676
NOTES
YOUNGEST TOBA TUFF: WORLD'S LARGEST KNOWN QUATERNARY ERUPTION:
ITS PETROLOGY AND IMPACT ON ATMOSPHERE AND CLIMATE*
A.R. NAMBIAR
Geological Survey of India, Op: Karnataka & Goa, Bangalore - 560 078
Email: nambiar-ar@ yahoo.co.in
EXTENDED ABSTRACT
Variously referred to as the Toba caldera, Toba caldera
complex, Toba volcanotectonic depression or simply Lake
Toba (100 km long, 31 km wide, 450 rn deep), this huge
volcanic centre in northern Sumatra, Indonesia is the largest
resurgent Quaternary caldera on Earth. Over the past
1.2 my, there have been four ash flow tuff eruptions from
the caldera complex. The youngest tuff (YTT) erupted at
-74 ka (Ninkovich et al. 1978) expelling an estimated
7 x 10 I s kg (or 2800 km3 dense rock equivalent, at
2500 kg m-') of rllyolitic magma (Rose and Chesner, 1987).
Eruption of YTT is responsible for the collapse structure
visible today. After this mega eruption, resurgent doming
of intracaldera YTT formed Samosir Island, which rises
750 m above Lake Toba. In addition, post-YTT lavas were
extruded at 4 sites. YTT consists of a11 extensive, mostly
non-welded outflow sheet with abundant pumice blocks,
and covers 20,000 - 30,000 km2 area.
Petrology of Toba Tuffs
Y'IT eruption was preceded by 35 km3 of Haranggoal
dacites tuff (HDT, 1.2 Myr BP), the 500 km3of Oldest Toba
Tuff (Om,840 kyr BP) and 60 k m b f Middle Toba Tuff
(MTT, 501 kyr BP) (Chesner and Rose, 1991). Field
relations and paleomagnetic sig~~atures
indicate that the
ash flows originated from calderas alternately situated at
the N and S ends of the depression. Magma erupted
during successive eruptions subsequent to HDT was
compositionally zoned, generally ranging from rhyodacite
to rhyolite. Collectively, the YTT, MTT and OTT are
referred to as quartz-bearing Toba tuffs and typically contain
up to 40 wt% crystals of quartz, sanidine, plagioclase, biotite
and hornblende. Minor minerals are magnetite, ilmenite,
allanite, zircon and fayalite. They represent crustal melts
that became compositionally zoned through extensive crystal
fractionation, which occurred at pressures of -3 kb or
depths of 10km. The magmas were not saturated with water.
Much of the crystallization of the quartz-bearing tuffs
occurred between 700' and 760 "C, but HDT erupted at
higher temperature of 8 17 "C.
Although similar in composition and mii~ei.alogy,the
OTT, MTT and YTT can be distinguisl~edby subtle
variations in their mineralogy, whole-rock and glass
compositions. Mechanical mixing of distinct puii+lice
composition indicates that multiple con1posi tional layers
were tapped simultaneously during the eruptions. Lowenergy ring fractures were respotisible for the cmplacc~nent
of OTT, MTT and YTT, which resulted in dense welding of
all units, except for the top of the youngest unit, and thick
accumulations of rhyodacitic magma in the collapsi~ig
calderas (Chesner, 1998). Recent seismic tomographic
investigations have suggested the presence of two melt
regions ('magma reservoirs') betwec~ithe lake and a
depth of 10 km (Masturyono et al. 2 0 1 ) . The tuffs and
lavas froin these two reservoirs are isotopically distinct. The
southern magma reservoir, which tapped OTT, YTT arid
post-YTT silicic lnvas at three widely separated sites are
uniform in Nd isotope composition. I n northern ~~eservoir
magmas (MTT and post YTT eruption at volcano
Tandukbenua), &Ndis almost one unit lower, and S7~r/s%r
is 0.00015 higher than in rocks from the Southern reservoir.
