Mainly summarized from Heiken and McCoy
(1984), Bond and Sparks (1976), Druitt and others (1989) and Friedrich
(1994). Part of the
masters' thesis by Tom Pfeiffer (1999), geology department
Aarhus, DK, under W. Friedrich.
The Minoan eruption happened around 1645 BC in
the Late Bronze Age. It was one of the largest plinian eruptions
in younger time. It erupted ca. 30-40 km3 rhyodacitic
magma and is ranked VEI=6 (Volcanic Explosivity Index after
Simkin and others, 1981). The eruption was followed by collapse
of the magma chamber that enlarged an existing caldera.
The height of the plinian eruption column is estimated 36-39 km
(Pyle, 1990). It dispersed tephra throughout the Eastern
Mediterranean and might have led to global climatic impacts. Its
deposits on Santorini consist of up to 50 m thick layers of white
pumice and ash.
The eruption destroyed an inhabited and
culturally high-developed island which perhaps might be the
origin of the Atlantis
legend as many scientists believe.
Since 1969 excavations near Akrotiri have brought to light an
important marine Cycladic town famous for its well-preserved and
The Minoan eruption has been studied in detail
and described by many authors. Among the most important works are
Fouqué (1879), Reck (1936), Bond and Sparks (1976), Pichler and
Kussmaul (1980), Pichler and Friedrich (1980), Heiken and McCoy
(1984) and Druitt and others (1989) (Reference list).
Reck (1936) described 4 major units as BO1,
BO2, BO3 and BO4, while Druitt
and others (1989) call these Minoan A, Minoan B, Minoan C and
Minoan D. Heiken and McCoy (1984) report thin basal units as BO0
that represent precursory volcanic activities before the main
Every unit corresponds to a distinct phase of the eruption with
its characteristic stile.
tephra fall unit BO0
Heiken and McCoy (1984, 1990) describe up to 4
very fine-grained yellow, orange-brown and/or light gray layers
of fine ashes, lapilli-sized rounded pumice and lithic fragments
that are present in southern Thera. Their thickness is in the
range of 1 to 4 centimeters.
They interpret them as air-fall deposits of phreatic and
phreatomagmatic activity from a vent near the present-day Nea
Kameni island that preceded the eruption with a short time
interval in the range of some months. Thus, they possibly
provided a warning to the inhabitants.
fine-grained yellowish precursory ash deposit (BO-0) overlying the
pre-Minoan soil as it appears in the quarry near Megalochori. A thin
veneer of the first pumice fall layer has been preserved from
||In places, only
the cemented yellow ash layer BO-0 has survived erosion and/or
quarrying. Here, it overlies directly a block of the red Cape Riva
ignimbrite (21 ka). Quarry of Megalochori.
The yellowish, 2-3 cm thick ash layer
BO-0 here seen in the left of the image with a rest of in situ
pumice fall deposit in the quarry of Megalochori.
eruption phase: Plinian pumice fall BO1
ash is found throughout the Eastern Mediterranean. The
easterly disperal axis of the tephra blanket indicates
the main wind direction at the time of the eruption.
After Friedrich (1994).
pumice fall deposit near Athinios, here about 5 m thick. Walter
Friedrich points to holes in the deposit that once were the trunk
and branches of a tree. In some of the holes partly charred wood
remnants are found, showing that the pumice was still hot enough to
burn the tree when it was falling.
Detail of a larger partly charred branch
in the big hole visible on the left photo. C-14 dating as well as
attempts to use the tree for dentrochronology are underway;
preliminary results of the C-14 anaylses confirm an age around
1700-1600 BC. For more information please contact
The first phase of the eruption is a typical
pumice fall-out deposit from an estimated 36 km high eruption
column. It ranges in thickness on Thera from 50-500 cm (Pichler
and Friedrich, 1980) or 10 to 600 cm (Heiken and McCoy, 1984). It
mantles uniformly the pre-Minoan surface which proofs its origin
as fall-out. A clear southeasterly trend in greatest
accumulations reflects the dispersal of the tephra-bearing
eruption column by strong atmospheric winds (Bond and Sparks,
The greatest thickness and the largest pumice clasts up to 30 cm
are found directly south of Fira in the pumice quarries. This and
the position of the isopach lines allow to define the vent. Its
position probably was somewhere west of Fira between Cape
Katofira and the present-day Nea Kameni island (Pichler and
The deposit is widespread throughout the
Eastern Mediterranean region. Finer particles were transported to
great heights and could be transported to long distances. The
deposit is clearly present in many deep-sea cores of the eastern
Mediterranean Sea and was found in locations on other islands and
in western Turkey (Watkins and others, 1978; Sigurdsson and
On Santorini the deposit consists mainly of
massive pumice and is only poorly sorted. In most sections,
however, it shows a slight reverse grading in clast size which
suggests that the eruption was increasing in violence with time
(Sparks and Wilson, 1990).
