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Temporal evolution and recurrence of large eruptions
Periods of quiescence, particularly between large-volume eruptions, last much longer than the few hundreds of years of recorded human history in Nicaragua. Assessment of the probability of occurrence of another such eruption has to rely on extrapolation of the past record of activity of the volcanoes assuming that the past behavior continues into the future. The temporal evolution of mass discharge and composition of the volcano/magma systems studied allowed us to extrapolate to their future behavior. In Nicaragua, Chiltepe is the most likely site of a future plinian eruption.
Here we focus on Masaya caldera and the Chiltepe peninsula - a volcanic complex hosting Apoyeque stratocone, Xiloa maar, and other now hidden vents - for which we have the most complete data on their eruptive histories. Apoyo caldera experienced two plinian eruptions about 25,000 years ago separated by a relatively short period of time (perhaps a few hundred years; Fig. 7a) but little is yet known of its earlier history. A simple way to estimate the time frame for a future eruption is to assume that a volcano follows a periodic pattern of eruptive activity, where the period is obtained by dividing the time over which a volcano was active by the number of eruptions that occurred. Masaya caldera produced four large eruptions during the past 6,000 years, indicating an average recurrence period of 1,500 years. Six eruptions during 15,000 years from the Chiltepe complex suggest an average recurrence period of 3,000 years but, since the last four eruptions occurred within the last 6,100 years, the recurrence period may have reduced to 1,500 years.
A more detailed impression about how a volcanic system evolved can be gained from the pattern in which erupted magma mass accumulated with time through a series of eruptions. In a steady-state system, larger erupted masses would be expected to be associated with longer repose times (Bacon, 1982; Wadge, 1982; Hill et al., 1998). Using the Chiltepe complex as an example (Fig. 7b), the cumulative erupted magma mass (added up starting from the Lower Apoyeque Tephra, Fig. 7a) plotted versus the age of each respective eruption (we guessed some ages since absolute age data are incomplete) produces a typical saw-tooth curve (short-dashed in Fig. 7b). On this saw-tooth curve, corners marked by white dots produce the bold line which shows how much magma has been produced up to a given time. The corners marked by gray dots produce a line that shows how much time passed after an eruption of a given magma mass until the next eruption commenced, and this line may be used to predict a time frame for the next future eruption. If eruptive activity at Chiltepe is controlled by a steady-state process (e.g., steady extension of the Nicaraguan depression), then the gray dots should lie on or close to a straight line (long-dashed line (1) in Fig. 7b; Bacon, 1982; Wadge, 1982). Line (1) is a linear regression through only the UAq to LT data. A better fit, however, is obtained for an exponential regression through the gray dots using all data (LAq to LT), yielding line (2) in figure 7b. This would imply that whatever process controls eruptive behavior at Chiltepe is not linear but exponentially accelerating. A forecast of the next future eruption can be made by projecting the cumulative erupted mass of the last eruption (Chiltepe Tephra, CT) up to either line (1) or (2). For the exponential regression, the next eruption should already have occurred ~200 years ago, whereas it would be expected in 6,500 years from the linear regression (Fig. 7b). Clearly, a more useful forecast requires a better understanding of the processes that control the evolution of the Chiltepe volcanic complex which has only produced eruptions of magnitude M>4 during the past few thousand years and is likely to continue doing so.
The data for Masaya caldera (Fig. 7c) comprise three moderately large eruptions (SAT, LCT, MTL; Fig. 7a), the unusually large Masaya Tuff eruption (MT), and a long repose period of ~30,000 years back to the Fontana eruption (which actually occurred outside the caldera; Wehrmann et al., this volume). The large variations in magnitudes and recurrence periods within this small number of data do not allow for extrapolation analogous to Chiltepe. Post-caldera volcanic activity since the last large eruption from the Masaya caldera has been cone-forming strombolian and phreatomagmatic activity and lava effusion (Walker et al., 1993), none of which was very hazardous outside the caldera. This may indicate that the Masaya magmatic system has been reduced to a state favoring less hazardous volcanism but it does not exclude that the system may return to a state of much higher activity.
Our stratigraphic work also constrained the temporal evolution of the smaller-scale eruptions at the Nejapa-Miraflores lineament crossing Managua city, where the youngest eruptions are <1500 years old. We demonstrated the occurrence of volcanogenic tsunamis in Nicaraguan lakes and discussed hazard implications. Basaltic plinian eruptions, presently considered as exotic events, contribute up to 50% of highly explosive volcanism in Central America and we have derived controlling eruption mechanisms from several dedicated case studies. We delivered special hazard maps to the Nicaraguan authorities including the newest research results (distribution and frequencies of quaternary fallouts, surges, and volcanogenic tsunamis) of the most dangerous eruptive centres Masaya and Apoyo Caldera as well as Chiltepe Volcanic Complex neighbouring all the very highly populated Managua-Masaya-Granada area.
Fig. 1: (a) Composite stratigraphic succession of deposits from highly explosive eruptions in west-central Nicaragua. Arrows at right indicate the production of pyroclastic surges (S) and pyroclastic flows (F) during these eruptions. Gray arrows at left mark the stratigraphic position of major phreatomagmatic (PM) eruption phases along the Nejapa-Miraflores lineament, in Managua city, and along the southwestern shore of Lake Managua. Our age data were determined by the Leibniz Laboratory for Radiometric Dating and Stable Isotope Research at Kiel University. (b) Cumulative mass of erupted magma versus eruption age for the Chiltepe volcanic complex. Ages of undated tephras are guessed considering the nature of intercalated deposits. (1) and (2) are linear and exponential regressions, respectively, through the gray dots as explained in the text. (c) Cumulative mass of erupted magma versus eruption age for the Masaya caldera.
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