Abstract
We discuss maar-diatremes and calderas as end-members on a spectrum of negative volcanic landforms (depressions) produced by explosive eruptions (note – we focus on calderas formed during explosive eruptions, recognizing that some caldera types are not related to such activity). The former are dominated by ejection of material during numerous discrete phreatomagmatic explosions, brecciation, and subsidence of diatreme fill, while the latter are dominated by subsidence over a partly evacuated magma chamber during sustained, magmatic volatile-driven discharge. Many examples share characteristics of both, including landforms that are identified as maars but preserve deposits from non-phreatomagmatic explosive activity, and ambiguous structures that appear to be coalesced maars but that also produced sustained explosive eruptions with likely magma reservoir subsidence. A convergence of research directions on issues related to magma-water interaction and shallow reservoir mechanics is an important avenue toward developing a unified picture of the maar-diatreme-caldera spectrum.
Highlights
Maar-diatremes and explosive calderas are two types of volcanic depressions that cut into their pre-eruption landscapes, and are surrounded by low-profile accumulations of erupted material
Recent field and experimental studies of maars and their underlying diatremes show that subsurface explosions may or may not erupt onto the surface, depending upon their depths and energies; shallow explosions are most likely to eject material, while deeper blasts brecciate magma and country rock and mix them through combined upward-directed debris jets and subsidence (e.g., White and Ross, 2011; Graettinger et al, 2014; Valentine et al, 2015)
The diatremes include pyroclastic material deposited on the crater floor and subsided to depth (e.g., Delpit et al, 2014), massive tuff breccias and lapilli tuffs which often occur in subvertical crosscutting domains and that have a wide range of juvenile and lithic clast proportions (e.g., Ross and White, 2006; Lefebvre et al, 2013), and small intrusions in a variety of forms (e.g., Valentine and van Wyk de Vries, 2014)
Summary
Maar-diatremes and explosive calderas are two types of volcanic depressions that cut into their pre-eruption landscapes (negative landforms), and are surrounded by low-profile accumulations of erupted material (note that non-explosive calderas, while important volcanic features, are not discussed here). Superficially they can be similar except in size, our understanding of their origins developed along two largely independent lines of research. We discuss these as end-members on a spectrum of explosive volcanic depressions. We consider the idealized maar-diatreme end-member ( more called maars elsewhere in this paper; Figure 1, Table 1) to be the product of repeated, discrete explosions caused by
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