**4. Vent locations and volcano morphology**

So far, ACVF preserves at least 47 (known) vents in a 2000 km2 area (**Figure 2**). 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 present-day water depth of no more than 13 m.

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 removal of the erodible ash and lapilli.

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

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

Mesozoic basement assemblages.

areas cannot be assessed and observed.

**3. Arxan-Chaihe volcanic field**

field covering an area nearly 2000 km2

lava flow units [1, 21].

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

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

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

The Arxan-Chaihe Volcanic Field (ACVF) is recognized as a monogenetic volcanic

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

[9, 10, 23]. Within this area, at least 47 vents


**225**

**Name onFigure 3**

14

Middle Pleistocene

47°33′00″N, 121°9′20″E

Circular Horseshoe

Composite Cone

10

Strombolian

182

1132 highland

15

Middle Pleistocene

47°37′17″N,

Circular Horseshoe

Composite Cone

25

Strombolian

172

120°50′30″E

ANER

16

Middle Pleistocene

47°35′35″N,

Circular Horseshoe

Spatter Cone

1

Hawaiian Strombolian

Hawaiian,

50

Strombolian

Strombolian

50

254

121°19′00″E

NWNPZ

17

Middle Pleistocene

47°14′8″N, 120°19′47″E

Circular Horseshoe

Collapsed or Breached

Scoria Cone

Fissure Oriented

Breached Scoria Cone

Collapsed or Breached

Scoria Cone

Composite Cone

8

Strombolian

Hawaiian,

35

Strombolian

Strombolian Strombolian Strombolian Strombolian Strombolian

196

131

38

73

20

126

1404 highland

18

Middle Pleistocene

47°14′31″N, 120°21′3″E

Long Horseshoe

1201 highland

19

Middle Pleistocene

47°24′25″N,

Horseshoe

120°46′49″E

WSLD

20

Middle Pleistocene

47°15′45″N,

Horseshoe

120°26′54″E

SFS

21

Middle Pleistocene

47°18′21″N, 120°29′4″E

Long Horseshoe

Fissure Oriented

Breached Scoria Cone

1153 highland

22

Middle Pleistocene

47°18′00″N,

Circular Horseshoe

Collapsed or Breached

Scoria Cone

120°27′50″E

WDC

23

Middle Pleistocene

47°20′2″N, 120°27′27″E

Circular Horseshoe

Collapsed or Breached

Scoria Cone

1213 highland

24

Middle Pleistocene

47°24′17″N, 120°37′8″E

Long Horseshoe

Collapsed or Breached

Scoria Cone

1308 highland

25

Middle Pleistocene

47°26′53″N,

Circular Horseshoe

Collapsed or Breached

Scoria Cone

120°48′32″E

1381.2 highland

26

Middle Pleistocene

47°30′18″N,

Circular Horseshoe

Collapsed or Breached

Scoria Cone

120°57′06″E

1246 highland

**Formation Age**

**Location**

**Map view shape**

**Type of volcano or main eruptive product**

**Area of lava flow**

**[km2**

**]**

**Main type of** 

**Relative** 

**height of** 

**edifice (m)**

**eruption**

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

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

205

