Concepción volcano is a steep symmetrical stratovolcano with a perfect shape and one of the most active volcanoes of Nicaragua. During the past 100 years, it has been in a near-constant state of small to medium ash explosions at irregular intervals. Together with its neighbor Madera volcano Concepción forms the NW half of the dumbbell-shaped island of Ometepe in Lake Nicaragua. Both volcanoes are connected by a narrow isthmus. Frequent explosive eruptions during the past decades have increased the height of its summit significantly (more than 100 m) and outdated the existing topographic maps. Because of the frequent ash eruptions, the upper half of the summit cone lacks vegetation. The basaltic-to-dacitic Concepción volcano contains a steep-walled 250 m summit crater with a higher western rim. Flank vents are aligned on N-S-trending fractures and contain chains of spatter cones, cinder cones, lava domes, and maars on the NW, NE, SE, and southern sides. Some of these vents are found down to Lake Nicaragua.

Volcán Concepción was constructed after Lake Nicaragua and rises 1600 m above a package of about 1000 m of lake sediments. The modern cone is built upon a caldera of an older generation of the volcano. A small remnant of this caldera forms a break in slope about halfway up the north flank.



Due to its particular setting above lake sediments rich in mobile clay, the volcano has been experiencing gravity-induced spreading of its edifice.



Geologic evolution

It has been proposed that Concepción undergoes cycles of magma evolution from basaltic to evolved dacitic and andesitic magma. It is possible that the current activity which is mainly basaltic is at the beginning of a 5th cycle.

1 Sub-lacrustine eruptions

The earliest volcanic layers are found on the northwest and southeast shores of Lake Nicaragua and overly directly mudstone. Magma intruded the crust and erupted basaltic rocks at the surface of the lake floor.

2 Building a first cone

A cone is built above the lake and its weight starts to deform the base, while pyroclastic sediments form an apron around the cone. Magma starts collects in a shallow chamber and evolves towards dacitic compositions during this phase.

3 Destructive phase

A major plinian explosive eruption causes the collapse of a small caldera, which is now buried, and the formation of several fault systems, which facilitate future eruptions. At the same time, the lateral spreading of the volcano is temporarily halted because of the redistribution of load. The pumice and ash layer from this eruption is known as the Tierra Blanca layer.

4 Second building phase

A new cone is built and magma evolves again with time. Magmas are erupted with greater ease than during the previous phase and evolve from

high-alumina basalt to silicic andesite. After reaching a critical mass, the sedimentary substratum start to subside and the volcano spreads. Lowered pressure and stress on the shallow reservoir seems to have prevented the magma to evolve further than andesite. Numerous intrusions form diapirs beneath the lower flanks.

5 Future spreading phase

The complex of intrusions at the base of the lake sediments will eventually begin to spread. The pure E-W extension observed today on the fault that cuts the summit could be a sign of this incipient creeping. This extension of may eventually decrease the stress on the crust sufficiently to allow the eruption of basaltic magmas. This phase may be beginning now.



Volcanic deposits

The prevailing trade winds carry most ash to the west, where they form thick layers, 50 m think at Moyogalpa 8 km west of the crater. Almost no ash is found in the coastal plain north and south of the volcano. Lava flows have mainly been deposited to the east.



Sources:

Borgia, A., van Wyk de Vries, B. (2003) "The volcano-tectonic evolution of Concepción, Nicaragua", Bull Volc, v. 65, pp. 248–266