Recent structural evolution of the Cumbre Vieja volcano, La Palma, Canary Islands: volcanic rift zone reconfiguration as a precursor to volcano ...
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Journal of Volcanology and Geothermal Research 94 Ž1999. 135–167
www.elsevier.comrlocaterjvolgeores
Recent structural evolution of the Cumbre Vieja volcano, La
Palma, Canary Islands: volcanic rift zone reconfiguration as a
precursor to volcano flank instability?
a,b,) b,c
S.J. Day , J.C. Carracedo , H. Guillou d , P. Gravestock b
a
Benfield Greig Hazard Research Centre, UniÕersity College London, Gower Street, London WC1E 6BT, UK
b
Department of Geography and Geology, Cheltenham and Gloucester College of Higher Education, UK
c
´ Volcanologica
Estacion ´ de Canarias, CSIC, La Laguna, Tenerife, Spain
d
´ CEA-CNRS, France
Centre des Faibles RadioactiÕites,
Received 10 May 1999
Abstract
The Cumbre Vieja volcano is the youngest component of the island of La Palma. It is a very steep-sided oceanic island
volcano, of a type which may undergo large-scale lateral collapse with little precursory deformation. Reconfiguration of the
volcanic rift zones and underlying dyke swarms of the volcano is used to determine the present degree of instability of the
volcano. For most of its history, from before 125 ka ago to around 20 ka, the Cumbre Vieja volcano was characterised by a
triple Ž‘‘Mercedes Star’’. volcanic rift zone geometry. The three rift zones were unequally developed, with a highly
productive south rift zone and weaker NE and NW rift zones: the disparity in activity was probably due to topographic-
gravitational stresses associated with the west facing Cumbre Nueva collapse structure underneath the western flank of the
Cumbre Vieja. From 20 ka to about 7 ka, activity on the NW volcanic rift zone diminished and the intersection of the rift
zones migrated slightly to the north. More recently, the triple rift geometry has been replaced at the surface by a
N–S-trending rift zone which transects the volcano, and by E–W-trending en echelon fissure arrays on the western flank of
the volcano. The NE rift zone has become completely inactive. This structural reconfiguration indicates weakening of the
western flank of the volcano. The most recent eruption near the summit of the Cumbre Vieja, that of 1949, was accompanied
by development of a west facing normal fault system along the crest of the volcano. The geometry of this fault system and
the timing of its formation in relation to episodes of vent opening during the eruption indicate that it is not the surface
expression of a dyke. Instead, it is interpreted as being the first surface rupture along a developing zone of deformation and
seaward movement within the western flank of the Cumbre Vieja: the volcano is therefore considered to be at an incipient
stage of flank instability. Climatic factors or strain weakening along the Cumbre Nueva collapse structure may account for
the recent development of this instability. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Cumbre Vieja volcano; volcanic rift zones; volcanic vents
)
Corresponding author. Tel.: q44-171-504-2212; fax: q44-171-380-7193; E-mail s.day@ucl.ac.uk
0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 1 0 1 - 8136 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
1. The Cumbre Vieja volcano, La Palma: a highly above sea level Ž6 km above the surrounding ocean
active and potentially unstable oceanic island vol- floor. and has a subaerial area of 220 km2 and a
cano subaerial volume of about 125 km3 , yet the oldest
dated rocks within it are only about 125 ka old
The Cumbre Vieja volcano forms the southern ŽGuillou et al., 1998.. There is also an unknown but
third of the island of La Palma. La Palma and the probably at least comparable volume of rock in the
adjacent island of El Hierro are the youngest islands submarine part of the volcano, which has grown
in the Canarian archipelago and are presently in a southwards from the preexisting northern part of the
‘‘shield building’’ phase of activity comparable to island ŽFig. 1.. It is likely that activity of the volcano
the present activity of the island of Hawaii ŽCar- began significantly before 125 ka. Carracedo et al.
racedo et al., 1998; Carracedo et al., 1999b-this Ž1999a, 1999b-this volume. discuss the stratigraphy
volume.. The Cumbre Vieja rises to almost 2 km and geochronology of the Cumbre Vieja and its
Fig. 1. Simplified geological map of La Palma.S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 137
relationship to the older parts of the island of La volcanoes where neotectonic structures, seismicity
Palma. A particularly important feature of the latter and geodetic data indicate lateral deformation that
is the presence of an older collapse scar, the Cumbre may be precursory to a future lateral collapse. Of
Nueva collapse scar, upon which the Cumbre Vieja these by far the best known and intensively studied,
is partly built. This contribution complements these partly because it is deforming so rapidly, is Kilauea
stratigraphic and geochronological studies by consid- Volcano, Hawaii ŽSwanson et al., 1976; Holcomb,
ering the structural evolution of the Cumbre Vieja. 1987; Lipman et al., 1988; Clague and Denlinger,
The structural study of the Cumbre Vieja de- 1994; Denlinger and Okubo, 1995..
scribed here was carried out in order to address Although Kilauea is commonly regarded as the
concerns raised by the 1949 eruption of the volcano type example of an unstable oceanic island volcano,
ŽBonelli Rubio, 1950., during which west facing it is in many respects atypical. The Hawaiian volca-
fault ruptures developed along the crest of the vol- noes are larger but less steep than many oceanic
cano. Following the recognition that giant lateral island volcanoes, with heights of up to 10 km above
collapses are a common feature of oceanic island the ocean floor and slopes averaging 5–108. In con-
volcanoes, as discussed below, the question has arisen trast, volcanoes such as the Cumbre Vieja; Teide, on
of whether this faulting might be a precursor to a Tenerife; and Pico do Fogo ŽFogo island, Cape
future giant lateral collapse of the western flank of Verde islands. rise 6 to 8 km above the surrounding
the Cumbre Vieja ŽCarracedo, 1994, 1996a,b.. This ocean floor but have average slopes between 158 and
paper also seeks to identify the timing and time more than 208. The maximum average subaerial
scales of any structural changes that have taken place slope of Pico do Fogo is no less than 288. Profiles of
within the Cumbre Vieja edifice in the past, using the Cumbre Vieja, Fogo and Kilauea, at different
the results of detailed mapping ŽCarracedo et al., scales but all with no vertical exaggeration, are
1997a. and highly accurate radiometric dating ŽGuil- compared in Fig. 2. The greater slope angles make
lou et al., 1998.. The work was coupled with geode- these islands intrinsically less stable, and also imply
tic monitoring of the volcano ŽMoss et al., 1999-this substantial structural differences between them and
volume.. Kilauea.
