# Distribution of rootless cones and landforms resulting from water-lava interactions at hagi in the Aðaldalur valley (Iceland)

Émeline Bourrier, Gaëlle Mathieu, Hervé Leyrit, Réjanne Le Bivic, Elsa

Ottavi-Pupier, Maxime Denis, Julien Duquennoy, Arnaud Dujanny

► To cite this version:

Émeline Bourrier, Gaëlle Mathieu, Hervé Leyrit, Réjanne Le Bivic, Elsa Ottavi-Pupier, et al. Distribution of rootless cones and landforms resulting from water-lava interactions at hagi in the Aðaldalur valley (Iceland). Jean-Yves Reynaud; Eric Armynot du Châtelet. Short Papers from the 19th French Congress of Sedimentology, Lille, 84, Association des Sédimentologistes Français, pp.9-12, 2025, 2-907205-83-8. 10.70665/WDLH3582. hal-05005185

HAL Id: hal-05005185

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Submitted on 1 Apr 2025

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# DISTRIBUTION OF ROOTLESS CONES AND LANDFORMS RESULTING FROM WATER-LAVA INTERACTIONS AT HAGI IN THE AðALDALUR VALLEY (ICELAND)

Émeline BOURRIER<fnref n="1" />, Gaëlle MATHIEU<fnref n="1" />, Hervé LEYRIT<fnref n="2" />, Réjane LE BIVIC<fnref n="2" />, Elsa OTTAVI-PUPIER<fnref n="2" />, Maxime DENIS<fnref n="2" />, Julien DUQUENNOY<fnref n="2" />, Arnaud DUJANNY<fnref n="2" />

Corresponding author: emeline.bourrier@gmail.com

**Abstract:** The region of Lake Mývatn and the Aðaldalur Valley in Iceland is known for its high concentration of rootless cones. To study the processes involved in the formation of those volcanic landforms, a 1.1 km<fnref n="2" /> study area rich in volcanic landforms was delineated near the Hagi guesthouse in the Aðaldalur valley. The rootless cones and other facies were referenced, and a high resolution orthomosaic and 3D model were produced using drone acquisition. A new classification of rootless volcanic cones and mounds was produced, as well as an architecture of the Hagi zone. This area is interpreted as a continuous transition of explosive interactions between lava and water saturated sediments followed by a basaltic flow due to depletion of water. The lava flow mechanically erodes some of the rootless cones.

**Keywords:** Hydrovolcanism, Iceland, Mývatn, Laxa lava, rootless cones

## 1 INTRODUCTION

The region between Lake Mývatn and the Aðaldalur Valley in Iceland is known for its high concentration of rootless cones. These are the result of local explosions at the surface of a lava flow, generated when it interacts with a shallow body of water (Thorarinsson, 1951) or water saturated sediments (Fagents and Thordarson, 2007). They are composed of two main types of volcanic ejecta: lapilli, with a diameter ranging from 2 to 64 mm, and spatter (> 64 mm), soft, plastically deformable bombs, which can aggregate and weld when hot (Boreham et al., 2018). They are not conventional volcanoes, as they are not directly fed by a magma chamber. The Younger Laxá Lava that forms these cones is a basaltic lava with a chaotic "aa" surface, which flowed from the Þrengslaborgir-Lúdentarborgir eruptive fissure towards the prehistoric lake of Mývatn, then followed the Laxá valley to Skjálfandi Bay in northern Iceland, covering a total distance of 63 km with an average thickness of ~ 17 m and a total area of around 220 km<fnref n="2" /> (Höskuldsson et al., 2010). A classification of crater cone morphology was proposed for the proximal sector around Lake Mývatn (Noguchi et al., 2016) and then modified by distinguishing hornitos (metric cones of spatter) and applied to the entire valley (Boreham et al., 2018).

The aim of this study is to identify the primary and secondary processes involved in the formation of volcanic landforms in the Aðaldalur valley (Iceland), a former glacial valley less studied than the area around lake Mývatn, located around 50 km from the eruptive fissure. Primary processes include the fragmentation of lava by interaction with water, as well as the sedimentary transport and deposition of the pyroclastic fall that forms the relief. Secondary processes include syn- to post-depositional remobilization of tephra. For this purpose, a 1.1 km<fnref n="2" /> study area rich in volcanic landforms was delineated near the Hagi guesthouse in the Aðaldalur valley.

