age from Cretaceous to Miocene (Borgia and van Wyk de Vries 2003). The youngest units in the region are the sediments of the Pliocene El Salto Formation, formed from the detritus of the Rivas anticline and Tertiary volcanics, and, above this, the lake sediments deposited by Lake Nicaragua, estimated to be up to 1 km in thickness (Borgia and van Wyk de Vries 2003).

Ometepe

Ometepe is ~275 km2 in area and is located in Lake Nicaragua, the largest lake in Central America, with an area of ~8,000 km2 (Freundt et al. 2007). Ometepe has a population of ~44,000 (Wilder 2010). The island consists of Maderas and Concepción volcanoes, connected by the Istián isthmus. Concepción is 31 km3 in volume (Carr et al. 2007b), rises ~1,600 meters above sea level (m.a.s.l), and is historically active with explosions and ashfall occurring as recently as 2010 (Wilder 2010). It has been studied extensively by van Wyk de Vries (1993) and by Borgia and van Wyk de Vries (2003).

Maderas is a small stratovolcano with a sub-conical shape (Grosse et al. 2009), a volume of ~30 km3 (Carr et al. 2007b), an elevation of 1,394 m.a.s.l., and a diameter of ~10 km (Borgia and van Wyk de Vries 2003). No historic activity is recorded. Borgia et al. (2000) state that Maderas has not erupted for at least 3,000 years. The flanks of Maderas, below 200–300 m.a.s.l., have largely been deforested for annual cultivation and pasture and contain more than 20 small towns and communities (Fig. 2). Above ~400 m.a.s.l., Maderas is covered by a cloud forest. The summit crater contains a small lake. A large part of the volcano remains forested and elevations above 850 m are protected as part of a natural reserve (Fig. 2).
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Fig. 2

Locations of towns and communities around Maderas and extent of forestation on Maderas. The natural reserve represents all land above 850 m. The location of a 1996 lahar is also mapped. Image © 2011 Digital Globe, © 2011 TerraMetrics, © 2011 GeoEye, © 2011 Google

Previous studies of Maderas

Geological studies of Maderas have largely been limited to investigations of structural features in which Maderas is viewed as an example of a volcano overlying a weak, ductile substratum that has undergone spreading (van Wyk de Vries and Borgia 1996; Borgia et al. 2000; Delcamp et al. 2008). The results of van Wyk de Vries and Borgia (1996) indicate that Maderas is a fast-spreading volcano with low collapse hazard where the elastic stress should be almost completely relaxed within the volcano. They also identified a slump feature on the southwest side of the volcano.

Mathieu et al. (2011) describe the deformation features on Maderas volcano with respect to a 135°-striking, right-lateral transtensional fault zone using analog models. This fault zone parallels the summit graben of the volcano. Their findings indicate that the regional stress field (transtensional fault) and local stress field (spreading) support the formation of a central conduit and leaf grabens around the base of the volcano. In addition, Funk et al. (2009) describe a 25-km-long and 5-km-wide fault zone with a half-graben structure and a strike identical to the 135°-striking fault zone on the volcano. Called the San Ramon Fault Zone, it lies southeast of Maderas in Lake Nicaragua and lines up with the graben feature on the volcano.

Data

Petrography

Handpicked samples and thin sections reveal that lavas from Maderas are largely porphyritic with most samples containing between 25 and 30 % plagioclase phenocrysts by volume. Less than half of the samples were vesicular, with most samples displaying less than 5 % vesicles by volume with the exception of sample MADERAS-007 which displayed 35–40 % vesicles. Ten thin sections were studied to determine modal percentages, describe textures, and document the degree of alteration (“Electronic supplementary material 1”). All rocks contain plagioclase, olivine, clinopyroxene, apatite, and opaque phenocrysts. Some also contain orthopyroxene, amphibole, and biotite. Overall, the mineralogy, textures, and zoning in Maderas lavas resemble those observed elsewhere in Central America (Carr et al. 1982, 2007a). A graphical summary of phenocryst mineralogy versus bulk SiO2 content for Central American volcanic rocks using data from Carr et al. (1982) is shown in Fig. 3. The ranges of SiO2 contents found in Maderas and Concepción lavas are plotted for comparison. Differences between Concepción, Maderas, and other nearby volcanoes are minor.
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Fig. 3