The south flank of Kilauea is also atypical in that
it shows semi-continuous, partly incremental Žco-
seismic. seaward movement which continues through
2. Precursors to lateral collapse at island volca- intereruptive periods ŽSwanson et al., 1976.. This
noes: is Kilauea typical? probably reflects the persistence of magma and duc-
tile, high temperature cumulates in the deeper parts
Since the collapse of the northern flank of Mt. St. of the Kilauean rift zones ŽDecker, 1987; Clague and
Helens at the start of the eruption of 18th May 1980, Denlinger, 1994.. In contrast, examination of the San
the lateral collapse of the flanks of large volcanoes Andres fault system on El Hierro indicates that there
has come to be recognised as a major process in their was at most a few tens of metres of slip on this fault
development and a severe volcanic hazard. The haz- system before sudden slip of about 300 m in an
ards are especially great in the case of lateral col- aborted lateral collapse event ŽDay et al., 1997.. This
lapses at oceanic island volcanoes, both because of implies that steep sided oceanic island volcanoes can
the exceptionally large volumes of these collapses become prone to catastrophic flank failure after only
and because they have the potential to generate giant a little precursory deformation. However, such fail-
tsunami with runup heights of hundreds of metres at ure is only likely to occur during eruptions or intru-
distances of hundreds of kilometres ŽMoore, 1964; sion events ŽElsworth and Day, 1999-this volume..
Moore and Moore, 1984.. In view of these potential This is in marked contrast both to Kilauea and to
consequences, the identification of the long term many stratovolcanoes, where deformation is signifi-
precursors to giant lateral collapses has become a cant even in intereruptive periods ŽVan Wyk de
critical problem to be addressed by studies of oceanic Vries and Francis, 1997. and collapse may occur
island volcanoes. Attention has focused upon active without magmatic activity ŽSiebert et al., 1987..138 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 Fig. 2. Topographic profiles through the Cumbre Vieja Žalong lines A–AU and B–BU in Fig. 1., Pico do Fogo and Kilauea-Mauna Loa compared. Note that the profiles are drawn at different scales but that all have no vertical exaggeration. It is therefore important to recognise more subtle reflects the distribution of underlying feeder dykes features that indicate that an oceanic island volcano and other intrusions, which are in turn controlled by such as the Cumbre Vieja is evolving towards, or the stress field in the volcano as originally shown by already in, a state of potential catastrophic lateral Anderson Ž1935.. In many cases, these vents are instability. In this contribution, we make particular concentrated into volcanic rift zones, and as a first use of the distribution and orientation of volcanic approximation the structural evolution of such a vents. As advocated by Nakamura Ž1977., the distri- volcano can be studied by considering the evolution bution of volcanic vents on the flanks of a volcano of its volcanic rift zones. It should be noted that the
S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 139
term ‘‘ volcanic rift zone’’ is used here in the strict pyroclastic units including scoria and spatter cones,
structural sense advocated by Walker Ž1993. rather phreatomagmatic lithic–scoria–ash breccias and air-
than the looser topographic sense. As discussed be- fall lapilli beds. It also contains a number of phono-
low, the topographic expression of a rift zone may be lite domes and lavas which are scattered over the
greatly complicated or even suppressed altogether volcano. The topography of the volcano is dominated
where it is developed on preexisting topography such by a north–south ridge which runs almost the entire
as the flank of an earlier volcano: for example, the subaerial length of the volcano and also extends
SW rift zone of Kilauea volcano would be excluded offshore for a few kilometres to the south of the
by the topographic definition because it does not island before bending to a south easterly direction
form a distinct ridge but instead rests upon the flank ŽMasson, pers. commun... The highest part of this
of Mauna Loa ŽHolcomb, 1987.. ridge ŽFig. 3. is everywhere above 1700 m elevation
Many components may contribute to the overall for a distance of some 5 km north to south, with a
stress field controlling the positions and orientations smaller but nevertheless strongly elongate central
of volcanic rift zones wsee discussion in McGuire area generally above 1900 m elevation around the
Ž1996.x. The result may be a complex stress field phonolite dome Nambroque. However, this highest
which varies both laterally and vertically, and area is formed of sequences of superimposed scoria
changes with time. It is therefore necessary to have and spatter cones, with subordinate lavas and phono-
detailed stratigraphic and geochronological control lite domes, similar to much of the rest of the axial
upon the history of changes in vent distribution and ridge and many other parts of the volcano. There is
also to exploit the additional information provided no central summit crater or feeder complex. Al-
by vent orientations. It has long been recognised that though the scattered phonolitic units may have been
elongate fissure vents are orientated along the trend fed by small and probably transient magma pockets
of the underlying feeder dykes. Tibaldi Ž1995. within the volcanic edifice, the near ubiquitous oc-
showed that the orientations of elongate scoria cones currence of lithospheric xenoliths and other petrolog-
and the positions of features such as low points on ical evidence indicates that the basic and intermedi-
the rims of the summit craters of the cones could ate magmas are erupted directly from reservoirs in
also be used to infer dyke trends. Dyke trends can the oceanic lithosphere, below the base of the La
also be deduced where multiple vents formed in the Palma edifice ŽKluegel et al., 1997; Hansteen et al.,
same eruption can be identified and linked, either 1998; Kleugel, 1998..
from historical records or from careful stratigraphic The existence of this summit ridge led to the
mapping of the eruption products, particularly pyro- suggestion that the Cumbre Vieja has only one rift
clastic sequences. We have used methods similar to zone and that the volcano is merely a southward
Tibaldi’s in the present work, but emphasise in overgrowth from the older volcanoes which form the
addition the importance of en echelon fissure and north of La Palma ŽAfonso, 1974; Ancochea et al.,
vent sets, since these can be used to infer changes in 1994.. However, consideration of the vent density
the orientation of the stress field along the trajectory distribution ŽCarracedo, 1994, 1996a,b. on the Cum-
of propagation of the feeder dykes, as discussed bre Vieja indicates that less topographically distinct
further below. NW and NE rift zones are also present on the flanks
of the volcano, and that the Cumbre Vieja may have
a triple rift or ‘‘Mercedes Star’’ ŽCarracedo, 1994.