Differences in composition between rocks associated with
the two magma reservoirs have also been recognized in
some trace elements like Rb, Ba, Ce, Y, Zr, T, Th etc.
Co-ignimbrite Ash Fall Deposits of YTT
The pyroclastic flows of youngest Toba eruption
generated enormous co-ignimbrite clouds of fine ash, which
drifted above both Indian Ocean and South Cl~inaSea and
deposited as tephra beds. Ninkovich (1979) described an
extensive rhyolitic ash horizon, up to 12 crn thick found in
deep-sea cores west and north of Sumatra and also Bay of
Bengal, and correlated with Youngest Toba eruption based
on inineralogy, K-Ar age (73.5 +/- 3 X 1 0 yrs) and oxygen
"'Lect~~r-e
de1lverc.d nt the nlo~irhlyt~iaetirzgof'flze Geological Societ
JOUR GEOL.SOC INDIA, VOL 63. J U N E 2004
NOTES
isotope stratigraphy. The ash layer occurs at marine oxygen
isotope stage (01s)5-4 boundary. Rhyolitic ash occurs on
land in Malayasia at many localities and has long been
attributed to Toba eruption on the basis of proximity,
mineralogy and relative age. Williams and Royce ( 1982)
first recognized rhyoli tic tephra predating late Pleistocene
alluvium occurring in Quaternary sediments of the Son
valley, north Central India alld later identified as derived
froin Toba (Rose and Chesner, 1987). At least 1% of the
Earth's surface has been estimated covered by Toba ash with
thickness of 10 cm or more. Based on the then known
dispersal of Toba ash, Rose and Chesner (1987, 1990)
estimated a minimurn mass of this tephra fall deposit as
2 x 1OI5 kg (a DRE volume of 800 km') and opined that
this value ]nay be much less than the true value, as the ash
fall deposits are [nost likely to be traced to much greater
distances than the known maximun~of 3 100 km.
Once, more people began looking for it, YTT turend up
further afield. Tephra layers, identified as Toba tuff have
been reported from nlany parts of the Indian subcontinent
(Middle Soi~eand Central Narmada Valley Basins, Kukdi,
Purna and Tapti Basins, Vamsadhara, Nagavali, Mahanadi
and BI-ahrnaniRiver Basins, many river basins of AP etc.)
(Acl~ai-yyaand Basu, 1993). Geochemical fingerprinting of
a comprehensive suite of samples, allowing comparison of
the Indian salnples to those from Toba, Malaysia and inore
importantly ODP 758 core, located halfway between
Sumatra and Sri Lanka that contains Oldest ancl Middle
Toba rephra as well as YTT (Dhen et a!. 1991), and fission
track dating of two samples of tephra from western
India demonstrate that all the presently known Toba
tephra occurrences in p e n i ~ ~ s u l aIndia
r
belong to the
Youngest Toba eruption (Shane et a]. 1995; Westgate et al.
1998).
Naii~biaret al. (1996) reported occurrence of late
Pleistocene difffused tephr-a layers in ilumber of deep-sea
sediment cores west of Laksl~adweepRidge and based on
geochemistry and morphology of glass shards correlated
them with YTT (Narnbiar and Suku~naran,2002). The
youngest Toba ash was identified in Arabian Sea cores
from the upper Indus fan and northeastern Murray Ridge.
Based on dls 0 stratigraphy, Schulz et al. (1998, 2000)
estinlate ages of 72,400 to 74, 600+5000 yrs BP for the
Toba ash in northeast Arabian Sea cores, in good agreement
with the ages centred at 74000+2000 yrs BP based on
KIAr, 4 0 ~ r / ' Yand
~ r fission track dating of glass shards
from ten+est~*ial
outcrops. These sites extend the distance
of the Toba ash occurrences to more than 4000 km froin
the source and the area of the ash blanket to > 4 x 10"n1*
(Schulz et al. 1998).