Phenocryst mineralogy of Maderas and Concepción volcanoes. Values of wt. % SiO2 for each mineral are from Carr et al. (1982) and represent common phenocryst mineralogy for Central America. Solid lines indicate that the mineral is usually present, dashed lines indicate that the mineral is sometimes present. ol olivine, cp clinopyroxene, op orthopyroxene, hb hornblende, bi biotite, pl plagioclase, qt quarts, kf potash feldspar, cu cummingtonite, mt magnetite, il ilmenite, ap apatite

Geochemistry

Whole-rock geochemical analyses of 18 samples, plus an additional six samples collected by Mathieu in 2009 (Mathieu 2010), were conducted at Laboratoire Magmas et Volcans at Blaise Pascal University, Clermont-Ferrand, France, using inductively coupled plasma atomic emission spectroscopy (Fig. 4; “Electronic supplementary material 2”). Thirty-four additional analyses from Maderas are also used to characterize the volcano (Benjamin van Wyk de Vries, unpublished data; “Electronic supplementary material 3”; Lindsay 2009), for a total of 58 whole-rock and trace element analyses (Fig. 4). The compositions of the Maderas lavas are compared to 54 lavas from Concepción volcano (Carr and Rose 1987; van Wyk de Vries 1993; Borgia and van Wyk de Vries 2003). The 24 new samples from Maderas volcano are being stored at Michigan Technological University and are available for further study.
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Fig. 4

40Ar/39Ar and geochemical sample locations by collector and rock type based on Le Bas et al. (1986). Note that some sample names are repeated (i.e. M1 and M1). Both van Wyk de Vries (unpublished) and Lindsay (2009) used the same naming system for their samples

Bulk composition of Ometepe lavas

Maderas erupted basalts, basaltic andesites, andesites and trachyandesites, and trachydacites (Fig. 5a). These rocks are slightly less alkaline than those at Concepción, where basalt, basaltic andesite, andesite, and dacite dominate (Fig. 5a). At Maderas, the whole-rock analyses indicate a typical basaltic to andesitic volcano with a mean SiO2 percentage of 54.4, a standard error of 0.540, and N = 58. This distribution is likely slightly skewed toward the mafic end as Lindsay (2009) selectively sampled mafic materials in order to study source processes. At Concepción, the distribution is similar with a mean of 55.2, a standard error of 0.611, and N = 54. The ranges of silica content for lavas from Maderas and Concepción are also similar to one another and to other Nicaraguan and Costa Rican volcanoes; however, Maderas displays a slightly more alkaline geochemistry than other volcanoes in this area (Fig. 5b).
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Fig. 5

Total alkalies vs. silica for Central America rocks based on Le Bas et al. (1986). a Maderas and Concepción lavas. b Nicaraguan and Costa Rican lavas (Carr and Rose 1987)

Incompatible elements

Maderas lavas have higher incompatible element abundances (K2O, Rb, Zr, Nb, and Th) at a given SiO2 content than Concepción lavas (Fig. 6). The degree of enrichment is about 20–50 %. Ba is the only incompatible element that, on average, has a lower abundance in Maderas lavas. Incompatible element abundances in Maderas and Concepción lavas per given MgO content are similar to lavas at other volcanoes in Nicaragua and Costa Rica (Fig. 7), although some more primitive mafic lavas (MgO > 6 wt.%) from Costa Rica have higher incompatible element abundances.
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Fig. 6

Plots of incompatible elements vs. MgO with regression lines for Maderas and Concepción volcanoes

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Fig. 7

Plot of incompatible trace elements vs. MgO comparing Maderas and Concepción to other Central American volcanoes

40Ar/39Ar dating

Nine samples were selected for 40Ar/39Ar analysis based on two main factors: lack of weathering and stratigraphic location. The goal was to obtain high precision dates demonstrating the entire age range of the volcano. Sample locations are shown in Fig. 4 along with the location of one previously analyzed sample by Carr et al. (2007b).

