Cameron and Reagan, 2002

Young volcanic tephra stratigraphy near the Nejapa crater
of Managua, Nicaragua.
Ian Cameron, Mark Reagan, Department of Geoscience, University of Iowa, Iowa City, IA 52246
Abstract
The youngest fallout deposit associated with the Nejapa-Miraflores alignment near
Managua, Nicaragua was erupted from a small crater just north of the much larger,
Nejapa crater. Its axis of greatest thickness extends to the northwest of this vent and north
of the Motastepe cone. This deposit covers an area of about 5 to 5.5 kilometers square to a
thickness of 10 centimeters or more. The deposit is charcoal grey and consists of basaltic
lapilli and ash. A variably thick layer of red-brown soil overlies the tephra. Directly
beneath the deposit is a brown soil that overlies the tephra associated with the Motastepe
cone. These soils were used as markers to find the deposit along road cuts and riverbeds
that are located closer to the vent. Farther from Nejapa, the soil underlying the young
tephra commonly appears atop a layer of white pumice that originated from the Apoyeque
Caldera.
Introduction
Bice’s (1985) characterization of the tephra stratigraphy near Managua, Nicaragua
focused on large and widespread deposits erupted from the major volcanoes in the Nicaraguan
alignment. Thus, many small tephra units that erupted from the complex of vents associated
with the Nejapa-Miraflores alignment remained relatively unstudied. The volcanoes along the
alignment are those closest to Managua, and thus, pose a significant threat to the population of
the city and surrounding towns. This study is concerned with identifying and mapping the
youngest such deposit, and discussing the implications of the nature of this particular deposit for
the hazards posed by the Nejapa-Miraflores volcanic complex.
Tephra A
The youngest deposit near Managua, here termed tephra A, has a depositional plume that
trends northwest from a small crater at the north end of the Nejapa crater complex. The axis of
the plume trends along the north flanks of the Motastepe cone and on towards Bello Amanecer.
It has a characteristic charcoal grey color with a relatively consistent thickness moving out
radially from the vent.
Because of Motastepes close location to the vent, the tephra layer is consistently around
50 centimeters in thickness on its north flank. In this area it is normally graded overall and
consists of decimeter-scale layers of charcoal grey lapilli with centimeter to millimeter scale
muddy interbeds. There are variations in the numbers of these fine ash layers that could be due
to small surges or syn-eruptive rainfall. Near Motastepe, the lapilli in the upper portion of tephra
A are typically a few millimeters in length, whereas towards the base of the unit the lapilli are 13 cm in length. The thickness of the lapilli layers grade downward from <1 cm to between 3 and
4 centimeters (Fig. 1a). The basal layer is the coarsest and thickest of the group. The overlying
soil is about half a meter thick and has a characteristic red-brown coloring. This soil is thicker
and darker in color farther from Motastepe. Beneath is another section of soil and then the
massive Motastepe unit.
Tephra A thins to the northwest from Motastepe. Because of weathering and landsliding
along its flanks, tephra A commonly is intermixed with runoff and air fall from Motastepe in this
area (Map, site 1b). The tephra unit dies out completely on the west side of Motastepe.
The tephra layer at its thickest, north of the Motastepe cone and northwest of the vent.
About 1 kilometer to the northwest of Motastepe the air fall consists of small lapilli and coarse
ash with interlayered fine-ash deposits. The uppermost tephra layer is made up of 0.5 - 2 mm
sized lapilli that are normally graded. Beneath this layer is an ash unit about 5 centimeters thick.
This ash unit covers a lapilli fall layer that is 2 to 3 centimeters thick (Fig. 1b). Beneath is a
transition back too medium sized, 1- 3 mm, lapilli that are normally graded similar to the first
lapilli fall layer. This entire unit overlies the rich red-brown soil. Between 1 and 2 km from the
vent, tephra A is characterized by a deposit of fine ash sandwiched between two coarse ash and
lapilli layers (Map, site 4). The overlying soil in this area is much thicker than on the Motastepe
flanks, and contains large clasts washed in from the surrounding volcanoes. Cross bedding in
some of the deposits from this area probably resulted from reworking of the tephra by a network
of streams that cut across the topography.
Near the crater, due north of Motastepe and due east of Laguna de Asososca, tephra A is
lacking. Airfall and surge deposits from Motastepe, however, are more than 5 m thick on the
south edge of this crater. A cinder cone consisting of bright reddish purple cinder underlies a 1
m thick orange soil beneath the Motastepe unit.