## 2 MATERIALS AND METHODS

### 2.1 Field work

A two-week field mission was carried out in July 2023. The morphologies of the Mývatn cones were compared with those of the Hagi zone to establish differences and similarities. Then, the cones in the Hagi zone were identified and referenced according to their pyroclastic products, color and angle of slope.

### 2.2 Drone and photogrammetry

The drone used for the acquisition was the DJI Phantom 4 Pro v2, equipped with an RGB optical imaging sensor. Acquisition was carried out with 75% longitudinal and 60% lateral overlap. 26 targets (or markers), whose coordinates were recorded using GPS, were randomly placed across the acquisition zone. These targets were used as Ground Control Point to achieve a precise georeferencing under 10 cm over the entire area. During data acquisition, the drone's speed was set at 3.5 m/s, and its altitude between 30 and 50 m

<footnote n="1">¹Institut Polytechnique UniLaSalle Beauvais, 19 rue Pierre Waguet 60000 Beauvais, France</footnote>

<footnote n="2">² Institut Polytechnique UniLaSalle, UPJV, B2R (GeNumEr), U2R 7511, 19 Rue Pierre Waguet, BP 30313, 60026 Beauvais, France</footnote>
(AGL). All photographs were taken from the same angle, at nadir, and the resolution was set at 20 megapixels.

The 4,500 photos captured by the drone were processed using Agisoft Metashape software. This software enables photogrammetric processing (SFM) of digital images to obtain a dense point cloud or textured model. A 12 cm resolution orthomosaic and a 3D model were obtained after processing.

# 3 RESULTS AND DISCUSSION

## 3.1 Classification

The classification established by Noguchi et al. (2016) was completed based on information gathered on the field (Fig. 1). Scoriaceous cones were classified into three categories: (i) Lapilli cones (LC), which were divided into two subgroups according to their slope angle: 30° (LC1), or less than 30° (LC2); (ii) lapilli and spatter cones (LSC), whose determining factor is the position of the spatter: on top of the cone (LSC1), or inside (LSC2); (iii) spatter cones (SC), with a slope angle close to 45° and a break of slope at the base of the cone (SC1) or without a break of slope (SC2). The main additions are (i) the creation of a second type of lapilli and spatter cone, and (ii) the definition of mound facies (LM, LSM, SM) describing craterless volcanic ejecta accumulations.

<figure><img src="image_2.png" /><figcaption>Figure 1. Rootless cones classification, based on the morphology and the tephra granulometry</figcaption></figure>

## 3.2 Zone facies referencing

The facies observed in the field were referenced in ArcGIS (Fig. 2A) and their distribution analyzed. A DEM was produced using orthomosaic data (Fig. 2B).

<figure><img src="image_3.png" /><figcaption>Figure 2. (A) Distribution of volcanic facies; (B) DEM of the zone.</figcaption></figure>

The northern zone is the richest in lapilli. This tephra platform is at least 4 m thick (Fig 2B). It is covered with scoriaceous cones (LC2, LSC1) with circular craters (Fig 2A). To the north-west, around ten lapilli cones,
sometimes with incomplete spatter crowns, are aligned in a west-east direction. In the northeastern part, smaller alignments, oriented north-south, have broadly similar granulometries and slope angles of less than 20°. On the eastern and southern edges of the platform, the more frequent facies are craterless LM or LSM (Fig. 2A).

The central zone has a lower topography (Fig. 2B) and is covered by dense vegetation. In this area, the few outcrops are those of pahoehoe lava tubes that trend north-westwards in the central plain and north-eastwards to the east of the tephra platform. The rare pyroclastic facies are spatter cones (SC) and lapilli and spatter cones (LSC) (Fig. 2A).

The southern zone is predominantly a lava sheet flow with a ropy surface and west-east oriented ogives (ridges) over several hundred metres. The pyroclastic cones are rare, isolated and generally replaced by craterless facies. At their contact, the orientation of the lava ogives changes, turn around each side of the cone. In addition, in the south-west area, the cone (LSC2 facies) with an elongated shape is a cone with a horseshoe-shaped crater open to the north-east that releases a flow at its base (Fig. 2A).

## 3.3 Interpretation and discussion

The facies architecture illustrates the gradual depletion of water in a wetland area and a river within the Aðaldalur glacial valley, following the continuous input of a voluminous flow of basaltic lava (Fig. 3).

