A direct line to the deep earth
The northern part of Hainan Island is covered by a volcanic field that is surprisingly young. This field, along with the Leizhou Peninsula to the north, forms the Leiqiong volcanic field, which covers about 8,000 square kilometers and contains at least 177 recognized volcanoes. The source of this activity is a thermal anomaly known as the Hainan mantle plume. Seismic tomography images show this plume extending deep into the Earth, possibly all the way to the core-mantle boundary.
The lavas that erupted here are primarily oceanic island basalts (OIBs), whose geochemical signatures give scientists a sample of the deep mantle. Isotopic analysis of these basalts indicates the plume is not a simple upwelling of hot rock; it seems to be a complex mixture that includes materials from ancient, subducted oceanic slabs that were drawn into the deep mantle. Petrological studies estimate the excess temperature of the plume to be between 140 and 200 Kelvin, with a diameter of around 80 to 160 kilometers.
A timeline of recent fire
The volcanic activity in the Hainan field spans a wide range, from the Miocene epoch (around 34.78 million years ago) to the very recent Holocene. While older eruptions created extensive flood basalts, the most recent activity has been concentrated in the Quaternary period. This period includes more explosive phreatomagmatic eruptions, which occur when magma interacts with groundwater, creating broad, low-relief craters called maars.
Scientists have used various methods, including Argon-Argon (Ar-Ar) and K-Ar dating, to establish a timeline of these eruptions. The most recent phase of volcanism occurred during the Pleistocene and Holocene epochs. The Ma'anling and Leihuling cones are among the best-preserved, part of a cluster of about 30 cones aligned along an east-west fracture. Some eruptions are recent enough to be recorded in historical documents. A small fissure eruption occurred at the Lingao cone in 1883, and another took place from the Nansheling ridge in June 1933. Geochronological data from some lava flows indicate eruptions as recent as 10,000 years before present.