61

Impact of Tephra Forming Eruptions on Human Beingsand the Environment
Hiroshi MACHIDA
Professor emeritus, Tokyo Metropolitan University
machida@andes.metro-u.ac.jp/QYP04721@nifty.ne.jp

Abstract
This paper firstly addresses that Quaternary tephrochronology should provide basic guidelines forpredicting future explosive volcanism, frequency and magnitude. Secondly, application of such studies
to archeology and human occupation of volcanic areas is explored using a case study of prehistoric
cataclysmic eruptions and their impacts on human beings and the environment. A large volcanic
eruption generated significant change in nature and culture in volcanically active area like Japan. The
principal mechanism concerned appears to be that of landscape change, which made large-scale
adaptation and readjustment necessary. The paper illustrates the wide range of research tools which
may be adopted to determine the relationship between an archeological site and the hazards present in
the natural environment.
Key words: archeology, AT, cultural discontinuity, dating, environmental change, eruptive history,
explosive volcanism, frequency, Japan, Jomon, K-Ah, Late Quaternary, magnitude,
microlithic, paleolithic, tephrochronology

1. Introduction
Volcanic ash (tephra) layers not only govern thenature of soil and landforms, but are also important ininterpreting past events in highly volcanic areas.
Such events include volcanic hazards and environmental changes of the past. This paper firstly emphasizes that Quaternary tephrochronology should
provide basic guidelines for predicting future explosive volcanism, because the tephras clearly indicate
the size and nature of past explosive events and how
frequently they happened. Secondly, application ofsuch studies to archeology and human occupation of
volcanic areas is explored using a case study ofprehistoric cataclysmic eruptions and their impacts onhuman beings and the environment. Joint studies
between tephrochronology and archeology are significantly useful for such interpretation. The Japan area
is selected here because many results have beenobtained.

2. Magnitude and Frequency of Very Large
Explosive Volcanism in the Late
Quaternary in Japan
Simkin et al. (1981) and Newhall and Self (1982)
proposed a volcanic explosivity index (VEI), which isa general indicator of the explosive character of an
eruption on the basis of bulk volume of tephra erupted

and the maximum height of the eruption column.
Eruptions can be assigned a VEI on a scale of 0 to 8.
The larger the VEI value the greater the amount ofdamage caused by an eruption. Eruptions with VEIof 3 or greater are ‘large eruptions’ causing severe
damage over extensive areas. In Japan explosive
activities with VEIs of 4 and 5 have happened several
times in historic times, but very large-scale activities
with VEIs of 6 or 7 have not taken place for the last
1,000 years. There is much evidence of very explosive activity, however, during prehistoric times.
Examples include widespread tephra sheets producedby large caldera volcanoes. Bulk volumes of suchtephras are calculated mainly on the basis of isopachmaps of fallout tephras and ignimbrite components.
All of these eruptions are the products of plinian andignimbrite-forming activity of VEIs of 6 and 7.
The Late Quaternary eruptive history of Japan isshown in Figs. 1a and 1b on the basis of the tephra
catalogue presented “Volcanoes and Tephras in theJapan Area” (See Table 1 on page 23). It is clear thateach caldera volcano produced major tephra layerswith VEIs of 7 once or twice and VEIs of 6 several
times during the last 130,000 years. It is also shownthat each large caldera has been formed not by a single
eruption but by multiple eruptions that have recurred
approximately every 10,000 years.
Volcanoes can be classified in terms of expected
maximum magnitude and frequency of eruptions

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H. MACHIDA

Fig. 1a

Late Quaternary large explosive eruptions with a VEI of 5-6 in Japan
Machida and Arai (1992), revised.