### *Updates in Volcanology – Transdisciplinary Nature of Volcano Science*


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

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**224**

**Name**

**Formation Age**

**Location**

**Map view shape**

**Type of volcano** 

**Area of lava flow**

**Main type of** 

**Relative** 

**height of** 

**edifice (m)**

**eruption**

**[km2**

**]**

**or main eruptive** 

**product**

**on**

**Figure 3**

1 Gaoshan 2 Yanshan

3

Holocene

SHG

4

Holocene

XDG

5

Late Pleistocene

WNPZ

6

Late Pleistocene

WSLZ

7

Late Pleistocene

BTH

8

Late Pleistocene

ZGS

9

Middle Pleistocene

47°30′25″N,

Circular Horseshoe

Composite Cone

3

Strombolian

178

120°52′00″E

JEG

10

Middle Pleistocene

47°27′19″N,

Long Horseshoe

Composite Cone

12

Strombolian and

170

Phreatomagmatic

Strombolian

168

120°38′36″E

TFL

11

Middle Pleistocene

47°27′51″N,

Long Horseshoe

Collapsed or Breached

12

Scoria Cone

120°43′34″E

SGS

12

Middle Pleistocene

47°23′23″N,

Circular Horseshoe

Composite Cone

11

Strombolian

122

120°44′00″E

TYC

13

Middle Pleistocene

47°32′2″N, 121°5′00″E

Circular Horseshoe

Composite Cone

20

Strombolian

154

1104 highland

Holocene

47°21′28″N,

Circular Horseshoe

Scoria Cone

50

120°37′30″E

47°20′28″N,

Circular Horseshoe

Scoria Cone

25

120°34′14″E

47°18′28″N,

Circular Horseshoe

Spatter Cone

5

Hawaiian

110

120°29′40″E

47°34′13″N,

Circular

Maar With Tuff

7

Phreatomagmatic

198

Ring, Base-Surge

Maar With Tuff

2

Phreatomagmatic

Phreatomagmatic?

150

Ring, Base-Surge

Tuff Ring,

Base-Surge

Collapsed or Breached

23

Strombolian

136

Scoria Cone

121°17′22″E

47°17′54″N,

Circular

120°20′10″E

47°35′18"N,

Ellipse

120°43′08″E

47°30′52″N,

Long Horseshoe

120°54′44″E

Holocene

47°22'N, 120°39′E

Circular Horseshoe

Composite Cone

5

Violent

362

Strombolian

Violent

233

Strombolian

Strombolian


*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**Table 1.**

**227**

(**Figure 4B**).

**Table 2.**

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

**Dating method**

08AES02 WNPZ - 5 K-Ar 0.162 ± 0.02 Ma Fan et al. [24] 07AES02 TFL - 10 K-Ar 0.246 ± 0.05 Ma Fan et al. [24] 07CH04 Chaoer River – lava flow K-Ar 0.269 ± 0.03 Ma Fan et al. [24] 08AES09 WSLZ - 6 K-Ar 0.45 ± 0.03 Ma Fan et al. [24] 08AES12 Budong River – lava flow K-Ar 0.587 ± 0.06 Ma Fan et al. [24] 07CH11 JEG - 9 K-Ar 0.743 ± 0.07 Ma Fan et al. [24] 07CH09 Dele River – lava flow K-Ar 1.365 ± 0.16 Ma Fan et al. [24] 08AES04 TFL - 10 K-Ar 2.3 ± 0.06 Ma Fan et al. [24] 08AES10 Budong River – lava flow K-Ar 6.7 ± 0.18 Ma Fan et al. [24]

NW223 Wuchagou – lava flow K-Ar 8.93 ± 0.64 Ma Liu et al. [25] NW214 Wuchagou – lava flow K-Ar 9.94 ± 0.63 Ma Liu et al. [25] ARS002 TFL - 10 K-Ar 0.34 ± 0.03 Ma Meng et al. [27] ARS003 Dujuan Lake – lava flow K-Ar 0.42 ± 0.10 Ma Meng et al. [27] ARS004 ARX - 33 K-Ar 0.53 ± 0.03 Ma Meng et al. [27] YL708 JEG - 9 tephra 14.2 ka Sun et al. [26]

**Age Reference**

K-Ar 0.34 ± 0.203 Ma Liu et al. [25]

14C 1990 cal a BP Bai et al. [11]