More than 90% of the deposit is coarse
white-pink rhyodacitic pumice (sizes typically 0,5-30 cm,
70,5-71,4% SiO2), rare crystal-rich pumice (52,5-63,6%
SiO2) and few cauliform-shaped clasts of gray
andesitic scoria (58% SiO2). Less than 10% are fine
ash and lithic fragments (Druitt and others, 1989).
No signs of magma-water interaction or breaks and changes in
eruption style occur; the eruption was a continuous dry event
driven only by magmatic gas (Sparks and Wilson, 1990). This adds
support to the view that the vent was subaerial and lay on the
supposed Pre-Kameni island (Druitt and Francaviglia, 1992).
In its uppermost part a thin (2-40 cm) layer of
very fine white ash occurs that Heiken and McCoy (1984) called
the "phreatomagmatic break". Its interpretation is the
first significant interaction of magma with sea-water that
intruded into the enlarged vent and announced the end of this
eruption phase: Base surge deposit BO2
The deposits of this phase consist of numerous
individual beds, mostly white layers of pumice lapilli bearing
ash, with abundant lithic blocks and fragments of up to 1-2 m
size. Up to 90% of the total volume is fine ash and well-rounded
pumice, the latter more present in proximal sections (Heiken and
McCoy, 1984). Strong variations in structure and composition are
present within the deposit. Lithic fragments and blocks reach up
to 20% of the volume and mark prominent beds (Heiken and McCoy,
Eroding contacts to the underlying phase 1 deposits,
cross-stratification, ripple-, dune- and antidune-structures and
bomb-sags implying ballistic transport for the larger blocks are
Large ballistic block from the second phase on top of Mt. Profitis Ilias, ca. 7
km away from the vent. Note that the block has split in two on impact and
penetrated into the first pumice fall layer BO-1.
Pichler (1973) was the first to interpret the
second phase deposits as pyroclastic base-surges produced by
phreatomagmatic explosions. As the result of cracks, fissures and
vent-erosion seawater entered the crater and produced violent
explosions that pulverized the magma and ejected large lithic
blocks. Similar explosions were observed during atom-bomb tests.
These produced fast-travelling, ring-shaped clouds that expand
and spread horizontally away from the center (Friedrich, 1994).
The bedding structures show that the bulk of the material was
transported laterally within individual flows moving at high
velocity (15-50 m/s) (Pichler and Friedrich, 1980). The deposits
are controlled by topography. Close to the vent they climb up
slopes of 10-30E and reach heights of 200-400 m (Heiken and
McCoy, 1984), whereas further away they thin out considerably and
are lacking on most of Mt. Profitis Ilias.
In the lowest parts of phase 2 up to 2 pumice air-fall beds are
present which are similar to the phase 1 fall-out unit. They show
that phase 1 activity continued for a while after the beginning
of phase 2, or that there was a period of fluctuation in erupting
style on the transition from dry to phreatomagmatic activity
(Heiken and McCoy, 1984).
Measuring the degree of pumice vesiculation, Wilson and Houghton
(1990) found that during the whole eruption magma had vesiculated
before the level of fragmentation, thus concluding that
water-magma interaction occurred above this level, in a shallow
depth of possibly a few hundreds of meters.
Literature data for phase 2 thickness differ
considerably. Pichler and Friedrich (1980) report 0,5-7 m, Bond
and Sparks (1976) and Heiken and McCoy (1984) give 0,1-12 m. The
difference could be explained by the gradual transition of BO2
into the phase 3 deposits as evident in many sections.
From the point of their greatest thickness between Fira and Cape
Athinios and from ballistic impact sags, Pichler and Friedrich
(1980) conclude a vent position similar to that of phase 1.
Heiken and McCoy (1984; in press) measured a number of flow
directions within that phase and found a more or less radial
pattern of flow-directions pointing from a central area south of
View of a 25m-high exposure of Minoan tephra in the quarry near Megalochori at
the caldera cliff. The first pumice fall phase BO-1 overlies the dark Minoan
paleosoil (lower middle of photo), followed by a thick sucession of BO-2
cross-bedded dune-forming surges and lithic-rich ash-flows of BO-3 (upper part
eruption phase: Ash-flows BO3
At most localities the base surge deposits
grade into a chaotic, unsorted, massive lithic-rich flow deposit
that is most prominent along the caldera cliffs.