Fig. 1b Late Quaternary large explosive eruptions of caldera volcanoes with a VEI of 6-7 in Japan
Machida and Arai (1992), revised.

based on Quaternary tephrochronology. Figure 2
shows the maximum magnitude of eruption that
occurred in the late Quaternary (ie., the maximum sizeof destructive eruption that has occurred so that it cantherefore be predicted that it will occur in the future),
the lapse of time since the last major eruption, and thefrequency of major eruptions (Machida,1990). It
must be remembered that long-quiescent caldera

volcanoes, such as Aso, Ata and Toya, with nosignificant explosive activity (VEI of 7) during these
80,000-110,000 years, are the most hazardous.
The recurrence of such a huge volcanic event
would be of catastrophic consequence to the highly
populated areas of modern Japan. The impact oflarge-scale pyroclastic flows and associated ash falls
on flora and fauna, as well as human beings, is diffi-

Impact of Tephra Forming Eruptions on Human Beings and the Environment

cult to predict. Incidentally, the Aso-4 pyroclastic
flow deposit of 85,000-90,000 years ago, one of thelargest tephra layers in the world, covers a land area of
30,000 km2, where approximately 10 million people
are living now. The effects of such extreme events inthe future will almost certainly result in cultural destruction and replacement over a vast area.

Fig. 263

Consequently such tephrochronological studies asmentioned here may be significant in the consideration of future eruptions and their effects in highly volcanic areas of the world. It would be instructive toknow, therefore, the impact of major past eruptions on
prehistoric communities.

Japanese volcanoes classified in terms of the maximum eruptive magnitude in the late Quaternary, lapse oftime since the last major eruption, and recurrence interval of major eruptive activity.
Machida (1990)

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H. MACHIDA

3. The Relationship between Tephra Layers
and Archeological Remains
As it is well-known, there are many cases where
archeological remains are found sandwiched within
the tephra layers. The tephra covers the humanbeings as they lived and their artifacts, and storesthem for a later date. We know of many prehistoric
settlements buried by tephra, where the volcanic
eruption destroyed the inhabitants’ lives directly or
indirectly. Dating the tephra can often tell us how
old the ancient remains are, while on the other handarcheological research on specific artifacts buried by atephra layer can often suggest an approximate age ofthe tephra-forming eruption. Thus, both fields ofresearch have influenced each other.
The common interests of both tephra research andarcheology lie in the issues of chronology, disasters
and environmental changes caused by the eruptions.
Tephrochronology of artifacts in deposits has been
carried out actively in Japan. However, little effort
has been made towards research on the impacts oreffects of volcanic eruptions on human lives and theenvironment using the archeological evidence foundin the tephra. Some of the basic information andtopics associated with these issues are discussed
below.
3.1 Chronological issues
One of the important points of excavating thearcheological sites in the volcanic ash soil is to discover the stratigraphic relationship between certain
time-marker tephras and cultural layers. As we havefound several widespread late Quaternary marker
layers throughout Japan, there should be a level ofsuch tephra virtually everywhere if preservation conditions are good. However, since such tephra consisting mainly of fine-grained glass shards is easily
weathered, it can sometimes hardly be identified by
the naked eye in the field. Therefore, careful observation and sampling are required for laboratory workon processing and examination.
Basically the archeological remains are not foundwithin the tephra layer but in the soil formed during
the dormant period between eruptions. But actually,
there are cases in which the stratigraphic relationshipbetween tephra and cultural layers is hard to recognize,
because either the artifacts have been moved afterwards by some natural or artificial agents, or thetephra has been reworked. Identification of the cause
is an important issue for discussion regarding chronology and paleo-environments.
a) Earlier Paleolithic archeological remains before
and after the Aira-Tn tephra eruption (26-29 ka)
Many studies and discussions have been made on
the problem of when the earliest human beings appeared in Japan and from where they came. Radiocarbon dating of hominid fossils on Okinawa island