3. Changes in the distribution and orientation of rift structure, like most other Canarian volcanoes.
volcanic vents on the Cumbre Vieja volcano The vent distribution evidence alone also permits a
further alternative, that of a single dominant volcanic
rift together with a sparse radial swarm of feeder
3.1. OÕerall geometry and stratigraphic subdiÕision dykes or vents centred on the highest parts of the
summit ridge around Nambroque ŽFig. 3..
The Cumbre Vieja volcano is dominated by se- Fig. 3 is a simplified version of the geological
quences of basic to intermediate alkaline lavas and map of the Cumbre Vieja ŽCarracedo et al., 1997a.,140 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
Fig. 3. Geological map of the Cumbre Vieja.S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 141 in which the following main stratigraphic units can palaeocliffs are less than about 40 ka old. The older be distinguished: platform-forming sequence formed between 20 ka Ž1. A cliff-forming sequence of lavas and pyro- and about 8 ka; all dated units of the younger clastic units intruded by phonolite cryptodomes and sequence are less than 7 ka old. The chronological lava domes. These rocks predate the end of forma- significance of these stratigraphic units means that tion of the palaeocliffs, formed in a period of intense the distributions and orientations of volcanic vents coastal erosion associated with low relative sea level. and intrusions in the three units can be compared in Rare dykes intruded into this sequence are exposed order to investigate the structural evolution of the in the high cliffs along the western coast of La volcano through time. Palma. Lava flows at the top of the cliffs can be traced inland and demonstrated to post date many of 3.2. Volcanic Õents and dykes in the cliff-forming the scoria cones, lava flows and phonolite lava domes sequence (125–20 ka) exposed high on the flanks of the volcano and along the summit ridge. The younger units on the volcano As noted above the cliff-forming sequence forms therefore form a thin veneer, less than 100 m thick, by far the largest part of the volume of the Cumbre on the cliff-forming sequences. Vieja edifice and also a significant part of the pre- Ž2. A platform-forming sequence, including sent surface area. Sufficient outcrop of cliff-forming phonolites and pyroclastic rocks but dominated by series rocks from all parts of the subaerial Cumbre lavas which have built up a series of lava deltas at Vieja volcano exists to constrain the distribution of the coast. This group of rocks is very widespread, volcanic vents at this period in its evolution. Unfor- especially in the northeast of the volcano, but only tunately, most of these vents are either partly buried forms a thin veneer on the older rocks. or sufficiently reduced by erosion for evidence of the Ž3. Within the platform-forming sequence it is alignment of the underlying dykes to be doubtful at possible to distinguish young lavas and scoria cones, best. A few exceptions to this rule, including elon- with associated pyroclastic units and at least one gate clusters of vents which are inferred to be coge- group of phonolite cryptodomes. These units are netic, are indicated in Fig. 4. In addition, a number morphologically very fresh, with little vegetation of WNW- to NW-trending dykes are exposed in the cover, and include both sub-historic and known his- west coast cliff section. These provide important toric eruptive units. Parts of the coastal platform, direct evidence for the orientations of feeder dykes especially on the west coast of the volcano, are to the upper part of the cliff-forming sequence in this formed by lavas of this unit. The individual eruptive area. Occurrences of scoria cones and surtseyan tuff units belonging to this group are discussed further in rings in these cliffs also give some indication of vent Section 3.4, below. distributions in the older, otherwise buried parts of These distinctions are primarily made on the rela- the cliff forming sequence. tionships of the rocks to coastal erosion features and The vents of the cliff-forming series occur in the obvious freshness of the most recent vents. several parts of the volcano. Six sectors can be Nowhere on the volcano are marked compositional defined for the purposes of description, converging changes or breaks in activity Žindicated by laterally upon the highest parts of the edifice around Nam- extensive terrestrial unconformities, weathering hori- broque. The approximate boundaries of these sectors, zons or distal ashfall sequences. present within the indicated by letters A–F, are shown in Fig. 4. sequences. Nevertheless, precise radiometric dating Sector A forms the axis or crest of the Cumbre ŽGuillou et al., 1998. has shown that these three Vieja from the south tip of the island to the summit units have chronological significance. The cliff-form- region. Exposures of cliff-forming series rocks in ing sequence formed between about 125 ka and 20 this part of the volcano are dominated by scoria and ka; most of the sequences exposed in palaeo-seacliffs spatter cones, with subordinate lapilli and phreatic up to 700 m high along the west coast were em- and phreatomagmatic ash units, a few phonolite placed between 125 ka and about 80 ka, although the domes and only very rare lavas. The pit crater of whole of the sequences exposed in the lower eastern Hoyo Negro, formed in 1949 ŽFigs. 6 and 14. and
142 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
Fig. 4. Distribution of volcanic vents and dykes during cliff-forming series activity. Sectors A–F discussed in text.S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 143
still some 150 m deep, exposes at least five discrete apparent trend. are substantially more degraded and
but superimposed scoria and spatter cones of the weathered than the other two. The other two cones
cliff-forming series, and only one small lava flow. show distinct NW–SE elongation indicative of
Vent elongations, where observed, indicate north– broadly NW-trending feeder dykes.
south alignment of feeder dykes in Sector A. It In Sector E, on the eastern and NE flank of the
appears to represent a very well developed volcanic volcano, elongation directions of individual scoria
rift zone. cones and chains of vents such as Montana Hoya
Sectors B and C, on the flanks of the southern Camacho appear to be more variable, changing from
part of the Cumbre Vieja, are not very well exposed NE in the northern part of the sector to easterly or
except in the coastal palaeocliffs. However, the latter even ESE in the south of Sector E. The elongation
include the highest palaeocliffs in the Cumbre Vieja directions and vent chains are consistently close to
and provide excellent sections through the cliff-form- the dip direction of the slope, but also tend to be
ing sequences in this part of the volcano. In strong offset slightly to the north of the local dip direction
contrast to sectors D and E, further north, neither throughout the sector by an angle of a few tens of
scoria cones nor tuff rings nor dykes are exposed in degrees.
