A landscape forged by fire
The Wudalianchi volcanic field is a geological rarity in northeastern China, covering over 500 square kilometers with 14 distinct volcanic cones. This landscape is exceptionally young in geological terms. The two most recent volcanoes, Laoheishan and Huoshaoshan, were formed by eruptions between 1719 and 1721. These events were significant enough to be recorded in official chronicles of the Qing Dynasty. The eruptions produced extensive lava flows, some pahoehoe (ropy) and some a'a (blocky), that covered approximately 65 square kilometers. This molten rock dammed a local river, creating the five interconnected lakes that give the area its name, "Wudalianchi" or "Five Great Connected Lakes". The entire field is a well-preserved collection of volcanic landforms, earning it the nickname "Natural Volcano Museum".
Beneath this recently active surface, a complex plumbing system exists. Geophysical surveys suggest the presence of a magma chamber at a depth of about 7 to 15 kilometers below the Weishan volcano area. This deep heat source is the main cause of the region's unique hydrological characteristics, even though there are no hot springs at the surface. Instead, the area is famous for its numerous cold mineral springs.
Reading the water's story
The cold springs of Wudalianchi are natural laboratories for studying post-volcanic processes. Though their temperatures are low, often below 10°C, their chemical makeup shows a connection to the Earth's mantle. The water is high in carbon dioxide, with a pH often below 6.5, and contains high concentrations of dissolved minerals like iron and silicon.
Scientists use isotopic analysis to find the origins of the water and its dissolved gases. The carbon dioxide in the springs has δ13C values ranging from -8.77‰ to -4.53‰, which is consistent with gas released from the Earth's upper mantle. This indicates that gases from the deep magma chamber are rising through faults and dissolving into the groundwater. This deep water, known as magmatic water, then mixes with meteoric water, rain and snow that has seeped into the ground.
Researchers also analyze strontium isotopes (87Sr/86Sr) to understand the interaction between the water and the surrounding volcanic rocks. The local potassium-rich basalt has a specific strontium signature, which is imparted to the water as it circulates. By comparing isotopic ratios in deep springs, shallow springs, and the lakes, scientists can model how much water is contributed by each source. This analysis shows that the lakes are fed primarily by the upwelling mineral springs, which are a mix of ancient magmatic fluids and recent surface water.