Bull Volcanol (2012) 74:2171–2185 DOI 10.1007/s00445-012-0658-1
RESEARCH ARTICLE
Syn-eruptive morphometric variability of monogenetic scoria cones Gábor Kereszturi & Gyozo Jordan & Károly Németh & Javier F. Dóniz-Páez
Received: 9 September 2011 / Accepted: 5 September 2012 / Published online: 29 September 2012 # Springer-Verlag 2012
Abstract According to Wood's model, morphometric parameters such as slope angle can provide valuable information about the age of conical volcanic edifices such as scoria cones assuming that their initial slopes range from 30° to 33°, measured manually on topographic maps, and assuming that their inner architectures are homogenous. This study examines the morphometric variability of nine young (a few thousand years old) small-volume scoria cones from Tenerife, Canary Islands, using high-resolution digital elevation models in order to assess their slope angle variability. Because of the young age and minimal development of gullies on the flanks, their morphometric variability can
be interpreted as the result of syn-eruptive processes including: (1) pre-eruptive surface inclination, (2) vent migration and lava outflow with associated crater breaching and (3) diversity of pyroclastic rocks accumulated in the flanks of these volcanic edifices. Results show that slope angles for flank sectors differ by up to 12° among the studied volcanoes, which formed over the same period of time; this range greatly exceeds the 2–3° indicated by Wood. The greater than expected original slope range suggests that use of morphometric data in terms of morphometry-based relative dating and detection of erosional processes and settings must be done with great care (or detailed knowledge about absolute ages and eruption history), especially in field-scale morphometric investigation.
Editorial responsibility: B. van Wyk de Vries Electronic supplementary material The online version of this article (doi:10.1007/s00445-012-0658-1) contains supplementary material, which is available to authorized users. G. Kereszturi (*) : K. Németh Volcanic Risk Solutions, CS-INR, Massey University, Private Bag 11 222, Palmerston North, New Zealand e-mail:
[email protected] G. Kereszturi : G. Jordan Geological Institute of Hungary, Stefánia út 14, 1143 Budapest, Hungary G. Kereszturi Department of Geology and Mineral Deposits, University of Miskolc, Miskolc, Hungary J. F. Dóniz-Páez Department of Geography, University of La Laguna, Tenerife, Spain Present Address: K. Németh Geological Hazards Research Unit, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia
Keywords Relative dating . Slope angle . Crater breaching . Cinder cone . Monogenetic volcanism . Digital elevation model . Tenerife
Introduction Morphometric studies of volcanoes commonly aim to characterize a wide range of eruptive and erosion-related processes (Inbar et al. 2011; Grosse et al. 2012). Scoria (or cinder) cones are one of the most frequent targets of such studies (Favalli et al. 2009; Bemis et al. 2011; Rodriguez-Gonzalez et al. 2012) due to their abundance on the Earth, occurring in monogenetic volcanic fields in almost all known geological settings (Wood 1980a, b; Connor et al. 2000; Rapprich et al. 2007; Valentine and Gregg 2008). The primary aim of morphometric studies on volcanoes (mostly on scoria cones) is to establish slope angle values, which are important parameters for assessing the relative age of the volcanic cone within a monogenetic volcanic field (e.g. Wood 1980a, b). A fresh scoria cone has been said to have initial slope angle values between 30° and 33°
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(Wood 1980a, b; Riedel et al. 2003), with these values decreasing systematically during the course of degradation (Wood 1980a, b; Dohrenwend et al. 1986; Hooper 1995). During cone formation and subsequent degradation, various processes can shape the morphology of volcanic edifices (Wood 1980a, b; Rodriguez-Gonzalez et al. 2011; Büchner and Tietz 2012; Kereszturi and Németh 2012). The syneruptive controlling parameters determine the initial geometry and properties of pyroclastic deposits. These processes include, for example, changing magma fragmentation style, migration or changing dimensions of a feeder dyke beneath the edifice, effusive activity or cone collapses and/or breaching due to intermittent explosions (Corazzato and Tibaldi 2006; Martin and Németh 2006; Kereszturi et al. 2011; Martí et al. 2011; Németh et al. 2011; Starkova et al. 2011), all of which can lead to the formation of complex volcanic landforms in terms of inner architecture, pyroclastic deposit types and morphometry. Such diversity of morphology and architecture is a source of pitfalls for morphometric parameterization and morphometry-based dating. On the other hand, the post-eruptive, erosional processes such as linear dissection of the flanks, landslides or sediment creep also play vital roles in the shaping of volcanic landforms after the cessation of volcanic activity (Hooper and Sheridan 1998). The number of both syn- and post-eruptive controlling processes make the interpretation of morphometry of scoria cones more complicated than it has been assumed by previous studies (Wood 1980a, b; Hooper and Sheridan 1998). In early studies, the initial geometrics of volcanic edifices were assumed to be similar, providing a basis for comparative morphometric studies (Settle 1979; Wood 1980a, b). In other words, previous studies interpreted the present morphometric characteristics entirely as the result of erosional processes governed by climatic conditions, assuming homogenous inner architecture and similar initial geometry (Porter 1972; Wood 1980a, b; Hooper 1995; Hooper and Sheridan 1998; Pelletier and Cline 2007). In order to test the variability of slope angle values in relation to syn- and post-eruptive controlling factors, nine young (≤Pleistocene) scoria cones in Tenerife, Canary Islands, are parameterized morphometrically. Scoria cones on volcanic islands such as Tenerife (Fig. 1) are located along volcanic rift zones in extensional stress fields (Carracedo et al. 2007; Geyer and Martí 2010). The nine studied scoria cones were selected on the basis of their known young (≤Pleistocene) ages (Carracedo et al. 2007) and their diverse pre-eruptive surface inclinations. As a consequence of the young age, the morphology of the studied cones has not significantly been modified by erosional processes as confirmed by the lack of vegetation or gullies (see for example Figs. 5, 6 and 7). Thus, the slope angles measured on the outer flanks of a scoria cone can be parameterized by using high-resolution digital elevation models (DEMs), allowing quantification of morphometric variability among
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monogenetic edifices. These young cones with limited erosional modification provide good opportunity to test the uniformity of initial geometry, one of the assumptions of morphometry-based dating and comparative studies on scoria cones (Porter 1972; Wood 1980a, b; Hooper and Sheridan 1998).