-
JOUR GEOL SOC INDIA, VOL 63, JUNE 2004
67 7
Volcanic glass and pumice fragments found in siliceous
abyssal sediments of the Central Indian Ocean Basin
(CIOB) south of the equator previo~lslyinterpreted variously
as product of intra-ai*c volcanis~n,Krakatau eruption and
Indonesian arc volcanism, are found compositionally
identical to the fdllout deposits of YTT (Pattan et al. 1999).
This correlation of ClOB ash with YTT extends the
distribution of glass shards 1500 km south of the previously
known fallout zone, which provides evidence for
bi-hemisphel+icdispersal of ash ft-om the Toba eruption.
Recently Song.et at. (2000) and Buhl-ing et a]. (2000),
on the basis of geochelnical characteristics and oxygen
isotope ages reported discovery of YTT in South China
Sea Basin (SCSB), which indicates an extended dispersal
of glass shards over 1800 k m northeast of the Toba
caldera, a direction opposite to what was previously
conceived, The dispersal of the ash cloud in both western
and eastern directions from the source indlca~estwo
contrasting wind directions ancl points to el-uption during
Northern Hemisphere summer monsoon season when
south-westerly winds pi-evail at low-to middle tropospheris
/
levels, transpol-tingash from lower parts of the co-igtlimbrite
clouds eastward into South China Sea and upper
tropospheric easterly winds spread Toba ash across
the Indian Ocean.
All reported ash beds 1-elatedto YTT are PI-edominantly
composed of glass shards, with some pumice fl-agmcntsand
crystals. Bubble wall shard forms domrnate, followed by
platy and blocky shards. Quartz and felrpars comprise the
bulk of cryqtals, followed by biotite, apatite, and amphibole.
All samples display very siinilar c h e ~ n ~ s t rfor
y major,
~ninorand rare earth elements. They plot within the high
sillca end of the rhyolite field i l l TAS diagram. Chondr~teilormalized REE profiles of YTT glasses from different
tephra beds are remarkably similar, showing steep slope foi.
LREE and a gentle reverse slope for HREE with pronounced
negative Eu anoinaly.
Reports of the distl-ibt~tionot'Toba ash published since
1993 till now show its rninimuni spread enconlpassed the
northeastern Arabian Sea (64" E), the Indian Ocean, 14" S
of equator, nol-thern India and Bangladesh north of the
equator and -1 1 3 O E in South China Sea. The minimum
area of the ash from the eruption, and tl~usits DRE
volume, are thus now substantially larger than previously
estimated.
Eruption Parameters: Intensity and Duration
The duration of YTT eruption estimated 1s 9-14 days
(Ninkovich et al. 1979). Applying this to the to~alYTT
volume, Rose and Chesner (1990) computed the average
678
NOTES
e~uptlonrate at about 8 x 1012g/s Taking the ~ntensltyof
Tobd eruptton as 7 1 X 1O9 kg s I , Woods and Wohletz (1991)
estlrnated the plume height for the YTT cloud as 32k5 km
Ramptno and Self (1993) cons~deredthls to be an underestimate, wggesting that for pedk lntenslt~esapproachtng
10'' kg s I, column helghts may have reached 40 krn
Impact on the Atnlosphere and Climate
C17t1calfor evaluation of Toba's potential cli~nat~c
itnpact
ale estimat~onof gaseous sulfur y~eldand the resulting
sulphate aerosol loading of the stratosphere Rose and
Chesner (1990) estrtnated gas contents of the YTT magma
to have been about 0 05 wt% H,S and 3 wt% H,O Scaltng
the H,S content to computed eiuptlve volume, they
est~matedan erupt~veemisslon of 3 5 x l o i 2g of H2S Thls
IS equivalent to about I x 10" g of sulphdte/wdter aerosol
Z~elinskiet a1 (1996) est~mdtedthe sulphatelwater aerosol
loadlng from the volcitnic sulphate concentrdtions
recorded at 71 kyr BP in GISP2 Ice core as 2 3-4 7 x 1012kg,
In reasonable agreement wlth the eailier estimate based on
rn~neralc hemlstry However, e s t mation
~
of sulfur emission
by YTT magma, based on extrapolatton of exper~mental
data 1s only around 3 5 x 10"' kg (Sca~lletet id 1998),
