Eruption Scenario Builder Based on the most Recent Fissure-Feed Lava-Producing Eruptions of the Arxan-Chaihe Volcanic Field (ACVF), NE China

Abstract

Fissure eruption is the most prominent type of Pleistocene to Holocene volcanism in Arxan-Chaihe Volcanic Field recording vent migration along fissures. This research is examined Sentinel Satellite Images to outline the youngest lava flows in the region in conjunction with field observations. Also, GIS-based analyses were performed with the aim to calculate the volumes of lava flows to determine the length of the lava flow emissions. Topographic cross sections and various geomorphological parameters (e.g., geomorphon and topographic position index) were used to reconstruct the pre-eruptive geomorphology of the region to simulate lava flow inundation using Q-LAVHA plug in the QGIS package. Pre-eruptive topography was created, and various simulations were used to obtain the best-fit lava inundation. This process yielded to estimate an average of 5 m lava flow thickness. The same parameters of the lava flow simulations were used to run on the post-eruptive topography to simulate future lava flow inundation. Results showed that the lava flows best simulate if they emitted along a NE–SE trending fissure between two young vent zones or in an extensive elongated area following the NW–SE trending valley axis initiated from the Yanshan vents.

1. Introduction

Lava flows in every volcanic field represent the major volcanic events and significant phases of volcanism [1]. The most distinctive characteristics of lava flows are their surface morphology. Such surface “shapes and patterns” not only represent the interests of tourism and scientific exhibitions but also reveal great value for interpretations of how volcanism occurred, potential indicators of petrogenesis, shifting of eruption phases, eruptive magnitudes (volumes and ejection pressures), and geochronology of how long an eruption last. In general, two types of the morphology of lava flows have been described in the literature: ʻaʻā type and pāhoehoe [1]. pāhoehoe lava flows commonly emerge during events of Strombolian style eruptions with low viscosity and high-temperature basaltic eruptions [2, 3]. This type of lava flow is usually known for its smooth surfaces and gentle undulations, with occasional hummocky surfaces and tumuli. In general, pāhoehoe lava flows can extend tens of kilometers in distance from their sources [3]. Small outflows alternatively emerge from the chilled crust of the surface and can feed small “toes”, which are approximately no more than few dm thick, several m long, and dm-to-m wide. Pāhoehoe lava flows are associated with low outflux velocity; this means the volumetric flow rates are about 2–5 m³/s and slow flow front velocities are approximately 1–10 m/hr. [3]. These behaviors of the emplacement are shown by the low flux velocity profile allowing the flows to develop a chilled crust keeping the melt able to move. Thus, the forehead of flows commonly proceeds moving slower than the lateral partitions of the flow body; this indicates not only the flow can maintain undisrupted with smooth and intact surfaces but also preserve “toe” structures while the flows keep moving. Like its counterpart, aa-type lava flows commonly present as extremely rough surfaces with spikes and swarmed gullies [1]. ʻaʻā lava type generally has denser inner parts. The emplacement behavior shows that the flows commonly have a thick (1–2 m) chilled shield surrounding the main lava body. The forehead of rough surfaces with clinkers can be broken while the flow moves [4, 5, 6]. The fallen part will eventually be buried by the bottom of the flows. Topography can affect significantly how the flow moves, and facilitate ponding or cascading effects in steep steps or behind obstacles or valleys. Pāhoehoe lava can also become rubble where chilled crusts can be broken apart and carried rapidly further. In this case, morphotypes could work better to describe the flows (e.g., [7]).

Following the regional tectonic trends, fissure eruptions always generate large volumes of lava, which form the continental flood basaltic lava provinces [8, 9]. Commonly, the lava provinces can stretch into hundreds to thousands of km² areas. Due to the volumes of the lava that ejected, a basaltic flood lava province usually was formed over long time, extreme in a period of 1 to 2 million years [10, 11]. The fissure eruptions generally are related to the continental rift zones, such as East African Great Rift [12]. The stretching or rifting property usually produces large lava volumes These geological settings are usually defined by monogenetic volcanic fields. They are defined by tens to hundreds of small vents formed due to mainly two eruptive styles, such as Strombolian, Hawaiian, and phreatomagmatic types [13]. Those small vents usually swarm and confine in a large region (e.g., thousands of km²). The occurrences of monogenetic volcanic fields are commonly shown by the linear trend of vents, clustering, or randomly distributed all strongly influenced by the crustal structure of the region where they erupted [14, 15]. In old continental crusts inherited structural elements commonly influence the vent distribution of monogenetic volcanic fields and can be controlled by the regional fracture zones, which could subsequently form fissure-aligned volcanic systems [14].

The products of monogenetic volcanism are commonly revealed as scoria cones, cinder cones, tuff cones, tuff rings, lava flows, and pyroclastic materials (formations of ashes and density currents). Those straight-generated and primitive volcanic products can be easily affected by the local environments and climates; in other words, the surface processes (Kereszturi et al. 2011). When the surface processes influenced the young volcanism, it could vastly change the outlines of the topography and landforms [16]. Also, the pre-existed landforms play an important role in forming subsequent volcanic landforms, such as hydrogeology-controlled lowlands and complexities of country rocks.

Modeling for lava flow emplacement is one of the interests of interpretations of lava eruptions in some ways that are beyond human records. Long lava flow commonly indicates flows extending approximately more than 100 km from their sources [17] (Stephenson et al. 1998). In general, long flows can generate rapid and insulated emplacements. The rapid model can expect lava flows exceeding 100 km from sources [17], with less than 0.5°C/km of chilling stages under the transporting velocity of approximately 2–15 m/s. The insulated models prefer flows outfluxing under low velocities [3, 12, 17], for example, 0.1–1.4 m/s. Also, insulated emplacements can be expected the thickness of flows to be no more than 23 m at the maximum, with effusive rates at about 8–7100 m³/s [3]. Slope datasets are the essence of all the above-mentioned assessments. The flows that were emplaced by rapid aspect should be distinguished by the channel-fed structures, for example, lava channels on the surfaces of ʻaʻā type, and expected as generated from short-lived lava fountaining. On the other hand, the insulated aspect can be expected to produce a range of inflated tube-feeding, and sheet pāhoehoe flows within a long distance from their sources/vents. Such flows are also marked as an indicator of a long-lived ponding system [3].