Geological settings The Canary Archipelago is located a few hundred kilometres from the NW coast of Africa. Of the seven volcanic islands forming the Canarian archipelago, Tenerife is the largest (2,034 km2) and highest (3,718 m asl). The first shield stage eruptive products of Tenerife (Fig. 1) are exposed as old volcanic massifs, e.g. Teno, Anaga or the Roque del Conde formed between 12 and 3.5 Ma (Ancochea et al. 1990; Thirlwall et al. 2000). The new phase of volcanism built up a large volcanic complex sitting on top of older remnants (Fig. 1), which was merged and partially covered by the eruption products of the Las Cañadas volcano (Ancochea et al. 1990; Carracedo 1994; Carracedo et al. 2007). The Las Cañadas volcano has been truncated by (1) lateral gravitational collapses (Carracedo et al. 2011) or by (2) multiple, vertical collapses following emptying of the shallow magma chamber (Marti and Gudmundsson 2000). Other important volcanic structures are the central edifice (Teide–Pico Viejo volcano) and its associated rift zones (Fig. 1) originating either from (1) swelling of the surface due to mantle ascent (Carracedo 1994; Carracedo et al. 2007) or from (2) spreading of the upper volcanic structure (Walter 2003). Recent volcanic activity and possible hazard are most likely linked to these rift systems, which host the majority of the subaerial monogenetic volcanoes studied in this paper (Carracedo et al. 2007; Doniz et al. 2008; Geyer and Martí 2010).
Materials and methods DEMs were created from the 5-m interval contour lines of the 1:5,000 Tenerife topographic map sheets (GRAFCAN 2009). A linear interpolation method specifically designed for contours was implemented with the ILWIS raster GIS software (Gorte and Koolhoven 1990). Linear interpolation is an exact interpolation method because it fits to the input data, i.e. contour lines (Gorte and Koolhoven 1990). This method rasterizes the digitized vector contour lines first with the contour elevation values; then, the unknown elevation values are interpolated. The interpolation is performed between the two nearest contour lines by using the shortest distances measured in both forward and backward directions on the rasterized maps (Borgefors 1984; Gorte and Koolhoven 1990). Selection of proper cell size is, however, crucial,
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Fig. 1 Simplified geological map of the island of Tenerife including the major shield edifices in the three corners of the present island, the deposits of Las Cañadas volcano and the location of the most recent
products of Pico-Viejo stratocone and its rift zones based on Ancochea et al. (1990), IGME (2003) and Carracedo et al. (2007). STR Santiago del Teide rift, DR dorsal rift, SVZ southern volcanic zone
because large cells containing two or more contour lines cannot resolve the original detailed topographic information. This loss of resolution and thus morphologic information is particularly likely on steep slopes with close contour line spacing. In order to retain the maximum morphological information carried by the contour lines, the appropriate grid cell size resolving all individual contours was determined by a trial-and-error approach of rasterizing the contours to 0.5-, 1and 2-m cell sizes. The cell size was regarded fine enough if the rasterized contour lines were separated by at least one undefined cell width. This was numerically tested by applying a simple neighbourhood operation in a 3×3 moving window. If none of the contour line raster cells in the map have neighbour cells with a different elevation value, then the tested grid cell size was defined as the right resolution for DEM interpolation. In this case, the 0.5×0.5-m grid cell size was found to properly resolve and represent the original contour line information, so the final DEM was interpolated from the contours to the 0.5-m grid cell size. In order to improve slope gradient calculation, the DEMs were smoothed with a 5×5 (2.5×2.5 m) moving average window (see Electronic supplementary material 1). A limitation of linear interpolation is that it creates flat areas within closed contour lines at hilltops and natural depressions, with cells of constant elevation value.