these cliffs. The sequences are formed by lavas Sector F in the north of the volcano lies between
derived from the crest of the volcano ŽSector A.. the ill-defined northern margin of Sector D and the
Thus, no dykes were emplaced radially to the sum- much older rocks exposed in the west-facing Cumbre
mit area of the volcano in directions to the SE and Nueva collapse scar. The southern part of the sector
SW. is covered by very young lava flows but it appears to
Sector D, to the west and north-west of the Nam- be characterised by a distinctly lower density of
broque area, is a broad zone with a large number of earlier scoriarspatter cones than characterises Sector
scoria and spatter cones in addition to a few phono- D. Scoria cones in the north of the Sector F are very
lite domes and the coastal dykes at El Remo. Where deeply weathered and morphologically degraded and
scoria cone elongation directions can be reliably may in fact be satellite vents of the older Bejenado
determined, they indicate underlying feeders trending edifice to the north ŽCarracedo et al., 1999a., project-
WNW to NW throughout the sector. Flow banding ing through a thin Cumbre Vieja sequence. Further-
and flow folds within the Los Campanarios phonolite more, gravel pits in the northern part of Sector F
dome in Sector D are also directed to the NW along expose thick sequences of alluvial sediments which
bearing 300 Žapproximately., suggesting a similar appear, along with lava flows to the south, to have
alignment of the underlying feeder. The concordant ponded within and filled a depression between the
WNW to NW trend of feeder dykes, exposed and Cumbre Nueva collapse scar and a barrier to the
inferred, is developed throughout Sector D. This is west along the eastern edge of Sector D. The nature
despite the WSW bearing of the southern part of the of this barrier is discussed further below.
sector from the summit region of the volcano around In summary, the activity of the Cumbre Vieja
Nambroque. The dykes exposed in the cliffs at El volcano during this period can be defined by six
Remo are consistently oriented at an angle of 20 to sectors, with an alternating pattern of vent Žand
408 northwards of the bearing that they would be underlying feeder dyke.-rich and vent-poor sectors.
expected to have if they were radial to the summit of Of the three sectors with abundant vents and inferred
the volcano. feeder dykes, the very narrow N–S zone ŽSector A.
Apparent north–south alignments of volcanic has consistently N–S-trending elongate vents and
vents in Sector D, such as the group Montana vent alignments; the broad NW zone ŽSector D. has
Todoque–Montana de la Laguna–Montana de Tri- WNW- and NW-trending elongate vents, vent align-
ana–Montana de Gazmira along the coast in the ments and dykes throughout, even in those parts
extreme north–west of the sector, are formed by which lie west and SSW of the summit area of the
cones of demonstrably different ages. In the case of volcano; and only the NE zone ŽSector E. shows a
the group of four cones noted above, Montanas de pattern that could possibly be considered to be par-
Triana and Gazmira Žat the northern end of the tially radial, although NE- to ENE-trending vent144 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
alignments are present throughout the zone including In contrast to the intense activity of the NE rift
those areas due east of the summit. This sectoral zone during this period, the NW rift zone, although
pattern indicates that despite the disparity in topo- marked by a number of vents spread over a broad
graphic expressions, the structure of the volcano area of sectors D and F Žas defined in Section 3.2.,
during the cliff-forming period is best described in did not produce as great an area of platform forming
terms of a ‘‘Mercedes Star’’ ŽCarracedo, 1994, lavas as in the NE rift. In particular, the distal part of
1996a,b. triple volcanic rift system. Possible reasons the NW rift zone appears to have become completely
for the disparity in productivity implied by the much inactive. Few vents of this age occur in the south of
greater topographic expression of the N–S rift zone Zone D indicating that the southern side of the NW
as compared to the other rift zones are discussed rift zone may also have become inactive. Vent elon-
further below. gation directions Žsee also Fig. 7, below. do, how-
ever, indicate that the underlying feeder dykes were
3.3. Volcanic Õents in the early platform-forming to the NW and WNW, as before 20 ka. These vent
sequence (20–7 ka) elongation directions, as in the NE rift zone, are
commonly oblique to the local topographic slope
Early platform-forming series rocks occur in all direction.
parts of the Cumbre Vieja. Their distribution is In comparison with the distribution of activity in
shown in Fig. 5, from which it is apparent that the the period prior to 20 ka, the intersection of the three
most extensive platform-forming sequences and the rift zones of the Cumbre Vieja volcano appears to
greatest numbers of eruptive vents are present in the have migrated northwards by at most 2 or 3 km in
southern and NE parts of the volcano. Rocks of this the period from 20 ka to 7 ka and the NW volcanic
age in the south of the volcano may be underrepre- rift zone appears to have undergone a marked de-
sented in Fig. 5 because of covering by younger cline in activity, although without a clear change in
lavas. The distributions of volcanic vents of this age its geometry. These subtle structural changes may
can be considered in terms of the three rift zones have been precursors to the more radical changes in
defined above in the cliff-forming series of rocks, the geometry of the volcano from about 7 ka on-
but two significant differences are apparent, in the wards.
NE and NW rift zones.
The vents of the southern, N–S aligned rift zone
from Nambroque southwards maintain their concen- 3.4. Distribution of historic and sub-historic (post 7
tration in Sector A along the crest of the ridge. N–S ka) Õolcanic Õents
elongation of many of these vents is also apparent.
The NE rift zone contains many vents of this age The distribution of the very youngest, morpholog-
and also has the most continuous coastal lava plat- ically freshest volcanic vents on the Cumbre Vieja
form in any part of the volcano. The vents show volcano is markedly different from those of earlier
consistent ENE to NE elongations and alignments of periods and is shown in Fig. 6.
multiple vents. The elongation directions and vent These vents include a number of prehistoric scoria
alignments again tend to strike slightly obliquely to and spatter cones and associated lava flows, charac-
the overall slope. However, the region of more east- terised by almost entirely unmodified morphologies
erly trending vents in the south of Sector E as and at most partial vegetation cover. The young ages
defined in Section 3.2 and Fig. 4 appears to have of a number of these eruptive units have been con-
been inactive after about 20 ka before present. In firmed by K–Ar andror C-14 radiometric dating
contrast the cover of platform forming lavas further ŽGuillou et al., 1998, except for La Malforada wun-
north, although thin, is almost complete and the published C-14 age of 1050 a B.P.; Carracedo et al.,
northern limit of Cumbre Vieja rocks is entirely 1999a. and Montana Quemada, previously dated us-
formed by platform forming lava flows. It therefore ing C-14 ŽHernandez Pacheco and Valls, 1982.x, as
appears that the axis of the NE rift zone may have shown in Table 1: locations of the vents listed in this
shifted significantly to the north after 20 ka ago. table are shown in Fig. 6. The majority of vents inS.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 145 Fig. 5. Distribution of volcanic vents during early platform-forming series activity. Sectors A–F discussed in text.
