Pelee, Martinique, West Indies

Eruption Style and History

The Pelee edifice has been volcanically active for 400,000 years. Its history includes the formation of multiple craters in close proximity to one another. Most assessments of volcanic hazard are based on the last 5000-6000 years. The stratigraphy for this period is accessible and has been well studied. Eruptions of Mt. Pelee alternate between dome growth-and-collapse (Pelean) and open crater pumice and ash flows (Plinian). Stratigraphically the two major styles have created alternating layers of dense andesite deposits and pumiceous deposits respectively. There have been 25 such alternations in the past 6000 years. There does not appear to be a correlation between the chemical composition of the deposits and the eruptive style. This suggests that gas content plays a key role. Volitile-rich magmas would erupt in Plinian activity while less volatiles would result in Pelean activity. For both styles, some eruptions commence with violent explosions while others are more effusive. Westerncamp and Traineau (1983) propose four total types of eruptions. They relate the presence or absence of initial explosive activity to the amount of meteoric water encountered near the surface.

Chemistry and Magma System

**Chemistry of products ??**
Two magma chambers are proposed based on geochemistry. In the proposed model, basaltic magma is fractionated in a lower (20-15 km) chamber to produce basaltic andesite and cumulates. This lower density magma could then rise to a higher chamber (13-10 km) where further fractionation creates andesites and dacites. The chemistry of this upper chamber varies systematically with time in a cycle separate from the eruptive style. Influxes of basaltic andesite magma into the upper chamber are likely to produce a density layered chamber with andesite magma at the top.

The introduction of new basaltic andesite magma into the upper chamber is thought to result in explosive Plinian eruptions of low-silica andesite. The last such eruption was 2200 years ago. Subsequent Plinian eruptions have had lower volume and extent separate by Pelean activity. Recent activity has been strictly Pelean. This implies that the last recharge of the upper chamber occurred 2200 years ago. The extrusion of gas-poor magmas in Pelean eruptions should continue until the chamber is recharged.

Local Seismicity

The seismicity of Pelee was addressed by Hirn et al. A temporary upgrade of the array in 1985 allowed shallow seismicity directly under the volcano to be observed. The array consisted of ~8 stations. They were able to locate events but focal mechanisms were not produced. Frequency analysis of shallow events showed that, despite their small magnitude, these events do not have high corner frequencies. This could indicate a volcanic as opposed to tectonic source. However, the lack of strong high frequencies could also be due to attenuation near the volcano.

Regional seismicity is moderate to low. Passive seismic studies would require long deployment.

Notes and Questions

• What has been done seismically since 1987? The observatory web page indicates that seismicity has been monitored closely but no array information is given and no references can be found. Hirn did not respond to emails concerning recent work.
• Could the two magma chamber model be confirmed via tomography?
• Given the presence of shallow events, could current locus of activity be determined and would this have an impact of hazard mapping?

Selected References

~mwest/antilles

Mt. Pelee, GSA Memoir 175, 1990

Westercamp and Traineau, The Past 5000 Years of Volcanic Activity at Mt. Pelee:

Implications for Assessment of Volcanic Hazards, JVGR, 17 (1983) 159-185

Hirn et al., Shallow Seismicity at Montagne Pelee Volcano. Bull. Vol. (1987) 49:723-728

JVGR, 38 (1989) Entire Issue