CC BY
59
УДК 551.442
DOI 10.29003/m876.0514-7468.2020_42_1/13-23
VOLCANIC CAVES IN VIETNAM
La The Phuca, Luong Thi Tuata, Hiroshi Tachiharab, Tsutomu Hondab, Bui Van Thomc, Nguyen Ba Hunga, Tran Minh Duca, Nguyen Trung Minha
a Vietnam National Museum of Nature, Vietnam Academy of Science and Technology b Vulcanospeleology Society, Japan c Institute of Geology, Vietnam Academy of Science and Technology
Volcanic caves or lava caves in Vietnam have been discovered and studied since 2007 up to now, mainly related to basaltic rock in The Central Highlands (Tay Nguyen) of the country. They are endogenic origin, formed during the cooling process of basaltic lava flows with low viscosity and volatile substances. Whole the lava cave system is embedded entirely in the Xuan Locformation, produced by the eruption of Chu B'Luck volcano in the Krong No district area, Dak Nongprovince. It is recognized as the biggest lava cave system in the Southeast Asia in term of the scale, and the length as well. In term of uniqueness, many of them contain typical lava formations such as: lava levees, lava shelves, lava linings, lava flow marks, lava balls, lava windows, lava seals, lava lakes, skylights, lava glaze, lavacicles, and lava tree molds, etc. These interior lava formations are convincing evidences for the lava cave's endogenic origin in the area. The age of all basaltic formations in The Central Highlands is about20.0-0.2 Ma, meanwhile, the age of basaltic rock of Xuan Loc formation related to volcanic cave is 0.7-0.2 Ma. Besides geological scientific significance, the lava caves also contain different values such as: archaeological value, biological diversity, etc. Therefore, volcanic caves in The Central Highlands are valuable and unique heritage resources. They are attractive destinations and special highlights in the local geopark and nature reserves and greatly contribute to science research, education and socio-economic development in the region. The lava cave system in Krong No area has been selected to propose as the most important KEY heritage in the Dak Nong Geopark's dossier, that was submitted to UNESCO in November, 2018 and waiting for becoming the Global Geopark Network's official member in the next April, 2020. The article shows an overview introduction about the volcanic caves and their related heritage types, which have been studied in The Central Highlands of Vietnam.
Keywords: Volcanic Cave/Lava Cave, Volcano, Basalt, The Central Highlands (Tay Nguyen), Vietnam.
Citation: Phuc L.T., Tuat L.T., Tachihara H., Honda T., Thom B.V., Hung N.B., Duc T.M., Minh N.T. Volcanic caves in Vietnam // Zhizn' Zemli [Earth Live]. 42 (1), 13-23 (2020). DOI 10.29003/m876.0514-7468.2020_42_1/13-23:
ВУЛКАНИЧЕСКИЕ ПЕЩЕРЫ ВЬЕТНАМА1
Ла Тхэ Фук, Луонг Тхи Туат, Хироши Тачихара, Цутому Хонда, Буй Ван Том, Нгуен Ба Хунг, Чан Минь Дук, Нгуен Чунг Минь
Вулканические пещеры, или лавовые пещеры во Вьетнаме, открытые в 2007 г. и изучаемые по настоящее время, связаны с базальтовыми породами в Центральном
1 Ла Тхэ Фук, Луонг Тхи Туат - Вьетнамский национальный музей природы, Вьетнамская академия наук и технологий; Хироши Тачихара, Цутому Хонда - Общество вулканоспелеологии, Япония; Буй Ван Том, Нгуен Ба Хунг - Институт геологии, Вьетнамская академия наук и технологий; Чан Минь Дук, Нгуен Чунг Минь - Вьетнамский национальный музей природы, Вьетнамская академия наук и технологий.
Автор-корреспондент: Нгуен Чунг Минь, nttminh@vast.vn.