At the University of Wisconsin–Madison, samples were crushed and sieved to 250–350 μm, and phenocrysts were removed via magnetic sorting or density separation using methylene iodide. Microphenocrysts that survived mechanical separation or groundmass which still showed evidence of alteration were ultimately removed by handpicking under a binocular microscope. Phenocryst-free groundmass separates were weighed and then wrapped in 99.99 % copper foil packets placed into in 2.5-cm-diameter aluminum disks with the 28.201 Ma Fish Canyon sanidine (Kuiper et al. 2008) which monitors neutron fluence. Samples and standards were irradiated at the Oregon State University TRIGA-type reactor in the Cadmium-Lined In-Core Irradiation Tube for 1 h.

At the University of Wisconsin—Madison Rare Gas Geochronology Laboratory, ~200 mg of groundmass packets was incrementally heated in a double-vacuum resistance furnace attached to a 300-cm3 gas cleanup line. Prior to sample introduction, furnace blanks were measured at 100-°C increments throughout the temperature range spanned by the incremental heating experiment and interpolated. Following blank analyses, samples were degassed at 550 °C for 60 min to potentially remove large amounts of atmospheric argon. Fully automated experiments consisted of nine to ten steps from 650 to 1,250 °C; each step included a 2-min increase to the desired temperature that was maintained for 15 min, followed by an additional 15 min for gas cleanup. The gas was cleaned during and after the heating period with three SAES C50 getters, two of which were operated at ~450 °C and the other at room temperature. Argon isotope analyses were done using a MAP 215–50 mass spectrometer using a single Balzers SEM-217 electron multiplier, and the isotopic data were reduced using ArArCalc software version 2.5 (Koppers 2002). Age uncertainties reflect 2σ analytical contributions, and ages were calculated using the decay constant of Min et al. (2000).

A summary of the results of the 40Ar/39Ar analyses is shown in Table 1. Three samples (MADERAS-007, MADERAS-008, and MADERAS-015) did not define an age plateau and are not included in the table. The remaining six samples revealed ages ranging from 70.4 ± 6.1 to 179.2 ± 16.4 ka. The ages for samples MADERAS-002, MADERAS-003, MADERAS-011, MADERAS-013, and MADERAS-018 are precise, with uncertainties of ±1.4– ± 9.2 %. The age determined for the sample MADERAS-004 is less precise (±17 %), which may be due to alteration. These ages are consistent with the previous age date obtained for Maderas of 76 ± 12 ka (Carr et al. 2007b)—also included in the table. Complete results of all 40Ar/39Ar age determinations are shown in “Electronic supplementary materials 49”.
Table 1

Summary of 40Ar/39Ar experiments

Sample

K/Ca total

Total fusion age (ka) ±2σ

40Ar/36Ari ± 2σ

MSWD

Isochron age (ka) ±2σ

Number, N

39Ar%

MSWD

Plateau age (ka) ±2σ

Geologic unita

MADERAS-002

0.43

68.1 ± 7.2

293.4 ± 5.5

0.31

73.2 ± 9.4

9 of 9

100.0

0.34

70.4 ± 6.1

Qpoba

MADERAS-013

1.88

85.1 ± 4.7

295.6 ± 2.4

0.28

85.1 ± 3.8

10 of 10

100.0

0.25

85.2 ± 3.1

Qpoa

MADERAS-004

0.13

125.8 ± 30.9

295.0 ± 1.7

0.23

136.5 ± 33.4

9 of 10

95.7

0.25

128.7 ± 22.2

Qprgb

MADERAS-003

1.69

157.7 ± 3.5

296.3 ± 7.0

0.23

157.1 ± 3.7

9 of 9

100.0

0.21

157.5 ± 2.2

Qpra

MADERAS-011

0.37

175.5 ± 8.6

295.5 ± 5.2

0.21

176.8 ± 9.1

9 of 9

100.0

0.19

176.8 ± 6.1

Qprba

MADERAS-018

0.17

183.9 ± 20.2

296.4 ± 1.8

0.32

166.1 ± 15.7

10 of 10

100.0

0.39

179.2 ± 16.4

Qprba

M10b

76.0 ± 12.0c

Qal

Ages were calculated relative to 28.201 Ma for the Fish Canyon sanidine (Kuiper et al. 2008) using decay constants of Min et al. (2000). Ages in italics are preferred. All uncertainties are given at 95 % confidence level

aGeologic unit refers to Fig. 10

bFor more information on sample M10, see Carr et al. (2007b)

cPlateau age for sample M10 was reported at 1σ in Carr et al. (2007b) but is reported as 2σ here