146 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
Fig. 6. Distribution of sub-historic and historic volcanic vents.
this youngest group are, however, the products of though minor intrusive activity at depth cannot be
historic eruptions, in 1585, 1646, 1677, 1712, 1949 excluded, this implies a very drastic reduction in the
and 1971. Some of these eruptions involved multiple rate of dyke-related extension across this rift zone. In
vents separated by distances of up to 3 km and it contrast, activity on the southern, N–S rift zone
should be borne in mind that, without the historical follows the same pattern developed in previous peri-
data, these vents would most probably have been ods, with most vents on the axial ridge. Recent and
mapped as the products of separate eruptions. historic activity in the general area of the old NW
Perhaps the most immediately apparent feature of volcanic rift zone has occurred on two distinct vent
activity in this most recent stage of the history of the alignment trends ŽFig. 7..
Cumbre Vieja is the complete absence of eruptions The prehistoric vents of La Barquita, Birigoyo
on the NE rift zone, in strong contrast to the intense and Montana Quemada lie on an approximately N–S
activity in that region in the preceding period. Al- trend. La Barquita and Birigoyo show definite ventS.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 147
Table 1 the case of the 1585 and 1949 vent complexes, these
Ages of pre-historic eruptive vents emplaced after the rift-zone trends are markedly to the south of the WNW to NW
reorganisation. See Fig. 6 for locations of vents. See text for
further discussion and references
alignments of vents in the old NW rift zone.
Ž3. Within all three groups of vents, individual
Eruptive vent Dating method AgeU Ž2 s errors.
vents Žor, in the case of the 1585 eruption complex,
Montana Quemada C-14 420"60 a
three trends within the vent complex as shown in
Nambroque II-Malforada C-14 1050"95 b
Las Indias K–Ar 3 ka"2 ka Fig. 9. are arranged en echelon. Elongation direc-
Montana de Fuego K–Ar 3 ka"2 ka tions of these individual vents are always offset to
C-14 3255"140 b the southwards Žor anticlockwise. with respect to the
3350"50 c overall trend of the vent alignment, and adjacent
Birigoyo K–Ar 6 ka"3 ka
vents are offset dextrally with respect to one another
a
C-14 age quoted by Hernandez Pacheco and Valls Ž1982.; as viewed along the alignment trend ŽFigs. 8 and 9..
confirmed by Guanche Žaboriginal. reports recorded by Spanish. In the case of the 1585 eruption complex at Jedey,
b
c
C-14 age determined by Kruger Analytical, USA. vents within each trend shown in Fig. 9 were linked
C-14 age determined by CEA-CNRS, Gif-sur-Yvette, France. by simultaneous events. The phonolite domes and
heterogenous hybrid lavas were erupted at the vents
elongation and rim breaches along a trend bearing
345, or slightly west of north. The eruptive fissure of
Montana Quemada is a N–S aligned elongate trough
implying a N–S-striking feeder dyke. The develop-
ment of broadly N–S aligned vents in this northern-
most part of the Cumbre Vieja suggests that the N–S
rift zone is propagating northwards from the older
summit region around Nambroque, thereby bisecting
the volcano.
The other group of vents occurs on the western
flank of the volcano and is represented by vents of
the 1585, 1712 and 1949 eruptions. These groups of
historic vents are distinctive in a number of ways:
Ž1. They form highly elongate fissures or fissure
alignments, with relatively little near vent construc-
tional relief in the form of scoria or spatter cones
Žalthough the 1585 eruption also involved the em-
placement of a number of juvenile phonolite domes,
Fig. 9.. The Llanos del Banco vent of the 1949
eruption is a sinuous trough, composed of multiple
overlapping vents, some 800 m long ŽFig. 8.; the
Jedey vent complex formed in the 1585 eruption is
about 1.5 km long and contains a dozen or more
overlapping vents ŽFig. 9.; and the 1712 eruption
involved seven or more discrete vents along an
arcuate but mainly WNW aligned trend extending
over a distance of some 3 km ŽFigs. 3 and 6..
Ž2. The overall trend of these fissures or fissure
alignments varies from WNW Žin the 1712 eruption.; Fig. 7. Comparison of vent elongation directions during the pre-7
to slightly north of west Žthe Jedey vent complex.; to ka and post-7 ka periods in the NW rift zone, summit region and
slightly south of west Žthe Llanos del Banco vent.. In adjacent areas.148 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
It should also be noted that this phenomenon is
confined to the western flank of the volcano and is
not observed in older vents anywhere on the volcano.
Vents of this period at the crest of the volcano ŽFigs.
6 and 7. do not show en echelon segmentation and
have a broadly N–S elongation direction, as noted
above. These vents were fed by dykes which retained
a N–S alignment Žand associated east–west exten-
sion and host rock s 3 . throughout their ascent to the
surface.
3.5. Analysis of the changes in Õent distribution and
orientation in terms of changing stress field compo-
nents
The variations in the stresses developed in differ-
ent parts of the volcano during the most recent
Žpost-7 ka. period can be understood in terms of the
Fig. 8. Geological sketch map of the 1949 Llano del Banco vents,
stresses which are predicted to develop within an
showing inferred feeder dyke orientations. edifice which is dominated by topographic-gravita-
tional loads; in other words, by the uneven distribu-
tion of its own weight. McGuire and Pullen Ž1989.
showed that in a ridge like edifice these stresses have
along the central trend whilst those to north and a characteristic pattern ŽFig. 11.. Across the crest of
south erupted only basic magmas. This implies the the ridge, extensional stresses are developed due to
simultaneous existence of three separate but overlap- the partially unbuttressed weights of the flanks
ping en echelon feeder dykes beneath this vent com- pulling in opposite directions. Dykes will therefore
plex. be emplaced parallel to the ridge crest in this region.
Ž4. In contrast to the slight obliquity of many In contrast, on the flanks themselves downslope
earlier vents to the local slope, best seen in the NE compression is developed and any extension will
rift zone ŽSections 3.2 and 3.3., the individual vents occur in the direction parallel to the topographic
in these three eruptions closely follow the local slope contours. Dykes emplaced into the flanks of the
direction. edifice will therefore be aligned downslope.
