Longgang volcanic field

Coordinates: 42°20′N 126°30′E / 42.33°N 126.5°E / 42.33; 126.5^[1]

Longgang is a volcanic field in Jilin Province, China. It is also known as Chingyu, Longwan Group or Lung-wan Group.^[2] This volcanic field contains over 164 individual centres in the form of crater lakes, maars and volcanic cones and covers a surface area of 1,700 square kilometres (660 sq mi). The field is forested and mostly undisturbed by human activities.

Longgang is located in an area of northeast China where the subduction of the Pacific Plate beneath the Eurasian Plate, along with other tectonic processes triggers volcanic activity. Some other volcanoes are found in the region, most importantly Changbaishan volcano.

Volcanic activity in the field goes back to the Pleistocene. Jinlongdingzi cone 350 AD or 460 AD suffered a major Plinian eruption, which was accompanied by ash fall and the extrusion of a lava flow. Future volcanic activity in the field may result in hazardous Plinian eruptions.

Geography and geological context

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 Lonngang are back-arc volcanoes associated with processes involving the sinking Pacific Plate slab.^[10] The Fushun-Mishan fault may further influence Longgang volcanism.^[11]

Geology and geomorphology

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, Jilin are located in the eastern part of the field.^[16][17] The area was historically sparsely habitated 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 / 42.03333°N 126.43333°E / 42.03333; 126.43333^[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]

Petrology

Rocks include basalt and trachybasalt.^[1] Olivine basalt, basanite and tholeiite have been found as well,^[5] with smaller amounts of trachyandesite.^[27] Tephras from different eruptions do not display significant differences in composition.^[28] Ultramafic xenoliths have been found in the rocks.^[11]

Phenocrysts include olivine, plagioclase and pyroxene. Alkali basalts have few vesicles and a gray colour. Basanites contain more olivine and less plagioclase/pyroxene phenocrysts.^[29] No non-basaltic magmas have been erupted at Longgang.^[30]

Partial melting of metasomatism-influenced mantle may be the origin of Longgang magmas. Some of the magmas underwent fractionation after their formation, but not all.^[31] The magmas appear to come from a magma chamber at a depth of 35–50 kilometres (22–31 mi),^[32] and also underwent fractional crystallization and assimilation of crustal materials.^[33]

Environment

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]

Eruptive history

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 (0.0037–0.0050 in/Ms).^[40]

References

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