Volcanic eruptions commonly reveal themselves as multi-phases and prolonged event during the syn-eruptive stages. Also, building a range of eruption scenarios can allow researchers to have a better view of the complexities of volcanic eruptions and volcanic uncertainties [18]. In general, the eruption scenarios contain four major aspects: eruption locations, types of eruptive phases, duration of a single phase, and occurrence and frequency of hazards. Eruption locations indicate short-lived and small vents with their distributions and a single large composite volcano, which produces long-lived and complex volcanic events. The transitions between different eruptive types show that the diversities of volcanism are the current and future preceding states of volcanoes. Durations of eruptive phases analyze the potential magnitudes of local volcanism and are considered either discrete or prolonged eruptions. Hazards’ frequencies and occurrence are considered as the influences of erupted gas, blast styles, pyroclastic density current-triggered syn-eruptive disasters, and post-eruptive lahar events.

In NE China, numerous mafic monogenetic volcanic fields formed through the Cenozoic. Among these fields, there are volcanic fields that had historic volcanic eruptions including lava effusions such as those known from Wudalianchi [19, 20] and the Arxan-Chaihe Volcanic Fields [21]. Two young volcanoes located in the southeast part of the Arxan-Chaihe Volcanic Field (ACVF), that is, Yanshan-the “triple vent” and Gaoshan (eastern side of YS), have been dated under the C¹⁴ method, which revealed the ages of those two vents about 1900–1990 cal a BP [21, 22].

The major aim of this research is to focus on providing the best possible eruption model to understand the potential impact of a similar eruption in the future based on the youngest eruptive event in the region that occurred approximately 2000 years ago in ACVF. To constrain this, we employed satellite images that show the surface successions of lava flows as a range of typical indicators of how to build the possible chronicle of the vent onset events and the subsequent ponding processes in the volcanic histories of ACVF. Thus we propose that the youngest eruption in the region took place along fissure-dominated vents. As the ACVF is part of the UNESCO Global Geopark network [23], volcanic hazard needs to be treated seriously and this work provides valuable geology-based information and lava flow simulation to envision the likely eruption scenario the region may face in a future volcanic eruption.

2. Geological settings

ACVF is located on the eastern side of Inner Mongolia, northeast of China (Figure 1a). The Great Xing’an Mountain is the basement holding ACVF’s volcanism as a distinctive geomorphological outline, which vastly distracts and draws the interests of both tourism and commercial behaviors.

Figure 1.

The general morphological aspect of the Arxan-Chaihe volcanic field (a) and its simplified geological architecture (b). Topography is based on SRTM 30 m resolution digital elevation model. Geology information is derived from (Wang et al. 2014). Please note that the young lava flow extent is greatly overestimated. Reconnaissance mapping indicated that many regions shown on this map covered by the youngest lava flows are not accurate and they are rather part of old lava flow fields. Maps are on WGS84 projection using NE China local coordinate system.

The volcanism of ACVF is the most targeted element of local interest, not only its rareness but also the value of research. ACVF is a distinctive volcanic field due to its distance away from Japan Subduction Zone, about 200 km. Such a long spatial span indicates that the general background of ACVF is mostly controlled by the intra-continental settings, which are influenced by a distant convergent plate margin [24]. ACVF is located on the western side of Song’liao Graben, which is an enormous geological and structural subsidence in the center of NE China in consideration of a rifting-zone environment. One of the conventional concepts is that the delamination of supracrustal layers takes place on the weak points so that the rifting processes can generate large subsidence areas, that is, grabens, on the local territories [25]. The subduction zone fueled the delamination processes, which manifested the increasing activities of rifting. The basement of ACVF is approximately formed in the middle to late Mesozoic eras. Previous pieces of research show that the major components of the basement of the Great Xing’an Range are composed of Mesozoic volcanic and granitoid materials. The dwelling elements such as zircon, whole-rock elements, and Hf isotopic composition indicate the properties of the Great Xing’an Range. Geochronology (mostly K-Ar and Ar-Ar techniques) shows that there are at least three stages of the evolutionary histories of the ranges. The felsic volcanic rocks were formed in the Middle-Late Jurassic periods about 174–148 Ma; intermediate and intermediate-felsic volcanic rocks were created in the Early Cretaceous intervals about 142–138 Ma, with no later than 125 Ma; normal felsic volcanic rocks were generated in the Early Cretaceous about 140–120 Ma, with the major volcanic events at 125 Ma. Upwelling mantle movements are the general factor of the compositions of Mesozoic volcanic rocks; this very well corresponds to the methods rifting system of Song’liao Graben [25].

The youngest activities of volcanism at ACVF occurred approximately 2000 years ago. Previous research shows that the lava flows cover the major areas of the Arxan UNESCO Global Geopark with several intervals of the ages. K-Ar method (whole rock) reveals the ages of the bulk lava flows in multiple vent locations of AVCF. Lava flows on both riverbanks of Halaha River erupted at about 0.587 ± 0.18 Ma [21, 26]. Among them, the youngest lava flows were generated from Yanshan’s “triple vent” about 1990 to 2000 years ago. All these flow or bulk volcanic rocks are basalt or trachybasalt. The mafic property makes those flows follow the specific rheology and emplacement mechanisms of forming a range of young landforms of volcanism [21, 26, 27].

Up to now, 47 vents have been recognized by various field trips or through satellite images (Li et al. 2021). The ACVF occupies an area of about 2000 km² in hill country (Figure 1a). Those vents are aligned from southwest to northeast, especially along the study area (Figure 1b). For instance, on the eastern side of Tianchi Lake, two paralleled fissures indicate the propagation processes during the syn-erupted stages (Figure 2a-d). This feature is also found on the eastern side of Dichi Lake (Figure 2a-d). From the satellite images, the two ends of ACVF are marked by Wusulangzi Lake in the southwest corner (Figure 2a-d) and Tongxin Lake in the northeast corner. Between those two lakes, at least 15 vents are aligned through a distinctive orientation, which is SW-NE. Thus, vent distributions of ACVF show that the local tectonic trends, such as fissure orientations, follow a general direction, which is SW-NE directed trend [23, 24].

Figure 2.