This systematic error can affect morphometric calculations such as slope statistics. In order to improve statistical analyses, topographic peaks within closed contour lines were not considered in further morphometric calculations, including slope angle statistics. These minor peaks are located around the crater rim, which anyway tends to be degraded early (Pelletier and Cline 2007; Valentine et al. 2007). The DEM's accuracy was characterized by the root mean square error (RMSE) (Fisher and Tate 2006) comparing the interpolated DEM elevation to independent spot heights digitalized from the topographic maps: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 P ZDEM Zref RMSE ¼ n
ð1Þ
where ZDEM is grid cell elevation, Zref is the reference spot height elevation and n is the number of reference points. In the case of the studied DEM, the RMSE was calculated separately for each studied volcanic cone, and it ranged from ±0.5 to ±3.8 m (see Electronic supplementary material 1). This is well within the original 5-m contour interval, confirming the appropriateness of the interpolation method.
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The gradient vector of a digital surface is characterized by its length, i.e. the slope (S) and a direction, i.e. aspect (A). These properties were calculated as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi S ¼ arctan fx 2 þ fy 2 ð2Þ A ¼ 180 arctan fy =fx þ 90 ðfx =jfx jÞ
ð3Þ
where A is the modified directional component of the gradient vector pointing in the down flow direction and measured clockwise from north. The partial derivatives (fx, fy) were estimated using the unweighted eight-point Prewitt operators: fx ¼ ðZ3 þ Z6 þ Z9 Z1 Z4 Z7Þ=6X and fy ¼ ðZ1 þ Z2 þ Z3 Z7 Z8 Z9Þ=6Y in a 3×3 moving window (Jordan 2007). Besides its smoothing effect, this method has the advantage of being identical to a leastsquares plane fit to the nine window elevations (Sharpnack and Akin 1969); thus, the linear Prewitt operators are particularly suitable for linearly interpolated surfaces. The slope and aspect values of a DEM are highly dependent on the resolution used (e.g. Zhou and Liu 2004; Deng et al. 2007; Dragut et al. 2011). In order to visualize changes of slope angle as a function of grid cell size, a series of DEMs were generated from the same contour lines in different resolutions from 0.2 to 20 m (Fig. 2A). The slope angles were calculated for only the outer flanks of the Fasnia edifice where the central tendency of slope angles is expected (Fig. 2B). The Smean and Smedian values showed a sharp decrease in their values at around 5-m resolution (Fig. 2), while the Smode showed no trend due to the multimodal slope angle histogram.
Fig. 2 a Change of slope angles (Smean, Smedian and Smode) as a function of grid cell size examined on the outer flanks of the Fasnia scoria cone from Tenerife. b The slope angle histogram for the same
The pre-eruptive surface or basement slope (Sbasal) and aspect (Abasal) were also calculated (Fig. 3). This pre-eruptive surface of the studied cones was modelled by a first-order trend surface, i.e. plane, fitted by the least-squares method to the digitalized spot heights (de Smith et al. 2009). To obtain the pre-eruptive surface, only those spot heights were used which were not located within the area of (1) the tephra blanket around the cone, (2) the cones' slope, (3) the associated lava flow fields and (4) the large valleys located near the analysed cones. Orthophotos, geological maps and field observations were used to delineate these areas. Morphometric parameters and geometry of monogenetic volcanoes, including slope angles on the outer flanks, can vary because of the pre-eruptive surface inclination (Tibaldi 1995; Kervyn et al. 2012). In the present study, the volcano outer flanks were classified into three types in relation to the slope of the underlying pre-eruptive basal surface. With this classification, inner intra-flank variability of the slope angle is detected. These flank sectors are called uphill (Fuphill), downhill (Fdownhill) and other (Fother) and were defined as (see in Fig. 4): Fdownhill ¼ Abasal 45
ð4Þ
Fuphill ¼ Abasal þ 180 45
ð5Þ
Thus, the Fuphill and Fdownhill slope facets are perpendicular (±45°), while the Fother are orthogonal (±45°) to the dominant pre-eruptive surface inclination. Scoria cones are mostly built up from loose, scoriaceous lapilli fragments
cone showing the unimodal distribution of slope values for grid cell size of 0.5 m, which is essential for accurate univariate analysis
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Fig. 3 Establishing the basal inclination by a first-order trend surface interpolated using the least-squares method and spot height data. Example is the Arafo scoria cone. a View of the volcanic edifice and its surrounding on an orthophotograph draped on the DEM. b and c Fitted first-order trend surface through the spot heights (red points) with the location of the outer flanks. The interpolated trend surface displays the general slope and aspect of the pre-eruptive surface
(Riedel et al. 2003). If the fragment size is in a narrow range for a pristine scoria cone (Wood 1980a, b), only a unimodal distribution of slope angles is expected. Thus, the central tendency statistics of average (Smean), median (Smedian) and mode (Smode) were used for the flank segments of each studied edifice. To identify inappropriate statistical slope angle descriptors, slope angle histograms were calculated and presented for each studied scoria cone. The calculated
slope angle values were also visualized as a function of slope aspect in the polar plots in order to detect geographical location of variance in slope angles on the outer flanks. The outer flanks were defined as the surface between slope break at the foot of the cone and the crater rim. In the delimitation of the outer flanks, 1:30,000 aerial photographs (with 40– 50-cm resolutions), 1:25,000 geological maps (IGME 2003) and field observations were used.