Results

Geological map and eruptive history of Maderas volcano

As a framework for geologically integrating the age dates, geochemical data, and field observations, a geologic map of Maderas volcano was constructed (Fig. 8). Previous geologic and structural maps were consulted (van Wyk de Vries 1986; van Wyk de Vries and Borgia 1996; Sebesta 2001; Mathieu et al. 2011). Topographic maps of the island, Google Earth images, and a 20-m DEM were also used in the mapping process. The new map is discussed below along with age dates to outline the basic eruptive history of the volcano.
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Fig. 8

Geologic map of Maderas volcano. The San Ramon Fault Zone (Funk et al. 2009) lies mostly outside the map area and has not been included. The 135° striking fault zone (Mathieu et al. 2011) is present across the top of the volcano

Existence of an older cone

Maderas has a flat summit with relatively steep western and eastern flanks. A slope map of the volcano (Fig. 9) indicates that areas west and east-northeast of the summit graben have the steepest slopes with average gradients of ~25° and ~24°, respectively. For comparison, the average gradient at Concepción is ~28°. The topography of Maderas is interpreted to have developed from an older cone that was eroded and breached by a graben. The east and west flanks apparently represent the slopes of the cone before faulting and are composed of the oldest exposed flows on the volcano. The depth of the erosional channels and gullies suggests that there have not been any new lava flows or deposits on the majority of these slopes for an extended period of time.
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Fig. 9

Slope map of Maderas volcano in degrees

40Ar/39Ar ages from the old cone indicate that it is older than ~129 ka (Fig. 8). Traverses into the deepest eroded channels on the old cone reveal that the edifice is composed of both lava flows and pyroclastic and debris flow deposits (Mathieu 2010). It is likely that the west side of the volcano received more pyroclastic material, similar to what is seen at Concepción (Borgia and van Wyk de Vries 2003), as the prevailing trade wind direction is east to west and this is likely to have been true throughout the life of the volcano.

Dominant structural feature: a cross-cutting graben

The dominant structural feature on Maderas is a graben striking 135° that cuts across the center of the volcano parallel to the Nicaraguan depression and the CAVF (Fig. 8). The main faults are normal and bound an asymmetrical graben with over 100 m of vertical displacement on the eastern fault and over 50 m of vertical displacement on the western fault. Down-dropping of the graben created a topographic low across the summit of the volcano (Fig. 10).
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Fig. 10

a Summit profile and cross-section of Maderas volcano in meters. Vertical exaggeration is ~1.5×. Light gray lines represent general orientation of volcanic deposits. The blue area represents alluvial deposits. b Location of A and A′ on Maderas volcano

After its formation, lava accumulated in the graben and flowed along the graben axis, creating a flatter topography to the north and south. The graben feature is delineated by differential erosion, where inside the graben younger deposits are less deeply incised. The faulting is estimated to have taken place at about ~100 ka but is not well constrained. Little or no eruptive activity is believed to have occurred during the graben’s formation as the faults are not covered by eruptive material.

It is also possible that the flat summit of the volcano could be explained by a large explosive eruption that removed the top of the volcano. However, no evidence was found to support this scenario. Neither large tephra deposits nor a remnant caldera was observed on the volcano.

Lava flows

Lava flows emplaced on the volcano mainly radiate from the central crater and are divided into pre-graben and post-graben flows (Fig. 8). Some flows were not sampled and are labeled as having unknown compositions on the map. A dashed line represents uncertainty about contacts between lava flows. Pre-graben flows are found on the east and west side of the volcano and are inferred to have been erupted between ~179 and ~100 ka. Lava compositions from the pre-graben flows vary from basalt to trachydacite. Post-graben flows emanated north and south from the summit crater along the graben. The compositions of these flows are basaltic to trachy-andesitic and their ages are ~80 to ~60 ka.