Sentinel satellite images of the Arxan – Chaihe volcanic field showing the distinct texture of the young lava flows. a) False color image; b) SWIR image; c) geology – Band 8, 11, 12; d) geology – Band 12, 8, 2. Maps are in WGS84 projection using geographical coordinate systems.

Present-day geomorphology of the region of ACVF is revealed by a range of post-eruptive landscapes with volcanic products. The currently available geological map (Figure 1b) shows an extensive lava-covered region marked as Holocene basalt. Interestingly, in this maps the three volcanic cone complexes, Gaoshan, Yanshan, and Dahei Gou marked as Middle Pleistocene basalts distinctly separated from the young lava flows (Figure 1b). The targeted two vent complexes (Yanshan and Dahei Gou) are scoria cones with welded cores and various clastogenic lava units indicating eruptions where Hawaiian and Strombolian style eruptions alternated over the activity. On the present-day Halaha riverbanks, the youngest lava flows extend over 10 km on average and are suspected to reach about 16 km SW from their source following the paleo-Halaha River valley (Figure 2a-d). The surface structures of the flows represent the two distinctive pāhoehoe and ʻaʻā types. Also, pāhoehoe flows preserve a range of ponded fabrics such as ponded lakes, lave tubes, whalebacks, and tumuli. Lava channels can be observed in a partially collapsed SW side of the Yanshan scoria cones.

3. Methods

Sentinel imagery is a ground-breaking aerospace technology, which was carried out by European Space Agency in 2014 (https://apps. sentinel-hub.com). The missions aim at the goals of agricultural monitoring, emergency management, land cover classification, and water quality. This research applies the satellite images depicted from the Sentinal-2 orbital system. The main reason for this research by utilizing the Sentinel images is that lava flows or products of volcanism can vastly reveal a range of different textures or visual effects of imagery on the satellite photography system compared to the textures of surrounding areas. Yanshan (YS) and Dahei Gou (DHG) are the two vents that generated the youngest morphologies of lava flows covering an area of at least about 70 km². Please note that on the geological map, the young lava flows are marked in an area significantly larger than the real extent of the lava flows suggested by our reconnaissance mapping. The images from the Sentinal-2 system reveal a series of flow textures under different observation methods (Figure 2a-d). Thus, the targeted two vents, that is, YS and DHG, and their eruptive products (mostly lava flows) might be outlined by systematical analyses of remote sensing and GIS methods in relation to the observed lava successions. DEM images downloaded from the ALOS-PALSAR dataset (https://asf.alaska.edu/data-sets/sar-data-sets/alos-palsar/) that offers digital elevation data with a resolution of 12.5 m. The accessed DEMs were reprojected to the WGS 84/UTM zone 51 N map datum and coordinate system. Using QGIS (version 3.26 – Tisler) and its SAGA and GrassGIS we created slope maps, hill shades, and topography position index maps to check the general geomorphological details of the region. Cross-sections were utilized to understand the general trends of the morphology of the study area (Figure 3).

Figure 3.

Slope map of the ACVF showing the characteristic texture of the region with scoria cones and extensive lava flows. Cross sections (lines with numbers on the map) revealed a very gentle sloping landscape upon the lava emplaced. Map is on WGS84 projection using NE China local coordinate system.

The Q-LAVHA plugin of QGIS software can provide a general model or a simulation with analyses of pre-eruptive and post-eruptive topography to interpret lava flow evolution histories and future hazard assessments [28, 29] (Mossoux et al. 2016). In general, the Q-lavHA program is a QGIS plugin that simulates probabilities of ʻaʻā lavas distributions from one or multiple distributed eruptive vents on a DEM satellite image [29]. The inserted models, such as probabilistic and deterministic models, can provide a range of calculations in relation to probabilities for lava flow propagation and terminal length under spatial aspects [28, 29, 30, 31, 32]. The confine of this program is the probabilistic steepest slope on parameters of spatial spreads. The corrective factors are the major algorithm to even the “pits,” which may jeopardize the lava modeling processes by obstacles from DEM images. To overcome this issue, the DEM upon the simulation runs need to go through a prescribed preparation outlined in the plug-in manual. We have completed these steps. Moreover, we created a pre-eruptive topography by removing the lava flows on the current DEM. To do this, we created a contour map based on the ALOS-PALSAR 12.5 m resolution DEM and then we manually modified – in a supervised fashion – the contour lines fitting them to the general morphology unaffected by the youngest lavas (Figure 4a and b).

Figure 4.

Contour map of the pre-eruptive (a) and post-eruptive (b) surface. Regenerated DEM of the pre-eruptive surface (c) was used to simulate lava flow emplacement. Post-eruptive DEM (d) was used to simulate future lava flow inundation. Maps are on WGS84 projection using geographical coordinate system.

Based on the new contour lines, we recreated a new pre-eruptive DEM (Figure 4c) and visually compared it to the current DEM (Figure 4d). To see the validity of the pre-eruptive topography we created slope maps for the pre- and post-eruptive-scenario (Figure 5a and b) as well as aspect maps (Figure 5c and d) to have visual control over the process. We decided to follow this manual process, in spite of it being more time-consuming as during the supervised process we had continuous connection to the landscape and we were able to self-evaluate the scenes based on our own field experiences. In the end, we also created pre- and post-eruptive topographic position index maps just to see how the lava flow removal affects this parameter (Figure 6a and b). Finally, we created a geomorphon map for the present-day situation (Figure 6c).

Figure 5.

Pre-eruptive slope map (a) shows the general trend of the landscape, while post-eruptive slope map demonstrates the rugged flow fields (b). Aspect maps of the pre-eruptive morphology (c) indicate a good trend to the general landscape characteristics. Aspect map of the post-eruptive surface (d) shows the rugged nature of the lava flow field. Maps are on WGS84 projection using geographical coordinate system.

Figure 6.

Pre-eruptive (a) and post-eruptive geomorphon maps show clearly the topographical differences the landscape received through the lava inundation. Codes refer to the following parameters: 1) flat; 2) summit; 3) ridge; 4) shoulder; 5) spur; 6) slope; 7) hollow; 8) footslope; 9) valley; 10) depression. Topographic position index map (c) of the post-eruptive (present-day) landforms show the variety of landforms the region has. Codes are: 1) canyons, deeply incised streams; 2) midslope drainages, shallow valleys; 3) upland drainages, headwaters; 4) U-shaped valleys; 5) plains; 6) open slopes; 7) upper slopes, mesas; 8) local ridges, hills in valleys; 9) midslope ridges, small hills in plains; 10) mountain tops, high ridges. Maps are on WGS84 projection using geographical coordinate system.