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Fig. 4 a Aspect-based flank segment delimitation method developed for scoria cones in Tenerife demonstrated using the Arafo scoria cone as an example. Note that the white parts of the classified aspect map are flat (i.e. zero slope) areas resulting from the linear interpolation method. These parts were discarded from the slope angle calculations. The inset diagram shows the graphical definition of flank sectors. b Local slope angle statistics for the Arafo scoria cone
Results Out of nine examined young scoria cones from Tenerife, only three examples are described in this section in terms of location, volcanology, morphology and slope angle characteristics for demonstration purposes. These three examples represent a wide spectrum of pre-eruptive inclinations from 4° to 11°. The rest of the studied scoria cones with detailed description and additional figures can be found in the Electronic supplementary material 2. Montaña Arafo The Arafo scoria cone (Fig. 1) is located on the floor of a large valley at the NE rift zone on the southern slopes of Tenerife. It is also a young volcanic vent formed in 1,705 AD,
and it is associated with the Sietefuentes, Fasnia and Arafo triple eruptions (Carracedo et al. 2007). The pyroclastic edifice is built up by scoriaceous lapilli and ash beds reflecting more explosive than effusive behaviour. The scoria cone is horseshoe shaped due to crater breaching (Fig. 5). The scoria cone of Arafo is in a special location in a valley, where the valley floor (pre-eruptive surface) has an 11° slope angle. The direction of this interpolated surface is facing towards the SE (129°; Table 1). The slope angles are generally the steepest among the cones of Tenerife according to our DEM-based calculations. The uphill (Smean 032°, Smedian 032°, Smode 033°) and other (Smean 031°, Smedian 0 31°, Smode 032°) flank sectors are steeper than the downhill sector (Smean 027°, Smedian 027°, Smode 022°, Table 1). The slope standard deviations are relatively low, between 3° and
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Fig. 5 Overview map of Montaña Arafo (left) with topographic profiles (inset profiles) along (A-A′ profile) and perpendicular (B-B′ profile) to the major axis of the pre-eruptive surface slope. The white
arrow shows the location of deepest point of the crater. Note that the contour interval is 5 m. Slope angle histograms (right) for the uphill, downhill and other flank sectors with the major descriptive statistics
5°, and have the lowest values in the Fuphill increasing towards Fdownhill (Table 1). The slope angle histograms show skewed and bimodal distribution, except the Fother, making accurate capture of descriptive statistics harder (Fig. 5).
high effusive activity in the waning stage of the volcanic edifice construction (Dóniz 2009). Boca Cangrejo is situated on a gentle pre-eruptive surface with an inclination of 7°. The cone is facing west (the aspect direction of the underlying surface is 288°; Table 1). The upper slope sector has a relatively low proportion of pixels (5 % from the total number of slope pixels), but it shows nearly the same results as the rest of the cone flanks (Table 1). Based on the slope angle histograms, the Boca Cangrejo scoria cone is one of the most complex in terms of morphometry because of the multi-modal and skewed distribution of the slope histogram (Fig. 6). This asymmetry of slope histograms is also represented by the large standard deviation, around 7° and 9° and the scattered values of slope angle parameters measured on the other (Smean 026°, Smedian 027°, Smode 029°) and downhill (Smean 022°, Smedian 023°, Smode 027°) flank segments (Table 1).
