**9. Evidence of violent Strombolian style eruptions**

Dry-eruption types of volcanism are the most common type of explosive volcanism in the ACVF. Large cones along the NE-SW trending ridge tops form a chain of cones that are vegetated and difficult to access. Most of these cones preserved edifice-building sections where ash, lapilli and bomb beds suggest normal Strombolian style volcanism. In the youngest known vent complex, the Gaoshan (High Hill) and Yanshan (Hill of Fire)-Triple vent large amounts of scoriaceous deposits form the cones and also spread across the landscape to form an extensive "ash plain" (**Figure 14**). The location of Gaoshan and Yanshan on the southeastern boundary of ACVF, is the location of several scoria cones representing the eruptive products of Strombolian style volcanism. In this section, Yanshan-Triple Vent form a more closely spaced vent system than those at Gaoshan inferred on the basis of the special distribution pattern of the volcanic products and the presence of multiple vents within one volcanic massif. The origins of Gaoshan are still being investigated.

Yanshan - Triple Vent, is located in the central part of ACVF (**Figure 2**). The highest peak in Yanshan is 1597 m above sea level. The total area of this "dry" hill is approximately 2 km2 , with a perimeter of nearly 5.3 km. On every side of the hill, the burnt forests cover the slopes of the hill; this was a fire hazard at the end of 1987, i.e. Black Dragon fire.

As **Figure 14** shows, the lava flows cover most of the area surrounding Yanshan. The slope on the right side of the cone complex reaches 44° due to the welded nature of the proximal scoria beds. Slope angles are lower in the eastern side of the cone complex where a breached section of the cone formed the lava outpouring point from where a lava flow at least 3.77 km long flowed into the Halaha River valley. Some 5 km from the source, scoriaceous ash and lapilli formed at least 1 m thick units suggesting the youngest eruption products were dispersed widely. This dispersal pattern is suggestive of a violent Strombolian, or sub-Plinian stage of eruption during growth of the edifice complex.

Outlines of the ridge crests of the three vents can be pointed out from various observation points. Vent 1 is a flattened, shallow scoria ring structure (**Figure 14A** and **B**). In the crater rim crest, a scoria deposit can be reached beneath

### **Figure 14.**

*The satellite view of Yanshan-triple vent. White lines mark the outlines of the three vents; red lines mark the outlines of the lava flow (LF). Yellow arrowheads show viewpoints from where Figure 15A–C was taken.*

**239**

**Figure 16.**

*intermittent more violent Strombolian phases (D).*

**Figure 15.**

*Basic Volcanic Elements of the Arxan-Chaihe Volcanic Field, Inner Mongolia, NE China*

*The detailed the field observations of Yanshan-triple vent. A view from the south shows a complex cone and an open crater toward the west. Rubbly pahoehoe and tumuli around the cone form a rugged surface morphology (A). The surrounding area is covered by thick ash and lapilli blanket of black, and reddish scoria (B). At least three vents can be identified forming the Yanshan-triple vent as it can be seen from Gaoshan peak (C). The ash plain trench just about 2 km east of the vents shows multiple tephra layers indicating repeated and sustained eruptions (D).*

*Yanshan - triple vent field details. A photo that was taken from the western rim of the vent 1 showing the overgrow of the vent 2 and 3 (A). The crater floor of vent 1 is filled with lava flow creating a rugged grasscovered morphology (B). Vent 3 is the source of the main lava flows outpoured from a breached section of the elongated (fissure-aligned) crater (C). The inner crater wall of the vent 3 exposes a set of agglomerates, welded scoria, spatter and localized clastogenic lava flows, all indicative for lava fountain dominated eruptions and* 

*DOI: http://dx.doi.org/10.5772/intechopen.94134*

*Basic Volcanic Elements of the Arxan-Chaihe Volcanic Field, Inner Mongolia, NE China DOI: http://dx.doi.org/10.5772/intechopen.94134*

### **Figure 15.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**9. Evidence of violent Strombolian style eruptions**