For the Q-LAVHA simulation, we tested all the in-built methods and we found the Euclidian simulation performed the best due to the general low slope angle at our lava flows emplaced (an average of less than 1 degree). Within the simulations, we increased the simulation distances from the measured maximum lava flow runout distances. As other researchers used Q-LAVHA on regions of low slope angles (e.g., flat surface) such as Paricutin in Mexico [33], we followed their recommendations to increase the simulation length well over 10 times of the measured lava flow length. In Paricutin, even 50 times longer simulation length has been applied. In our work, we find the simulation distance about 25 times longer than the average lava flow runout distances provided good results, hence in our work we followed this simulation distance (e.g., 250 km for a 10 km long, or 100 km for a 4 km long lava flow).

The terminal length is considered as FLOWGO Model [31] through the definitions of fixed length values, a function of statistical length probabilities, and thermal-rheological properties of open-channel lava flow [7]. While Q-LAVHA offers the FLOWGO simulation, it requires numerous physico-chemical parameters [19]. As we do not have most of those data, we have not explored the FLOWGO to the full extent, instead run some tests with likely parameters drafted from variable literature [34, 35, 36]. As field evidence indicates that the eruption, at least in the Yanshan vent complex reached high intensity in times as evidenced by the presence of abundant clastogenic lava flows, we applied high magma mass flux rates, mafic composition, high-temperature conditions, and wide flow channel parameters.

The lava flow simulation method was performed by Q-LAVHA within QGIS. Three different modeling concepts have been carried out: points, a line, and a polygon [29]. A single point or multiple points are the envisions of vents that probably erupted flows around Yanshan areas. Lines are the simulations of fissure vents. Polygons imply vent swarms. For the simulation processes, the preparation of DEM images is necessary. The resolution of a DEM image used is approximately 12.5 m in pixel sizes. Then, the contour map was treated in a supervised fashion, essentially adjusting the contours where the lava flow clearly acts as an addition to the morphology. Utilizing the GrassGIS plugin of QGIS is the next step to rebuilding a DEM image with a new corrected pre-eruptive contour line. It is also important that all the maps must be projected properly to UTM coordinate with the NE China sections.

In the end, we applied 5 m average lava flow thickness and run the flow simulation on the current topography to see potential lava inundation in case a similar eruption take place in the same vent locations as those created in the 2000 AD lava flow fields. As mentioned above, several methods of Sentinel Imagery have been applied to the areas of YS and DHG.

False-color imagery (Figure 2a) aims at observations of at least one non-visible wavelength to image Earth, which is generally composed of red and green bands in a very popular recognition of images. False-color imagery is most used to assess plant density and healthy conditions since plants reflect near-infrared and green light while they absorb red. Cities and exposed grounds are gray or brown, and water appears blue or black. In this image, the youngest lava flows appear as green color regions with specific flow-banded patterns. Ash is shown up in pink smooth pattern that fills topography lows. From ash, loose alluvial deposits can be difficult to distinguish but their surface pattern is slightly different.

Short wave infrared composite (SWIR) measurements (Figure 2b) can estimate how much water is present in plants and soil as water absorbs SWIR wavelengths in the optical spectrum. Shortwave-infrared bands are also helpful in distinguishing between cloud types (e.g., water clouds vs. ice clouds), snow and ice, all of which appear to be white in visible light. In this classification, vegetation appears in green gradients, soils and infrastructures are in brown gradients, and water appears to be black. Newly burned lands are strongly reflected in SWIR bands, making them visible for mapping fire events. Each rock type differently reveals shortwave infrared light, making it possible to map geology by comparing different colors of reflected SWIR light. Barren lava flow surfaces appear in pink tones reflecting the quick depletion of moisture from porous, dark lava surfaces.

Geology 8, 11, and 12 composite uses both shortwave infrared (SWIR) bands 11 and 12 to differentiate among different rock types (Figure 2c). Each rock and mineral type reflects shortwave infrared light differently, making it possible to map out geology by comparing reflected SWIR light. Near-infrared (NIR) band 8 highlights vegetation, contributing to the differentiation of ground materials. Vegetation in the composite appears red. The composite is useful for differentiating vegetation and land, especially geologic features that can be useful for mining and mineral exploration. In the Arxan region, young lava flows appear bright blue and differ significantly from the background brownish-reddish (mostly dry vegetated) regions (Figure 2c).

Geology 12, 8, and 2 composite use shortwave infrared (SWIR) band 12 to differentiate among different rock types. Each rock and mineral type reflects shortwave infrared light differently, making it possible to map out geology by comparing reflected SWIR light. Near-infrared (NIR) band 8 highlights vegetation, and band 2 detects moisture, both contributing to the differentiation of ground materials. The composite is utilized for finding geological formations and features (e.g., faults and fractures), lithology (e.g., granite and basalt), and mining applications. At Arxan, the image shows clearly the young lava surfaces against the vegetated background (Figure 2d).

4. Results

4.1 Lava flow morphology and the sources of the youngest AD2000 eruption

Field observations have already determined that YS (Figure 7a) and DHG are the two large scoria cone complexes that generated significant morphologies of lava flows.

Figure 7.

a) Yanshan complex scoria cone from the south. Note the steep slope and the wide-open crater toward the SW. b) Extensive ash plain over lava flows indicate that explosive eruption took place after the first effusive stage. C) the broad and deep crater of Yanshan is surrounded by steep crater rim formed by clastogenic lavas.