Жизнь Земли 42(1) 2020 13-23 13
нагорье (Тхайнгуен) страны. Они эндогенного происхождения, образовались в процессе охлаждения базальтовых лавовых потоков с низкой вязкостью и летучими веществами. Вся система лавовых пещер полностью встроена в породы формации Xuan Loc, образовавшиеся в результате извержения вулкана Chu B'Luck в районе Krong No, провинция Dak Nong. Она считается самой большой системой лавовых пещер в Юго-Восточной Азии как по масштабам, так и по длине. С точки зрения уникальности, многие пещеры содержат типичные лавовые образования, такие как: лавовые запруды, лавовые полки, выравнивания лавой, следы течения лавы, лавовые шары, окна в кровле лавового потока, лавовые пробки, лавовые озёра, световые люки, стекловатая корка на лаве, лавовые сферолиты и формы лавового дерева и т. д. Эти внутренние образования являются убедительным доказательством эндогенного происхождения лавовых пещер в этом районе. Возраст всех базальтовых образований в Центральном нагорье составляет 20,0-0,2 млн лет, в то время как возраст базальтовых пород формации Xuan Loc, связанных с вулканическими пещерами, составляет 0,7-0,2 млн лет. Помимо геологического научного значения, лавовые пещеры также имеют археологическую ценность и значимы с точки зрения биологического разнообразия. Поэтому вулканические пещеры в Центральном нагорье являются ценными и уникальными ресурсами природного наследия. Они являются привлекательными местами для посещения и особыми достопримечательностями в местном геопарке и заповедниках и вносят значительный вклад в научные исследования, образование и социально-экономическое развитие региона.
Система лавовых пещер в районе Krong No была выбрана в качестве наиболее важного наследия KEY в протоколе Геопарка Dak Nong, который был представлен ЮНЕСКО в ноябре 2018 г. и, как ожидается, в апреле 2020 г. станет официальным членом Глобальной сети геопарков.
В статье приведён обзор сведений о вулканических пещерах, которые были изучены в Центральном нагорье Вьетнама, и связанных с ними типах природного наследия.
Ключевые слова: вулканическая/лавовая пещера, вулкан, базальт, Центральное нагорье (Tay Nguyen), Вьетнам.
Ссылка для цитирования: Phuc L.T., Tuat L.T., Tachihara H., Honda T., Thom B.V., HungN.B., Duc T.M., Minh N.T. Volcanic caves in Vietnam // Zhizn' Zemli [Life of the Earth]. 42 (1), 13-23 (2020). DOI:
Поступила 04.09.2019/ Принята к публикации 20.02.2020
Introduction. The Generally, caves in Vietnam mainly consist of two types: karst caves and volcanic caves, which are distributed in two rock types: limestone and volcanic rock, respectively. In this paper only volcanic cave in volcanic rock will be mentioned. Volcanic rock in Vietnam has a lot of composition, from basalt, andesite to rhyolite. However, the volcanic caves are only related to basaltic rock, and they are also called "lava cave". The area covered by basaltic rock in Vietnam is not large, around 32,000 km2, occupies approx. 8 % of the Vietnam's mainland, mainly distributed in 5 provinces of The Central Highlands (Tay Nguyen), along the coast of East Sea, and some small areas in the north and central parts of Vietnam. The study results and isotopic age analysis showed that the Neogene-Quaternary basaltic formations distributed in The Central Highlands and some adjacent areas have the age of from 20.0 Ma to 0.2 Ma [6]. Basalt eruption activities in The Central Highlands have been divided into two series: early series (approx. 20-6 Ma with the main composition of quartz tholeiite, insignificant alkaline basalt. The surveying results show that volcanic cave hasn't been found in this series. Meanwhile, the late series (approx. 6.0-0.2 Ma) contains mainly alkaline basalt. During eruption process of the volcanoes, there were many long rest-time periods, even lasting one million years [4, 5]. 14
Fig.1: Distribution map of the Cenozoic Basaltic formation related to volcanic caves in Vietnam (based on Vietnam Geological map on scale 1:200 000).
Рис. 1. Карта распространения кайнозойской базальтовой формации, связанной с вулканическими пещерами во Вьетнаме (на основе Геологической карты Вьетнама М 1: 200 000).
