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Paleomagnetism and Volcanoes


There is now considerable evidence that the eruptive activity of volcanoes is characterised by subsequent “cycles” displaying similar features among them, both in term of timing and type of eruptions. Understanding in which part of this framework should be considered the present-day activity of volcanoes is an important part to constrain the future history and hazard.
Unravel in detail the past history of a volcano requires a careful dating of the various volcanic products exposed along the flanks of the volcano. Dating is classically obtained on target flows by isotope (mostly K/Ar and Ar/Ar) dating. However, given the “spot” character of these dates (which are expensive and time-consuming), there are normally only few clue- age points reported. 14C dates of soils interbedded within the flows are also used, but soils are uncommon in volcanoes characterized by high eruption rates. Conversely, the comprehension of the rate of eruptive activity would require a detailed dating of all (or almost all) the flow exposed.
In the last two decades, there has been an increasing use of paleomagnetism to provide the ages of volcanics emplaced by the Italian active volcanoes during the last centuries or millennia. When lavas (and other volcanics) cool, they faithfully record the direction parameters of the magnetic field acting in that moment. In fact, when the temperature of the rock lowers below the Curie temperature of the ferromagnetic minerals, the rock acquires a thermoremanent magnetization parallel to the ambient magnetic field. Such magnetization is “frozen” in the rock even when the direction of the magnetic field subsequently changes. As known by over three centuries, the directional parameters of the geomagnetic field (declination and inclination) vary continuously and rather quickly. For instance, the magnetic declination at Etna was about –17° about two centuries ago, while today is exceeding +2°.
In order to infer dates, the paleomagnetic directions gathered from the exposed volcanics are compared to relocated reference directions from given age windows, derived from independent reference curves of the paleosecular variation (PSV) of the geomagnetic field. The availability of high-quality PSV reference curves (which are valid for wide regions such as Europe, but not at a global scale) is critical to soundly perform such “paleomagnetic dating” method.
For the Mediterranean region, valuable PSV reference data come from direct geomagnetic field measurements (last four centuries), archeomagnetism (several data sets from different countries of Europe and Middle East extending back to 2000-8000 yr BP), and paleomagnetism of sedimentary cores drilled in Holocene-Pleistocene lakes. Reference European PSV data for the Holocene relocated at Stromboli are shown in fig. 1 (from Speranza et al., 2008).

Figure 1 Reference European PSV data for the Holocene relocated at Stromboli are shown 
(from Speranza et al., 2008).

Since 2003, the paleomagnetic laboratory of Rome, in collaboration with volcanologists from INGV, has started studying the PSV of the geomagnetic field recorded in lavas from Stromboli (fig. 2) and Etna (fig. 3).  

Figure 2 Paleomagnetic sampling sites from Stromboli volcanics (from Speranza et al., 2008).

 

Figure 3 Paleomagnetic sampling sites from Etna lava flows (from Speranza et al., 2006)

The results allowed an unprecedented dating of many lavas from both volcanoes, and helped unravelling their eruptive history during the last centuries/millennia (fig. 4 and 5). For details see Speranza et al. (2004; 2005; 2006; 2008).

Figure 4 

 

Figure 5 

 


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