14C 1900 cal a BP Bai et al. [11]

sections of large scoria cones. Tuofengling Lake (Camel Humps Lake in Chinese) is a good example for such volcano edifice architecture (**Figure 4B**). This volcano hosts an elongated lake suggesting that this volcano might be a complex volcano with multiple vents along the lake axis. This volcano appears different in its edifice architecture from Tongxin Volcano. In Tongxin Volcano, the country rocks are exposed in the inner basement hosting the crater lake while in Tuofengling, no basement is exposed. The volcanic edifice surrounding the crater lake is entirely volcanic in origin and part of a larger volcanic edifice. The preserved and accessible sites exposing typical lava spatter, agglomerate, scoriaceous deposits and various clastogenic lava flows suggest a largely magmatic explosive eruption style being responsible for the formation of them. However, from observations of the crater rim, at least two different volcanic deposits were found. The majority is typical of a scoria and spatter cone complex, while in the basal sections thick pyroclastic successions indicate explosive hydrovolcanism occurred as recorded by chilled pyroclast-dominated units predating the formation of the main central cones that were later disrupted giving space to form a broad and elongated crater within them

*Absolute dating results for the volcanic rocks of the ACFV [11, 24–26]. Samples marked as "lava flow" derived* 

The youngest volcanic sites of the ACVF, i.e. approximately 2000 years ago [11], demonstrate a typical volcanic landform considered to be a common form of volcano types across the ACVF. Yanshan Hill marked as a single volcanic edifice, in

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

Nt202 Tianchi forest farm – lava

flow

Yanshan – 2 - under scoria fall

Yanshan – 2 - under scoria fall

*from extensive flow fields without exact information from their source vent.*

**Location, Code (on Table 2) or Type**

**Sample No.**

*Parameters for previously studied volcanoes in the ACVF.*


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

### **Table 2.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**226**

**Name**

**Formation Age**

**Location**

**Map view shape**

**Type of volcano** 

**Area of lava flow**

**Main type of** 

**Relative** 

**height of** 

**edifice (m)**

**eruption**

Strombolian Strombolian Strombolian Strombolian

76

138

60

66

**[km2**

**]**

**or main eruptive** 

**product**

**on**

**Figure 3**

27

Middle Pleistocene

47°24′23″N,

Circular Horseshoe

Collapsed or Breached

Scoria Cone

120°42′28″E

NSYH

28

Middle Pleistocene

47°14′56″N,

Circular Horseshoe

Collapsed or Breached

Scoria Cone

120°22′13″E

1259 highland

29

Middle Pleistocene

47°14′56″N,

Circular Horseshoe

Collapsed or Breached

Scoria Cone

Collapsed or Breached

Scoria Cone

120°22′40″E

1338.6 highland

30

Middle Pleistocene

47°27′30″N,

Horseshoe

120°35′18″E

W 1286

highland

31

Middle Pleistocene

47°27′39″N,

Horseshoe

Collapsed or Breached

Strombolian Strombolian

175

45

Scoria Cone

Collapsed or Breached

Scoria Cone

Scoria Cone

25

Strombolian

130

120°35′49″E

E 1286 highland

32

Middle Pleistocene

47°19′09″N,

Horseshoe

120°25′00″E

1325 highland

33

Middle Pleistocene

47°19′00″N,

Semi-Round

120°24′16″E

ARX

34

Middle Pleistocene

47°30′44″N, 121°2′35″E

Horseshoe

Composite Breached

6

Strombolian

204

Scoria Cone

Spatter Cone

8

Hawaiian,

117

Strombolian

HXD

35

Middle Pleistocene

47°31′18″N, 121°3′32″E

Cone

1221 highland

**Table 1.**

*Parameters for previously studied volcanoes in the ACVF.*

*Absolute dating results for the volcanic rocks of the ACFV [11, 24–26]. Samples marked as "lava flow" derived from extensive flow fields without exact information from their source vent.*

