Longgang volcanic field

Northeast China is the site of major Cenozoic volcanism.[3] The Pacific Plate subducts beneath the Eurasian Plate, generating volcanic activity.[4] This subduction process 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] 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,[6] and the sole volcano to erupt felsic magma.[7] Tephra from this volcano has been found in the Longgang field.[8] Other volcanoes in the wider region include Jingbo Lake, Keluo, Wudalianchi and Xianjindao. The last of these is in North Korea.[6][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.[9] 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.[10] The Fushun-Mishan fault may further influence Longgang volcanism.[11]

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][12] 9[13] or 8 maars are found in the field's western half.[14] These maars have depths ranging between 15–127 metres (49–417 ft).[15] The towns of Houhe, Jilin and Jingyu are located in the eastern part of the field.[16][17] The area was historically sparsely inhabited and it was turned into a national forest park in 1992.[18]

Jinlongdingzi is a 240 metres (790 ft) high,[19] 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.[20][1] Another volcano formed by basalt and tephra is known as Dayizishan.[21]

A number of crater lakes surrounded by forests exist in the Longgang field.[22] 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.[20] This lake was formed by phreatomagmatic activity.[13] Similar research has been performed on other maars.[15] This maar is surrounded by a 10–119 metres (33–390 ft) high rim composed of pyroclastic material.[23] 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.[24] 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).[23] Erlongwan maar covers a surface of 0.3 square kilometres (0.12 sq mi).[25] Other maars are Dalongwan, Donglongwan, Longquanlongwan, Nanlongwan and Sanjialongwan.[22]

The volcanic field is constructed on a basement formed by rocks of Archaean age. The field is located at the edge of the North China Craton.[15] At Erlongwan, this basement is formed by the Anshan Migmatites and represented by amphibolite, gneiss and quartzites.[26]

Vegetation around Sihailongwan lake consists of broadleaf-coniferous forest.[14][20][23] Some lava flows erupted from Jinlongdinzi now feature vegetation distinct from the pre-flow vegetation, including Abies nephrolepis and Betula platyphylla.[34] Human influence is limited.[22]

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.[23]

Volcanism occurred over three different phases, the Xiaoyishan period 2,150,000 - 750,000 years ago and the Longgang period 680,000 - 50,000 years ago. The third period occurred during the Holocene.[12] The oldest volcanic rocks are 27.3 million years old.[5] Most centres formed between 680,000 and 50,000 years ago.[35]

Tephra layers found in lakes indicate that explosive volcanism was common in the field during the late ice age.[28] Dayizishan and Diaoshuiu formed 71,000 ± 9,000 and 106,000 ± 13,000 years ago, respectively.[36] Varve chronology has indicated the occurrence of eruptions 11,460 and 14,000 years before present.[37] 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.[38]

The youngest eruption occurred about 350 AD or 460 AD, forming Jinlongdingzi and causing ash fall east of the cone. This basaltic ash formed black layers in lakes.[37][1] These are known as the Sihai layers and cover a surface area of 330 square kilometres (130 sq mi).[19] 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.[19] This eruption was probably forceful enough to damage the surrounding vegetation and trigger wildfires.[39] A long lava flow was erupted from the western flank of the cone, flowing at least 26 kilometres (16 mi) and into lake Dalongwan. A number of trees were encased in the lava and dates obtained on them indicate that the long lava flow is older than the one that entered lake Dalongwan.[34] They may thus be products of separate eruptions.[37] The youngest date mentioned about the field is 785 years before present.[5]

Volcanic activity at Jinlongdingzi may still constitute a danger, especially Plinian eruptions.[37] 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.[19][21] Hot springs are found in the area,[5] and uplift on the ground at a rate of 3–4 millimetres per year (0.12–0.16 in/year).[40]