An eruptive history of Maderas volcano using new 40Ar/39Ar ages and geochemical analyses
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- Kapelanczyk, L., Rose, W.I. & Jicha, B. Bull Volcanol (2012) 74: 2007. doi:10.1007/s00445-012-0644-7
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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.
Keywords40Ar/39Ar datingEruptive historyVolcanoMaderas
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.
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 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).
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.
Bulk composition of Ometepe lavas
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).
Summary of 40Ar/39Ar experiments
Total fusion age (ka) ±2σ
40Ar/36Ari ± 2σ
Isochron age (ka) ±2σ
Plateau age (ka) ±2σ
68.1 ± 7.2
293.4 ± 5.5
73.2 ± 9.4
9 of 9
70.4 ± 6.1
85.1 ± 4.7
295.6 ± 2.4
85.1 ± 3.8
10 of 10
85.2 ± 3.1
125.8 ± 30.9
295.0 ± 1.7
136.5 ± 33.4
9 of 10
128.7 ± 22.2
157.7 ± 3.5
296.3 ± 7.0
157.1 ± 3.7
9 of 9
157.5 ± 2.2
175.5 ± 8.6
295.5 ± 5.2
176.8 ± 9.1
9 of 9
176.8 ± 6.1
183.9 ± 20.2
296.4 ± 1.8
166.1 ± 15.7
10 of 10
179.2 ± 16.4
76.0 ± 12.0c
Geological map and eruptive history of Maderas volcano
Existence of an older cone
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
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 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.
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.
Evolution of SiO2 at Maderas
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
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.
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.
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.