YS, The Triple Vent, preserves three distinctive scoria cones with visual successions from the observations of satellite images (Figure 2a-d). As Figure 2a-d depict, YS composes of three distinct volcanic edifices overlapping each other with a total area of 2.2 km². Fieldworks indicate that the highest elevation (approximately 1597 m above sea level) is observed on the top of the jointing point between the southeast one and the southwest one (Figure 7b). Scoria caps cover the top of the triple vent. The steepest slope is located on the southern flank of the edifice, which is nearly 47 degrees due to the welded nature of the proximal scoria and spatter beds. A breakage merges on the southwestern cone with outpouring lava flows that form a range of influxes at least 3.77 km long into the broad fluvial valley of the Halaha River. Scoria ash and lapilli formed deposits that blanketed at least 1 m thick units in an area mostly to the east of the Yanshan vents (Figure 7c). The Sentinel images reveal that ash plain extends about 4 km from the Yanshan volcanic complex and traces of valley accumulated ash up to 10 km from the source are likely based on the satellite image pattern. The eastern side of the Yanshan edifice is truncated and a hummocky surface can be traced about a km from the cone flank indicating an early collapse of the vent toward the east. The rafted cone fragments were subsequently covered by the ash plain (Figure 2a,b and d). Lava flows preserve a range of distinctive features of morphologies, such as rubbly pāhoehoe (Figure 8a), rugged surface textures (Figure 8b), lava channels, and tumuli. Similar to YS, DHG is located in the southwest direction of YS, about 5 km.

Figure 8.

a) Typical distal lava flow field along the Halaha River valley, about 8 km from the source. b) Typical proximal lava flow field in the upper basin just east of the Yanshan volcano group.

The total area of the vent is approximately 2 km². In the areas of lava flows influenced by DHG, raft-shaped spatter sections and large slabs of lava that randomly pack in chaotic nature are shreds of evidence of the outpouring of lava from the vent. Inside the crater areas, individual tumuli, ramped-up lava rubble/talus, and large piles of ʻaʻā lava blocks form a range of rough surface topography. Lava flows along the crater margin preserve several meter-long cracks parallel to the crater margin. These zones, as mentioned above, are represented as fractures along the inflated and ponded intra-crater, where ponded lava collapsed upon the partial evacuation of the large crater. DHG is composed of at least three major nested crater systems (Figure 2b and d), which indicate vent migration, crater infill, and sudden releases of lava forming a pit-like crater system. Those extended lava fields and morphologies of the vents have drawn interest in eruption histories and geoconservation purposes [23, 24].

General observations on satellite images such as the Sentinel image sets reveal a complex lava flow emplacement history. Three vents of the YS are clearly overlapping each other. On the basis of the overlap, a relative chronology can be established. The northern edifice formed first followed by a new edifice grown in its southeastern flank. Later on, a third edifice was built on the western margin of the second cone. It is evident that the northern, first cone likely suffered a collapse event and a large part of the edifice was rafted away, forming a hummocky landscape in the eastern regions. It is also evident that the majority of the youngest lava flow is not covered by ash, hence the main ash-producing eruption, inferred to be sub-Plinian (based on the estimated extent of its deposits), was followed by the main lava effusion toward the west and from a small fissure just NE from the first Yanshan cone.

The main lava flow can be distinguished into at least five satellite image patterns not including the valley filling flow segment inferred to be derived from Dahei Gou (Figure 9a). The patterns are very similar in each sentinel image; hence they are likely to be reasoned from some geological feature. The main lava flow has a whirlpool-like pattern (Figure 9b and c), indicating flow movement and interaction with obstacles like tumuli; pressure ridges formed slightly earlier in the same flow field. All the sentinel images show that the main lava flow from YS made into the paleo-valley of the Halaha River. They are not covered by ash; hence they clearly represent the youngest eruptive event in the region. The whirlpool-like pattern on the main flow indicates a higher portion of the lava margin on its N-NE side of the confining valley than on the S-SW side, suggesting that the flow likely made a curved move anticlockwise (Figure 9a). The elevation difference between the two edges of the flow (on profile 2) is about 40 m indicating a slightly westward inclined surface on what the lava emplaced (over 3660 m, about 40 m elevation difference yield to a slope of no more than 1 degrees, a very flat landscape). A low slope angle means that the lava emplaced in a very gentle sloping landscape, so no wonder it generated some ponding once entered the main flow channels of the paleo-Halaha River. On the NE-SW sections of the main YS lava flow, the flow thickness is estimated to be less than 10 m in proximal areas but in the flow edges the flow became thin, often only around 1–2 m thick. On the basis of the geological observation, a 5 m average flow thickness is a realistic estimate. In the thickest part of the lava flow fields and some ponded sections, the lava thickness may reach 20 m in localized sections. Topography profile from YS and to the far end of the lava field in the SE, over 16,000 m distances, 225 m drop has been recorded, yielding a less than 1° slope again. A low slope angle also indicates relatively thin lava coverage, especially in distal regions which the direct observations confirmed. DHG longitudinal topography profile across the lava flow emitted from DHG also shows a clear drop of 94 m over 6638 m distance, yielding less than a 1° slope. This section forms the main DHG flow part. The satellite image pattern shows the textures of flows from DHG smoother than the ones of Yanshan flows, indicating that the main DHG flow might predate YS. About 1186 m above sea level, a clear 2–4 m drop, and a topography gap was recognized that separated a younger satellite image pattern suggesting that a young flow probably YS origin formed the axis of the river valley fill flows.

Figure 9.

a) Distinct lava flow regions of the youngest lava flow of Arxan. b) Whirlpool-like lava flow pattern on false color (c) and geology band 8, 11, 12 sentinel satellite images. Maps are on WGS84 projection using geographical coordinate system.

Flow thickness is estimated to be similar to YS and fixed to an average of 5 m. DHG topography profile perpendicular to the main flow axis indicates about 7 m higher lava surfaces on the N-NE side of the flow channel, suggesting that the flow slightly climbed in the southern valley margin, just as expected by the movement of the flow from DHG. YS lava flow initiated about 125 m higher than DHG (1381 m versus 1256 m). Judging that both flows emplaced on flat areas with less than 1° slopes, it is expected that flows, if similar effusion rates are expected, will go further distances rather than go higher elevations (16,000 m vs. 6638 m, YS vs. DHG, respectively). Following the above-mentioned logical steps, it can be assumed that at low lands, some mixing and interflow of lava flows take place, what we recognized as different satellite image textures. From field investigations, we can see that young lava flows are covered by an ash plain of about 2–4 m thick scoriaceous ash in the SE of YS. Light color patterns on satellite images indicate that the ash covers are extensive in the SE of YS and likely reach over 10 km from their source. Field mapping confirms that beneath the ash cover, young lava flows to fill the valley in the SE of YS with a new channel of lava flow probably not thicker than 5 m. These lava flows formed before the ashfall. Satellite image textures indicate that lavas likely erupted and formed after the main ash fall events toward the NW, feeding main YS flows to the Halaha River valley. Thin ash coverages were recorded in NW of YS, in elevated regions and beneath some proximal flows. Satellite image textures indicate that young lava flow erupted from a fissure and filled the valley between YS and Gaoshan and some local lows SE from YS. As Gaoshan is fully covered by ash, following the points mentioned above, it can be stated that the fissure formed after the main ash fall event. In summary, Sentinel images reveal different textures from various methods of waveband observations. Those textures might be the indicators of lava successions and possible eruption histories of major territories of ACVF.