Montaña Boca Cangrejo The eruptions of Boca Cangrejo (Fig. 1) took place on the centre of the NW rift zone in 1,492 AD based on historical descriptions (Romero 1991; Carracedo et al. 2006). The historic documentation is also supported by recent radiocarbon age determinations providing an age range of 1,440 and 1,660 AD for the cone's activity (Carracedo et al. 2007). The eruption of the Boca Cangrejo scoria cone includes cone emplacement as well as emission of an extended aa, pahoehoe and block lava flow field that covers a large part of the island (8 km2) on the southern part of the Santiago del Teide rift (STR) with a length of about 10 km. The Boca Cangrejo scoria cone consists of successive beds of lava spatter, scoriaceous lapilli and ash deposits. This volcanic complex has a multiple vent system with at least three larger, wellpreserved craters (Fig. 6). Two of these craters are ring-like explosion craters, and three are breached probably due to the
Montaña Fasnia Montaña Fasnia (Fig. 1) is located on the south edge of the NE rift and formed in 1,705 AD (Carracedo et al. 2007). It comprises two well-distinguished, highly asymmetrical
preHolocene
Güímar
115
317
142
250
92
277
330
182
7
155
238
153
354
148
97
70
272
97
245
328
243
84
238
187
160
2
187
335
58
333
174
328
277
Max
28.5 3.27
STD
28
29
31
22
29
b
a
5.12
28.3
31
27
31
19
30
20
a
33
29
a
0.0a
Smode
23
28
Smean
26
26
2.69
26.8
429,473 (31.1 %) 27
38,885 (19.8 %) 27
62,430 (62.4 %) 30
44,448 (9.9 %)
187,272 (50.9 %) 24
12,242 (4.8 %)
a
99,485 (22.6 %) 31
598 (0.2 %)
a
0.0 (0.0 %)a
N of cells
Fother
2.74
27.2
28
27
30
27
24
27
31
23
29
Smed
Largest difference between flank segment slope angle values
No representative proportion of pixel numbers (under 5 % of the total population of pixels)
3.39
28.7
29
28
31
22
29
25
a
a
25
32
32
29
a
a
29
0.0a
0.0a
Ave
1
3
4
6
7
7
11
14
24
Smed
Smean
Basal Fuphill
Slope angle (°)
All values are in degrees, except for the number of cells
1,910 AD
290
895± 115 years BP 1,705 AD
65
148
13
1,706 AD
264
129
63
Chinyero
Fasnia
Boca Cangrejo Garachico Reventada
58
283
7
Min
Min
Max
Fuphill
Fdownhill
288
3,933± 212 years BP 1,705 AD
Chío
232
Basal
Slope aspect (°)
1,492 AD
1,798 AD
Chahorra
Arafo
Age
Edifice
Table 1 Results of the aspect-based flank sector delimitation of studied locations from Tenerife
3.99
27.1
30
24
29
27
25
29
32
19
29
Smode
Smean
22
27
21
21
108,721 (55.5 %) 656,889 (47.6 %)
3.09
24.5
26
26
18,537 (18.5 %) 30
345,244 (76.9 %)
84,964 (23.1 %) 22
231,432 (52.6 %) 170,785 (67.1 %)
184,081 (68 %)
55,323 (32.2 %) 26
N of cells
Smed
3.33
24.6
27
26
30
21
23
23
27
19
26
Fdownhill
5.1
24.3
29
27
28
19
23
27
22
15
29
Smode
Smean
294,106 (21.3 %)
2.63
26.7
28
48,415 (24.7 %) 27
19,166 (19.1 %) 30
59,266 (13.2 %) 25
95,953 (26.0 %) 26
108,993 30 (24.8 %) 71,380 (28.1 %) 25
116,758 27 (67.8 %) 85,973 (31.8 %) 22
N of cells
Fwhole
2.81
27.0
28
27
30
25
27
26
31
22
27
Smed
3.79
27.3
28
27
31
27
27
27
32
19
29
Smode
1,380,468 (100 %)
195,821 (100 %)
100,133 (100 %)
448,958 (100 %)
359,316 (100 %)
254,412 (100 %)
439,910 (100 %)
270,652 (100 %)
2
3
1
5
7
3
5
8
2
2
1
6
6
4
5
10
29
2
3
3
8
7
9
11
14
29
Smean Smed Smode 172,081 (100 %) 28
N of cells
Differenceb
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Fig. 6 Montaña Boca Cangrejo. For details, see the Fig. 5 caption
cone-like landforms which were probably breached by extensive effusive activity. This edifice associated with a fissure eruption has a highly elongated profile with the dimensions of 200 m width and 1,200 m length (Fig. 7) with multiple craters. The edifice is built up by mostly lapilli and bombs and spatter-dominated beds. The aa lava field associated to the edifice covers an area of 0.52 km2 (Dóniz 2009). Fasnia cones are significantly breached from the south, and the substrate dips towards 142° (SE direction) with a relatively low basal slope angle (4°; Table 1). The slope angle variations among the flank sectors are relatively low. All of the calculated slope angles are within the range of 28° to 31° with standard deviation between 3° and 4° (Fig. 7). All of the slope histograms show distinct unimodal behaviour; however, some skewed distributions can be found in the cases of the Fother and Fdownhill flank sectors (Fig. 7).