sections of large scoria cones. Tuofengling Lake (Camel Humps Lake in Chinese) is a good example for such volcano edifice architecture (**Figure 4B**). This volcano hosts an elongated lake suggesting that this volcano might be a complex volcano with multiple vents along the lake axis. This volcano appears different in its edifice architecture from Tongxin Volcano. In Tongxin Volcano, the country rocks are exposed in the inner basement hosting the crater lake while in Tuofengling, no basement is exposed. The volcanic edifice surrounding the crater lake is entirely volcanic in origin and part of a larger volcanic edifice. The preserved and accessible sites exposing typical lava spatter, agglomerate, scoriaceous deposits and various clastogenic lava flows suggest a largely magmatic explosive eruption style being responsible for the formation of them. However, from observations of the crater rim, at least two different volcanic deposits were found. The majority is typical of a scoria and spatter cone complex, while in the basal sections thick pyroclastic successions indicate explosive hydrovolcanism occurred as recorded by chilled pyroclast-dominated units predating the formation of the main central cones that were later disrupted giving space to form a broad and elongated crater within them (**Figure 4B**).

The youngest volcanic sites of the ACVF, i.e. approximately 2000 years ago [11], demonstrate a typical volcanic landform considered to be a common form of volcano types across the ACVF. Yanshan Hill marked as a single volcanic edifice, in

### **Figure 4.**

*Four types of volcanic geoforms identified in ACVF, such as (A) tuff ring; (B) complex cone; (C) scoria/cinder cone; (D) tumuli.*

fact, consists of at least three preserved volcanoes, informally named as the Triple Vent forming a volcanic complex. This volcano is a nested and closely spaced scoria cone edifice (**Figure 4C**). The main edifice is a steep-sided scoria cone with slope angles about 45°. The elevation of this volcano is about 1590 m above sea level. The volcanic complex was the source of extensive lava flows that reached 4 km to the west and captured the Halaha River causing major landscape modification and damming of the fluvial system. The lava flows show inflation and deflation features, development of pressure ridges, tumuli and outbreaks leading to the formation of a complex laterally changing surface flow morphologies ranging from rubble pahoehoe to aa lava morphotypes. Such large and complex volcanoes are the main volcanic landforms along with the main structurally controlled vents and with NE-SW trending structural elements. The complexity and estimated edifice volumes, as well as the large volume of associated lava flows of these volcanoes, suggest these eruptions changed the actual erupting points within km-ranges, were likely long-lasting events and were capable for sustained eruptions such as sub-Plinian or violent Strombolian style eruptions. This is supported by the extensive ash plain mapped around Yanshan.

Across the field, there is evidence of ponded lava flows, leaving behind a range of ponded lakes, clastogenic lava flows or agglomerates (**Figure 4D**). Based on the previous studies and evidence from 2019 fieldwork, the vents in ACVF are mostly fissure-controlled.

## **5. Evidence of explosive hydrovolcanism**

Explosive hydrovolcanism is the result of magma and water explosive interactions [28]. Phreatomagmatism is a term reserved for magma and groundwater interaction commonly involving molten-coolant interaction [28]. In the ACVF,

**229**

**Figure 5.**

*away from the lake). Brown letter "g" indicates "granite".*

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

pyroclast textures and deposit characteristics indicate that in low-lying areas, along fluvial valleys, phreatomagmatic explosive eruptions took place (**Figure 2**). While shreds of evidence of sustained phreatomagmatic explosive eruptions that formed maar or tuff ring volcanoes alone are not common, evidence of intermittent phreatomagmatic phases during eruptions are compelling. Evidence for phreatomagmatism can be seen in the form of quenched pyroclasts, glassy and angular texture of juvenile particles and relative abundance of country-rock fragments as accidental lithics in the pyroclastic successions. Indirectly, volcanic landforms may also be linked to the occurrence of phreatomagmatism in the course of volcanic eruptions; however, typical "wet", phreatomagmatic landforms

