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

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.

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].

#### **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.*

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.

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

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

#### **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.*

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.
