On the nature and consequences of super-eruptions - Volcanism and Global Environmental Change - page 22

(e.g. Matthews et al ., 2012 ). For instance, Youngest Toba Tuff ash has been
identi ed at several terrestrial sites in India; in many deep-sea sediment cores,

some as far distant as the Arabian Sea, 4000 km away; and in cores taken from the
bed of Lake Malawi (Lane et al ., 2013 ). Studies of the overlying and underlying
deposits at such sites offer the potential to investigate the regional environmental
impacts of the eruption. The minimum estimate of the mass of the tephra fall-
out is 2 10 15 kg (based on a dense magma volume of 800 km 3 ; Rose and

Chesner, 1990 ).
While in many cases a vast proportion of the dust from super-eruptions will
have settled in the oceans, in some others, such as past paroxysms of Yellowstone
caldera (located mostly in Wyoming, USA), very wide continental areas are likely
to have been blanketed in ash (Izett and Wilcox, 1982 ). Such widespread layers
can represent invaluable chronological markers in archaeological and palaeoenvir-
onmental contexts (e.g. Lane et al ., 2014 ).

2.5.2 Ignimbrites
The deposits of pyroclastic density currents form some of the most dramatic
landscapes around super-volcanoes. They are typically referred to as ignimbrite ,
and are predominantly composed of ash and pumice. The parent ows are charac-

terized by high temperature ( > 550 C), often evident in the deformation and
sintering of clasts that form a welded ignimbrite or sillar . Ignimbrites can reach
hundreds of metres in thickness outside the caldera but as much as a kilometre
or two inside. They often form plateaux that become deeply incised, generating
complex and spectacular topography with deep, steep-sided canyons.

2.5.3 Reworked deposits
The quantities of tephra rapidly deposited on the landscape are clearly very
substantial in the case of super-eruptions. In the areas inundated by pyroclastic
currents, entire valleys may become choked, resulting in substantial disruption of
drainage systems. Distal deposits of tephra fallout, which can cover vast areas,
can also have signi

cant hydrological impacts that leave clear signals in the
sedimentological record ( Figure 2.2 ; Gatti et al ., 2013 ; Williams et al ., 2009 ).

2.6 Impacts of super-eruptions
The global climatic impacts of super-eruptions have been widely debated, espe-
cially in the context of the Youngest Toba Tuff eruption (re

ecting, in particular,
interest in its consequences for extant human populations). However, it has proven