Mount Etna eruptions of the last 2,750 years:: revised chronology and location through archeomagnetic and 226Ra-230Th dating

A careful re-examination of the well-known written documents pertaining to the 2,750-year-long historical period of Mount Etna was carried out and their interpretation checked through the high-accuracy archeomagnetic method (> 1,200 large samples), combined with the Ra-226-Th-230 radiochronology. The magnetic dating is based upon secular variation of the direction of the geomagnetic field (DGF) and estimated to reach a precision of +/- 40 years for the last 1,200 years, and +/- 100 to 200 years up to circa 150 B.C. Although less precise, the Ra-226-Th-230 method provides a unique tool for distinguishing between historic and prehistoric lavas, which in some cases might have similar DGFs. We show that despite the abundance of details on ancient historical eruptions, the primary sources of information are often too imprecise to identify their lava flows and eruptive systems. Most of the ages of these lavas, which are today accepted on the geological maps and catalogues, were attributed in the 1800s on the basis of their morphology and without any stratigraphical control. In fact, we found that 80% of the historically dated flows and cones prior to the 1700s are usually several hundreds of years older than recorded, the discrepancies sometimes exceeding a millennium. This is proper the case for volcanics presumed of the 1651 east (actually similar to 1020), 1595 (actually two distinct flows, respectively, similar to 1200 and similar to 1060), 1566 (similar to 1180), 1536 (two branches dated similar to 1250 and similar to 950), 1444 (a branch dated similar to 1270), 1408 (lower branches dated similar to 450 and similar to 350), 1381 (similar to 1160), 1329 (similar to 1030), 1284 (similar to 1450 and similar to 700), 1169 or 812 (similar to 1000) eruptions. Conversely, well-preserved cones and flows that are undated on the maps were produced by recent eruptions that went unnoticed in historical accounts, especially during the Middle Ages. For the few eruptions that are recorded between A.D. 252 and 750 B.C., none of their presumed lava flows shows a DGF in agreement with that existing at their respective dates of occurrence, most of these flows being in fact prehistoric. The cinder cones of Monpeloso (presumed A.D. 252) and Mt. Gorna (394 B.C.), although roughly consistent magnetically and radiochronologically with their respective epochs, remain of unspecified age because of a lack of precision of the DGF reference curve at the time. It is concluded that at the time scale of the last millennia, Mount Etna does not provide evidence of a steady-state behavior. Periods of voluminous eruptions lasting 50 to 150 years (e.g., A.D. 300-450, 950-1060, 1607-1669) are followed by centuries of less productive activity, although at any time a violent outburst may occur. Such a revised history should be taken into account for eruptive models, magma output, internal plumbing of the volcano, petrological evolution, volcano mapping and civil protection.