**2. Geological setting**

ACVF is located about 2000 km away from the Japan subduction zone (**Figure 1**). Thus, the background settings are generally under controls by intra-continental settings influenced by a distant convergent plate margin.

From the satellite map, ACVF is located toward the southeast end of Great Xing'an Range (also known as Da Hinggan Mountains; both names are legitimate, but this paper utilizes the name as Great Xing'an Range). The highest ridges in ACVF are nearly 1500 m above sea level. The ACVF is located near the current political boundaries between China, Russia and Mongolia (**Figure 1**). The regional fault systems align NE–SW with the vents of ACVF, suggesting that the volcanic field is associated with major supracrustal weakness zones or even be linked to rifting and/or reactivation of Mesozoic structural zones separating major tectonic terrains (**Figures 1** and **2**). The rifting history of the region occurred in two major phases [12] around the Songliao Graben (**Figure 1**). It is believed that Songliao Graben subsidence is related delamination of the lithosphere and thinning of the upper crust of NE. China [12–15]. One of the conventional concepts is that the ascending asthenosphere caused by subduction of the down-going slab into the upper mantle initiated and sustained the delamination processes that were manifest as rifting in NE. China [1]. ACVF is located on the west flank of Songliao Graben (**Figure 1**). Hence the regional tectonic setting, characterized by the fault-controlled features, resulted fissures and NE-SW orientation and alignment of vents of (**Figure 2**). These are correlated with macro scale the tectonics of NE

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

**Figure 3.**

*entered to various publications.*

*A GoogleEarth map showing the volcanic locations from where basic volcanological information is available. The names and abbreviations also correspond to the common name the locations* 

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

**Figure 3.**

*A GoogleEarth map showing the volcanic locations from where basic volcanological information is available. The names and abbreviations also correspond to the common name the locations entered to various publications.*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

origin that covered an area of at least 13 km2

builds spatter cones and small scoria cones (**Figure 2**).

influenced by a distant convergent plate margin.

youngest.

**2. Geological setting**

provide evidence of the important role of explosive hydrovolcanism, fissure-fed eruptions, Strombolian-style eruptions and various large-volume lava outpouring to the volcanic architecture of the field. We demonstrate that the Tongxin Lake near Chaihe township (**Figure 2**) is a tuff ring of explosive phreatomagmatic

Volcano were probably the most violent in the ACVF indicating the importance of externally-driven explosive hydrovolcanism during the eruptive history of ACVF. In contrast, the youngest known eruption site at Yanshan (C14–2040 +/−75; 1960+/−70; 1990+/−100; 1900+/−70, BP) [11] is a nested scoria and spatter cone with three distinct vents forming a volcanic complex producing scoria-fall, agglutinated and clastogenic lavas. This location demonstrates probably the most common type of volcanic eruption in the ACVF. They are also the most voluminous and the

The region at and nearby Dichi Lake vent produced the smallest eruptive volumes in ACVF (**Figure 2**). The crater wall of Dichi Lake is composed of lava flows. These were disrupted by an explosive event that left produced angular breccias as a pyroclast ring around the now water-filled crater, best defined as a maar volcano. From Dichi Lake, however, a NE–SW trending fissure exposing a chain of vents gradually

The majority of the volcanoes in the western side of the ACVF, closer to Arxan are clearly volcanoes that erupted through magmatic explosion and effusive processes and formed lava spatter cones, spatter ramparts, scoria cones and associated lava flows. Large, elongate craters in this region is filled by water, for example Tuofengling (**Figure 2**). Tuofengling was first interpreted as a scoria cone, but recent field mapping has revealed it is a complex volcanic cone with a basal tuff ring capped by a scoria cone complex. The field-based data presented here provide evidence of one of the largest Pleistocene phreatomagmatic explosive eruptions in the ACVF that formed a maar within a closed intramountain basin (**Figure 2**).

