**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.

## *Eruption Scenario Builder Based on the most Recent Fissure-Feed Lava-Producing Eruptions… DOI: http://dx.doi.org/10.5772/intechopen.109908*

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.

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


*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.*

**88**

*Eruption Scenario Builder Based on the most Recent Fissure-Feed Lava-Producing Eruptions… DOI: http://dx.doi.org/10.5772/intechopen.109908*

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.


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**).

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,

#### **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.*

*Eruption Scenario Builder Based on the most Recent Fissure-Feed Lava-Producing Eruptions… DOI: http://dx.doi.org/10.5772/intechopen.109908*

**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.*

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.

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.

### **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 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.*

*Eruption Scenario Builder Based on the most Recent Fissure-Feed Lava-Producing Eruptions… DOI: http://dx.doi.org/10.5772/intechopen.109908*

#### **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**).

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.

#### **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.*

## *Eruption Scenario Builder Based on the most Recent Fissure-Feed Lava-Producing Eruptions… DOI: http://dx.doi.org/10.5772/intechopen.109908*

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.