The most typical and complete site where evidence for explosive hydrovolca-

*Typical pyroclastic deposit characteristics associated with explosive phreatomagmatic eruptions at the ACVF are linked to accidental lithic-rich pyroclastic breccias, inferred to be phreatomagmatic shower curtain deposits (A), the abundance of cored/loaded (B) and partially melted accidental lithics in chilled lavas (C), and typical angular and low vesicularity juvenile pyroclasts forming typically graded beds in a combination of shower and pyroclastic density current-derived beds (D, captured on the south of Tongxin Lake, about 12 km* 

nism, phreatomagmatism has been identified in the ACVF is Tongxin Lake (**Figures 2** and **3**). Tongxin Lake has been recognized as a large tuff ring, partially draping over the rugged mountainous region (**Figure 2**). Two significant outcrops on the west flank of the ring margin have revealed deposits primarily formed by sustained phreatomagmatic eruptions. Dune-bedded, cross-stratified and wavy, accidental lithic-rich pyroclastic successions were observed and inferred to be the product of a combination of pyroclastic density currents and fallout. The succession of interbedded coarse-grained and fine-grained units suggest rapid changes in particle concentration, transportation style and interaction with microtopography. The majority of the pyroclasts are dense (i.e. low vesicularity), angular to subangular with delicate chilled margins. Accretionary lapilli are common in the fine ash beds. Features indicative for phreatomagmatism can be found at other sites, such as Tuofengling/Camel Humps Lake, southwestern side of Tianchi/Heaven Lake, and in sporadic outcrops south of Wusulangzi Lake (**Figure 2**). The deposits at Tuofengling Lake form a basal unit beneath the dominant scoria/cinder deposits. This unit is composed of a range of indurated parallel bedded, pyroclastic beds abundant in angular and dense lapilli within fine ash matrix. The average grain sizes from the deposits of Tuofengling is greater than the majority of the deposits recorded at Tongxin Lake (**Figure 5A**), and they are also more indurated than those

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

are rarely preserved at ACVF.

from Tongxin Volcano.

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

pyroclast textures and deposit characteristics indicate that in low-lying areas, along fluvial valleys, phreatomagmatic explosive eruptions took place (**Figure 2**). While shreds of evidence of sustained phreatomagmatic explosive eruptions that formed maar or tuff ring volcanoes alone are not common, evidence of intermittent phreatomagmatic phases during eruptions are compelling. Evidence for phreatomagmatism can be seen in the form of quenched pyroclasts, glassy and angular texture of juvenile particles and relative abundance of country-rock fragments as accidental lithics in the pyroclastic successions. Indirectly, volcanic landforms may also be linked to the occurrence of phreatomagmatism in the course of volcanic eruptions; however, typical "wet", phreatomagmatic landforms are rarely preserved at ACVF.

The most typical and complete site where evidence for explosive hydrovolcanism, phreatomagmatism has been identified in the ACVF is Tongxin Lake (**Figures 2** and **3**). Tongxin Lake has been recognized as a large tuff ring, partially draping over the rugged mountainous region (**Figure 2**). Two significant outcrops on the west flank of the ring margin have revealed deposits primarily formed by sustained phreatomagmatic eruptions. Dune-bedded, cross-stratified and wavy, accidental lithic-rich pyroclastic successions were observed and inferred to be the product of a combination of pyroclastic density currents and fallout. The succession of interbedded coarse-grained and fine-grained units suggest rapid changes in particle concentration, transportation style and interaction with microtopography. The majority of the pyroclasts are dense (i.e. low vesicularity), angular to subangular with delicate chilled margins. Accretionary lapilli are common in the fine ash beds.

Features indicative for phreatomagmatism can be found at other sites, such as Tuofengling/Camel Humps Lake, southwestern side of Tianchi/Heaven Lake, and in sporadic outcrops south of Wusulangzi Lake (**Figure 2**). The deposits at Tuofengling Lake form a basal unit beneath the dominant scoria/cinder deposits. This unit is composed of a range of indurated parallel bedded, pyroclastic beds abundant in angular and dense lapilli within fine ash matrix. The average grain sizes from the deposits of Tuofengling is greater than the majority of the deposits recorded at Tongxin Lake (**Figure 5A**), and they are also more indurated than those from Tongxin Volcano.