These features indicate that the feeder dykes to The occurrence of the downslope trending fissures
these eruptions show a consistent pattern of rotation in en echelon arrays ŽFig. 10. can also be understood
and segmentation with depth. This is depicted in Fig. in terms of this model if the old triple rift geometry
10. At depth, the feeder dykes to these eruptions has persisted at depth in the form of dyke swarms.
appear to strike NW or WNW, along the trend of the Dykes propagating upwards from this region, with its
old NW rift system. The extension direction Žand radially symmetric stress field and tangential mini-
implied minimum principal stress s 3 in the host mum principal stress directions ŽCarracedo, 1994.,
rock. associated with dyke emplacement is broadly into the region immediately beneath the flank of the
NE–SW, or NNE–WSW. As the dykes propagate volcano would experience a rotation of s 3 and
towards the surface the extension direction and im- therefore a rotation of their preferred alignment into
plied s 3 in the rocks through which the dyke tips Žrespectively. contour parallel and downslope direc-
are moving consistently rotate into a N–S direction, tions. In contrast, N–S aligned dykes propagating
parallel to the local topographic contours, resulting upwards directly beneath the crest of the ridge would
in rotation of the propagation plane and segmenta- experience no change in principal stress directions in
tion of the dykes in the sense observed. the host rock and thus no change in orientation.S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 149
Fig. 9. Geological sketch map of the 1585 Jedey vents showing lava flows and inferred feeder dyke orientations. The area of this map also
contains complex sequences of pyroclastic deposits erupted from the numerous vents active during the eruption.
This model does, however, raise a problem with horizons, distal ash layer packages or erosional un-
respect to the distribution of vents and vent align- conformities that can be correlated across sectors of
ment pattern earlier in the history of the volcano. the volcano ŽCarracedo et al., 1999b-this volume.
Prior to about 7 ka ago, as discussed above, a triple indicates that at no earlier time in the history of the
rift pattern was established at the surface of the volcano has a rift zone abandonment, comparable to
volcano and numerous vents were emplaced in direc- the recent abandonment of the NE rift zone, taken
tions oblique to the local topography, most notably place. Although crosscutting ŽNE- and East-trending.
in the NE rift zone but also in the NW rift zone. This sets of vents exist at the top of the cliff forming
pattern was developed throughout the earlier history series in the southern part of Sector E ŽSection 3.2.,
of the volcano. The absence of major weathering no sets of en echelon fissures comparable to those150 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 Fig. 10. 3-D perspective sketch showing inferred subsurface geometries of recent dykes beneath the N–S crest of the Cumbre Vieja and under the western flank of the volcano. developed in the historic west flank eruptions have Furthermore, the topography of the volcano has been found. The abandonment of the NE rift zone changed little since about 20 ka ago when cliff and other events since 7 ka therefore appear to be erosion ended. The magnitudes and orientations of unique in the history of the Cumbre Vieja. the topographic-gravitational components of the Fig. 11. Sketch showing near-surface stress directions and fissure orientations in a volcanic ridge dominated by topographic-gravitational stresses Žafter McGuire and Pullen, 1989..
S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 151
stress field within the volcanic edifice can therefore boundary zone has developed between them which is
have changed little in the period of rift zone reorgan- structurally weak and thus inefficient at propagating
isation. Whilst some influence of changes in the stresses from one to the other.
topographic stresses over time upon the changes in The extinction of the NE rift zone in contrast to
vent distribution and orientation patterns cannot be the development of en echelon fissure eruptions on
excluded on the basis of this argument, it appears the west flank of the volcano implies an asymmetry
that the primary cause of the reconfiguration of the in the structure of the volcano from 7 ka onwards.
eruptive vent distribution and orientation pattern must The east flank appears to have become a relatively
lie in changes in the other components of the overall rigid buttress. The northward propagation of the
stress field. N–S rift zone can then be understood in terms of
The possibility that the reconfiguration may be synintrusive movement of the western flank of the
due to development or disappearance of a significant volcano away from this rigid buttress, the movement
magma reservoir within the volcanic edifice can be being accommodated within the weakened zone. One
excluded on two grounds. Firstly, as noted above possible explanation for the extinction of the NE rift
there appears to be no large magma reservoir within zone and the northward propagation of the N–S rift
the edifice, the bulk of the magmas ascending in- zone is therefore that the weakened region is to be
stead from lithospheric depths of the order of 7 to 11 found only under the western flank of the Cumbre
km ŽHansteen et al., 1998.; and secondly, there is no Vieja. The location Žparticularly the depth., extent
overall major change in the compositions of the and nature of this weakened region can be con-
Cumbre Vieja rocks at the time of the reconfigura- strained by consideration of the pre-Cumbre Vieja
tion. The one group of rocks whose genesis may substrate upon which the volcano has grown, and the
involve low pressure fractionation, the phonolites, faulting which took place along the crest of the
occur from at least 56 ka onwards ŽGuillou et al., volcano during the 1949 eruption. These are exam-
1998.. The triple rift geometry developed through ined in the following two sections.
the majority of the history of the volcano is therefore
best interpreted in terms of doming above a deep
magma body as proposed by Carracedo Ž1994;
1996a,b. for other Canarian volcanoes. The develop- 4. The pre-Cumbre Vieja geology of southern La
ment of the triple rift geometry and the consequent Palma
obliquity of the NE and NW rift zones to the local
topography is therefore critically dependent of the As noted in Section 1, the Cumbre Vieja is only
efficient transmission of doming stresses from the the youngest component of the island of La Palma.
region at depth where they are developed to the The north of the island is formed by an earlier shield
upper part of the volcano. volcano, the Taburiente–Cumbre Nueva edifice
In view of the development of en echelon vent ŽCarracedo et al., in press.. In the last stages of
geometries on the western flank of the Cumbre Vieja growth of this volcano a highly active N–S-trending
Žand implied subsurface dyke segmentation., our volcanic rift zone developed on its southern flank.
