QUANTIFYING THE TEMPO OF LATE QUATERNARY VOLCANISM ALONG ARCS AND RIFTS: CHALLENGES, AND PROSPECTS (Invited Presentation)

Reconstructing the tempo of volcanism in arcs and rifts presents several major challenges; not least, how to integrate the products of eruptions of different style and size first at the volcano-level, and then at the scale of the arc- or rift-segment.

In this presentation we review approaches based on studies of young volcanoes in three settings: the Aegean, the Andes and the Ethiopian Rift. In the South Aegean Volcanic Arc, volcanoes lie in zones of crustal extension above a subduction zone. Some individual centres (notably, Santorini) have well characterized and long eruption histories, with events ranging from individual lava-dome (0.01 – 0.1 km³) to caldera-forming eruptions (> 30 – 60 km³). There is growing observational evidence for the modulation of the rates of eruptive activity by seawater loading on glacial to interglacial timescales.

The Southern Volcanic Zone of the Andes is a continental arc system comprising multiple stratovolcanoes, minor volcanic cone fields and a few complex calderas. Volcanoes in the southern reaches of the zone are closely spatially associated with a regional strike-slip fault system. Many of these volcanoes were extensively glaciated during the last glacial maximum, so detailed eruption histories are typically known only from their post-glacial products. Volumetric growth rates estimated both from whole-edifice morphometry, and from eruption rates of tephra provide constraints on longer-term and short-term rates of magmatism, and evidence for modulation of eruptive fluxes by both tectonics and secular changes in glacial loading.

The Main Ethiopian Rift is the type example of a magma-assisted continental rift. The rift axis is punctuated with regularly spaced silicic caldera complexes and central stratovolcanoes, interspersed with large fields of small mafic scoria cones. Ongoing field studies have established a framework for the late Quaternary volcanic history of this rift system, and place bounds on the rates and styles of volcanism. Most central volcanoes in the MER host large mid-Pleistocene calderas, with voluminous ignimbrites of trachyte to peralkaline rhyolite composition. In contrast, post-caldera activity at most centres marks a sharp decline in eruptive flux, with eruptions of peralkaline rhyolite obsidian flows, domes and pumice cones.