Publication Date:
2021-12-06
Description:
Voluminous rhyolitic eruptions from Toba, Indonesia, and Taupo Volcanic Zone (TVZ), New Zealand have
dispersed volcanic ash over vast areas in the late Quaternary. The w74 ka Youngest Toba Tuff (YTT)
eruption deposited ash over the Bay of Bengal and the Indian subcontinent to the west. The w340 ka
Whakamaru eruption (TVZ) deposited the widespread Rangitawa Tephra, dominantly to the southeast
(in addition to occurrences northwest of vent), extending across the landmass of New Zealand, and the
South Pacific Ocean and Tasman Sea with distal terrestrial exposures on the Chatham Islands. These
super-eruptions involved w2500 km3 and w1500 km3 of magma (dense-rock equivalent; DRE),
respectively.
Ultra-distal terrestrial exposures of YTT at two localities in India, Middle Son Valley, Madhya Pradesh,
and Jurreru River Valley, Andhra Pradesh, at distances of 〉2000 km from the source caldera, show a basal
‘primary’ ashfall unit w4 cm thick, although deposits containing reworked ash are up to w3 m in total
thickness. Exposures of Rangitawa Tephra on the Chatham Islands, 〉900 km from the source caldera, are
w15e30 cm thick. At more proximal localities (w200 km from source), Rangitawa Tephra is w55e70 cm
thick and characterized by a crystal-rich basal layer and normal grading. Both distal tephra deposits are
characterized by very-fine ash (with high PM10 fractions) and are crystal-poor.
Glass chemistry, stratigraphy and grain-size data for these distal tephra deposits are presented with
comparisons of their correlation, dispersal and preservation. Using field observations, ash transport and
deposition were modeled for both eruptions using a semi-analytical model (HAZMAP), with assumptions
concerning average wind direction and strength during eruption, column shape and vent size. Model
outputs provide new insights into eruption dynamics and better estimates of eruption volumes associated
with tephra fallout. Modeling based on observed YTT distal tephra thicknesses indicate a relatively
low (〈40 km high), very turbulent eruption column, consistent with deposition from a co-ignimbrite
cloud extending over a broad region. Similarly, the Whakamaru eruption was modeled as producing
a predominantly Plinian column (w45 km high), with dispersal to the southeast by strong prevailing
winds. Significant ash fallout of the main dispersal direction, to the northwest of source, cannot be
replicated in this modeling. The widespread dispersal of large volumes of fine ash from both eruptions
may have had global environmental consequences, acutely affecting areas up to thousands of kilometers
from vent.
Description:
Published
Description:
54–79
Description:
4.3. TTC - Scenari di pericolosità vulcanica
Description:
JCR Journal
Description:
restricted
Keywords:
Toba eruption
;
04. Solid Earth::04.08. Volcanology::04.08.99. General or miscellaneous
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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