Central crater and lateral vents

Knowledge about rock materials near Maderas’ summit is limited by strong weathering and heavy vegetation cover in an area receiving orographic rainfall. The central vent of Maderas is located within the graben along its west side (Fig. 8). On the west side of the central crater is a debris avalanche deposit, while the east side of the crater appears to contain deeply weathered lava flows.

Two apparent vents are located on the northeast side of the volcano. One is located in an area known as Punta Gorda and the other is in southeast of Punta Gorda near the town of El Corozal (Fig. 8). The northern half of Punta Gorda has a semi-circular shape and a topographic high towards the northern edge of the point and is cut by a fault. Near the northern coast and at the topographic highs is a lava flow with large boulders of lava. The largest are ~1.5 m in diameter, with an average diameter of ~40 cm. The southern area on Punta Gorda has a semi-circular shape and a flat central crater enclosed by a rim. This rim is highest on the southwest side (~100 m.a.s.l.). A phreatomagmatic deposit is located in this same area as well as on the eastern side along with a lava flow (Mathieu 2010).

We propose that a lava flow descended from the summit crater of the volcano and built a lava delta into the lake in the area of Punta Gorda. This delta collapsed in the south and reacted with water, creating a littoral explosion crater or rootless vent. This interpretation is supported by lava flows that have been mapped upslope of Punta Gorda in this study and by Sebesta (2001).

A similar situation is thought to have occurred near El Corozal to the southeast where another semi-circular, flat crater is located near phreatomagmatic deposits. A lateral vent, located upslope at about 200 m.a.s.l, is thought to have erupted a trachy-andesitic to trachydacitic lava near the lakeshore. When this flow reached the water, it reacted with it, forming another explosion crater.

Another lateral vent that erupted several flows of basaltic to trachy-andesitic lavas is located on the northwestern slope of the volcano above the community of El Tistero. The map of Sebesta (2001) indicates that this vent is a maar feature; however, no explosion crater was found during field observations of this area. As the graben does not appear to cut this lateral vent, we propose that the vent formed after the graben. One other possible vent located in this study is Punta el Delirio on the north-northwest side of the volcano.

Alluvial deposits

Large alluvial deposits are found around the base of Maderas volcano with the most voluminous deposits on its east and west sides (Fig. 8). The majority of these deposits are found downslope of the extensively eroded older cone. They likely originated from the erosion of the older cone in the form of lahars and fluvial deposits. Other depositional features include an alluvial fan to the south, streambeds where sediments have been deposited, and lacustrine deposits from Lake Nicaragua on the Istián isthmus between Maderas and Concepción.

Discussion

Evolution of SiO2 at Maderas

Figure 11 illustrates the geochemical history of the volcano and reveals no change in SiO2 over time. It does show that a range of lava compositions (basalt to trachy-andesite) has been erupted throughout the life of the volcano. Samples were arranged using age dates and stratigraphy (pre-graben flow or post-graben flow; Fig. 8). Then, within each unit, flows known to be stratigraphically younger or older than dated flows were added. The remaining flows with geochemical data were then placed. In some cases, uncertainty exists about the relative ages of these flows. We conclude that Maderas shows no tendency to become more silicic with time but produced variable compositions during its history.
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Fig. 11

Plot of wt. % SiO2 versus lava flow age (see text for relative stratigraphic relationships of samples not directly dated). Error bars for undated samples represent stratigraphic boundaries with other flows when appropriate

40Ar/39Ar ages

Phases of volcanism

40Ar/39Ar ages of Maderas lavas range from 179.2 ± 16.4 to 70.4 ± 6.1 ka. We propose that Maderas experienced at least two separate periods of volcanism. First, growth of an older cone and eruption of the pre-graben flows occurred from ~179 to ~100 ka (Fig. 8). Pre-graben flows are judged to be older than the central graben as they do not appear to have been constrained by the graben which channeled later volcanism. The second phase of volcanism occurred after ~100 ka and the formation of the central graben. The youngest dated flow, which is ~70 ka, lies outside of and near the graben structure, an expected result if enough lava had accumulated within the graben in this area to have flowed over the side of the structure at the time of its eruption.