wh~chis 2-3 orders of lnagn~tudelower than the prev~ous
estimates
Based on scdling up fi om slnallei el uptlons and computer
models, conservative stiatospheiic de~osolload~ngof
1 x 10" g from YTT el uptlon is predicted to have caused
a "volcanic wlnter" with a global cool~ngof 3" to 5OC for
several years, and ieglonal cooling up to 1 5 O C (Ramplno
and Selr, 1992, 1993) But according to Oypenhelmer
(2002), the globally avel aged surface coollng estimate of
3"-5°C IS probably too hlgli , a f~gurecloser to 1 C appeals
more realistic The eruption occur1ed durlng the stage 5a-4
trdnsition of the oxygen tsotope leco~d,a time of rapld
Ice growth and falltng sea level Ramp~noand Self (1992)
suggested that the Toba eruption might have greatly
itccelerdted the qhift to glaclcll cond~tlonsthat was alleady
ilndei way, by lnduc~ngperennial snow cover and rncreased
sea-ice extent at cens~tlveno~thernlat~tudes
Oxygen isotope stud~esof sediments lrnmediately above
YTT layel i n a South Ch~ndSea (SCS) core suggest that
sea-surface tempe~aturesdropped by -1°C (Song et a1
2000), though these could reflect cond~t~ons
10s or 100s
year after the event Huang et al (2001) also recorded an
abrupt 1°C dlop I n the tempeiJtuie of SCS lrnmedlately
above the Toba dsh, which lasted for about 1 kyr They
t~nplicatethe follow~ngchain of events polar cooling,
expanston of northern heinlsphe~eIce sheets, Increaying
lntenslty of the East Asian rnonqoon, coolrng of Chlna aiid
\
-
trop~calPactflc and reduct~onof atmospher~cwater
concentrat~onand concluded that such plocesscs could
have acted to prolong the short term cool~ngrnir~atedby tlie
Toba event The h~ghestfieque~lc~es
of ~ce-rafteddebri$ In
the 95,000 year long record of the North Pacrfic Ocean were
recorded at 72 ka (Kot~lanlnenand Shackleton, 1995)
In the Greenland GISP2 Ice core, the largest dmount 01
volcanic sulfur rn the 110,000-year recoid occu~s dt
71+5 ka dnd IS attr~butedto Tobcl (Z~elinqk~
et al 1996)
The1e also occurs a large ECM spl ke dt this period, which
too indicates a volcanlc source The ~nasslvesulfur peak
spans SIX-sevenyea1 s, confirmtng the or1g~nalestl tnates of
a sulfur aerosol-induced S I X year volc;tn~cwintet proposed
by Ra~nplnoand Self (1992, 1993) <indma1 ks the tcrinlnation
of the ~nterstadlalof D - 0 event 20 1h ~ dl
s clmat~cvolcdnic
event IS followed by 1000 years of the dbsolutely lower ice
core oxygen lsotope ratlos of the last gldc1d1perlod I n other
words, for 1000yeas lmmedlately foilowiiig the Tobcl event,
the earth WI tnessed teinper atur es relentlessly colder than
duting the last glacial max~mumat 18-21 ka The f~rst200
years of thls stddral event &remarked by ~ncreasedcdlc~um
deposit~onindrcating unusually htgh amounts of windblown dust, probably due to dect eased vegetation cove1
and exposu~eof sediments as sea levels dlopped Ze~lrnski
et a1 (1996) esttmated temperature h o p of >G°C ovcr
Greenland The interstad~alportion of D - 0 evcnt that la5tcd
f o ~-2000 years following this coldest millennrutn sliows
that Toba was not directly responstble for the onset of the
last glacial, which occurs In the tce coie 68 ka (Zielinski
et a1 1996), though leaves open the poss~bil~ty
that ~t 1s
lmplicdted i n the 1 kyr cold pellod prxol to ~ntetstadldl
19 (Oppenhe~mei,2002) But ~nspect~on
of the Iilghresolutloi~oxygen isotope I ecord of Lang ~t dl ( I 999) shows
that tlie cold event at 7 I ka was cons~stentlycoldest for its
entire du~ation,and had the longest dulatton of all quch
events In the 110 ka Ice Cole record (Ambl ose, 2003) The
ice Cole geoche~nrcallecord shows that the yatte11.1of
enhanced cool~ngtor seveidl centui les aftel the el u p l ~ o ~ ~
of Toba does not occur in other stadia1 events (L~el~lisk~,
2000)
The Vostok ice core ftom Antarctica shows d n slurninurn
peak, indlcat~ngenhanced wind blown dust at -71 k y r
and an inci ease rn non-sea salt sulfur in the core '11 the stage
