**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.

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
