Longgang volcanic field

Northeast China is the site of major Cenozoic volcanism.[3] The Pacific Plate subducts beneath the Eurasian Plate,[4] along with back-arc spreading and continental rifting triggers volcanic activity in the region. Geothermal resources are also widespread.[3]

Longgang belongs to a group of volcanoes in the Changbai Mountains.[5] 200 kilometres (120 mi)[6] farther east lies the Changbaishan volcanic field, including Changbaishan volcano on the China-North Korea border.[4] This volcano is the most active and dangerous in the area,[7] and the sole volcano to erupt felsic magma.[8] Tephra from this volcano has been found in the Longgang field[9] and tephra deposits at Longgang have been used to reconstruct volcanism in the wider region.[10] Other volcanoes in the wider region include Jingbo Lake, Keluo, Wudalianchi and Xianjindao. The last of these is in North Korea.[7][3]

Seismic tomography of Longgang volcano has evidenced a 200 kilometres (120 mi) low-velocity anomaly, which tilts north beneath this depth until a depth of 400 kilometres (250 mi). Some neighbouring volcanoes feature similar anomalies.[11] Based on these structures, it has been concluded that Changbaishan and Longgang are back-arc volcanoes associated with processes involving the sinking Pacific Plate slab.[12] The Tan-Lu fault zone[13] and the Fushun-Mishan fault may further influence Longgang volcanism.[14]

The field covers a surface area of 1,700 square kilometres (660 sq mi) with 164 individual centres.[5][4] It contains lava flows, over 150 cinder cones and 8 tuff rings.[1][15] 9[16] or 8 maars are found in the field's western half.[17] These maars have depths ranging between 15–127 metres (49–417 ft).[18] The towns of Houhe, Jilin and Jingyu are located in the eastern part of the field.[19][20] The area was historically sparsely inhabited and it was turned into a national forest park in 1992.[21]

Jinlongdingzi is a 240 metres (790 ft) high,[22] horseshoe-shaped cone formed by lava bombs and scoria(999 metres (3,278 ft) elevation, 42°2′0″N 126°26′0″E[2]), it also known as Gold Dragon Peak.[23][1] Another volcano formed by basalt and tephra is known as Dayizishan.[24]

A number of crater lakes surrounded by forests exist in the Longgang field,[25] which formed when water filled volcanic craters[26] Among the volcanic centres of Longgang, the 0.39 square kilometres (0.15 sq mi) large Sihailongwan maar southeast-east of Jinlongdingzi has been used for dating volcanic activity through sediment layers in the lakes.[23] Similar research has been performed on other maars.[18] This maar is surrounded by a 10–119 metres (33–390 ft) high rim composed of pyroclastic material[27] and was formed by phreatomagmatic activity.[16] 0.85 square kilometres (0.33 sq mi) large Hanlongwan is a dry maar north of Jinlongdingzi and is surrounded by a 10 metres (33 ft) high rim.[28] Xiaolongwan maar is surrounded by a 5–100 metres (16–328 ft) high crater rim and covers a surface area of 0.079 square kilometres (0.031 sq mi).[27] Erlongwan maar covers a surface of 0.3 square kilometres (0.12 sq mi).[29] Other maars are Dalongwan, Donglongwan, Longquanlongwan, Nanlongwan and Sanjialongwan;[25] the Gushantun peatland is a former maar as well.[30] Lava flows from the field dammed the Hani River during the late Pleistocene, forming a lake that later developed into the Hani peatland.[31] Overall, more than 100 crater lakes and lava-dammed lakes and over 20 peatlands in maars occur in the Longgang volcanic field.[26]

The volcanic field is constructed on a basement formed by rocks of Archaean age,[18] such as the Anshan Migmatites consisting of amphibolite, gneiss and quartzites[32] form the basement.[31] The field is located at the edge of the North China Craton[18] which underlies the volcanic field.[31]