However, notably, volcanic caves only distribute in the youngest eruptions of this late series and firstly, they have been discovered by La The Phuc et al. in 2007 in the field survey of the project «Researching geological heritages for establishing the Geopark and environmental protection in Trinh Nuoc waterfall, Cu Jut district, Dak Nong province, Vietnam», sponsored by UNESCO (2007-2008). Up till now, there are only about 60-70 caves located on the map in two areas: mainly in Krong No district, Dak Nong province (more than 50 caves); some others in Dinh Quan and Tan Phu districts, Dong Nai province (around appx. 11 caves), and the less in Lam Dong and Dak Lak province, but not typical ones. Only 22 caves have been measured and mapped in detail, including 20 caves in Krong No district, Dak Nong province, and 2 caves in Tan Phu and Dinh Quan districts, Dong Nai province. The C7 lava cave in Krong No is the longest one with the length of 1066.5m long. Although, they are all not too long in comparison with some of their partners in South Korea, Japan or USA, but have been recognized as ones of the longest and the most unique ones in Southeast Asia [2, 4, 7]. Since 2017, volcanic caves in The Central Highlands of Vietnam have been surveyed and researched comprehensively on all three fields: geological, biodiversity and cultural values. However, only the overview information about volcanic caves in Vietnam will be presented in this paper, meanwhile the results in detail on each field will be mentioned in subsequent papers.
Basic literature, methodology, regional geology.
Basic literature. The basic literature for the research on the volcanic caves in Vietnam includes: geological maps at scale 1:200 000 to 1:50 000; topographic maps; satellite maps at scale 1:50 000 to 1:10 000; satellite maps (Google Earth, Spots, Landsat) and Flycam images (Drone); thematic reports on basaltic rocks, and regional geomorphology, and analysis results of samples collected in volcanic caves.
Methodology. Inheritance data method. The purpose of this method is to inherit the results of previous works to study the new related research issues. On the basis of collecting basic geological survey results in various scales, related materials of the cave geomorphology study on volcanic rock/basaltic rock distribution in Vietnam. A distribution map of basaltic rock and related caves will be established. This map is the basis for seeking, discovering and recognizing volcanic caves on the field.
Sociological investigation method. The purpose of this method is to collect information directly related to the volcanic caves in order to discover and establish caves. The survey form is conducted by interviewing local people directly and taking questionnaires. The survey results have been also marked on the map for field research work.
Remote Sensing image interpretation method. The purpose of this method is to contribute to the field surveys. All photographs (satellite images, airplane image, etc.) in volcanic rock distribution area will be analyzed and interpreted to identify potential areas of caves and field of research work. This method combined with the sociological investigation method will become a valuable data resource for the field of research work and collecting original materials for research.
Investigation, field survey method. Investigation, field survey method will be conducted after the results of the above three research methods are available. Volcanic caves will be surveyed and established through:
• Direct observation, general description of geographical location, scale of volcanic cave;
• Mapping, taking photos, collecting samples and original materials of all kinds;
• Surveying, researching and establishing cave heritage in the field, evaluating scientific values, aestheticism, assessing safety condition, etc. contribute to preparing the scientific profile for volcanic cave heritage.
Sample analysis methods. The purpose of this methods is to determine composition of the bed rock in caves, composition of the cave interior formations, etc. to study the features and the cave geological development history. The analytical methods include:
• Petrographic analysis;
• Chemical composition analysis;
• Radio-isotopic analysis.
Statistic and classification methods. Each cave has some different typical features, containing different heritages. Based on the survey and specialized research results, caves will be classified according to the 3 research fields, include: geological heritage values, biodiversity, cultural heritage as follows:
- In term of Geology: Geological heritages of caves have been classified according to the GILGES system (Global Indicative List of Geological Sites) of UNESCO. Value of geological heritage has been evaluated according to the following criteria:
• Outstanding scientific, educational, aesthetic or economic value;
• Scale and outstanding features of scale;
• Convenient level of transportation, population and service organization when recognized and put into operation;
• Safety of heritage before natural and social impacts;
• The prospect of spiritual and economic benefits is obtained when protected and exploited to promote values.
- In term of Biology: biodiversity will be classified according to the specialties of biology.
- In term of Culture: Cultural heritage in volcanic caves (including archaeological sites or historical relics) and collected samples during the fieldwork and excavation will be classified and evaluated in accordance with The Cultural Heritage Law of Vietnam.
Results.
