About Cerro Negro (Nicaragua) volcanic eruptions:
Cerro Negro is an active volcano in the northwest
part of Nicaragua, one of a series of active volcanoes found in this
stretch of the Pacific Ocean's "Ring of Fire." Subduction
or recycling of the oceanic crust of the Pacific Ocean floor underneath
Central America eventually leads to the formation of magma deep underground,
which rises and erupts, forming a volcano. A special type of volcano,
called a stratovolcano (or composite volcano), is typically produced
by subduction. These volcanoes are explosive in nature, and are composed
of both lava flows and layers of airborne debris. There are many hazards
associated with stratovolcanoes, including volcanic mudflows and debris
flows (called lahars) that surge down valleys, dense clouds of hot
ash and volcanic fragments that rush across the ground, and ash and
other tephra that fall out of the sky.
Cerro Negro is quite different from a typical
subduction-related stratovolcano. Cerro Negro is not very large compared
to a stratovolcano like Mount Rainier in Washington State or Pinatubo
in the Philippines. As its name implies, this volcano is made of very
dark rocks (basalts), rather than the salt and pepper rocks (andesites
and dacites) that are more commonly found in stratovolcanoes. Cerro
Negro is mostly made of black cinders, with lesser lava flows, and
is classified as a cinder cone. Thus the only hazard associated with
Cerro Negro is the airfall tephra. Nonetheless, 200,000 people live
in Leon just 20 km to the west of Cerro Negro; 9 people were killed
by tephra (through building collapse) in the 1992 eruptions.
Hill et al (1998) have analyzed the very recent
eruptive history of Cerro Negro in order to quantitatively evaluate
the hazards associated with airfall tephra from a basaltic cinder
cone. The geologists measured the volume of airfall tephra associated
with 23 different eruptions dating from 1850 through 1995. Each eruptive
event produced a blanket of tephra whose thickness decreases as you
go away from the volcano; lots of thickness measurements are therefore
required to determine the actual volume of an eruption. The data give
the cumulative volume (in cubic kilometers) of tephra with time; the
graph shows the same information, however the earliest two eruptions
were not plotted.
Students can regress the data using a linear model,
to find the typical eruption rate for Cerro Negro during the last
century (Hill et al omitted the first two data values in their regression).
Students should look closely at the units for the eruption rate; can
they convert this rate to cubic meters per year? The correlation coefficient
for the linear model is quite high, suggesting a near steady state
eruption mechanism over this time period. If Cerro Negro is a steady
state volcano, then its relatively uniform and regular behavior could
be used to make fairly reliable forecasts of both maximum eruption
size and maximum time interval between eruptions. How would students
test the assumption of steady state behavior?
Source: Hill BE, Connor CB, Jarzemba MS, La Femina
PC, Navarro M and Strauch W (1998), 1995 eruptions of Cerro Negro
volcano, Nicaragua, and risk assessment for future eruptions; Geol.
Soc. Amer. Bulletin, v. 110, pp. 1231-1241.
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