has revealed that they are dated at 32,000 to 16,000
yr.14C BP (e.g., Ono et al., 1999). On the other handearlier paleolithic tools were excavated from volcanic
ash soil with a 14C age of about 30,000 yr. BP or
younger at several archeological sites in central Japan.
Recent studies, however, show a need for revision andrefinement of these ages because calibration techniques have progressed from conventional to calendrical ages (Stuiver et al., 1998).
Aira-Tn tephra (AT), the most importanttime-marker of the Paleolithic age in Japan, wasformerly dated at 21,000-22,000 yr.14C BP byconventional radiocarbon methods (Kigoshi et al.,
1972; Machida and Arai, 1976), but was redated at ca.
24,500 yr.14C BP by accelerated mass spectrometry
radiocarbon methods (e.g., Ikeda et al., 1995).
However, it has been further calibrated to a calendricalage of 26,000-29,000 cal yr.BP by combined data
between marine oxygen isotope dates and calibrationfrom AMS radiocarbon dates based on high resolution
U-series ages.
As the earliest paleolithic tools are found in soil
significantly below AT tephra, they are estimated to bedated 35,000 years or older, roughly speaking.
Calendrical age determination in this time range isstill a subject of further research together with thepaleo-environment at that time. We have not yet
obtained useful data for calibration older than ca.
30,000 years ago (e.g., Beck et al., 2001). Also,
correlation of oxygen isotopic fluctuation with theterrestrial sequences is incomplete in Stage 3.
AT tephra is found on the boundary between theearlier knife-blade culture Phase 1 and the later Phase
2 in Japan (Ono et al., 1999). Although this eruption
could have disrupted ecosystems on a fairly large
scale, well beyond the direct impact zone, we have asof yet no abundant evidence that any prolonged occupational or cultural hiatus may have occurred. The
diversification of stone tools, however, became clear
in two major cultural regions, Northeastern Japan andSouthwestern Japan. The historical duality in Japanese culture, eastern and western, may have become
obvious since then.
b) Some chronological issues from the Latest
Paleolithic culture to the Oldest Jomon ceramic
cultureArcheological stratigraphy shows that the latePaleolithic cultural phase from ca.18 ka to ca.15 ka ischaracterized by the spontaneous development of two
industries (forms of workmanship). One is characterized by small points, centered in central Honshu.
The other is characterized by trapezoids found in
Kyushu. These cultures were followed by the microlithic culture, the final stage of the Japanese Paleolithic. It took part in the micro-blade producing
trend that spread over eastern Eurasia and North
America and was introduced into Japan via Sakhalin
and the Korean Peninsula. The microlithic cultureImpact of Tephra Forming Eruptions on Human Beings and the Environment

was followed by the oldest ceramic Jomon culture,
which is characterized by flat based and undecorated
pottery (Ono, et al., 1999).
During this period some regional marker tephra
layers have been described with a variety of roughly
calibrated radiocarbon ages. Kirishima-Kobayashi
tephra (Kr-Kb) of ca.16-17 ka in southern Kyushu,
Asama-Yellow (Kusatsu) (As-Y) of ca.15-16.5 ka incentral Honshu, Eniwa-a (En-a) tephra of ca.19-20 ka
in Hokkaido are representative markers immediately
before the microlithic industry.
Nantai-Imaichi
tephra (Nt-I, S) of ca.14-15 ka is one of the markers
produced during the microlithic culture phase innorthern Kanto. Towada-H tephra (To-H) of ca.15ka
is important because it lies just above the microlithic
culture. The calendrical age calibration of thosetephras has not yet been fully completed and therefore
requires urgent study for establishing a high-resolution
chronology of the cultural changes during this period.
c) Significance of K-Ah tephra as an importanttime-marker for the Jomon culture
No widespread tephras have been found forcorrelating or dating Jomon archeological cultures
earlier than K-Ah tephra (7,300 cal. BP), which occurs
extensively from southwestern Japan to central
Honshu. This is not only an excellent time-marker
for the earliest and early Jomon ceramic sequence but
also an indicator of significant cultural changes overan extensive area in western Japan. The catastrophic
impact of this tephra-forming Kikai eruption is briefly
described below. The analysis was carried out via
collaborative studies between tephrochronology andarcheology.
Machida (1984) discussed the magnitude of this
volcanic impact on prehistoric society and the environment. Combined evidence from volcanology andarcheology indicate a major cultural discontinuity
between the pre-eruption (Earliest Jomon) ceramic
culture phase and the post-eruption (Early Jomon)
phase.
Immediately before the Kikai eruption,
Kyushu was occupied by an indigenous ceramic
culture represented by the Kyushu-Kaigara-mon stylepottery (identified by the use of shell scraping) and by
the Oshigata-mon style pottery (distinguished by aroller pattern) as shown in Fig. 3a. These ceramic
types are not found in the soil above the K-Ah tephra
and therefore may have been terminated as aconsequence of the eruption. Archeological researchsuggests that the post-ash ceramic cultures, the
Jokon-mon-style and the Sobata-style, were derived
from outside the region (Fig. 3b).
It appears that people of the pre-eruption Jomon
culture in southern Kyushu either dispersed to thenorthwestern part of Kyushu where the volcanic
devastation was not so severe, or perished totally inthe areas of greatest impact (Fig. 3b). In addition,
there is plentiful evidence that the eruption andassociated strong earthquakes and tsunamis induced