ACVF is located about 2000 km away from the Japan subduction zone (**Figure 1**). Thus, the background settings are generally under controls by intra-continental settings

From the satellite map, ACVF is located toward the southeast end of Great Xing'an Range (also known as Da Hinggan Mountains; both names are legitimate, but this paper utilizes the name as Great Xing'an Range). The highest ridges in ACVF are nearly 1500 m above sea level. The ACVF is located near the current political boundaries between China, Russia and Mongolia (**Figure 1**). The regional fault systems align NE–SW with the vents of ACVF, suggesting that the volcanic field is associated with major supracrustal weakness zones or even be linked to rifting and/or reactivation of Mesozoic structural zones separating major tectonic terrains (**Figures 1** and **2**). The rifting history of the region occurred in two major phases [12] around the Songliao Graben (**Figure 1**). It is believed that Songliao Graben subsidence is related delamination of the lithosphere and thinning of the upper crust of NE. China [12–15]. One of the conventional concepts is that the ascending asthenosphere caused by subduction of the down-going slab into the upper mantle initiated and sustained the delamination processes that were manifest as rifting in NE. China [1]. ACVF is located on the west flank of Songliao Graben (**Figure 1**). Hence the regional tectonic setting, characterized by the fault-controlled features, resulted fissures and NE-SW orientation and alignment of vents of (**Figure 2**). These are correlated with macro scale the tectonics of NE

(**Figure 2**). Eruptions from Tongxin

China. Volcanoes of ACVF overlie from Mesozoic basement rocks, such as granites and metamorphosed sediments [16–20]. The eruptive products are interfingered with various Quaternary sediments that accumulated in intra-mountain basins and along fault-controlled valley networks (**Figure 2**). The granitoid basement is strongly fractured, intruded by younger mafic to intermediate dyke swarms and usually covered by thick surface material derived from erosion and in situ weathering. Basement rocks are abundant in pyroclastic rocks as accidental lithics, especially those formed during explosive magma and water interaction. These lithics are preserved as xenoliths in individual or cored bombs in pyroclastic breccias or as accidental lithics of ash and lapilli in pyroclastic density current (PDC) deposits and within lava flows. Among the country-rock debris, there are low-grade metamorphic rocks or meta-sediments (commonly referred informally as "mudrocks") in minor amounts within the pyroclastic beds. These rocks are also part of the Late Mesozoic basement assemblages.

The region around ACVF includes two main fluvial systems (Halaha River and Chaoer River) and several lacustrine systems (**Figures 2** and **3**). The lacustrine systems likely been formed after major lava flows diverted the fluvial channels. For example, a lake formed when a lava flow from the Yanshan – Gaoshan volcanic systems blocked the Halaha River approximately 2000 years ago (**Figures 2** and **3**) [11]. Major structural zones facilitated the storage of groundwater that was driven downward to the lowlands where springs and/or channels developed between major lava flow units [1, 21].

Annual precipitation is about 450 mm; the average temperature is around −2.7°C in the range of −25.1°C in January to 16.8°C in July with about six months of the year below 0°C. These data indicate that ACVF lies within typical subarctic conditions with a strong monsoonal influence [22]. Data relating to paleo-monsoon conditions in NE China are rarely found. However, research based on the lacustrine sediments found in the bottom of crater lakes in ACVF show that around the Last Glacial Maximum (LGM), about 18,000 BP, there was a significant enlargement of the northern grassland areas and warm periods are recorded during mid- Holocene (10000–6000 BP) [21]. These stages were influenced by the East Asian Monsoon. More recently the areas in forest are enlarging; this might indicate influence from the warm periods of mid-Holocene [21]. The present region is covered by typical subarctic forest (Betula), grass and shrubland. Volcanic landforms are heavily vegetated, and large and continuous exposures are rare, making geological mapping challenging. Soil formation is intense and even a seemingly young volcanic landforms are covered by thick Holocene sediments or typical sub-arctic massmovement generated cover beds. The sub-arctic environments and high latitude lead to frozen ground for half the year (Oct to Apr) and only five months when the ground is not frozen (mid-Apr to Sep). A major wildfire in 1987, the Black Dragon fire event, caused widespread destruction of the vegetation around Yanshan-Triple Vent and Gaoshan. Nowadays, that area of forest and vegetation are regenerating and nearly cover the volcanic sites. This means that during field trips some of the areas cannot be assessed and observed.

## **3. Arxan-Chaihe volcanic field**

The Arxan-Chaihe Volcanic Field (ACVF) is recognized as a monogenetic volcanic field covering an area nearly 2000 km2 [9, 10, 23]. Within this area, at least 47 vents have been identified so far; however, this is likely a minimum number [9]. ACVF has been experiencing high erosion triggered by dramatic temperature changes and fluctuating surface water runoff. Two major fluvial systems within ACVF, the Halaha

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tions to potential volcanic hazard have not yet been studied extensively.

