China is rich in volcanic geological relics, and 25 national geoparks with volcanoes as the theme have been built. Each Geopark has its own unique volcanic landscape, which not only has a high ornamental value, provides us with a good place for leisure and entertainment, but also can better protect the geological relic landscape and ecological environment. More importantly, they are important witnesses to the evolution of the earth's history, a window through which we can spy on the earth's movement, and a natural place for popular science education in geology.

Under the influence of the neotectonic movement, five submarine volcanic activities occurred in the coastal-shallow sea environment around Weizhou Island. 1.42 million years ago, Weizhou Island played the symphony of Weizhou Island volcanic eruption. at that time, the eruption took place in the crater of Henglushan volcano, and the basaltic lava overflowed from the volcanic cone to flow around, forming a shield-shaped volcanic topography with high and low surrounding areas in the center of Weizhou Island. At present, the basalts formed in this period are mainly hidden below sea level, but some basalts can be seen along the intertidal zone when the tide goes out in the western corner and Houbeitang in the north of Weizhou Island. they are the founders of Weizhou Island. 36000 years ago, volcanic activity was dominated by exhalation magma eruption, which formed typical low-level crater and thick basement wave accumulation, experienced weathering and marine erosion, and shaped the geomorphological landscape of Weizhou Island today.

Xijiao Village, the upper right is the base wave accumulation, and the lower left is the intertidal basalt.

Crater

The volcanic activity of Weizhou Island has multiple eruption centers, and the crater is located in Henglushan and Nanwan respectively, which belongs to coastal-shallow sea submarine eruption and accumulation, so the shape of crater and cone is not as obvious as the volcanic eruption on land. Under the influence of weathering and wave erosion, the preservation of the original topography is not complete.

Mount Etna, Italy

Nanwan crater, located at the southern tip of Weizhou Island, belongs to a low-level crater and is nearly round in shape. The west, north and east are low-level volcanic rings, the south of the crater has been washed away by the waves, and there is a headland on the east, which is named Pig Tsai Ling because it looks like a naive pig. The depression in the center of the volcano is submerged by the sea, forming what is now a semi-annular bay-South Bay, with a diameter of about 2 kilometers. On the east, north and west of the Huokou Wall are nearly vertical cliffs 20-50 meters high, which are composed of base wave deposits caused by jet magma eruption and volcanic blocks, volcanic breccia and volcanic elasticity caused by magmatic eruption. The cliff retains rich and intact volcanic rock profiles and various eruptive structures, recording the process of volcanic eruption. Because there are agglomerate rocks, volcanic breccia and a large number of volcanic bombs a few meters to more than ten meters thick in the area of Pig Tsai Ling and the west, the volcanic debris from the crater to the outside has gradually changed from coarse to fine, thus it is inferred that there is a crater in Nanwan Port.

Panoramic view of Nanwan crater

The Henglushan crater is located on a hill 600 meters northwest of Henglu Mountain on Weizhou Island. it is a shield-shaped volcanic cone in physiognomy. due to the lush geomorphological vegetation, the crater can not be directly observed, but according to boreholes and other data, the volcanic rock in the middle of the shield-shaped low hill is 370 meters thick and the outward volcanic rock is gradually thinning, so it is speculated that there is a crater.

Base wave deposits and magmatic blasting eruptions

The thick base waves and volcanic debris formed by the Nanwan volcanic eruption formed a half-ring wall dozens of meters high around the crater. The tuffaceous sandstone is stacked in centimeter thin layers into a spectacular base wave accumulation, reflecting the characteristics of fire mountain eruption. The base wave accumulation extends to the northern part of the island, and the thickness gradually decreases. When the base wave accumulation reaches the seaside of the west corner village on the west coast, the base wave accumulation is a thin layer of tuffaceous sediment with a thickness of only 2 meters, and then gradually disappears to the north and replaced by coral reefs.

Extremely thick base wave accumulation profile on the north side of Jinan Bay

The eruptions formed by magmatic blasting mainly include volcanic tuff, breccia, agglomerate and other volcanic rocks of different sizes and volcanic debris such as volcanic elasticity and volcanic slag, as well as a small amount of volcanic lava flow. The Sea platform of crocodile mouth and piggy island is formed by strong fusion of the above magmatic blasting eruption products in a semi-plastic state.

Magmatic eruption product profile of alligator mouth

Volcanic bomb

A number of volcanic bombs and a large number of impact craters can be seen on the crocodile mouth Sea platform (figure / picture worm)

Volcanic bomb: commonly seen on Sea cliff and Sea platform along the Nanwan crater, the shapes are spindle-shaped, spherical, ellipsoidal, twisted or pancake-shaped, etc. They vary in size, ranging from a few centimeters to about 1 meter. The composition is basaltic lava, the intact volcanic shell is vitreous, the internal stomata are developed, and the stomata change from large to small from the inside to the outside. Due to long-term weathering and sea water erosion, some volcanic bombs have left only impact craters in their original position.

Figure 1: spindle-shaped volcanic bomb; figure 2: spherical volcanic bomb; figure 3: ellipsoidal volcanic bomb; figure 4: pore distribution inside volcanic bomb

Staggered bedding and prosodic bedding

Prosodic bedding on the section of Qiwucaitan

Staggered bedding is widely developed near the Nanwan crater, especially in the area of Dishuidanping. The cause is that the pyroclastic surge is a low-density fluid, which is mainly composed of magma and surrounding rock debris carried by liquid water and water vapor. Because the proportion of liquid water is usually high, and the blown debris is transported and accumulated in dispersed particles, clear structural signs of hydrodynamic transport and accumulation can be seen in the accumulation. such as staggered bedding, rhythmic bedding and so on. When the layer reason staggered bedding changes into horizontal bedding, it shows that the sedimentary environment of the formation changes from shallow water area to deep water area. Obvious rhythmic bedding can be observed on the Wucaitan section, which is composed of several volcanic eruption cycles. The upper part of each cycle is fine clastic tuff with well-developed layered structure, and the lower part of the cycle is coarse clastic tuff, which is composed of volcanic gravel, volcanic rock blocks, volcanic elasticity and so on. This rhythmic layer of clastic grain size indicates that it was pulsating in the early stage of Nanwan volcanic eruption. In this eruption of a mixture of magma and water, the intensity of each explosion is from strong to weak, so different grain sizes of debris are formed. When moving debris of different sizes, if the force that drives the debris forward is less than the settling force of the debris, the debris will be deposited. Because the settling force of large-grained debris is usually greater than that of small-grained debris, large-grained debris usually sinks to the bottom of the accumulation layer first, thus forming these rhythmic layers with particle size changes.

Figure 1, 2, 3: staggered bedding; figure 4: prosodic bedding and differential weathering

Climbing bedding

The Crocodile mouth is located at the southwest end of Weizhou Island, and the interbedded occurrence of base wave accumulation with volcanic breccia, agglomerate rocks and volcanic elasticity can be observed on the southernmost sea erosion cliff section, which clearly shows the characteristics of jet magma eruption. In addition, the unique climbing bedding can also be observed in this section, the bedding formed by the base wave accumulation is inclined to the South Bay Sea, and the base wave accumulation climbs outward along the inner wall of the fire mouth from