favoured explanation for the changes that have taken The remnants of the resulting topographic ridge form
place since about 7 ka on this flank of the volcano is the present day Cumbre Nueva ridge ŽFig. 1.. The
that the deep triple-rift stress field has become de- west flank and axis of the rift zone were removed
coupled from the near surface stress field so that the about 560 ka ago by a giant lateral collapse directed
latter is now dominated by the essentially unchanged to the south and west. Work presently in progress
topographic-gravitational stresses while the triple rift ŽCarracedo et al., 1999b-this volume. indicates that
stress field persists at depth ŽFig. 10; note the dis- the axis of the Cumbre Nueva rift zone lay several
tinctly oblique dilation directions of the en echelon kilometres to the west of the present day ridge:
fractures relative to the deeper part of the dyke certainly, since the present Cumbre Nueva ridge is
feeding them.. The development of decoupled stress composed of east dipping rift flank lavas with only a
fields in these two adjacent regions implies that a few dykes, it must be significantly to the east of the152 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
position of the old rift zone axis. The altitude of the lapse structure beneath the Cumbre Vieja. In the
rift crest is therefore likely to have been 2000 to absence of subsurface geological data, both rift and
2500 m above present sea level. The collapse scar, collapse structure have previously been assumed,
whose floor is largely at or below present sea level conservatively, to extend only a short distance south
ŽCarracedo et al., 1999a. has since been partly in- of their present outcrop Že.g., Ancochea et al., 1994;
filled, mostly by the growth of the Cumbre Vieja Carracedo, 1994.. However, there is a geometrical
volcano. This has completely infilled the southern problem associated with this interpretation which is
part of the collapse scar and buried the southern end illustrated in Fig. 12.
of the Cumbre Nueva escarpment. The level crest of the present day Cumbre Nueva
A major question in the geology of La Palma is ridge is at an elevation of about 1450 m. In addition,
therefore that of the southward extents of the Cum- the dips of the lavas at the crest of the ridge are
bre Nueva rift zone and of the Cumbre Nueva col- approximately perpendicular to its trend: there is
Fig. 12. Map showing predicted southern extents of 1500 m contour on Cumbre Nueva volcano prior to collapse, with variation according to
position of Cumbre Nueva rift zone axis indicated.S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 153 very little if any along ridge component of dip. Carracedo, 1994; Carracedo et al., 1997b; Day et al., When allowance for post collapse erosion is made, 1997.. The presence of a southward extension of the the ridge crest therefore closely approximates to the Cumbre Nueva collapse scar beneath some or all of position of the 1500 m contour at the time of the the westward flank of the Cumbre Vieja volcano is collapse. The curvature of the Cumbre Nueva ridge also indicated by recent imaging sonar mapping of indicates that the original trend of this contour to the pre-Cumbre Vieja debris avalanche deposits that ex- south would have been to the SW, but nonetheless it tend further south along the SW submarine flank of would not have curved back to the NW until it La Palma than can be accounted for by the previ- intersected the axis of the Cumbre Nueva rift zone to ously inferred extent of the Cumbre Nueva collapse the west. Depending on the precise position of the ŽUrgeles and Masson, pers. comm... Cumbre Neuva rift zone axis, as shown in Fig. 12, The southern limit of the Cumbre Nueva collapse Cumbre Nueva rocks would have originally occurred scar must therefore underlie most if not all of the at elevations of 1500 m or more as far south as the region in which the en echelon vents have been region immediately east of the village of Jedey, and emplaced ŽSection 3.4; Figs. 6 and 7.. The exact perhaps as far as El Remo on the present day coast- depth of the collapse structure below the surface and line. Instead, younger Cumbre Vieja rocks occur at the height of the buried collapse scar are not well around 1000 m elevation to the east of Jedey, and at defined, but for present purposes the critical point is sea level at El Remo ŽFig. 3.. A minimum of be- the inferred presence of a west-dipping collapse scar tween 500 m and 1500 m Žvertical thickness. of the beneath the western flank of the Cumbre Vieja, at or Cumbre Nueva sequence has therefore been removed below present sea level ŽFig. 13.. Depending on its from these areas. position and orientation, this collapse scar may be This indicates that the Cumbre Nueva collapse associated with one or more structurally weak litho- extended further south than has previously been sup- logical units: remnant debris avalanche deposits; a posed. A possible geometry of the crest of the head- collapse scar sediment fill sequence; and hyalo- wall is indicated in Fig. 12. Collapse structures with clastite units at the base of the filling volcanic a similar asymmetric scalloped geometry occur in sequence. Direct evidence for the presence of post the adjacent island of El Hierro Žthe El Golfo col- collapse sediments comes from boreholes on the lapse structure and the San Andres aborted collapse: northern side of the Cumbre Nueva collapse ŽCar- Fig. 13. E–W cross-section through Fig. 12 Žalong line A–AU of Figs. 1 and 2. showing inferred position of collapse scar, collapse scar fill sequence and original Cumbre Nueva topography.
154 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
racedo et al., 1999a.. Beneath the younger ŽBejenado east and the older Bejenado volcano to the north
and Cumbre Vieja. lavas, these cross unconsolidated ŽSector F of Section 3.2..
and structurally weak alluvial fan breccias, clay ma-
trix rich debris flow deposits and scree deposits
which directly overlie the unconformity representing 5. The 1949 fault scarp: first surface rupture
the floor of the collapse scar. associated with instability of the western flank of
The postulated southern extension of the Cumbre the Cumbre Vieja?
Nueva collapse structure therefore implies that struc-
turally weak lithologies are present beneath the west- The 1949 eruption of the Cumbre Vieja volcano,
ern flank of the Cumbre Vieja edifice. The presence lasting from 24th June 1949 to 30th July 1949,
of such units would account for decoupling of deep involved two eruption sites: a N–S cluster of vents
and shallow stress fields within the western half of in the summit region of the volcano on the N–S rift
the Cumbre Vieja and thus for the geometry of the system and, as discussed in Section 3.4, an en eche-
structural asymmetry of the volcano that has devel- lon vent system on the western flank of the volcano.
oped since 7 ka ŽSection 3.4.. However, this hypoth- Amongst the syneruptive phenomena ŽBonelli Rubio,
esis does not account on its own for the development 1950. were locally intense seismicity and the devel-
of this asymmetry only in this recent period: this is opment of surface ruptures, principally in the period
discussed further below. 1str2nd July 1949, along a west-facing normal fault
The presence of pre-Cumbre Vieja topography system in the region between the two eruption cen-
beneath the volcano, on the scale shown in Figs. 12 tres. As discussed in Section 1, Carracedo Ž1994;
and 13, may also account for the unequal develop- 1996a; b. proposed that these faults might represent
ment of the three rift zones throughout its history. As an incipient stage of instability of the west flank of
noted above Žsee Fig. 11., McGuire and Pullen Ž1989. the Cumbre Vieja, and a precursor to a flank collapse
show that topographic-gravitational stresses favour in the future. They may also provide an explanation
emplacement of dykes along ridge crests, especially of the structural reconfiguration of the volcano dur-
adjacent to steep, unstable cliff faces. An example of ing the Holocene.
