Journal of Volcanology and Geothermal Research
Morphometry of scoria cones located on a volcano flank: A case study from Mt. Etna (Italy), based on high-resolution LiDAR data
Introduction
Scoria cones are the most common and most uniform volcanic landforms as demonstrated in previous key papers of Porter (1972), Settle (1979) and Wood, 1980a, Wood, 1980b. Due to the great number of such cones in large monogenetic volcanic fields (typically developed on flat areas and associated with a single or a few more silicic, voluminous volcanic centres; Connor and Conway, 2000), they provide a good opportunity for quantitative morphometrical studies (e.g. Scott and Trask, 1971, Porter, 1972, Bloomfield, 1975, Settle, 1979, Wood, 1980a, Wood, 1980b, Martin del Pozzo, 1982, Hasenaka and Carmichael, 1985, Moriya, 1986, Tibaldi, 1995).
In most of these studies, standard morphometric parameters H (cone height), Wco (cone width) and Wcr (crater width) were used. Based on these parameters, Porter (1972) was the first to conclude that H/Wco is constant (~ 0.18) for a great number of Hawaiian (Mauna Kea) cones, a relationship that has later been confirmed and further supported by many examples worldwide (e.g. San Francisco Volcanic Field, Arizona, Wood 1980a; Paricutin and Michoacán-Guanajuato fields, Mexico, Hasenaka and Carmichael, 1985; Nunivak Island, Alaska, Settle, 1979; Etna, Italy, Settle, 1979; Fuji, Japan, Moriya, 1986). Settle (1979) and, in more detail, Wood (1980b) also demonstrated that H/Wco decreases with time due to erosion. For old (some Ma) scoria cones especially under arid–semiarid climates, where linear erosion is limited, areal redistribution of scoria (i.e. lowering of cone and enlargement of basal diameter) results in a progressively smaller H/Wco ratio down to 0.08 (Wood, 1980b, Hooper and Sheridan, 1998).
These examples of scoria cones studied by previous authors are located mostly on a flat surface (monogenetic volcanic fields), but they can also be found on gently to moderately dipping planes (large volcano flanks: Mauna Kea, Hawaii; Etna, Italy; Mt. Cameroon, Cameroon; Nyiragongo, Democratic Republic of Congo, etc.). The slope of the flank can have a considerable effect on the emplacement of a cone. For example, Tibaldi (1995) pointed out that the direction of crater breach is strongly controlled by the substrate slope dip if its inclination is > 9°.
However, none of the above authors focussed on the geometrical consequences to H, Wco and their ratio in the case of emplacement on a volcano flank. There are at least two problems to be addressed: (1) how to calculate the (relative) height of the scoria cone on a dipping plane, and (2) what are the morphometrical consequences of burial by subsequent, regular lava overflows.
In this paper we investigate the scoria cone field of Mt. Etna volcano by using new (September 2005) high-resolution LiDAR data, interpolated on a 2 m stepped DEM (Favalli et al., 2009). The Etnean cones are located from 400 m to higher then 3000 m a.s.l. on a large central volcano having variously dipping flanks (Fig. 1). Although these cones are all Holocene in age, their morphology, in most cases, has been significantly modified by the subsequent effusive activity of Etna, and to a less extent by erosion (related to the high altitude and the Mediterranean climate). All these factors result in significant changes of cone shape that may be detected by high-resolution morphometry.
Section snippets
Geological background of Etna's flank eruptions
Mt. Etna volcano, located on the east coast of the island of Sicily, Italy, has a basal diameter of about 40 km and an elevation of about 3350 m. It evolved on the continental crust of eastern Sicily at the tectonic boundary marked by the subducting Ionian oceanic slab (Gvirtzman and Nur, 1999). Its structural dynamics are principally characterized by volcanic spreading, which results in an overall seaward movement of its eastern sector, accomplished mostly by movements along fault systems
The high-resolution DEM of Etna
Airborne altimetric LiDAR (Light Detection and Ranging) data have been used to generate a high-resolution (2 m step) digital elevation model (DEM) of most of Mt. Etna flanks from data acquired during a LiDAR survey at Mt. Etna in September 2005 (Favalli et al., 2009). The LiDAR survey consists of more than 2.57 × 108 scattered topographic points. The points are distributed in thirty-four NNE–SSW trending strips covering most of our study area, i.e. a large part of Etna's northern, western and
Results
In accordance with the list of morphometric parameters in Table 1, all the obtained data are presented in the Supplementary Table. Using the data base, a number of plots, both standard and new, have been constructed.
In Fig. 6, volume and average slope of basal plane of scoria cones vs azimuth (i.e. geographic position on Etna's flank) are plotted, also displaying the shape and age classes we established. Large cones (> 1,000,000 m3) tend to be well formed, and are located on relatively
Discussion
Variations in cone volume and basal slope of Etna's scoria cones, as Fig. 6 testifies, have no direct relationship to their position in terms of rift zones. There is no relationship to ages either, although for the W rift, only a few age constraints are available. Given this uniformity, there is no reason to consider cone morphometry with respect to structural control. Apparently, volume is only controlled by elevation: with increasing altitude (i.e., steeper basal slope), cones tend to be
Conclusions
In our work, 135 scoria cones of Etna were investigated by high-resolution morphometry. The flanks of Etna have various slopes up to ~ 20°, on which the scoria cones are scattered largely corresponding to the NE, W and S rift zones. However, as the volume, H/Wco, and age distributions of cones show, these parameters are uniform in the different zones, i.e. there is no direct structural control on them. On the other hand, smaller cones tend to occur toward the summit, probably in relation with
Acknowledgments
The research was supported by fundings from the MIUR Project ‘Sviluppo Nuove Tecnologie per la Protezione e Difesa del Territorio dai Rischi Naturali’ and from the Italian Dipartimento Protezione Civile to the Istituto Nazionale di Geofisica e Vulcanologia. We thank A. Tibaldi and an anonymous reviewer for their revision.
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