Geological characteristics of the volcanic cave area. The volcanic caves spread out in the area, which consists of the following formations: terrigenous sediments of the Middle Jurassic La Nga formation, basaltic rocks of the Pliocene - Quaternary of Tuc Trung and Xuan Loc formations, and Quaternary friable sediments [6].
La Nga formation (J2ln). La Nga formation is widely exposed in 5 provinces in The Central Highlands (Tay Nguyen) and the northern part of Dong Nai province, the north and northeast part of Binh Phuoc province. The complete section consists of 3 sets [6]:
Set 1: mainly gray, strip siltstone, weathered yellow, easy to crumble into small pieces; and clay in black, smooth surface, alternating thin layers of yellow gray sandstone, dark gray silty sand containing plant monuments. The thickness is about 250-300 m.
Set 2: Black, thin layered siltstone, the surface layer has many pyrite crystals alternating with shale and thin layers of sandstone with white mica scales stripes in stone. Siltstone contains much plant debris. The thickness is about 100 m.
Set 3: Small, gray, light gray, yellow gray, medium-small grained sandstone mixed with gray, yellow gray, silty sand, siltstone and few thin layers of clay. In some places, sandstone of this set contains much plant debris. The general thickness of this formation is about 700-800 m.
The upper part of this formation is shallow coastal marine sediment. The formation is unconformably covered by Pliocene - Quaternary basaltic rocks of Tuc Trung and Xuan Loc formations [6].
Tuc Trung formation (3N2 - Q1tt). Basaltic rocks of Tuc Trung formation are widely distributed in The Central Highlands. Petrographic composition includes: basalt olivine -augite, basalt olivine - augite - plagioclase. Basaltic rock is from gray to blue gray, dark gray; with solid or hole block structure; popular porphyritic architectures with dolerite substrate, slab, augite or hyalopilitic. This type of basalt is nearly saturated alkaline, in which sodium alkali dominated over potassium. Tuc Trung formation's basalt covers the weathering surface of La Nga formation's sediment, 20-70 m thick. No cave is found in Tuc Trung formation.
Xuan Loc formation (3Q12xl). Basaltic rocks of Xuan Loc formation are distributed in great amount in the southern provinces of The Central Highlands. They are closely related to volcanic structures that were well preserved. Relating to erupting rocks in the form of lava flows, there is a considerable amount of tuffs, volcanic ash attached to volcanic cones. Petrographic composition includes: basalt olivine, basalt olivine - augite, basalt olivine - augite -plagioclase. The rocks are micro-granular or subtle, gray, dark gray, dense or gaping masses, porphyritic structures with micro-dolerite, chelate or hyalopilitic. General thickness is about 20-90 m. The survey results show that volcanic caves are mainly distributed in the rocks of this formation [2, 4, 7].
Quaternary deposits (Q):
- Upper Pleistocene sediment (Q^). The formation of late Pleistocene sediments is composed mainly of pebbles, gravel, sand with good round grinding in the lower part and clay,
patchy yellow powder in the upper part. Sediments are mainly alluvial origin, distributed along the valleys of rivers and streams, creating secondary level terraces with fairly flat surface, relative height of 10-15m. These sedimentary deposits are about 8-9 m thick [5, 6].
- Holocene deposits (Q2). The formation of Holocene sediments has 2 types of origin, mainly rivers and lakes. The sedimentary formations of river origin are distributed mainly along two sides of rivers in the form of primary level terraces, with relatively 5-7 m high. The composition of sediments includes sandy clay, coloured silty clay and curdy laterite with about 5-6 m of thickness. The sedimentary formations of the lagoon (marsh) are scattered in some valleys between mountains. The main components are clay silt rich in plant humus-peat [5, 6].
In addition, there are also undivided Quaternary sediments (Q) in the area with eluvial, deluvial, proluvial origin distributed scatteredly on the high river terraces [5, 6].
Distribution characteristics of volcanic caves. Distribution characteristics. Until now, volcanic caves only have been found in basaltic rock of Xuan Loc formation (0.8-0.2 Ma), which considered as the youngest porous basaltic rock of Vietnam [4]. The caves are unevenly distributed, often into strips, reflecting the direction of lava flows, closely related to volcanic eruptions, 300-15 000 m from the crater and centripetal to the crater.