65

significant landform changes over a vast area
following a major decline in natural productivity
throughout the region (Machida and Sugiyama, 2002).
d) Chronological issues of the late Jomon and later
PeriodMany regional and local tephra layers have been
useful for correlating and dating archeological sites ofthe late Jomon and later periods in Japan. This rangeof periods covers the recent 3,000 years. Much higher resolution age determination is required, with temporal resolution that is at least decadal and ideally,annual or seasonal because we have to reconstruct thedetailed history of volcanic activity, environmental
change and human history for this recent time span.
However, this is not easy work as shown in thefollowing case study on the Baegdusan-Tomakomai
tephra that is currently being focused on.
Baegdusan-Tomakomai tephra (B-Tm) occurs inboth northern Japan and the northern part of the
Korean Peninsula and relates to the youngest eruptionof Baegdusan on the border of North Korea and China(Machida et al., 1990; Machida and Arai, 1992). It is
one of the largest eruptions during the last 2,000 years
in the world. The stratigraphic relationship of thistephra with the immediately underlying Towada-atephra (possibly the AD 915 eruption of Towada
caldera, northern Honshu) and with archeologicalremains suggests that the Baegdusan eruption
occurred in the early half of the 10th century. Microscopic observation of the outermost bark of trees
buried in the related ignimbrite suggests that theeruption occurred from autumn to winter (Machida
and Mitsutani, 1994). The widespread occurrence of
this tephra offers significant potential for reconstructing and correlating historical chronologies in Korea,
China and Japan. Also the widespread tephra fallout
and flow should have had severe consequences forhuman beings and the environment in proximal areas.
Today we can still observe such traces of the impact
on vegetation and land-use around the volcano.
Therefore detailed age determinations have been madeby various methods, but consensus among authors has
not fully been obtained (Table 1).
Generally speaking tephra layers are useful forcorrelation between archeological sites, for establishment of chronologies, for cross-checking ages determined by various methods and for explaining the relationship between environmental and cultural changes.
In particular, we expect to have useful information if
we carry out archeological research focusing on thecharacteristics of tephra as a trigger of environmental
changes.
3.2 Influence of tephra eruption on human beingsand the environment
We know of some natural catastrophic events that
may have caused cultural changes or reformations, orabandonment of areas in historic and prehistoric

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H. MACHIDA

cultural history. Historians and archeologists areconventionally apt to explain that most of suchchanges were caused by some human social problems,and have not researched well enough to see whether
any natural events induced human changes. Even if
there had been some problems within the humansociety, many of them could be considered to haveresulted from complex processes involving bothnatural events and human history.
A tephra forming eruption is one of the greatest
natural events that could cause historical changes. If
we focus our research on large explosive volcanism aswell as its impact on the ecosystem and human society,we can much better understand past changes.
When we think of a large explosive eruption, thefirst thing that comes to mind is that the tephra covers