So far, ACVF preserves at least 47 (known) vents in a 2000 km2

From these 47 vents, only 35 were investigated in more detail and assigned to some volcanic geoforms (**Figure 3**, and **Table 1**). The age of the volcanoes were mostly assigned by their relative stratigraphy (**Table 1**) as sporadic and random absolute age dates are available [10, 24–26] only from a handful of identified volcanoes (**Table 2**). As absolute age dating using radiometric tools such as K-Ar or Ar-Ar methods are problematic in young mafic volcanics, most of the data derived from lava flows and coherent lava as pyroclasts from volcanic edifices. Lava flows however shows that volcanism was spread through time in the wider ACVF region (**Table 2**). This means that the available age data should be viewed with care and many data may not representative for an eruption age of the volcano located in the vicinity of the sampling point but shows ages of earlier lava flows. The volcanic landforms identified in ACVF include tuff rings, scoria/spatter cones, complex volcanic cones and tumuli structures, respectively (**Figure 4**). Tongxin Volcano (**Figure 4A**), is the largest tuff ring preserved in the ACVF with a rim to rim diameter of 1.4–1.1 km. The volcanic edifice is sandwiched between cliffs of basement rock (granite and metavolcanics) forming a typical intramountain basin that has been gradually filled from the north by an alluvial fan (**Figure 4A**). Bedrock is exposed about 306 m above the present-day crater lake surface that is commonly flanked with debris. The average elevation of the ring boundary is approximately 800 m above sea level. The central bottom of the lake is nearly flat with the

The volcanic edifice itself is partially stripped off due to surficial erosion and the thickest tephra succession preserved in its western side is about 22 m thick. Due to thick soil cover (commonly over 2 meters thick, and impenetrable by manual trenching) and grass cover, the pyroclastic deposits associated with the former volcano commonly form only a thin drape of ash and lapilli. In well-protected areas in foothills, however, deposits have been identified over 10-meter thickness about 2–3 km away from the volcano. As preliminary field mapping showed, most of the deposits derived from Tongxin Volcano accumulated in a broad braided river system of the Chaoer River and post-eruptive fluvial processes are likely responsible for the

There are no similar, large preserved tuff rings known from the ACVF, however, small phreatomagmatic volcanoes are suspected or been associated with the basal

area (**Figure 2**).

**4. Vent locations and volcano morphology**

present-day water depth of no more than 13 m.

removal of the erodible ash and lapilli.

River and Chaoer River, facilitate surface erosions (**Figures 2** and **3**). They rework the volcanic materials and modify the general topography. Within ACVF volcanic landforms typical of a monogenetic volcanic field, such as tuff rings, scoria/cinder cones, fissures, lava flows, as well as ponded lava flows, have been recognized [9, 10]. This great diversity of monogenetic volcanic landforms gives the region its high geoheritage value and was the basis for protecting of volcanic landforms by the establishment of a geopark network in the region through intensive work since 2004. The western side of the ACVF became part of the UNESCO Global Geopark Network in 2017 (http://www.unesco.org/new/en/natural-sciences/environment/earth-sciences/ unesco-global-geoparks/list-of-unesco-global-geoparks/china/arxan/). The recognition of the region's volcanic geoheritage in the highest level shows the importance of the ACVF as an intraplate volcanic field that is far from active plate margin processes and still recognized as an active volcanic region [9]. The ACVF is one of the lesser known volcanic regions of the world, and it's physical volcanology and the implica-

*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*

River and Chaoer River, facilitate surface erosions (**Figures 2** and **3**). They rework the volcanic materials and modify the general topography. Within ACVF volcanic landforms typical of a monogenetic volcanic field, such as tuff rings, scoria/cinder cones, fissures, lava flows, as well as ponded lava flows, have been recognized [9, 10]. This great diversity of monogenetic volcanic landforms gives the region its high geoheritage value and was the basis for protecting of volcanic landforms by the establishment of a geopark network in the region through intensive work since 2004. The western side of the ACVF became part of the UNESCO Global Geopark Network in 2017 (http://www.unesco.org/new/en/natural-sciences/environment/earth-sciences/ unesco-global-geoparks/list-of-unesco-global-geoparks/china/arxan/). The recognition of the region's volcanic geoheritage in the highest level shows the importance of the ACVF as an intraplate volcanic field that is far from active plate margin processes and still recognized as an active volcanic region [9]. The ACVF is one of the lesser known volcanic regions of the world, and it's physical volcanology and the implications to potential volcanic hazard have not yet been studied extensively.