this is provided by the dykes across the back of the In view of the hazard implications of this, the
Valle del Bove on Etna, running parallel to the top of faults were mapped in detail by the first author with
the headwall cliff face ŽMcGuire et al., 1991.. A the aims of determining:
similar relationship would exist for the buried seg- 1. their age relationships with respect to other events
ment of the Cumbre Nueva collapse scar and dykes in the eruption Žin conjunction with a reexamina-
feeding vents along the present line of the N–S rift tion of eyewitness accounts.;
zone of the Cumbre Vieja. In contrast, dykes beneath 2. their surface geometry including amounts and
the NW rift zone would run obliquely across the directions of displacements, and thus inferring
WSW dipping floor of the Cumbre Nueva collapse their subsurface geometry;
scar, while those beneath the NE rift zone would run 3. whether or not they were associated with lo-
obliquely across the intact eastern slopes of the older calised fumarolic activity that might indicate the
volcano. In order to propagate to the surface they presence of shallow intrusions along their length;
would therefore have to overcome a component of 4. whether the major surface ruptures identified by
the downslope compressive stress indicated in Fig. Bonelli Rubio were in fact only a component of a
11. The overall effect of these various topographic- broader and more distributed deformation field
gravitational stresses would therefore be to promote with deformation taking place on many smaller
activity on the N–S rift and partially suppress it on and less obvious structures.
the other two, throughout the history of the volcano. A map of the fault system showing displacements
Further suppression of the topographic expression of measured on the mapped fault strands is shown in
the NW rift zone in particular can be attributed to the Fig. 14; the chronology of the eruption based on the
development and infilling of a sediment and lava accounts of Bonelli Rubio and others is summarised
trap between it and the Cumbre Nueva scarp to the in Table 2. It should be noted that the fault systemS.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167 155
extends much further north than was thought — 5.1. Field obserÕations of the 1949 fault system
Bonelli Rubio considered that the northern limit was
at the northern end of the Llanos del Agua — and Although vegetation growth and reworking of
that the displacements along the fault have not previ- 1949 deposits have obscured the fault system in
ously been measured. places, the gaping fissures with vertical offsets of the
Fig. 14. Map of the 1949 fault system and eruption sites.156 S.J. Day et al.r Journal of Volcanology and Geothermal Research 94 (1999) 135–167
Table 2
Chronology of 1949 eruption, primarily based on Bonelli Rubio Ž1950., Martin San Gil Ž1960. and Monge Montuno Ž1981.; see text for
discussion of these and other accounts, which are in part contradictory
Date Eruptive activity Seismic activity
22nd February – ŽEarliest date of seismicity recorded in Martin San Gil Ž1960.;
Bonelli Rubio does not mention any activity before 21st June.
25th March – Strong earthquakes in south of La Palma,
with damage to lighthouse.
21st June – Two strong earthquakes; many smaller felt earthquakes
22nd–23rd June – Lesser seismic activity
24th June Start of eruptive activity at Duraznero Žcontinuing Moderate seismicity, continuing to 6th July
at lesser intensity to 6th July.. Activity of
phreatomagmatic, vulcanian to strombolian type
1st, 2nd July Žactivity at Duraznero continuing at moderate level. Two very strong earthquakes, felt throughout island Žstrongest
earthquakes of entire eruption.
6th July Strong vulcanian explosion at southernmost vent of Seismicity ceased temporarily after strong explosion
Duraznero, followed by diminution of activity Žnote:
other accounts have this diminution in activity not
occurring until 8th July.
7th July – Strongly felt seismicity
8th July Opening of Llano del Banco fissure and
commencement of eruption of lava at high rate
Žcontinuing until 26th July.
Strong felt seismicity accompanying opening of vent Žnote: other
accounts have this vent opening occurring near-aseismically.
9th July–11th July Weakly felt seismicity
12th July Opening of Hoyo Negro vents; Duraznero Vent opening accompanied by two strong earthquakes
vents remaining inactive. Explosive
Žmainly vulcanian. activity continuing at
Hoyo Negro until 22nd July
13th–14th July Frequent earthquakes; last on 14th accompanied by dilation of
fissure between Duraznero and Hoyo Negro
21st–23rd July End of explosive activity at Hoyo Negro Ž22nd July. Intermittent, weak to moderate seismicity
30th July Brief resumption of eruptive activity at Hoyo Negro; Weak earthquakes at start of Duraznero fissure eruption
opening of Duraznero fissure north of Duraznero
crater and short but intense fire-fountain eruption
walls observed and photographed by Bonelli Rubio to some metres deep bounded by vertical walls is
on and after July 6th 1949 ŽBonelli Rubio, 1950. are preserved in places ŽFig. 15a,b.. A different geome-
still recognisable along much of the length of the try occurs where the fault system cuts fine grained
fault system. The walls, especially where formed by phreatomagmatic ashes and alluvially reworked ashes
loose scoria rather than more cohesive spatter or from the prehistoric explosion craters of Crater El
rubbly lava, have partially collapsed and filled the Fraile and Llanos del Agua III, Žto the east and west
fissure in many places. However, a narrow trough up of the Duraznero fissure and Hoyo Negro, respec-
Fig. 15. Field photographs of the 1949 fault system. ŽA. View of the fault scarp Žca. 2 m high. at the northern end of the fault system, as
seen from the west. ŽB. Gaping fissure at the extreme southern end of the fault system, viewed down the length of the fault from the north.
Note Hoyo Negro lithic ash and 30th July lapilli deposits on hangingwall Žwestern. side of fault, and absence of these deposits on the
degraded footwall side. One metre length of tape for scale. ŽC. 1949 Fault-related fissures along western side of Llanos del Agua, cutting
yellow phreatomagmatic ash of prehistoric Crater El Fraile eruption. Fissures filled with grey Hoyo Negro lithic ash from early phase of
Hoyo Negro eruption and covered by undisturbed Hoyo Negro deposits.You can also read