5a-4 suggests a possible tnitioi cooling from 111cieclsed
cloud cover of -0 6" C
Evidence for other atmosphet I C perturbat~ons lsing
from the Toba eruption has come to I~glitfrom tcuther ice
Cole analysis Yang et a1 ( I 996) who recorded time 1e\olve~I
concentratlons of chlollde, nltrate and the ritt~oot CI to
Na' In the GISP2 ldyet found thc pulse of ae~osolfitllo~~t
-
-
-
JOUR GCOL SOC INDIA VOL 67 JUNE, 2004
NOTES
coinciding with depletions of chloride and nitrate and an
accompanying low ratio of C1' to Na+ indicating Toba's
strong impact on tropospheric chemistry.
Impact on Terrestrial Environment
Rose and Chesner (1990) Iikened the aftermath of
the Toba eruption to an 'enormous fire' covering up to
30000 km2, arising from the widespread ignition of
vegetation. The hot tephra, lava and gases would have
exterminated all life in the irnrnediate vicinity of the Toba
crater. Gathome-Hardy and Harcourt-Smith (2003) question
the environmental impact, because Mentawi island, located
350 km southwest of Toba, has a primate and termite fauna
that was apparently not destroyed by hot debris from the
eruption. However, no direct effects should be expected,
because this island, as well as rest of Sumatra and Java,
was directly upwind of, and thus insulated from the
eruption (Ambrose, 2003).
If Tambora, the largest historic eruption (20 km3 DRE)
caused the year without a summer in 1816 and global crop
failures and famines (Ramaswamy, 1992), Toba could have
been responsible for six years of relentless volcanic
winter, substantial lowering of plant biomass and disastrous
faniine (Rampino and Am brose, 2003). All aboveground
tropical vegetation would have been killed by sudden hard
freezes, and a 50% die-off of temperate forests is predicted
from hard freezes during the growing season (Rampino and
Ambrose, 2000). Long pollen sequences from Java and
marine sediments in Indonesia and paleobiogeographic
evidence indicate large parts of SE Asia were cieforested at
this time (Flenley, 1996). Rampino and Ambrose (2000)
argued that in oceans, the tephra sedinienting through the
water column would have scavenged out nutrients. This
effect combined with reduced sunlight reaching sea level,
and changes in atmospheric circulation, could have limited
surface productivity in the oceans. But it has also been
suggested that ash fallout can actual1y fertiIize the oceans
by supplying macronut~~ients
and 'bioactive' trace metals
(Frogner et a1.,2001).
The climatic effects of volcanic eruptions can have
severe consequences for huinan populations. Virtually
all studies of' the genetic structure of living human
populations that have analysed the history of past population
JOUR GEOL.SOC.IND[A, VOL.63, JUNE 2004
67 9
size, have identified a significant late Pleistocene human
population bottleneck - a dramatic reduction, followed by
expansion from a very small population size in Africa
approximately 70 kyrs ago. The highest estimates of
effective population size during the bottleneck are about
10,000 reproductive females (Sherry et al. 1997), and
Harpending et al. (1993) provided estimates as low as 5003000 females. Famine caused by the Toba super-eruption
and volcanic winter provides a plausible hypothesis for late
Pleistocene human population bottlenecks (Ambrose, 1998,
2003), though according to Gathorne-Hardy and HarcourtSmith (2003), it is unlikely that Toba super-eruption caused
a human, animal or plant population bottleneck. Six years
of volcanic winter, followed by 1000 years of the coldest,
driest climate of the late Quaternary may have caused low
primary productivity and famine, and thus could have
decimated most Modern man's populations, especially
outside of isolated tropical refugia (Ambrose, 1998).Many
local human populations at higher latitudes and in the path
of the ash fallout may have been completely eliminated.