Dry-eruption types of volcanism are the most common type of explosive volcanism in the ACVF. Large cones along the NE-SW trending ridge tops form a chain of cones that are vegetated and difficult to access. Most of these cones preserved edifice-building sections where ash, lapilli and bomb beds suggest normal Strombolian style volcanism. In the youngest known vent complex, the Gaoshan (High Hill) and Yanshan (Hill of Fire)-Triple vent large amounts of scoriaceous deposits form the cones and also spread across the landscape to form an extensive "ash plain" (**Figure 14**). The location of Gaoshan and Yanshan on the southeastern boundary of ACVF, is the location of several scoria cones representing the eruptive products of Strombolian style volcanism. In this section, Yanshan-Triple Vent form a more closely spaced vent system than those at Gaoshan inferred on the basis of the special distribution pattern of the volcanic products and the presence of multiple vents within one volcanic massif. The origins of Gaoshan are still being

Yanshan - Triple Vent, is located in the central part of ACVF (**Figure 2**). The highest peak in Yanshan is 1597 m above sea level. The total area of this "dry" hill is

the burnt forests cover the slopes of the hill; this was a fire hazard at the end of 1987,

As **Figure 14** shows, the lava flows cover most of the area surrounding Yanshan.

The slope on the right side of the cone complex reaches 44° due to the welded nature of the proximal scoria beds. Slope angles are lower in the eastern side of the cone complex where a breached section of the cone formed the lava outpouring point from where a lava flow at least 3.77 km long flowed into the Halaha River valley. Some 5 km from the source, scoriaceous ash and lapilli formed at least 1 m thick units suggesting the youngest eruption products were dispersed widely. This dispersal pattern is suggestive of a violent Strombolian, or sub-Plinian stage of

Outlines of the ridge crests of the three vents can be pointed out from various observation points. Vent 1 is a flattened, shallow scoria ring structure (**Figure 14A** and **B**). In the crater rim crest, a scoria deposit can be reached beneath

*The satellite view of Yanshan-triple vent. White lines mark the outlines of the three vents; red lines mark the outlines of the lava flow (LF). Yellow arrowheads show viewpoints from where Figure 15A–C was taken.*

, with a perimeter of nearly 5.3 km. On every side of the hill,

**238**

**Figure 14.**

investigated.

approximately 2 km2

i.e. Black Dragon fire.

eruption during growth of the edifice complex.

*The detailed the field observations of Yanshan-triple vent. A view from the south shows a complex cone and an open crater toward the west. Rubbly pahoehoe and tumuli around the cone form a rugged surface morphology (A). The surrounding area is covered by thick ash and lapilli blanket of black, and reddish scoria (B). At least three vents can be identified forming the Yanshan-triple vent as it can be seen from Gaoshan peak (C). The ash plain trench just about 2 km east of the vents shows multiple tephra layers indicating repeated and sustained eruptions (D).*

### **Figure 16.**

*Yanshan - triple vent field details. A photo that was taken from the western rim of the vent 1 showing the overgrow of the vent 2 and 3 (A). The crater floor of vent 1 is filled with lava flow creating a rugged grasscovered morphology (B). Vent 3 is the source of the main lava flows outpoured from a breached section of the elongated (fissure-aligned) crater (C). The inner crater wall of the vent 3 exposes a set of agglomerates, welded scoria, spatter and localized clastogenic lava flows, all indicative for lava fountain dominated eruptions and intermittent more violent Strombolian phases (D).*

a c. 50 cm thick soil. On vent 3, a clear crater wall collapse, and rafting from where the main lava flow escaped the crater are recorded (**Figure 16C**). The lava ponded in the crater and flowed out of the ring structures forming an outlet. The width of the breakage is narrow (**Figure 16C**), not more than about 250 m, which means the flux of the lava flows may reach a threshold value triggering the collapse of the edifice sector. The crater rim of the main crater composed of agglutinate, clastogenic lava and densely welded scoria beds (**Figure 16D**).