Discussion Evidence of morphometric variability All of the studied scoria cones are less than a few thousand years old (Carracedo et al. 2007; Doniz et al. 2008). Consequently, their morphometric characteristics such as slope angle should be ≥30° according to previous observations for
fresh and pristine scoria cones, e.g. the Merriam age group is characterized by 30.8±3.9° from the San Francisco Volcanic Field, Arizona (Wood 1980a, b; Hooper and Sheridan 1998). In contrast to this expected high value, the slope angles of the studied cones vary from 22° to 30° for Smean and from 21° to 30° for Smedian measured as an average of flank sector values. Individual flank sectors are in the same range (Table 1). The Smode values are not presented here because of their high variability (see Fig. 2). This variance in slope angles of young scoria cones on Tenerife is unexpected. Based on the young examples, the morphometric variability exists: (1) among the volcanic edifices, i.e. the slopes of young cones are not always in the same range, and they do not always have the expected slope ≥30°, and (2) within a single edifice, i.e. there are differences between the dominant values of various flank sectors. Morphometric variability among the volcanic edifices has an important consequence for morphometric analysis, because a narrow variability of slope angle and initially similar geometries are the bases of the comparative morphometric studies, including morphometry-based dating (Settle 1979; Conway et al. 1998; Sucipta et al. 2006) or erosional studies on volcanoes (Wood 1980a, b; Bemis et al. 2011; Inbar et al. 2011; Fornaciai et al. 2012). This key assumption is not, however, supported by this study of young scoria cones from Tenerife. This is in agreement with recent
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Fig. 7 Montaña Fasnia. For details, see the Fig. 5 caption
studies indicating inaccuracies in morphometric-based dating (Kereszturi and Németh 2012). Similar variances have been documented for cones that have not reached their final, well-developed size and geometry (Bemis et al. 2011; Kervyn et al. 2012) and/or collapsed or were truncated during eruption (Kereszturi and Németh 2011; Németh et al. 2011). Fine-scale morphometric variability among flank segments within a single edifice is also documented by this study. These segments correspond to flank sectors along and perpendicular (±45°) to the main axis of the terrain slope (±45°) and show flank–slope differences of as much as 12° (Fig. 8), which is about one third of the entire range of the natural spectrum of angles of repose of loose, granular scoriaceous volcano flanks (Wood 1980a, b; Riedel et al. 2003). To detect and visualize such changes as a function of position within individual edifices, polar plots were made (Fig. 8), with emphasis on the location of the expected ‘average’ values, i.e. 30.8 ± 3.9° (Wood 1980a, b). The expected average slope angle is fulfilled by the Fuphill segment of the Arafo scoria cone (Figs. 5 and 8). This flank segment has a low standard deviation (3°) and is in the expected ≥30° range. The overall geomorphic state of the Arafo scoria cone is interesting because there is an increase in the standard deviation towards the Fdownhill sector, from 3° to 5° (Fig. 8). The observed increasing slope segment
variability results from the combination of inclined basement and the effusive activity and/or flank collapse (Figs. 5 and 8). Origin of morphometric variability The morphology of scoria cones can change rapidly (years to decades) after eruption (Inbar et al. 1994; Fornaciai et al. 2010; Inbar et al. 2011). Nevertheless, these secondary modifications including deflation by wind, small gravitational landslides or rain splash erosion (Wood 1980a, b; Hooper 1999) on the flank of a young volcanic edifice (≤ka) do not extremely alter the original morphology (Pelletier and Cline 2007; Fornaciai et al. 2010). The morphometric variability detected among these young cones through slope angle values resulted from syn-eruptive rather than post-eruptive processes. Such morphometric variability may be related to either non-volcanic and/or volcanic processes including (1) pre-eruptive surface inclination, (2) vent migration and associated crater breaching, (3) effusive activity and associated crater breaching and flank collapses as well as (4) variation of the properties of the pyroclastic successions, e.g. grain size, degree of welding and agglutination. From these controlling factors on initial cone morphology, the effects of pre-eruptive surface inclination and
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Fig. 8 Slope angles as a function of slope aspect visualized in a polar plot (black dots). Red arrows display the overall dip direction of the pre-eruptive topography. Black curves with small arrows in the polar plots show the approximate location of lava outflows from fissures during the evolution of each studied scoria cone. The solid and dashed
red circles are equivalent to the generally used 30.8° slope angle and its error range of ±3.9°, respectively, expected for young volcanic edifices according to Wood (1980a). Slope dots inside and outside the dashed red circles represent morphometric variability from the expected values for a fresh scoria cone
of effusive activity are hardly distinguishable from each other because most of the lava flows left the cones at the base of their downhill flank (Fig. 8). These lava outflows and associated processes such as flank collapses (Holm 1987; Németh et al. 2011) may have affected the final morphometry of the studied volcanic edifices by partially removing, rotating and gradually distorting the flank segment(s), as on the Arafo scoria cone (Figs. 5 and 8). Flank collapses and subsequent rebuilding of cones represent a more complex eruption history than may be typical for pure
symmetric cones (Holm 1987). The pre-eruptive surface inclination helps to develop such collapse and breaching, as indicated here by the preferred extrusion of lavas from the cones' downhill flanks (Fig. 8). A larger degree of morphometric variability can be expected with pre-eruption topographic slopes exceeding 5° (Table 1) which is in agreement with the field-based observation at the Los Morados scoria cone in Argentina (Németh et al. 2011) and with the LiDARbased characterization of parasitic cones from the flank of Mt. Etna (Favalli et al. 2009).
