The main body of Magong City is basalt volcanic lava. Most of the lava all ejects from shallow sea to land mainly in the way of fissure ejection. Separated lava flow might connect together as a wild tableland. For example, on the wave erosion platform of the east side of Tongpan Islet, there is an arched land similar to a mouth of volcano in the shape of lotus blossom that was broken into blocks through the crust movement. The islands and islets are like square mountain terrain surrounded by steep cliffs with flat surface, and there are tall and steep pillar basalt ground around. Such as Ssuchiao Islet over the west sea area of Snake Head Mountain, or Fuching and Tungpan Islet over the South Ocean are all typical examples.

We call the it volcanic rock, when magma pours out of the surface of earth then cools down and becomes rock. The lava flows of Penghu area belong to basalt of volcanic rock. It ejects quietly, not so sticky and has a great mobility. Most situation is like this: magma flows out of surface along the fissure of layers, then move toward lower place along the landscape, then becomes lava tableland after cooling and solidification. The main minerals of basalts are pyroxene, plagioclase, and few olive rocks. It can be divided into dense and hole-shaped basalt. The former is often in grayish-black and the later has many stomas with big color variation, for example, Wenshih is formed in this kind of basalt.