The cave is divided into 2 types based on the groundwater level: dry and wet caves. The dry caves are caves above the current groundwater level, so not flooded. The wet caves are caves under the current groundwater level, therefore they are often flooded. However, the current researches have been only focused on the dry caves, which their entrances expose on the surface; but the wet, flooded underground caves haven't been surveyed yet due to lack of access equipment.
Cave characteristics. Volcanic cave entrances in the study area have many types, including: circle, semi-circle, oval, triangle, etc.
In term of origin, volcanic cave entrances there have two types of origin: primary and secondary. Primary entrances are often vertical and deep due to their formation mechanism from gas escape - explosion [4]. The scale of this cave type is usually small with a circle entrance from 3-7 m in diameter, and the depth of 23-25m (Fig. 2, Fig. 3). The secondary entrances are formed by the roof collapse of the cave (Fig. 4), especially in the places where the cave roof consists of thin and weak basalt layer. Most volcanic cave entrances in the study area of Vietnam are secondary origin, except some ones of P20, P8 caves.
Fig. 2. The vertical primary entrance with the 25m depth of P20
cave in Krong No district [3]. Рис. 2. Основной вертикальный вход в пещеру Р20 глубиной 25 м в районе Кронг-Но [3].
Fig. 3. The P20 cave entrance seen from the
cave floor [3]. Рис. 3. Вход в пещеру P20, видимый с пола пещеры [3].
Fig. 4. Secondary entrance formed by roof collapse in C1 cave, Krong No
district [3]. Рис. 4. Второй вход в пещеру С1, образовавшийся в результате обвала крыши, район Кронг-Ho [3].
The length of the caves varies from 81.0 m-1066.5 m, reaching a series of Southeast Asian records about the length of lava caves: the 1st, 2nd, 3rd positions, all belong to volcanic caves in Krong No district, Dak Nong province, Vietnam [2, 4, 7].
The depth of the cave (or the cave roof) has quite large variance, usually ranging from 0.5 m to few meters, especially there are some caves with several tens of meters such as cave P8 and cave P20 in the north and northwest of Chu B'Luk volcano, Krong No district, Dak Nong province [4].
Branching and flooring features. Branching is quite common, the level of branching from simple to complicated. The openness/distance between branches depends on features of the ancient terrain surface. The flatter ancient terrain is the larger the branching distance is as in the map of cave C7 in Krong No distr. (Fig. 5). In narrow ancient valleys the less branching is, the narrower branch distance is, even no branching such as in cave P20 map in Krong No distr. (Fig. 6).
Fig. 5. Outline of Cave C7 in Krong No Fig. 6. Outline of Cave P20 in Krong No
district, Dak Nong province. Cartograpy: district, Dak Nong province. Cartograpy: Kat-
Yuriko Chikano, Hirohisa Kizaki (NPO Vulca- suji Yoshida (NPO Vulcano-Speleological-Soci-
no-Speleological-Society, Japan). ety, Japan).
Рис. 5. Схема пещеры C7 в районе Рис. 6. Схема пещеры P20 в районе
Кронг-Но, провинция Дак Нонг. Карто- Кронг Но, провинция Дак Нонг. Картогра-
графия: Юрико Чикано, Хирохиса Кизаки фия: Кацудзи Йошида (НПО Вулкано-Спе-
(НПО Вулкано-Спелеологическое Обще- леологическое Общество, Япония).
ство, Япония).
The flooring features of the volcanic caves are complicated in both width and depth. Some caves show only one floor such as cave C8, cave C9, cave P11, etc. (Fig. 7), meanwhile there are some caves with 2-3 floors such as cave C0, cave A1, etc. [4]. Especially, in cave C7, tube-in-tube structure with several cave floors could be amazingly observed (Fig. 8).
Studying the distribution characteristics and structure of lava caves will contribute to demonstrate the close relationship between material composition and hydrodynamic characteristics of lava flow, ancient terrain characteristics and tectonic activities in the area. This relationship will be mentioned in another paper.