### **Figure 5.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

fact, consists of at least three preserved volcanoes, informally named as the Triple Vent forming a volcanic complex. This volcano is a nested and closely spaced scoria cone edifice (**Figure 4C**). The main edifice is a steep-sided scoria cone with slope angles about 45°. The elevation of this volcano is about 1590 m above sea level. The volcanic complex was the source of extensive lava flows that reached 4 km to the west and captured the Halaha River causing major landscape modification and damming of the fluvial system. The lava flows show inflation and deflation features, development of pressure ridges, tumuli and outbreaks leading to the formation of a complex laterally changing surface flow morphologies ranging from rubble pahoehoe to aa lava morphotypes. Such large and complex volcanoes are the main volcanic landforms along with the main structurally controlled vents and with NE-SW trending structural elements. The complexity and estimated edifice volumes, as well as the large volume of associated lava flows of these volcanoes, suggest these eruptions changed the actual erupting points within km-ranges, were likely long-lasting events and were capable for sustained eruptions such as sub-Plinian or violent Strombolian style eruptions. This is supported by the extensive ash plain

*Four types of volcanic geoforms identified in ACVF, such as (A) tuff ring; (B) complex cone; (C) scoria/cinder* 

Across the field, there is evidence of ponded lava flows, leaving behind a range of ponded lakes, clastogenic lava flows or agglomerates (**Figure 4D**). Based on the previous studies and evidence from 2019 fieldwork, the vents in ACVF are mostly

Explosive hydrovolcanism is the result of magma and water explosive interactions [28]. Phreatomagmatism is a term reserved for magma and groundwater interaction commonly involving molten-coolant interaction [28]. In the ACVF,

**228**

mapped around Yanshan.

**5. Evidence of explosive hydrovolcanism**

fissure-controlled.

**Figure 4.**

*cone; (D) tumuli.*

*Typical pyroclastic deposit characteristics associated with explosive phreatomagmatic eruptions at the ACVF are linked to accidental lithic-rich pyroclastic breccias, inferred to be phreatomagmatic shower curtain deposits (A), the abundance of cored/loaded (B) and partially melted accidental lithics in chilled lavas (C), and typical angular and low vesicularity juvenile pyroclasts forming typically graded beds in a combination of shower and pyroclastic density current-derived beds (D, captured on the south of Tongxin Lake, about 12 km away from the lake). Brown letter "g" indicates "granite".*

Bombs and accidental lithics in the eruptive products from Tongxin Volcano are commonly cored/loaded and/or granitoid lithics are fused or partially melted (**Figure 5B** and **C**). Cauliflower-shaped bombs with xenoliths are strong evidence for country-rock entrapment prior to the explosive disruption of the clasts during an eruption. Among the shell of the juvenile materials, large amounts of lithic debris are intercalated within. All these lithics are irregular-shaped, and, the juvenile materials are pasted onto the surface, chilled and form into the cauliflowershaped bombs or juvenile pyroclasts. The general shape of those juvenile pyroclasts is sub-angular to angular. They probably came into contact with external water and became chilled, fragmented in brittle fashion to form angular, low vesicular pyroclasts. Pyroclastic successions, which are rich in accidental lithics and contain large volumes of chilled, angular juvenile pyroclasts commonly form unsorted, and graded beds intercalated with fine tuff that also contains accretionary lapilli (**Figure 5D**). These textural features and outcrop scale sedimentological characteristics are common among phreatomagmatic pyroclastic successions. Commonly, the fine-sized grains indicate the explosive phreatomagmatic fragmentation was accompanied by blasts; on the contrary, the coarse-grained and moderately sorted beds imply fallout origin.
