Black Pearls in the Ocean:The Basalt of Penghu

From a distance, when sailors travel from the South China Sea into the Taiwan Strait, the first thing they see-glaring into their eyes-is the sun shining off impressive basalt columns in the ocean. These upright pillar-shaped basalt formations, which rise as high as 60 or 70 meters, glitter like fish scales, reflecting the sunlight between the blue sky and the vast sea. The blinding light reflecting off the textured surface is both mysterious and moving.

Lava story

Between eight and 17 million years ago, in the southern part of the Taiwan Strait, magma forced its way through fractures in the earth's crust. The lava poured through to the surface, accumulating and covering the earth in large and small piles, creating the impressively beautiful Penghu archipelago.

Compared to an intense volcanic eruption on land, when an entire mountain turns red and a huge cloud of smoke and debris rises up into the heavens, the eruptions around Penghu were much smaller. The effect was rather more like liquid flowing through cracks. The lava flowed through, covering up the area all around, forming into flat or low-gradient tablelands tens of meters deep called "mesas."

"As far as the explosive power of volcanoes goes, the lava eruption was much more moderate," says Tsao Shuh-jong, section chief of the Division of Original Geology in the Central Geological Survey. In addition, basaltic lava has a relatively low silicon dioxide content, so it is correspondingly less sticky. Flowing basalt thus forms very different shapes from the lava of a typical volcanic eruption.

The most interesting natural spectacles related to Penghu's basalt are the cliffs consisting of "pillars" created by fractures (or joints) running through the rock. Tsao Shuh-jong explains how the basalt got this columnar shape: "After the lava stopped flowing, the molten rock began to cool. At the top and bottom ends there were polygonal cracks caused by shrinking, which eventually penetrated through the rock to form closely-packed hexagonal vertical pillars. In areas where the surface has risen, you can sometimes see groups of basalt pillars in the shape of a fan or angled like toppled dominoes."

Turbulent history

In fact, basalt formations are not that unusual around the world. Well-known sites include the Columbia River Plateau that encompasses three of the larger states in the US, the Deccan Plateau in northwest India (which dates back to the Cretaceous period), and the Emei Mountain basalt rock in China. "But it is rare to see a series of basalt formations spread across the sea like the 67 islands of Penghu," says National Taiwan University geographer Wang Shin.

Penghu's basalt formations are very unique within Taiwan itself. The lava pushed up by volcanic activity around the main island of Taiwan was mainly composed of andesite. Molten andesite has a relatively higher content of silicon dioxide, so is more sticky. Thus it more readily forms into conical mountains such as the Tatun volcanic mountains of northern Taiwan. The basalt rocks of Penghu form a stark contrast to the geology of the main island of Taiwan.

History has its ups and down. Volcanic activity, whatever the type, eventually ceased to play any role in Taiwan.

Eight million years ago, the east coast of Taiwan was at the very edge of where the Philippine Sea Plate was crashing into the Asian mainland. As compression along the edge of the Eurasian Plate increased, fracturing of the crust in the Taiwan Strait came to an end. Channels for magma to reach the surface were sealed, and volcanic activity was locked out. All that remains is the basalt that the sea has been continuing to erode for tens of millions of years.

After millions of years of wind and sea erosion, many of the pillars on the basalt islands have been worn down and are now covered with mud. More recently, human activity has made it difficult to distinguish what is so interesting about these basalt islands. Only by approaching cliffs near the edge of the sea or visiting uninhabited peripheral islands can one clearly see the basalt columns with their vertical fractures running from top to bottom.

In order to protect Penghu's precious basalt formations, as well as the countless sea birds that pass by here, in 1993 the Council of Agriculture, acting on suggestions from academics, declared a group of islands in the southeast part of Penghu which rarely see human activity to be the Penghu Columnar Basalt Nature Preserve. The basalt islands, like black pearls across the Western Pacific, are glittering assets of the common heritage of mankind.

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The columnar basalt of the Penghu archipelago was formed out of cooling basalt lava. (photo by Huang Li-li)