Implications of ages for shorelines at Maderas and Concepción

Maderas and Concepción volcanoes differ in that Maderas exhibits a drowned shoreline whereas Concepción’s shorelines appear to be rising. At Concepción, raised beaches and deformed beds form a deformation belt around the base of the volcano (Borgia and van Wyk de Vries 2003), whereas at Maderas there are few beaches and much of the shoreline is rocky. This difference in shorelines may be due to the differences in the ages of the two volcanoes. Concepción has historic activity (Diez et al. 2006) and one lava sample has an age of 19 ka (Siebert et al. 2010). The rise of shorelines around Concepción is believed to be caused by loading of volcanic material, causing spreading of the ductile lake sediments on which the volcano sits (Borgia and van Wyk de Vries 2003).

The drowned shorelines at Maderas, which is no longer loading volcanic material, imply that it is subsiding. This could be explained by the reaction of the underlying lake sediments to the overlying edifice since the eruptions stopped over the last several tens of thousands of years. Collapse of the magma chamber below the volcano could be another explanation for subsidence.

Comparison of age dates to other Central American volcanoes

The duration and age of volcanic front volcanoes in Central America is not yet well constrained. Few volcanoes have been extensively sampled for geochronology. Figure 12 plots dated samples from various volcanoes in Central America. Numerous recent eruptions have been dated for most volcanoes, but evaluation of the duration of volcanism is difficult due to sparse age dating for older eruptions.
https://static-content.springer.com/image/art%3A10.1007%2Fs00445-012-0644-7/MediaObjects/445_2012_644_Fig12_HTML.gif
Fig. 12

Ranges of age dates analyzed for Central American volcanoes. White symbols represent Guatemalan volcanoes, black symbols represent Nicaraguan volcanoes and gray symbols represent Costa Rican volcanoes. Age dates from Bardintzeff and Deniel (1992), Vallance et al. (2001b), Carr et al. (2007b), Escobar-Wolf et al. (2010), Siebert et al. (2010, 2011)

Carr et al. (2007b) show that the onset of active volcanoes of the volcanic front in Nicaragua has taken place more recently than ~400 ka. This is based on the oldest dated Nicaraguan volcanic front lava sample, from Telica volcano, that has an age of 330 ± 20 ka. Costa Rican volcanism is thought to have begun at ~600 ka. In Nicaragua, the age when volcanism began is much less constrained due to the location of the volcanoes within the Nicaraguan depression where earliest flows are buried by sediments (Carr et al. 2007b). Guatemalan volcanoes display age patterns similar to those in Costa Rica. Extensive age date sampling at Santa Maria revealed ages ranging from 438 ka (Singer et al. 2011) to 103 to 35 ka (Escobar-Wolf et al. 2010), with a climactic eruption sequence beginning in 1902. Volcanism at the Fuego–Acatenango volcanic complex ranges in age from over 230 ka to the present, while Fuego may represent an overall duration of less than 30 ka (Vallance et al. 2001) and Pacaya has age dates extending back to 553 ka (Vallance et al. 2001).

In all of these cases, the duration of volcanism is unclear because there are few dated samples. Fuego’s young volcanism since 30 ka has piled on an edifice that is 230 ka or older (Vallance et al. 2001). While Santa Maria is mostly younger than 103 ka, it is built on cones that have ages ranging from 163 to 438 ka (Singer et al. 2011). Only a few volcanoes have numerous dates, so the duration of currently active cones is unconstrained and could be a few tens of thousands of years. It is unclear whether the maximum ages of cones like Telica suggest that the volcano has been continuously active since 300 ka or if the volcano’s current activity is only the latest of several periods of concentrated activity, each of which could be viewed as a separate volcano. It is also unclear at Maderas, which has undergone at least two phases of volcanism, if it represents two volcanoes or one. Despite these uncertainties, the lack of age-dated materials younger than ~60 ka at Maderas is surely significant to hazard potential and it contrasts markedly with Concepción which is known to have had dangerous historic activity.