Fig. 3aGeographical extension of the regional phases of the earliest Jomon ceramic culture immediately before the
Kikai-Akahoya eruption.
Machida (1984)

Fig. 3b

Geographical extension of the regional phases of the early Jomon ceramic culture after the Kikai-Akahoya
eruption.
Machida (1984)

Impact of Tephra Forming Eruptions on Human Beings and the Environment

67

Table 1 Baegdusan-Tm ash ages estimated by various methods.
Estimate age (AD)Method or evidence
14

Reference

945 ~ 984 (969±20)

Wiggle-matched C dating with an AMS for
charred wood buried in the fallout tephra

Horn and Schmincke (2000)

937±8

Wiggle-matched 14C dating with an AMS for
charred wood buried in the ignimbrite

Nakamura et al. (2002)

916 ~ 936 (1σ)

Wiggle-matched 14C dating with an AMS for
charred wood buried in the fallout tephra

Machida & Okumura (in press)

937 ~ 938

Lacustrine varve chronology in Lake Ogawara
based on the underlying Towada-a tephra age ofAD915

Fukusawa et al. (1997)946

Old record of rumbling and ash fall? in the Kinki
districtHayakawa and Koyama (1998)

911 ~ 946 (2σ)

the ground surface and directly destroy the ecosystemin the surrounding area, but we cannot ignore the factthat it can affect the global climatic system.
Many observations and studies have indicated that
explosive eruptions put large amounts of volcanic
aerosols into the atmosphere, and the parasol effectdecreases the temperature of the lower troposphere forseveral years (Lamb, 1970). Even relatively small
eruptions, such as the Agun eruption of 1963, produce
massive amounts of volcanic aerosols such as SO2 andaffect the climate greatly (Palais and Sigurdsson,1988). Since small-scale eruptions have higher frequencies, they could sum up to have a larger scale
influence. There is much research on volcanic eruptions that affected the ‘Little ice age’ climate (Lamb,1970). Volume estimation of past volcanic gas bygeochemical methods could be an important step forenhancing this research (Sigurdsson et al., 1985).
Joint studies among tephra research, archeology
and paleontology, etc. on the 7300 cal. BP Kikai
eruption, give a basis for a solution as to what kind ofand how much damage this explosive volcanism may
have caused to prehistoric human communities andenvironments as described above. Analysis of recent
phenomena would also be instructive. Many eventsshow us that pyroclastic flows and surges result inmuch more serious consequences than tephra fallout.
That is why it is firstly important to identify theprocess of tephra formation at archeological sites.
On the other hand, for fallout tephra, it is important todetermine the season of eruption and its original
thickness to identify how much damage it caused tothe ecosystem. However, there are only a handful ofcase studies to answer these questions. We needbasic research on many individual aspects, such as
different climates and vegetations that would affectthe situations: what kinds of processes followed after
the ecosystem had been destroyed by the volcanic
event? Needless to say, it is also important to consider the social organization, that differs from age toage. Large-scale eruptions occur less frequently andresearch in the historic era is not enough. Therefore,
we need to use several different methods of analysis

and find out what impacts they had upon humansocieties as well as ecosystems.
The above descriptions somehow may give anegative impression of the impact of tephra forming
eruptions on humans and the natural environment, but
over a long-term perspective, we can find positive
aspects as well. In fact, tephra provides a parent
material for forming soil and increasing the productivity of crops. Pyroclasitc flows change ragged hills
and valleys into flat surfaces and plateaus.
Those who live in a volcanic country have to copewith the tephra, use it, and become adapted to it, andmany are unaware its benefits. This is true not only
regarding eruptions or tephra, but also regarding every
natural catastrophe. We have to recognize this paradox where natural ferocity always comes with natural
benefits.
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