What "island genes" are hidden in Hainan? Uncovering the geological "epic" of the volcanic coast

(Outstanding popular science works of Hainan University's "Popular Science in Chinese Universities" - Graphics and Texts)

The geological epic of the volcanic coast: decoding the time and space code of Hainan's 10,000-year-old lava

On Hainan Island, located in the southern tip of my country, volcanoes and volcanic rocks record the geological changes of this land for billions of years. They are masterpieces of nature and vivid witnesses of the history of earth evolution. Volcanoes write the book of the earth and forge the genes of islands.

On the vast land of Hainan Island, volcanoes are not only the recorders of geological evolution, but also the creators of the symbiosis of all things. Hainan's volcanic activities have gone through four eruption periods: the Eocene, Miocene, Pleistocene, and Holocene. The skeleton of this island has gradually taken shape in the tempering of magma and time.

In the Eocene, the global plate movement was active, the crust in the South China Sea region was unstable, and the volcanic activity in Hainan was weak. Small-scale magma upwelled, forming a small amount of basalt eruption and small volcanic deposits in some areas, laying the material foundation for subsequent volcanic activities. When time entered the Miocene, the tectonic hand of the earth began to reshape this sea area - the tearing of the Leiqiong Rift allowed the magma to find an eruption channel, and the volcanic cone group composed of quartz tholeiitic basalt pierced the sea level, forming quartz tholeiitic basalt and olive tholeiitic basalt, shaping many volcanic landforms such as volcanic cones and volcanic terraces, changing the regional topography and land and sea pattern. If the first two stages are the prologue of the volcanic epic, then the Pleistocene ushered in the most magnificent chapter. During the Pleistocene, the world was in a period of alternation between the glacial and interglacial periods. During this period, more than 100 volcanoes in northern Hainan awakened one after another, and Hainan's volcanic activity entered its heyday. A large amount of magma erupted rapidly in various forms, forming more than 100 dormant active crater groups in northern Hainan, creating typical volcanic landforms such as the Shishan volcano group. They are like the eyes of the earth, silently gazing at the flow of time. Ma'anling Volcano is a typical representative of them. Its unique double crater shape, one large and one small, nestles against each other, as if telling an ancient story. The Holocene plate movement was relatively stable, and the Hainan crust still had a certain degree of activity. There were still volcanic eruptions in the early days. For example, Ma'anling Volcano has erupted twice in the past 10,000 years, forming small volcanic deposits and lava flows in the local area, transforming the surrounding topography and providing unique geological landscapes and research samples. In the laboratory of the Hainan Provincial Bureau of Geology, a laser ablation mass spectrometer analyzed volcanic glass inclusions. The data showed that the last eruption of Haikou Shishan Volcano was about 10,000 years ago. The eruption column was 25 kilometers high and the pyroclastic flow covered an area with a radius of 15 kilometers within 8 minutes.

The afterglow of the Holocene is still lingering, and the laser ablation mass spectrometer in the geological laboratory reveals that in the shocking eruption ten thousand years ago, the 25-kilometer-high volcanic cone left a fiery mark on the lattice of basalt. It is also these solidified flames that gave birth to the most exquisite geometric code of nature. When the hot magma meets the calm laws of physics, a perfect form that amazes mathematicians is born. When volcanic lava cools, it forms basalt, but naturally formed basalt has regular hexagonal columnar joints. There are many mathematical principles behind this: based on the principle of minimum energy, the total side length of regular hexagons is the smallest when they are densely laid on a plane, which can make the surface energy of the system the lowest and achieve a stable state; from the perspective of symmetry and balance, hexagonal columnar joints are highly symmetrical, with uniform forces and physical properties in all directions and stable structures; according to the principle of geometric growth, in an isotropic medium, when the crystal growth interface evolves, the growth rate in all directions of the three-dimensional space is balanced, and it is easy to form a hexagonal columnar shape; according to the principle of stress distribution, the hexagonal columnar structure can evenly distribute the thermal stress generated by the cooling of the lava, avoid stress concentration, and promote the appearance of this joint form. Under an electron microscope, the pores of basalt show a fractal structure with a porosity of up to 35%. According to the "Qiongzhou Prefecture Records" of the Qing Dynasty, local people called volcanic rocks "honeycomb stones". It was also because of its high porosity that Hai Rui of the Ming Dynasty used porous basalt to make "volcanic air-conditioned rooms" that were warm in winter and cool in summer.

Not only that, these natural holes are also storage for condensed water and a source of inspiration for the wisdom of our ancestors. In the volcanic rock fields of Yongxing Town, ancient wisdom and modern technology are having a dialogue across time and space. In the "infiltration irrigation field" paved with basalt fragments by 72-year-old Grandpa Wang, this "stone-building field" technique inherited from the Song Dynasty has been rejuvenated due to the porous characteristics of volcanic rocks. The pitaya roots can autonomously find condensed water in the cracks of the rocks, and the yield per mu is 40% higher than that of conventional planting. In the experimental field of Hainan Academy of Tropical Agricultural Sciences not far away, an unmanned tractor equipped with the Beidou system is sowing super rice seeds cultivated in volcanic rock matrix-trace elements extracted from basalt, which makes the selenium content of rice increase by 3 times. This is the latest movement of the dance between rocks and life. If nitrogen-fixing bacteria are cultivated with crushed volcanic rocks, the organic matter content of degraded red soil can even be increased by 5 times in 3 years.

The gift bestowed by the volcano continues to sublimate at the height of human civilization. The 759 basalt components of the Meilang Sister Towers perform a duet of mechanics and aesthetics with a pure mortise and tenon structure. The Meilang Sister Towers can withstand winds up to level 16. At the same time, due to the natural armor formed by the carbonated surface of the volcanic rock, which can increase the acid rain resistance of the ancient building by 70%, the Sister Towers are also more resilient in the acid rain erosion.

Figure 1 Meilang Sister Tower

The lava tunnels created by the flow of magma weave a fantasy world underground. In the cave system connected by cavities, the huge dome formed by volcanic action and the stalactites deposited by dripping water coexist, which is a mixture of two cave forms, witnessing the encounter of two geological forces across thousands of years.

Figure 2 The 72 Caves of Haikou Volcano

When the morning light dyes the volcanic coast of Lingao Cape red, iron oxide and silicon dioxide depict blue-green rust spots on the rock strata, which is a medal awarded by time. In the Hainan Provincial Geological Museum, VR equipment is taking children through thousands of years to reproduce the magnificent scene of volcanic eruptions; at the exploration site 30 kilometers away, engineers are cracking the code of the ancient volcanic channel, trying to transform the former path of magma into a future gas storage. From the wisdom of the ancestors who built fields with stones to the exploration of contemporary scientists to crack the rock code, from the cracks in the infiltration fields to the positioning signals of the Beidou system; from the mechanical inheritance of mortise and tenon structures to the innovative use of underground space, this island cast by volcanoes has always been writing a magnificent epic of the co-evolution of man and nature.