Implications for geologic hazards

Concepción volcano has erupted frequently in the past century (Diez et al. 2006). While the most recent eruptions have been VEI = 1 or VEI = 2, earlier accounts indicate that more violent eruptions have occurred. In comparison, the new ages from Maderas imply that it has likely not been active for tens of thousands of years, with the youngest determined age date being 70.4 ± 6.1 ka.

The apparent lack of activity at Maderas since ~60 ka indicates that the probability of an eruption in the immediate future is low. However, a future eruption cannot be ruled out. Santa María volcano in Guatemala erupted catastrophically in 1902 after a repose that may have lasted ~30 thousand years (Escobar-Wolf et al. 2010). A future eruption at Maderas may create a similar scenario as the closed vent of the volcano would require a major deformation to reopen.

Fortunately, there are often signs of unrest before an eruption, especially after long reposes. Before the Santa María eruption, there was a marked seismic warning lasting for months (Anderson 1908). Precursory deformation or increased seismicity associated with movement of magma below Maderas would likely be detected by the inhabitants of Ometepe. While Maderas is currently not being monitored for deformation, the INETER website indicates that a seismic station is located on its southwest side and two other seismic stations are located near Concepción.

Additional geologic hazards on Maderas are earthquakes and lahars. Ometepe is located in a seismically active region of the world along the CAVF and near the subduction zone of the Cocos and Caribbean plates. Also, Maderas itself is crossed with a number of faults indicating past seismic activity (van Wyk de Vries and Borgia 1996; Mathieu et al. 2011; Fig. 8) and continued spreading of the volcano could lead to more seismic events.

Lahars and landslides are also a concern for the inhabitants of Ometepe. As shown in the geologic map (Fig. 8), much of the base of Maderas is covered in alluvial deposits. Some of this alluvium has been deposited in the form of lahars during extreme rainfall events. Such a lahar occurred on September 27, 1996, devastating the town of El Corozal on the northeast flanks of Maderas (Fig. 2) (Smithsonian Institution 1996). There remains a risk in these areas for future lahars to occur, especially as climate change may bring about more extreme weather events (McBean 2004). The older cone is likely the source of these deposits as they are largely located downslope of the older cone.

Conclusions

While the petrology and age of Maderas are comparable to those of nearby volcanoes, it has other characteristics that make it atypical. Maderas is located within a large lake, near a large step in the frontal arc axis, and its geochemistry, which ranges from basalt to trachydacite, is more alkaline than other Nicaraguan and Costa Rican volcanoes. Its geomorphology is also dominated by a central graben not seen elsewhere in the CAVF. These characteristics make Maderas a unique and important volcano to study.

New 40Ar/39Ar ages range from ~200 to ~60 ka and indicate that Maderas underwent at least two phases of volcanism: the construction of the initial cone and pre-graben volcanism prior to ~100 ka and post-graben volcanism after ~100 ka. These phases were separated by the formation of the central graben which constrained the dispersal of lavas erupted during the last phase of volcanism within its boundaries.

These ages suggest that Maderas may not have erupted for ~ 60 thousand years and future volcanic eruptions are not considered an immediate hazard. However, earthquakes and lahars are considered as significant potential hazards. Tropical storms, especially those with high rainfall rates, could lead to dangerous debris flows beneath Maderas’ steep flanks. Continued seismic monitoring should take place on the island.

Acknowledgments

This study was supported by US National Science Foundation PIRE Grant #0530109. The costs of the first author’s time in the field at Maderas were paid by the US Peace Corps. We thank Lucie Mathieu for allowing us to use samples that she collected from Maderas and Ben van Wyk de Vries for sharing his expertise of Nicaraguan volcanoes and chemical analyses of many samples from Ometepe. Heather Cunningham is thanked for her support in preparing the samples for 40Ar/39Ar analysis.

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© Springer-Verlag 2012