5. Discussion

5.1 Eruption scenarios and possible magnitudes of the events

From the previous research, YS and DHG are two volcanoes formed by effusive or even violent Strombolian eruptions. Successions of lava flow distributed in surrounding areas of these two vents indicate a range of typical histories of syn-eruptive stages and volcanic edifice constructions.

DHG vent opened first in the youngest eruptive events and formed a scoria cone in the NE, gradually shifting fissures toward the SW. Lava outpour from the SW vent of DHG. At least three closely spaced vent/volcano formed, most of them still preserving young volcano morphology. Satellite image textures indicate that some lava flows spill over the SW vent of DHG and feed the main DHG flow. It cannot be ruled out that the flow was not fed from some western flank fissure in the SW vent of DHG, but the satellite image pattern is more indicative of spillover. The dense vegetation cover over the main DHG flow indicates some soil formation over the lava flow surface. Some ash recorded on top of DHG flow indicates that subsequent, probably early Yanshan-sourced ash covered the main flow and the mixed flows in the far valley. The main SW crater is a 1 km wide and about 50 m steep pit crater filled with fresh lava that shows similar satellite image textures to the youngest lava flows and no sign of ash cover. This indicates that this fresh lava must have been emplaced after the main ash falls from YS.

Shortly after the emplacement of the DHG main flow, the complex YS vent system formed. While earlier Gaoshan was assigned to be part of the Yanshan vent system, based on the satellite image pattern and the general morphological architecture of the edifice, Gaoshan is clearly the oldest landform of the YS region as it has erosional gullies typical for older landforms in the region. It is also completely covered by ash inferred to come from YS.

The YS first vent y1 (Figure 2) is the northern edifice, a scoria cone with relatively fine-grained deposits. It is not easy to establish if this vent sourced any lava or not. It seems that its SE side might have suffered some collapse as some hummocky surface was observed in the SE of this edifice about a km from its rim, which is in the right position to have some rafted cone there. In addition, in the same scar region, the second set of vents formed y2. This volcanic edifice reached about 100 m elevation and formed a steep scoria cone. The deepest point of the funnel-shaped crater is only about 25 m above the lava fields in the east. This cone must have been active for a long time to build such a substantial size of cone. The cone also evolved in at least two phases as magma withdrawal must have created a crater. After rejuvenation, it formed an intra-crater cone that nearly filled the original crater zone but never grew out of it. This eruption phase was also purely explosive, but its deposits were likely to form localized deposit piles within the major crater and probably the outer flank. This complex explosive activity produced ash plumes that deposited ash that covered earlier lava flows. Such flows are probably emitted due to magma withdrawal during the main cone growth phase. The eruption subsequently built the third volcano just SW from the y2, defined as y3. The y3 built a scoria cone as well, forming an attached cone nearly as high as y2. Gradual SW-ward shift of the activity gradually built another edifice that subsequently changed its activity dominated by Hawaiian-style lava fountaining and building a complex spatter system. This side of the volcanic complex probably suffered some collapse and rafting, letting the magma find its way out toward the west feeding the main lava flow of YS, reaching the Halaha River valley about 16 km away from the emission point.

At the time of the main lava emission, explosive activity was ceased or limited only to small lava fountains and/or localized ash emission toward the east, as the young lava flow surface has no ash cover. Probably at the same time (not really possible to establish relative chronology), a small fissure opened between YS and Gaoshan and emitted a flow that filled the depression just east of the YS system and between Gaoshan and YS. This event also postdates the ash fall event and is likely the youngest phase of the eruption. Interestingly, the Sentinel image textures in the DHG crater also exhibit very young lava morphology, raising the question that DHG probably had experienced an intermittent lava effusion phase that partially refilled the crater. Thus, a complex fissure is an aligned eruption sequence that puts DHG and YS on the same time horizon. Gaoshan is likely part of an older phase of eruptions. Thus, it can be assumed that this eruption was really an extensive event that occurred in a structural alignment (fissure or fault?) about 15 km in length.

The calculations of the estimated volumes of lava pouring can reveal how long the eruption events last. The lava flows are unlikely to form more than 20 m in thickness in ponded regions on valley floors. The estimated volumes of lava emplacements are calculated by a range of standards, such as the eruption types of other volcanic fields (e.g., Mt. Etna in 2001 and Hawaii in 1985). So far, the field identifications and observations, or even classifications, have already yielded an outcome that YS and DHG were formed by a series of violent lava fountaining or effusive eruption events. Also, the steps on the field are indicated that the slope of lava fields is relatively flattened. Thus, the simulations carried out on YS and DHG from the volcanoes with this similar eruptive style can be considered to be valid assumptions of eruption scenarios. In Table 1, the rates of lava emplacements are based on different scenarios from varieties of volcanic fields in the world.

