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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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 Cerro Negro (Nicaragua) volcano eruption history source: Hill et al (1998) cumulative volume in cubic kilometers eruption year cum. vol. 1850 0.006 1867 0.010 1899 0.011 1914 0.012 1919 0.012 1923 0.039 1929 0.039 1947 0.051 1948 0.051 1949 0.051 1950 0.068 1954 0.068 1957 0.074 1960 0.095 1961 0.095 1962 0.096 1963 0.096 1968 0.115 1969 0.115 1971 0.142 1992 0.152 1995 0.152 1995 0.160

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