Vegetation around the Hani peatland[31] and Sihailongwan lake consists of broadleaf-coniferous forest.[17][23][27] Peatlands have developed in some parts of the field.[31] Some lava flows erupted from Jinlongdinzi now feature vegetation distinct from the pre-flow vegetation, including Abies nephrolepis and Betula platyphylla.[40] Human influence is limited.[25]

Airmasses come from central Asia during winter, while in summer southeasterlies from the Pacific Ocean dominate. The climate is overall cool.[31] The average air temperatures at Xiaolongwan and Sihailongwan are 4 °C (39 °F), with the lakes freezing between November and April. Average precipitation is 760 metres (2,490 ft), mostly during summer[27] and makes the climate monsoonal.[31]

The Longgang volcanic field contains climate records that have been used to reconstruct the former climate of the region, both in the form of deposits in lakes[41] and in peatlands.[42] Some of these records may go back over 12,000 years.[43] Fluctuations of the strength of the winter and summer monsoons have been reconstructed from the peatlands.[44] Two cold periods may coincide with two stages of Heinrich event 1 over 10,000 years ago.[45] During the Medieval Climate Anomaly the climate was moister than during the Little Ice Age.[46] Past environmental changes, such as peat growth[47] and red tides in lakes, also leave records.[48]

Volcanism occurred over three different phases, the Xiaoyishan period 2,150,000 - 750,000 years ago, the Longgang period 680,000 - 50,000 years ago and a third period during the Holocene.[15] The oldest volcanic rocks are 27.3 million years old.[5] Most centres formed between 680,000 and 50,000 years ago.[49] Dayizishan and Diaoshuiu formed 71,000 ± 9,000 and 106,000 ± 13,000 years ago, respectively.[50]

Tephra layers found in lakes indicate that explosive volcanism was common in the field during the late ice age.[34]Varve chronology has indicated the occurrence of eruptions 11,460 and 14,000 years before present;[51] the first may have occurred at Jinlongdingzi where a second eruption occurred between 10,340-10,250 years ago.[52] A tephra layer dated 15 BC - 26 AD is chemically similar to the one of Jinlongdingzi and may stem from an eruption in the Longgang field.[53]

The youngest eruption occurred about 350 AD or 460 AD at Jinlongdingzi;[51][1] although another date mentions an eruption 785 years before present[5] and a lava flow that entered lake Dalongwan may be only a few centuries old.[51] The 350/460 AD eruption of Jinlongdingzi caused ash fall east of the cone. This basaltic ash formed black layers in lakes[51][1] and is known as the Sihai layers; it covers a surface area of 330 square kilometres (130 sq mi).[22] This eruption was the second largest eruption of a Chinese volcano in the past 2000 years,[4] with a total volume of 0.024 cubic kilometres (0.0058 cu mi) of tephra that fell from a 7,000–8,000 metres (23,000–26,000 ft) high eruption column.[22] It was probably forceful enough to damage the surrounding vegetation and trigger wildfires.[54] A long lava flow was erupted from the western flank of Jinlongdingzi and went on for at least 26 kilometres (16 mi).[40]

Volcanic activity at Jinlongdingzi may still constitute a danger, especially Plinian eruptions.[51] A new eruption may result in damage to property or fatalities especially east of the cone and the field is thus considered a potentially active volcano.[22][24] Hot springs are found in the area,[5] and there is ongoing uplift at a rate of 3–4 millimetres per year (0.12–0.16 in/year).[55] Seismic tomography shows evidence of crustal melts persisting beneath Longgang.[56] The 2008 Wenchuan earthquake triggered seismicity in the Longgang volcanic field.[57]

Rocks from the Jinlongdingzi eruption have been used to simulate the properties of lunar rocks for applications in lunar exploration.[58]

• PRELIMINARY VOLCANIC HAZARD ZONATION IN JINLONGDINGZI VOLCANO, LONGANG VOLCANO AREA, JILIN PROVINCE, CHINA