Release from the bottleneck could have occurred either at
the end of the hypercold phase, coinciding with D - 0 event
interstadial 19 around 70 ka, or 10,000 years later, at the
transition from cold oxygen isotope stage 4 to warmer
stage 3, as late as 60 ka. In this climate-inducedbottleneck
scenario (Volcanic WinterIWeak Garden of Eden Model
proposed by A ~ ~ ~ b r o1998)
s e , the surviving populations
would have expanded simultaneously when favorable
climates returned. The largest populations surviving from
this catastrophe should have been found in the largest
tropical refugia, and thus in equtorial Africa. High genetic
diversity in modem Africans may thus reflect a less severe
bottleneck rather than earlier population growth.
Volcanic winter and the instant Ice Age may help resolve
the central but unstated paradox of the recent African origin
of humankind: if we are all so recently "Out of Africa",
why do we not alf look more African? Because the volcanic
winter and consequent environmental changes would have
reduced populations to levels low enough for founder effects,
genetic drift and local adaptations to produce rapid
population differentiation,causing the peoples of the world
look so different today. In other words, Toba may have
caused modern human races to differentiate abruptly only
70, 000 years ago, rather than gradually over one million
years (Ambrose, 1 998).
NOTES
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+
ANNOUNCEMENT
Geological Society of India
Annual General Meeting - 2004
and
National Seminar on Deltas of India
The Annual General Meeting (AGM) of the Geolog~calSociety of I n d ~ afor 2004 will be
held at the rnvrtat~onof the Department of Geology, Andhra Un~versltydurlng 2-4 November 2004, at
Visakhapatnarn A natlonal semlnar on 'Deltas of Ind~a'w~thspecial emphasis on the natural resources
of deltas (011 and natural gas) w ~ l lbe held concurrently, organrzed by the Department of Geology,
Andhra Un~vers~tySc~ent~sts
rnterested in partlc~pat~ng
in the national semlnar may please contact
Prof K L V Ramana Rao, Head of the Department of Geology, Andhra Untvers~ty,Vlsakhapatnarn - 530 003,
Phone: (0891) 275487 1 Ex t 243, (0891) 2734883, Ernail: lvrkoya@yahoo corn
During the Annual Convention, it is customary to hold a sesslon for presentat~onof results
of recent and ongolng research, espec~allyby younger researchers, who are requested to contact
Shrl S V Snkantla, Secretary, Geolog~calSoc~etyof Ind~a,No 63, 12thCross, Gavlpuram, PB No 1922,
Bangalore-5600 19 Email: gsoctnd @bgl vsnl net In,Telefax: 080-26613352, Phone: 080-26522943
A.U. GEOLOGY DEPARTMENT ALUMNI MEET
Durlng the National Seminar on Deltas of lnd~a(2-4 November 2004) ~t1s envrsaged to hold a meetlng
of the Alumnl o f the Geology Department of the Andhra Un~vers~ty
at Vlsdkhal~atnamdur~ngone of the
evenlngs Former students of the Department are requested to send updated addresses (~ncludingtelephone
number and Ernall address) of their own and other alurnnl known to them to Prof K L V Ramana Rao,
HOD, Department of Geology, Andhra Unrvers~ty,Visakhapatnarn-530 003, Email: Ivrkoya@yahoo corn
JOUR GEOL SOC INDIA VOL 63 JUNE 2004