Lava flow	Surface area [m2]	Average estimated thickness (m)	Volume (km^3)	Estimated emplacement time – Negros de Aras (Chile) (m^3/s): 14	Estimated emplacement time Negros de Aras (Chile) (m^3/s): 113	Estimated emplacement time Nyíamuragira 2006 (m^3/s): 145	Estimated emplacement time Mt. Etna 2001 (m^3/s): 30	Estimated emplacement time Lentiscal (m^3/s): 100	Estimated emplacement time Hawaii 1985 (m^3/s): 2
Small YS (y2)	2,845,217	3	0.00853565	7.056589782	0.874267761	0.68132591	3.293075231	0.987922569	49.39612847
Thin mixed lava (yd mix)	12,014,075	4	0.0480563	78.82095929	4.922187398	3.835911558	18.5402392	5.562071759	278.103588
Thin YS (y-d)	7,363,514	4	0.02945406	4.325035232	3.016844477	2.35105811	11.36344753	3.409034259	170.451713
Ash covered YS (y1)	8,537,547	3	0.02561264	68.54108424	2.623385878	2.044431753	9.881420139	2.964426042	148.2213021
Major DHG (d)	10,923,970	8	0.08739176	14.7572615	8.951138971	6.975715198	33.71595679	10.11478704	505.7393519
Mayor YS (y)	37,086,705	8	0.29669364	232.6958946	30.38897493	23.68244253	114.4651389	34.33954167	1716.977083
Total	78,771,028		0.49574405	406.1968246	50.77679941	39.57088506	191.2592778	57.37778333	2868.889167
				In realistic		In min	In realistic		In max

Table 1.

Estimated eruption scenarios of YS and DHG that were calculated by different standards from similar volcanic fields. Color codes are referred to the information outlined in the main text.

From Table 1, it can be assumed probably about 14–30 m³/s interval of the realistic one (green color). These two values mean that the eruption periods lasted about half a year to one and a half years with the continuous development of the entire flow field. However, considering the distinctive flow fields, it can be inferred that major effusive phases, either explosive phases or quiet time, have taken place. Overall, the possible assumption can be estimated that a similar eruption probably took a few years with distinctive explosive phases and separate lava effusion stages from vents along the main structural zones.

The 1983 eruption on Hawai’i was fed by effusion rates of up to 22–44 m³/s, and flows extended 7 km² to form a 6 km², 100 × 10⁶ m³ flow field (Trusdell 1995). In contrast, the 1985 eruption (also in Hawai’i) was fed by effusion rates of 0.5–4.5 m³/s, which resulted in flows extending 1.8 km to form a 2.2 km², 19 × 10⁶ m³ flow field (Harris et al. 1997). Effusion rate also appears to control the basic flow dynamics. In Hawaii, effusion rates determine the manner in which flows are emplaced. Effusion rates at 120 m³/s produce rapidly advancing channelized ʻaʻā lava flows, and effusion rates of approximately 20 m³/s (but typically more than 5 m³/s) produce slowly advancing tube-fed pāhoehoe flows (Rowland and Walker 1990). The lava flows at ACVF are somewhere between. In the upper flow regime, they are more like aa-type of lava flows with lots of slabs and rubbly pahoehoe; once they reach the valley floor, they slow down, inflate, and make whaleback features. Considering that the region is very flat, it can be imagined that the flow had to go at a reasonable speed (higher effusion rate) to retain heat to make the lava able to advance. In the end, the flow advanced over 16 km from its source, and even in DHG, the flow reached nearly 7 km. This is a large number and requires a relatively fast-moving flow.

5.2 Lava flow simulation

Sentinel images in different observation methods show the different textures of lava flows around YS and DHG. Those textures indicate the different lava batches, which were systematically emplacing and overlapping each other. As mentioned above, the flow thickness is estimated to be about 5 m on average. While flows accumulated on a very flat surface (less than 1 degree), the program needs a substantial L value of simulation distance to put in as the measured lava runout distance [33]. If given a specific runout distance of the flows, for example, 10 km, the L value would be 10 km. Thus, if given 25 times of runout distance, the L value is actually 250 km. Eventually, the best simulation outcome is 250 km for the lava flow runout distance as the modeling pattern successfully covers the estimated flow areas (i.e., from YS). Field works have already proven that the slope of the flow areas is low, which might be no more than 1°. Such large volumes of flow can only be pushed on the flattened surface by a high effusion rate. Also, the low value of flow thickness with such an effusion rate that leads to large coverage areas of lava flows is approximately 5 m. Eventually, 5 m thickness of the flow pluses and 10 m of the given buffer thickness can help the modeling process switch on the quadrant, and the “16-point” aid makes sure that the simulation does not stop on flat surfaces [29]. The best modeling pattern was created by Euclidean Length, which is that each iteration stops when the flowline reaches the specified Euclidean Length (m). The Euclidean represents the crow-fly distance between the point where the simulation starts and the front of the flow line. This calculation way can let flow patterns freely distribute in confined areas. From the above-mentioned modeling processes, geological implications can be:

1. The flow thickness is probably the best to fix at 5 m, knowing that this might be higher up to 10 in proximal areas or far less in the far end of the flow lobes. This envision is very well corresponded with geological observations in two seasonal field works.

2. The lava flows probably cannot be in the 3 m range as they are too thin, and the model stops too early, whatever parameters that are fitted in the program.

3. The flows also cannot be more than 10 m range as that creates edifice-like flow patterns with far less lava flow runoff.

Especially applying another single point marking as the vent location shows the model can reach the far north and northeast boundaries of the flow. This newly assumed emission point was put on the territory of a whirlpool-like feature that was apparent in satellite images (Figure 9b and c), which is suspected to be a buried vent beneath the lava flows.

In addition, this location also falls in the zone where another SW-NE fissure may exist lying on the old Tianchi Lake fissures. The points simulations likely provide the flow from YS in the flow inundation areas (Figure 10a-d).

Figure 10.

Lava flow simulations from four different point sources; a) from y1; b) from y2; c) from y3; d) from y4. Maps are on WGS84 projection using geographical coordinate system.

However, the problem is that this simulation cannot simulate the flow flowing through the Halaha River channel, which has a lower elevation than YS’s. In order to solve this issue, one possibility is that those fields had been inundated by flows from DHG earlier.

The line simulation yields a distinctive outcome that may indicate YS and DHG were lying on the same fissure (Figure 11a and b). If the program is carried out YS and DHG within a 1 km wide, 7 km long fissure line, also let the program select random vents along this assumed line with an average distance of about 500 m spacing. In this way, almost the entire lava flow area is covered by the simulated pattern. Thus, the line simulation may imply that the Triple Vent and DHG probably erupted at the same time along a fissure about 1 km wide and 7 km long. Furthermore, flow thickness is no more than 5 m on average.

Figure 11.