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Importance of morphometric variability The abovementioned syn-eruptive processes lead to morphometric variability both among individual volcanic edifices and between individual flank segments within the same edifice. Such initial morphometric variability has important implications for interpretation of morphometric characteristics of volcanic edifices, cluster of cones or even volcanic fields. Because there is morphometric variability even among young, uneroded scoria cones, edifice ages and their general implications for eruption histories of individual edifices or groups cannot be inferred. Comparison should be restricted to edifices with strict age control, knowledge on eruption history and pyroclastic composition as well as information on the initial geometry of the edifice. The studied cones were sorted in decreasing order of slope angles of the three flank sectors with the Fasnia or Arafo cones having the highest slopes. These steep cone flanks would be traditionally interpreted as a sign of young age, but they are not the youngest edifices (Table 1). More interesting is that the pre-Holocene cone of Güímar has higher flank slopes (26–28°) than the 600- and 300-yearold edifices including Boca Cangrejo (22–25°) or Garachico (22–28°) (Table 1). Similarly, the morphometric signatures of the Güímar and Chinyero scoria cones are in the same range between 29° and 26° in spite of their age difference of over 10,000 years (Table 1). These observations show that the same morphological state of an edifice can be reached either during eruption or as a consequence of post-eruptive processes. In this case, the 29° and 26° slopes for Güímar reflect a slight modification by erosional processes (e.g. weathering and soil formation), while the fresh (100 years old) Chinyero is formed by syn-eruptive processes and shows no evidence of weathering or soil formation. A key inference from our work is that comparative morphometric investigation of monogenetic volcanoes without knowledge of the initial geometry cannot be used to infer relative ages of well-preserved cones. These distorting effects of the initial geometry fade away by the final stages of the erosion, when more compact and erosion-resistant layers become exposed on the flanks of a scoria cone (Kereszturi and Németh 2012), but this usually requires a million years (Wood 1980a, b; Kereszturi and Németh 2012) or, in some cases, as long as 40 My (Rapprich et al. 2007; Büchner and Tietz 2012). Relation of morphometric variability with basement inclination An increasing difference can be identified among the three overall slope descriptors (Smean, Smedian and Smode; Fig. 9), which is related to the occurrence of multi-modal and skewed slope angle histograms (see Figs. 5, 6 and 7 and
Fig. 9 Graph showing the maximum morphometric variability between slope angles derived from flank segments of scoria cones as a function of pre-eruptive surface inclination. See the text for detailed explanation
Electronic supplementary material 2). These differences coincide with the overall inclination of the pre-eruptive surface, and it is characterized by a moderate positive, Pearson correlation with correlation coefficients of 0.61 for Smean, 0.67 for Smed, and 0.97 for Smode. These values were calculated without Mt. Chio and Mt. Chahora due to their missing uphill slopes. If Mt. Chio and Mt. Chahora were included, the correlation coefficient between the maximum slope angle difference between flank segments and the pre-eruptive inclination is ≥0.90. For the largest difference in slope angles between flank segments as a function of preeruptive inclination, three groups were recognized (Fig. 9). The analysed volcanic edifices in the present study are located on a pre-eruptive surface with inclination of 0–5° (‘plain’), 5–10° (‘gentle’) and over 10° (‘steep’). The scoria cones located on plain, sub-horizontal preeruptive surfaces (up to 5°) have the lowest differences between the Fuphill, Fdownhill and Fother sectors in the mean, median and mode slope angle values (Fig. 9). The largest differences among slope angle parameters are usually less than 3° (Table 1). Examples of this type of cone are the Montaña Güímar, Fasnia and Chinyero. This similarity observed in the slope angles of flanks can be a result of (1) the almost plain-like pre-eruptive terrain and/or (2) the homogenous distribution of pyroclasts in certain slope sectors. The homogenous distribution of pyroclasts can also be the result of a horizontal pre-eruptive surface. For example, cones formed on a horizontal pre-eruptive surface are less likely
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to have slope collapse and/or breaching when compared to those that erupted onto a pre-eruptive surface with high basal inclinations exceeding 4° (Németh et al. 2011). Subhorizontal pre-eruptive topography helps the formation of a well-developed edifice with symmetrical cone geometry, though it may still be modified by wind, vent migration and/or lava outflow and associated breaching. This finding is in agreement with experimental modelling results (Tibaldi 1995; Kervyn et al. 2012). The scoria cones built on gently sloping pre-eruptive surfaces with inclinations of 5–10° are characterized by greater variation of slope angle differences between flank segments, from 3–9° (Fig. 9 and Table 1). Three studied cones are of this type, the Garachico, Reventada and Boca Cangrejo scoria cones. These differences are high enough (e.g. exceeds the overall accuracy of the input topographic data) to cause problems for morphometry-based dating and comparative studies of scoria cone degradation. The scoria cones situated on pre-eruptive surfaces with inclinations of ≥10° are represented by the Chahorra, Chío and Arafo scoria cones. These examples formed on 24°, 14° and 11° pre-eruptive surfaces, respectively (Fig. 9). In this group, two of the studied edifices have no uphill-facing flanks. All of these examples have breached crater morphology, where the direction of breaching is in accordance with the direction of inclination. The difference between slope sectors is the largest for the Chahorra cones and ranges up to 28° due to the missing uphill flanks (Table 1). In addition to the missing uphill-facing slopes, the extremely elongated morphology of the Chío scoria cone may also be related to (1) the high degree of pre-eruptive surface inclination, (2) formation of fissure rather than a ‘point-like’, well-localized vent and/or (3) late-stage effusive activity.