Characteristics of cave interior formations. The interior formations of lava cave contain information reflecting exactly its formation mechanism, characteristics and divergence of lava flows, physical, chemical and hydrodynamic characteristics of lava flows, flow direction, interaction between lava flows with each other and with the surrounding environment, etc. Lava trace marks and lava shelves are remnants of lava flows through the cave. Origin lava
ЩШ;
Fig. 7. The sole floor of cave P11 in Krong No district, Dak Nong province. Photo: Luong Thi Tuat.
Рис. 7. Одноуровневая пещера P11 в районе Кронг Но, провинция Дак Нонг. Фото: Луонг Тхи Туот.
Fig. 8. Complicated tube-in-tube structure in cave C7, in Krong No district, Dak Nong province. Photo: La The Phuc.
Рис. 8. Сложная структура «труба в трубе» в пещере С7 в районе Кронг Но, провинция Дак Нонг. Фото: Ла Зе Фук.
cave entrances or skylights are remnants of the escaped-gas domes during cave forming. Lava falls are remnants of the later lava flows into the cave through a lava window or a skylight, etc. In addition, there are many other unique features in the lava cave that could not be mentioned in the limited frame of this paper.
Relevant heritage. Heritage related to the volcanic cave of Vietnam are rich and diverse, including geological heritage, biodiversity, cultural heritage. Each volcanic cave itself is a real geoheritage site. In each cave, there are many geoheritage types according to the GILGES classification of UNESCO, including: cave interior formations (stalactites, lava flow, etc.) -rocky heritage; fossils (lava tree mold) - paleontological heritage; fissures/faults - tectonic heritage, etc. Cultural heritage in the cave has two types: archaeological and historical relics. In these caves, there are many endemic species of photophilic and scionphyte creatures. Each of these heritage types will be presented in the next special papers.
Heritage values. Volcanic cave is a mixed heritage, with many educational, aesthetic and economic values. The cave's characteristics and geological heritage in volcanic cave are valuable evidences on the geological context, cave formation mechanism, development history and stage in the eruption process, paleogeographic characteristics, interaction of lava flows with the surrounding environment, hydrodynamic characteristics of lava flow, etc. Archaeological sites in volcanic caves in Vietnam include the following types: residence relics, factory relics and burial relics, with many scientific and practical values, unique and rare in volcanic caves founded in Southeast Asia as well as in the world. It can be said that the geological, cultural heritage and biodiversity in volcanic caves in Vietnam are useful for research on archeology, anthropology, residential development history in The Central Highlands socio-cultural history flow and many other related issues. Each cave is a natural visual toolkit for teaching, learning and studying geology, volcanic cave biodiversity, archeology and socio-cultural development history.
Discussions.
Volcanic cave origin and formation mechanism. Obviously, cave in karstic rock (limestone) is exogenous origin. However, caves in volcanic rocks have two types of origin: mainly endogenous and rare exogenous. The endogenous volcanic cave is formed during
the eruption thanks to the principle of gas concentration to create an empty cavity and shrink volume when lava source drain out of lava tube. When the hot lava is low in viscosity, rich in volatile substances flows on the terrain surface, its surface exposes to the air (above) and covers the soil (below). Therefore, lava flow is firstly cooled down and solidified to form tube-shaped arches, while gas accumulates in the upper part of the lava tube dome, and hot lava still flows inside that tube (Fig. 9). The amount of gas released from the lava rich in volatile have been continuously increased by the amount of gas generated by the thermal interaction between lava and the environment. This amount of gas has been concentrated in the lava tube dome, creating a cavity in the tube and increasing the pressure of lava flow. When the eruption stops, the supply of lava and heat have been cut off, lava flow drains out toward downstream, then volume shrinks, gravity is balanced and all systems cool down to form a lava cave [1].
Fig. 9. Lava tube formation [1]. Рис. 9. Формирование лавовой трубки [1].
In some rare cases, there may also be existed exogenous caves in volcanic rock regions, which are related to exogenous processes such as weathering, erosion, and formed after a long period of volcano eruption inactivity. Those exogenous caves occur in the areas, where basalt eruption activities (especially the central eruption/explosive eruption) happened with several alternation eruptions with different compositions. During each explosion, it is firstly volcanic ash, next is the basaltic rock. The eruptions overlap with each other will create an alternation between the layers of volcanic ash and solid basaltic rock, with different physical properties: interstratification between volcanic ash with the loose structure and basaltic rock with the solid structure. As a result, the loose ash layers will be firstly destroyed and washed out due to exogenous processes (weathering, eroding, etc.), create exogenous volcanic cave in some volcanic regions.