Lava flow simulation through a fissure between Dahei Gou and Yanshan (a) or a rectangle shape area of vents between Dahei Gou and Yanshan (b). Fissure eruption along an NW-SE axis valley centerline (c) and along a rectangle shape area (d) near Yanshan. Maps are on WGS84 projection using geographical coordinate system.

In addition, we explored the simulation if we envision a fissure from Yanshan toward the Halaha River valley and simulating a fissure eruption event (Figure 11c) and treating the region as a potential vent zone within a rectangular region (Figure 11d). Both modeling was able to reproduce the upper flow fields of Yanshan. The vent area model however generated a potential scenario that lava may have overspilled from the Yanshan valley to Dahei Gou which is a unique but apparently not impossible scenario (Figure 11d).

On the basis of the simulations, we fixed the lava flow thickness at 5 m and applied the same parameters we used on the current DEM to test how a lava flow would behave if future eruptions would take place from the same vent. This is an unlikely situation within monogenetic volcanic fields but not unknown. In addition, the two main vent complexes clearly show geological evidence that they are amalgamated complex edifices where subsequent eruptions took place at least in the vicinity of the previous vents.

The four-vent simulation on the current topography created a lava flow field nearly completely covered the lava fields in the Yanshan and upper Halaha River Valley (Figure 12). It is clear that such eruptions would produce enough lava flow to disrupt the two main roads crossing the region.

Figure 12.

Set of lava flow simulations run over the current topography (post-eruptive) simulating lava effusion from the four vents along the NW-SE trending valley near Yanshan. Shaded relief map (a) with roading, post-eruptive DEM with roading (b), and GoogleEarth satellite imagery with roading showing the potential inundation if effusive eruptions would take place from y1, y2, y3, and y4 vents in this time sequence.

5.3 Implications for the geopark

Applying the simulation to a theoretical fissure opening between Dahei Gou and Yanshan produced very extensive lava flow fields that clearly would be a devastating event for the operation of the geopark (Figure 13).

Figure 13.

Simulating lava effusion along a fissure running between Dahei Gou and Yanshan on a post-eruptive DEM (a) and shaded relief (b) maps showing an extensive lava inundation that would fill the two parallel NE-SW valley within Dahei Gou and the Yanshan group situated. Maps are on WGS84 projection using geographical coordinate system.

Surprisingly, if we envision a vent swarm within a rectangle area in the Yanshan valley, it is likely that lava will enter the Dahei Gou valley and be able to produce extensive flow inundation, posing a substantial volcanic hazard for the geopark (Figure 14).

Figure 14.

Lava flow inundation simulation applying rectangle shape vent zones along the NE-SW axis valley within the Yanshan group sits. On the digital elevation model (a) it is clearly visible that lava flows can reach the Dahei Gou valley and the maximum run-out distance of the flow can reach the broad alluvial valley near to the local tourism center of Tianchi township. On the shaded relief, (b) and Google earth satellite image (c) illustrate well the potential extent of the lava flows and its impact on the infrastructure. Maps are on WGS84 projection using geographical coordinate system.

ACVF is located in a territory of a UNESCO Global Geopark, which was established in 2016 [23]. The annual tourism visitation rate is high, especially in high season and Chinese holidays, hence the volcanic risk is evident. The infrastructures of this geopark are mostly well constructed but lava flow inundation simulations showed they are beyond the potential destruction zones. The current lack of volcanic hazard management in consideration can be a critical issue not only for wealth engagements but also for the safety of local people.

As mentioned above, ACVF is still an active volcanic field; YS and DHG are the two major vents in magnificent scales (e.g., volumes and areas). DHG and YS are observed from the Google satellite images and located from the main Tianchi Town, about 12.6 km and 17.3 km, respectively. From the observations of slope maps, the town is on the west side of these two vents, and elevations on the western side of ACVF are generally lower than the elevations on its east. Satellite images show that lava flows are the basement of constructions in relation to the town. The Halaha River cuts through from the western end of ACVF and then flows to the south, which is also the southern side of the town. Assessments and evaluations from Table 1 are sufficient indicators that a possible effusion rate of lava flows might be a significant parameter of local risk management. Under the relatively high effusion rates and considered flattened surfaces of local territories, basaltic flows always influenced tremendous areas surrounding the vents. The total area of flows in targeted destinations of ACVF is approximately 90 km². In comparison to the Hawaii eruptions in 2018, the flow areas generated from YS and DHG are less than the mentioned one, which was about 144 km². Another thing is that a 90 km² area of lava flows has already succeeded the total area of Wudalianchi flows, which is approximately 65 km² [37, 38]; this could mean that the potential hazards from ACVF are needed to pay attention to most aspects of safety prospects. Local geomorphology shows that YS and DHG were formed in intra-mountainous settings. Valleys are the confines for the lava flowing. The low viscosity properties of basaltic lavas make the hazard areas even more dangerous than other areas with open topography; specifically, the town was built in the central part of the valley bottoms. ACVF is a national geopark in an active volcanic zone; this is very different from other volcanic geoparks that are commonly far away from the major vents. The low population in the region prevents generating primary interest in volcanic hazards within the community. In addition, people have low information and understanding of volcanic hazards hence the Arxan UNESCO Global Geopark could be an excellent avenue to pass knowledge on the volcanic hazard to the local communities and visitors.

6. Conclusion

This preliminary research about lava flows erupted from YS and DHG tries to bring a new insightful result of lava evolutionary histories and subsequent hazard evaluations. Sentinel imagery plays an essential role in this research. Calculations for the lava volumes and effusion rates are the major outcomes of this research for the first time providing geologically validated and modeled lava flow eruptive volumes for ACVF. GIS-based techniques provided new information on the nature and extent of lava flow inundation. Applying STRM, ALOS-PALSAR Digital Elevation Models and applying GIS techniques to analyze the morphological assets of the ACVF provided a complex framework to simulate lava flow inundation. Lava flow simulations in concert with direct field observations revealed that future lava flow effusion would generate significant lava flow infill along the NE-SW trending valley within Dahei Gou and Yanshan volcanoes sit. As these lava flow likely would reach the main transport routes of the region, complex volcanic hazard studies and probabilistic estimates are needed to mitigate future volcanic hazards. Future research is needed to concentrate on the chronologies of lava successions, and predictions of future eruptions along with the local fissures which formed the lava flows in ACVF.