It consists of fine ashes, pumice and 25-30% lithic fragments and
blocks up to 10 m or more in diameter. It thickens strongly into
topographic lows (Friedrich, 1994). Along the caldera rim it
reaches maximum thickness of 40 m (Pichler and Friedrich, 1980),
55 m on southern Thera (Heiken and McCoy, 1984) and thins out
rapidly at greater distances. On the Profitis Ilias Mountain it
is missing (Heiken and McCoy, 1984).
Within the matrix of usually very fine vesiculated ash occasional
degassing pipes occur. This implies emplacement with 3
components: pyroclasts, liquid water and gas (steam) (Sparks and
Wilson 1990). Lithics and larger pumice blocks sometimes show
concentrations that can be traced, horizontally and some few
hundreds of m downflow. They define individual flow units (Sparks
and Wilson, 1990).
Only a few of the larger lithic blocks show
clearly ballistic impact sags; most of the lithics were obviously
transported by flows (Sparks and Wilson, 1990). The flows must
have been very energetic, as they are present on slopes as steep
as 30E on the upper flanks of Micro Profitis and Megalo Vouno
Mountain and because many intraclasts are sheared in the flow
direction (Heiken and McCoy, 1984).
Studies from Wright (1978) and McCleeland and Thomas (1990)
report different results concerning emplacement temperatures.
Some of the deposits seem to have been emplaced cold, others at
temperatures up to 400E C.
The origin of the phase 3 deposits is
Bond and Sparks (1976) interpret the deposits as mud-flows due to
apse of a giant tuff-ring built up by the surge flows.
Pichler and Friedrich (1980) consider them rather ash-flows that
were produced from an extremely enlarged vent and like
"boiling milk" poured over the caldera-rim. The large
amount of lithic fragments is seen as the result from beginning
Heiken and McCoy (1984, p.8454) recognize a multiple
emplacement facies comprising:
- ballistic (proximal, on the caldera rim),
- base surge (proximal, in the south-western corner of the
- mud-flows (distal, on the outer flanks of most of Thera and all
of Therasia) and
- slumps (distal, on steep slopes of the Profitis Ilias
eruption phase: Ignimbrite or reworked (?) BO4
Phase 4 deposits differ from phase 3 by a
subtle color change from white to cream colored; they are finer
grained with smaller lithic blocks and pumice clasts; the total
lithic concentration, however, is significantly higher (34-50%)
(Bond and Sparks, 1976). Phase 4 contains abundant different flow
units usually marked by concentrations of normal-graded lithics
or erosional contacts (Bond and Sparks, 1976).
On the caldera rim the deposits are thin (0,7-2 m, Heiken and
McCoy, 1984), if present at all, but form thick fan-shaped units
up to 40 m thick on the coastal plains (Bond and Sparks, 1976).
Their origin is controversial:
Pichler and Kussmaul (1980), and Friedrich (1994; in press)
believe that they are reworked phase 3 deposits. Processes like
intensive flooding during and after the caldera collapse,
rainfall, tsunamis, wind, agriculture and other factors could
have eroded and washed away large parts of the Minoan tuff.
Especially where it originally was deposited on steeper slopes,
it would be easily removed by such processes and deposited in the
coastal plains. On the outer flanks of Profitis Ilias, Mikro
Profitis Ilias and Megalo Vouno Mountain the effects of erosion
are visible: the Minoan tuff has virtually disappeared.
Bond and Sparks (1976), Heiken and McCoy (1984) and Sparks and
Wilson (1990) argue that the deposits are ignimbrites, hot
gas-rich and highly fluid pyroclastic flows that only came to
rest "on low gradient (1-2° ) slopes" (Bond and
Sparks, 1976, p. 7). The eruption-sequence from Plinian fall-out
over phreatomagmatic deposits to ignimbrite-forming styles is a
frequent observed feature.
They support their interpretation by the presence of typical
lag-breccia, gas-segregation pipes, the existence of many
individual flow units which follow the topographic valleys, and
occasional co-ignimbrite ash-fall deposits interbedded within the
flow units described by Sparks and Walker (1977). Furthermore,
studies of Wright (1978), Downey and Tarling (1984) and
McCleeland and Thomas (1990) indicate hot emplacement
temperatures from 200 to 400E C.
The primary magmatic origin of the phase 4 deposits is questioned
by some authors (e.g. Friedrich, 1994, 2000).