<figure><img src="image_4.png" /><figcaption>Figure 3. The depletion of interactions between water and large basaltic lava flow</figcaption></figure>

Two periods can be distinguished in the development of the lava flow in the study area. In the first phase, the lava's arrival on the river and wetland triggered intense explosive activity, creating a platform of pyroclastic fall (tephra blanket), from which numerous tephra cones (LC) emerge and tend to cover their neighbours. The gradual reduction in the amount of water during explosions led to a reduction in fragmentation, resulting in the formation of a spatter layer at the top of the lapilli cones (LSC). This zone is well preserved in the northern sector north of the road. In a second phase, following dewatering, the continuous flow of lava partially covered this new relief from the south, before being diverted on either side to continue its flow towards the north of the valley. Thus, in the southern sector, the greater frequency of scoriaceous mounds (LM, LSM) on the edges in contact with the flow results from the deformation of the first cones and their mechanical erosion by the continuous flow of basaltic lava, which explains the disappearance of the craters. According to this interpretation, the tephra relief acts as an obstacle that can be deformed by the flow. The latter also induces greater deformation of the basaltic flow surface, materialized by the fragmentation of the solidified crust into blocks and the formation of metre-scale pressure ripples (ogives) (Fig. 3).

Rather than aa facies (Thorarinsson, 1951), the lava exhibits a ropy pahoehoe facies emplaced as a sheet flow with a low enough to allow the formation of a stable upper crust on the surface of the pahoehoe flow but high enough to deform the crust as the lava progresses (Boreham et al. 2018). The preservation of surface ogives makes it possible to distinguish the path of the various lobes that bypassed isolated cones or groups of lapilli cones. The V-shape of the lava lobes surrounding the cones may be typical of an interaction between the relief and a fluid basaltic lava flow (Fig. 3).
4 CONCLUSION

The new classification of rootless volcanic cones and mounds allows us to propose an architecture for the Hagi zone. This is interpreted as a zone of explosive interaction between lava and water saturated sediments. Following the initial strong explosions that produced the lapilli cones (LC) covering the lapilli platform, the water supply to the wetland was progressively reduced, limiting the explosive character to a spatter cover at the top of some cones (LSC) and the formation of spatter cones (SC). The lava flow then flowed around these new lapilli topography, partially covering and deforming it. The results provide new insights into the development of rootless cones and the secondary processes that may have affected them.

In the future, non-circular or asymmetrical morphologies of cones, excluded from this initial classification, could be studied to refine the classification of cones and to complete the distribution of volcanic facies. Other areas in the Aðaldalur valley will have to be studied to complete description and to explain (1) the phenomenon of water-lava interaction, (2) the origine of transverse alignments of cones and (3) the detour of lava by a new obstacle. In addition, the search for Martian analogues could enable us to distinguish a scenario of water or ice source depletion during the setting up of large basaltic flows.

5 ACKNOWLEDGMENT

The authors are grateful to La Fondation Dassault Systèmes for the financing, APEX UniLaSalle Beauvais for the support provided for this work, and Rina Noguchi and Armann Höskuldsson for their help and support, for his review.

6 REFERENCES

Boreham, F., Cashman, K., Rust, A., Höskuldsson, Á., 2018. Linking lava flow morphology, water availability and rootless cone formation on the Younger Laxá Lava, NE Iceland. Journal of Volcanology and Geothermal Research, 364, 1-19.

Fagents, S., Thordarson, T., 2007. Rootless volcanic cones in Iceland and on Mars. The Geology of Mars: Evidence from Earth-Based Analog. Cambridge University Press, pp. 151-177.

Höskuldsson, Á., Dyrh, C., Dolvik, T., 2010. Grænavatnsbruni og Laxárhraun yngra. Fall Meeting JFI 2010, Ágrip erinda Jaðfræðafélag Íslands, 65 bls.

Noguchi, R., Höskuldsson, Á., Kurita, K., 2016. Detailed topographical, distributional, and material analyses of rootless cones in Myvatn, Iceland. Journal of Volcanology and Geothermal Research, 318, 89-102.

Thorarinsson, S., 1951. Laxárgljúfur and Laxárhraun. A Tephrochronological Study. Geografiska Annaler, 1-2, 89 p.