On the basis of document about lava cave in the world and the research results of a series of lava caves in Krong No district show that: lava caves in the area have endogenous origin with the suitable formation mechanism mentioned above with 3 evidences: Firstly, stalactites in the cave are typical for endogenous origin, which are composed of basalt rocks to form caves. Secondly, cave interior formations (lava levees, lava shelves, lava trace marks, lava linings, lava balls, lava seals, lava windows, lava falls, pahoehoe, lava A'a, ect.) have typical endogenous features and homogeneous composition of basalt rock forming the cave. Thirdly, traces of exogenous processes such as weathering, eroding haven't been found in the caves; except some exogenous/secondary stalactites, formed by seeping water along cracks down from the roof.
The relationship with bedrock. Volcanic caves in Vietnam have been found to be related to the youngest foam/porous basalt rocks (0.7-0.2 Ma) [4], so what about the older ones? Based on the formation mechanism, the lava has low viscosity, rich in volatile substances af-
ter erupting, cool down to form foam basalt rock, porous basalt rock containing caves. It may be noticed that signs of volcanic cave in the field are the foam basalt rock and porous basalt rock. In the older basalt formations than the Quaternary one, they also have the foam basalt rock, porous basalt rock, but no cave was found there; it was weathered and collapsed possibly due to poor conservation capacity. On the other hand, there are quite large gaps (1-2m) at a depth of several tens of meters in some boreholes in the basalt rock area of Neogene age in The Central Highlands. These gaps may be underground caves - this is an issue that needs to be studied to clarify the relationship between the volcanic cave and bedrock both in composition and age.
It was thought that "the earlier caves" in the oldest basaltic formations in The Central Highlands there may be collapsed due to weathering and bad conservation conditions.
About conservation and exploitation for socio-economic sustainable development.
Volcanic caves in Vietnam have great heritage potential, valuable to foster tourism to obtain high benefits. The heritage values in the volcanic caves have been studied in detail by scientists of Vietnam Museum of Nature, including: natural heritage (both geological heritage and biodiversity) and cultural heritage (archaeological and historical relics) for conservation on-site and socio-economic sustainable development exploitation, with the principle: Conservation for exploitation and exploitation for conservation. Volcanic caves have been divided into different purposes, including: studying natural sciences (geology, natural development history, etc.), studying social sciences and geo-culture (archeology, anthropology, ethnic evolution history, etc.), popular tourism and limited tourism developing. Before tourism developing, every cave must be required studying to assess the safety, implement necessary treatments (such as preventing roof collapse, etc.) to ensure safety for visitors. Besides on-site conservation to comprehensively exploit all heritage values, the protection of the surrounding environment is also strictly required for conservation and sustainable development purposes.
Conclusion.
1. Volcanic caves in Vietnam were discovered since 2007, and had been affirmed endogenous origin. They were formed from basaltic lava flows with low viscosity and volatile substances, erupted from craters aged 0.7-0.2 Ma.
2. Distribution characteristics, stratified branching, interior formations in the lava cave depend on the ancient terrain surface features, lava flow composition, physical-chemical properties and hydro-dynamics of lava flows, interaction between lava flows with surrounding environment and between lava flows in different eruption phases of the volcano.
3. Besides the outstanding geological value, volcanic caves in Vietnam contain many unique and rare values on biodiversity and cultural heritage (prehistoric archeology). It is considered as a special mixed heritage, non-renewable resource and very sensitive to human activities and nature impact as well. Therefore, it needs to be comprehensively researched for protection, conservation, management and exploitation for sustainable socio-economic development.
4. It's a very short time since 2017 up to now for studying all comprehensive values of the volcanic cave system in The Central Highlands. Therefore, it needs investment, research cooperation and development with domestic and international organizations/individuals in all related fields.
Acknowledgments. The paper was supported by the Project TN17/T06 of The Vietnam National Programme Tay Nguyen in the 2016-2020 stage.
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