Conclusions The nine studied young cones in Tenerife are characterized by wide ranges of slope angles for individual edifices and for individual flank sectors making morphometric parameters unique for every cone studied. Such variation in slope angle values could be as high as 12°, which is significantly higher than indicated early studies (Porter 1972; Wood 1980a, b). Due to the known young geological age (mostly Holocene), the observed morphometric variability can be interpreted as the result of mostly syn-eruptive processes such as (1) pre-eruptive surface inclination, (2) vent migration, (3) lava outflows and associated crater breaching and (4) differences in the properties of pyroclastic rocks preserved in the flanks of the volcano. The studied young cones are seen to have variations in slope angles of certain flank segments when the preeruptive inclination gets steeper. The observed morphometric
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variability may become significant on cones on basal slopes ≥5°, as suggested by recent studies (e.g. Németh et al. 2011; Kervyn et al. 2012). This shows that the basement inclination is an important factor in triggering flank collapses and subsequent re-construction processes, leading to complex eruption histories and often complex geomorphology of the resultant volcanic edifice. This diverse nature of edifice growth possibly makes the internal architecture of a scoria cone complicated, which plays an important role in the subsequent degradation. The observed wide range of morphometric variability caused by syn-eruptive processes raises the question about the interpretation of morphometric parameters. Previous studies interpreted morphometric parameters of monogenetic volcanic edifices in two ways: (1) primary, geometric modifications triggered by volcanological processes (Tibaldi 1995; Corazzato and Tibaldi 2006; Doniz et al. 2008; Bemis et al. 2011; Fornaciai et al. 2012; Kervyn et al. 2012) and (2) secondary, post-eruptive surface modifications caused by erosional processes during the degradation (Wood 1980a, b; Hasenaka and Carmichael 1985; Dohrenwend et al. 1986; Hooper 1995; Hooper and Sheridan 1998; Inbar and Risso 2001; Sucipta et al. 2006; Favalli et al. 2009; Negrete-Aranda et al. 2010; Rodríguez et al. 2010; Fornaciai et al. 2010; Inbar et al. 2011; RodriguezGonzalez et al. 2011; Kereszturi and Németh 2012). However, morphometric characteristics as documented here are too complex to interpret in a single way because they could bear both primary (i.e. results of syn-eruptive processes) and secondary (i.e. results of erosion-related processes) information. Theoretically, the likelihood of syn-eruptive processshaped morphometric values is higher in the first stage of the degradation history than after a few thousand years or even million years after the eruptions ceased. Further studies should target the understanding of the meaning of morphometric parameters at certain stages of erosion. It seems that using a large number of scoria cones of varying age is inappropriate for detailed geomorphic studies. Without strict age control, simple calculations of slope angles can merge the effect of various geological/volcanological and erosion processes which in turn can lead to morphometric misinterpretation. Thus, the simple global interpretation of morphometric characteristics based on a large number of scoria cones should be avoided. The presented methods are suitable to detect morphometric irregularities found on the flanks of monogenetic scoria cones. In addition to the flank sector analysis, slope angle histograms proved to be useful for controlling the accuracy of statistical estimates. Also, polar plots efficiently describe the morphometric variability of small-volume, monogenetic volcano flanks. In this study, flank sectors located parallel to the main direction of the pre-eruptive basal surface have multi-modal and asymmetric slope distributions suggesting
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a control of basal tilt. The combination of such techniques allows a more detailed investigation of the late-stage, syneruptive morphometric evolution of young scoria cones. Acknowledgments Gabor Kereszturi would like to thank the PhD Research Fellowship offered by Volcanic Risk Solutions, Institute of Natural Resources, at Massey University (New Zealand). This research was also partly supported by the Department of Geology and Mineral Deposits, University of Miskolc, Hungary. The authors are grateful for the topographic maps and orthophotos of the Gobierno de Canarias (GRAFCAN). The comments and discussion with Mike Tuohy and Matthew Irwin (Massey University) and the comments by the editors, Benjamin van wyk de Vries and James White, as well as the two anonymous journal reviewers improved the quality of the manuscript.
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