Ultra-distal tephra deposits from super-eruptions: Examples from Toba, Indonesia and Taupo Volcanic Zone, New Zealand

Abstract

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 ∼74 ka Youngest Toba Tuff (YTT) eruption deposited ash over the Bay of Bengal and the Indian subcontinent to the west. The ∼340 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 ∼2500 km³ and ∼1500 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 ∼4 cm thick, although deposits containing reworked ash are up to ∼3 m in total thickness. Exposures of Rangitawa Tephra on the Chatham Islands, >900 km from the source caldera, are ∼15–30 cm thick. At more proximal localities (∼200 km from source), Rangitawa Tephra is ∼55–70 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 (∼45 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.

Introduction

Rhyolitic tephra deposits associated with large-magnitude volcanic eruptions provide crucial age control for geological sequences and are therefore a valuable tool for stratigraphic, geomorphological and archaeological studies. This paper investigates the characteristics of distal tephra deposits associated with two super-eruptions (Volcanic Explosivity Index, VEI ≥ 8, with volumes >500 km³; Newhall and Self, 1982, Mason et al., 2004): the ∼74 ka Youngest Toba Tuff (YTT) eruption, Sumatra, Indonesia; and the ∼340 ka Whakamaru Group Ignimbrites (hereafter referred to as the Whakamaru super-eruption), Taupo Volcanic Zone (TVZ), New Zealand. Quaternary tephra deposits from both eruptions are widely used as tools for regional correlation of sedimentary archives. Distal ash deposits from such eruptions are particularly valuable as they can be deposited up to thousands of kilometers from the vent, forming discrete layers in sedimentary sequences that are useful for chronological purposes providing they can be traced to a specific eruption of a particular volcanic source (e.g., Self and Sparks, 1981). Information obtained from distal deposits, such as thickness and grain-size, can also be used to constrain eruption parameters (e.g., Pyle, 1999, Pyle et al., 2006). It is difficult to estimate tephra volumes associated with these large-magnitude eruptions and usually rough approximations are adopted, which inevitably involve extrapolation based on empirical observations. The application of computational models of ash dispersal enables production of alternative estimates for erupted volumes.

This paper describes and reports characteristics of the ultra-distal deposits of the two super-eruptions. The analysis uses data from proximal and distal exposures of both tephra units to test whether the correlations of distal deposits to the particular eruptions are robust; and constrain eruption parameters using the HAZMAP (Macedonio et al., 2005) ash dispersal model. These new constraints on ash dispersal from super-eruptions provide detail on areas affected. This paper also discusses impacts such volumes of ash might have had on the environment and on human health.

Glass shards, which typically comprise >95% of distal deposits, usually have characteristic compositions, which enable the matching of deposit and eruption. Detailed major- and trace-element compositional data were collected on glass shards from both distal and proximal outcrops to assess the chemical variation within and between deposits, and ensure that correlations were correct. Physical parameters of the deposits at ultra-distal terrestrial sites, such as grain-size and tephra thicknesses, are documented to help understand reworking of the ash at these sites. These data are coupled with tephra thickness data from sedimentary archives located up to thousands of kilometers from source (e.g., references listed in Table 2, Table 3), and input into the HAZMAP model to help constrain tephra dispersal, estimate eruption parameters, and gain insight into eruption plume dynamics and ash deposition.

The potential for other large caldera-forming eruptions to occur, with global consequences (e.g., from Uturuncu, Bolivia; Sparks et al., 2008), makes it imperative that possible climatic and environmental effects are investigated. Data and ash dispersal models of these two different super-eruptions provide insight into ash transport, and help evaluate the impact of large volumes of fine ash on the environment and human health at great distances from the source volcano.

The ∼74 ka YTT super-eruption is the youngest of four large eruptions in northern Sumatra, with the earlier three being the ∼1.2 Ma Haranggoal Dacite Tuff, ∼840 ka Oldest Toba Tuff (OTT), and ∼501 ka Middle Toba Tuff (MTT) (Chesner et al., 1991). It formed the 100 × 30 km Toba caldera (Fig. 1; Rose and Chesner, 1987, Chesner, 1998), erupting ∼2800 km³ (DRE) of rhyolitic magma – making it the largest eruption to have occurred globally during the last 2 Ma (Rose and Chesner, 1987, Rose and Chesner, 1990, Chesner et al., 1991). The minimum mass of the ash fall deposit has previously been estimated as 2 × 10¹⁵ kg, equivalent to 800 km³ DRE, representing ∼30% of the total mass of erupted material (Rose and Chesner, 1987). The eruption resulted in deposition of thick ignimbrite sequences proximally (covering up to 30,000 km² of Sumatra; Rose and Chesner, 1987, Bühring et al., 2000) and widespread distribution of ash westward over the Indian subcontinent and the Arabian Sea (Schulz et al., 1998, Pattan et al., 1999, Pattan et al., 2001), and eastward over the South China Sea (∼1500–1800 km NE of Toba; Bühring et al., 2000, Song et al., 2000, Liang et al., 2001; Fig. 1a).