Directly to the west of the vent next to the Army installation (Map, sites 11-13) the
topsoil has been nearly washed off leaving 20 centimeters of tephra A exposed at the top. The
tephra bed is made of 1-3 mm lapilli on top that grades downward to colluvial ash up to 0.1 cm
in diameter. Under this layer is a light brown soil unit with beds about 10 centimeters thick. At
this location, four of these ash beds are interlayered with brown soil, and underlain by red soil
and the Motastepe unit (Fig. 1c).
A thick, coarse rhyolitic pumice deposit generally underlies Tephra layer A in the
northwest portion of the field area. The pumice consists of white angular to sub-rounded
fragments that are about 1-3 centimeters in length with sparse phenocrysts of plagioclase and
hornblende. It probably is the youngest rhyolitic air-fall deposit from the Apoyeque volcanic
complex (the Jiloa pumice of Bice, 1985). The thickness of the Jiloa pumice unit ranges from 45
to 60 centimeters thick in the northwest part of the study area. Directly under the pumice is
another much finer tephra bed (Map, site 10). This tephra layer contains very fine lapilli and
small to medium sized clasts (Fig. 1d).
About 6 kilometers northwest of the apparent vent, tephra A consists of <10 centimeters
of fine ash. This layer is overlain by about 20 centimeters of the red-brown soil. The Jiloa
pumice is quite a bit thicker here, 50-55 cm, and is made up of a diverse range of sizes, from
<1cm to 3cm in diameter. The pumice is a dirty white-grey color in this area. The depositional
plume of the Jiloa Pumice extended to the southeast (Bice, 1985) where it intersected tephra
deposits from the Nejapa alignment. (Map, site 17).
Other Volcaniclastic Deposits
The two closest deposits to the vent are the Motastepe cone to the northwest and the
cinder cone to the north. The cinder cone is made up of rough surfaced, reddish purple, vesicle
rich cinders. They range in sizes from 3-6cm to 10-15m. This cinder is overlain by a 1-m thick
orange soil beneath the Motastepe unit. The Motastepe unit consists of layered units of ash and
tephra. It is a massive deposit with layers that range from a half of a meter to more in thickness.
The unit is a charcoal grey color but also has lighter grey and black layers in it. Its maximum
thickness is about 100 m at the core of the Motastepe cone. The shape of the cone and its size
give the impression that it erupted relatively quick and consistently. The Apoyeque pumice
covers the Motastepe cone.
Summary and Implications
The youngest tephra unit of the Nejapa Miraflores alignment, tephra A, was vented from
a crater immediately north of the Nejapa crater. Its axis of greatest thickness extends to the
northwest of the vent. It covers an area of about 5 kilometers and has a thickness of 10
centimeters or more. The tephra has a characteristic charcoal grey color with lapilli that range
from fine to medium size, 0.5-3 mm. It largely is a fallout deposit, although bedding
irregularities and variation in sorting suggest that some of the tephra was deposited from
pyroclastic surges within one or two kilometers of the vent. Some of the deposits appear to be
reworked, perhaps by syn-eruption or shortly post-eruption rainfall.
Tephra A appears to be the only significant volcanic deposit that is younger than the Jiloa
Pumice of Bice (1985) and erupted from the portion of the Najapa-Miraflores alignment that lies
west of Managua. The Jiloa pumice was dated at 6,590 radiocarbon years BP (Bice, 1985) which
converts to about 7,500 calendar years BP (using program CALIB rev. 4.3 by Suiver and
Reimer, 2000). Thus, it appears that only one significant eruption of the Nejapa-Miraflores
occurred in the past 7,500 years. This tephra, as well as older basaltic tephras originating in the
portion of the Nejapa-Miraflores alignment and investigated here are all have significantly
smaller volumes than tephras originating from Apoyeque and Masaya (e.g. Bice, 1985). The
small volumes and infrequent eruption frequency along the Nejapa-Miraflores alignment near
Managua suggest that the hazard posed to Managua by this alignment probably is less than the
hazard posed by the larger but less proximal volcanic edifices of Masaya and Apoyeque.
Acknowledgments
We are grateful to the staff at INETER in Managua, Nicaragua, for allowing us to use
their facilities and providing us with logistical assistance. Jorge Cruz, and Jessica ToledoRasmussen were both invaluable field partners.
Reference
Bice, David C. “ Quaternary volcanic stratigraphy of Managua, Nicaragua: Correlation and
source assignment for multiple overlapping plinian deposits” in Geological Society of American
Bulletin, 96: 553-566, April 1985.