Distal YTT was first documented in Malaysia by Scrivenor (1930) and subsequently reported in the Bay of Bengal (Ninkovich et al., 1978a, Ninkovich et al., 1978b, Ninkovich, 1979, Gasparotto et al., 2000) and the Indian subcontinent (Acharyya and Basu, 1993, Shane et al., 1995, Westgate et al., 1998, Pattan et al., 1999). YTT deposits in the Middle Son Valley, Madhya Pradesh, India, were first identified by Williams and Royce (1982) and Basu et al. (1987). Further south at Jwalaparum, Kurnool District of Andhra Pradesh, the tephra was first reported by Rao and Rao (1992) and geochemically identified as YTT by Petraglia et al. (2007). Localities of deposits referred to here are listed in Table 1 and mapped in Fig. 1.

Analyses of glass compositions of tephra throughout the Indian peninsula provide evidence for direct correlation with Toba (Shane et al., 1995, Shane et al., 1996, Westgate et al., 1998). The magmas that drove the OTT, MTT and YTT eruptions were, however, compositionally similar (Smith et al., 2011) and the glass chemistry normally used to distinguish between eruption units cannot be relied on solely to identify chronostratigraphic markers in their distal deposits. Correlations using biotite compositions confirm, however, that the ash found in Malaysia and in archaeological sites in India was from the ∼74 ka YTT eruption (Smith et al., 2011).

The Rangitawa Tephra unit (formerly the “Mt Curl Tephra”; Froggatt, 1982, Kyle and Seward, 1984, Froggatt et al., 1986, Wilson et al., 1986, Keall, 1988, Kohn et al., 1992) has previously been linked to the ∼340 ka Whakamaru super-eruption from the TVZ on the basis of glass shard major-element chemistry, ferromagnesian mineralogy, and similarity of paleomagnetic dates and zircon fission-track ages (Kohn et al., 1992, Alloway et al., 1993, Pillans et al., 1996, Lowe et al., 2001).

The TVZ in the central North Island of New Zealand is the locus of numerous caldera volcanoes that have been active for the last ∼2 Ma (Wilson et al., 1995, Wilson et al., 2009). In addition to the Whakamaru super-eruption, there have been three other super-eruptions from the TVZ: the ∼26.5 ka Oruanui (∼530 km³; Wilson, 2001, Wilson et al., 2006), 1.06 Ma Kidnappers/Rocky Hill pair (Schipper, 2004), and ∼1.2 Ma Ongatiti (Briggs et al., 1993, Houghton et al., 1995, Brown and Smith, 2004). The Whakamaru super-eruption is the largest eruption known to have occurred in New Zealand (Brown et al., 1998), erupting a total volume of >1000 km³ of rhyolitic magma, which covered 13,000 km² of the central North Island in ignimbrite (Wilson et al., 1986). The volume of ashfall dispersed across the Pacific Ocean is estimated to be ∼700 km³ (Froggatt et al., 1986), which was deposited primarily to the southeast of the source caldera. However, significant deposits are also recorded to the northwest of the vent area in the Waikato region and >500 km beyond New Zealand at DSDP Site 591 in the Tasman Sea (Fig. 2; Nelson et al., 1985, Lowe et al., 2001).

The preserved proximal Whakamaru super-eruption deposits, the Whakamaru Group Ignimbrites, are divided into five ignimbrite units that may represent separate eruption phases possibly separated by periods of quiescence: Manunui, Te Whaiti, Whakamaru, Rangitaiki, and Paeroa Range Group Ignimbrites, and contain four distinct magmatic groups based on pumice composition, referred to here as types A, B, C and D (Wilson et al., 1986, Brown et al., 1998, Matthews et al., In press). The relationship between the separate ignimbrites has not been constrained, although Wilson et al. (1986) suggested that there were three eruption phases (1: Manunui/Te Whaiti; 2: Whakamaru/Rangitaiki; and 3: Paeroa/Rangitawa as a final phreatoplinian phase), and that all ignimbrites were erupted over a period of <10 ka.

The Rangitawa Tephra represents an important stratigraphic marker in mid-Pleistocene marine and terrestrial sequences in New Zealand and has been fission-track dated at ∼340 ± 40 ka (near the end of OIS 10; Kohn et al., 1992, Pillans et al., 1996, Lowe et al., 2001, Holt et al., 2010), corresponding to the age of the Whakamaru eruption. The crystal-rich nature of the fall deposit is thought to preclude it from being a simple co-ignimbrite ashfall (Kohn et al., 1992), although previous work has been unable to establish how this fallout is related to the ignimbrites. Using grain-size analysis, glass chemistry, and insight from the modeling, the aim is to establish which phase emplaced the Rangitawa Tephra.

Distal tephra correlatives in the TVZ have been mapped by Manning (1996) using mineralogy and glass chemistry to identify source volcanoes. Localities of deposits referred to here are listed in Table 1, and mapped in Fig. 2, Fig. 3. The type locality is the Rangitawa Stream exposure (Site R024; Fig. 2) near Mt Curl, Wanganui region, North Island, New Zealand (Te Punga, 1952). The most distal on-land exposures of the Rangitawa Tephra are preserved on the Chatham Islands (Holt et al., 2010; Fig. 2, Fig. 3) where it is present as a distinctive macroscopic tephra horizon within terrestrial deposits, commonly preserved within peat and outcrops at coastal exposures. Distal ash deposits resulting from the large Oruanui eruption from Taupo caldera (Wilson, 2001, Wilson et al., 2006), the Kawakawa Tephra, are also preserved on the Chatham Islands (Holt et al., 2010).