Volcanic landscapes in the active weathering environment
Penghu islands possess various volcanic landscapes which are the oldest one in Taiwan and their volcanic landscapes are not much distorted by ad hoc tectonic shifts. The formation of Penghu basalts represents the event that crustal deformation occurred in the East China Sea and the South China Sea between 17.4 million and 8.2 million years ago with successive volcanic eruptions. Different levels of basalt lava flow erupted onto the surface through different geologic time periods and then formed today’s surface structures and landforms in Penghu. The oldest rock strata of the Penghu region are located in Huayu, farthest to the west in Penghu. Huayu is the only islet composed of andesite lava flow (Volcanic Island Arc type). The rest of Penghu islands are composed of basalt lava flow. During the Quaternary period, besides on the affection of sea erosion, the laterite layer was developed in the surface horizon of soils followed with the crustal uplift and subsidence. Notably, in Penghu, there are no high mountains to withstand wind. Thus Penghu islands have a high frequency of sand-laden wind blowing and bad weather. That combination of volcano tectonic processes and weathering by wind and ocean waves forms attractive diversity in landforms and landscapes of Penghu. Consequently, except for the common cape bays, we can also see wave-erosion platform, wave-erosion cliff, wave-erosion cavity, sandy coast and rocky coast landforms along the coast area.
The Penghu formation
The Penghu Formation is the main type of basalt. There are four layers of lava flow and the oldest one at the lowest layer is about 17 million years old. We can observe easily three lava layers at many places. After weathering, the basalt became soil. The later lava flows covered the top of the soil and caused the organic matter to burn into black carbonated materials and sometimes sulfur was left on the surface of the soil layers.
The palaeo-soil layer at Penghu Marine Geopark
External processes (endogenesis, exogenesis)
There are two types of earth external processes. The first type is called en earthquake, tectonic movement, and volcanic activities. The second type is exogenous processes, such as rainfall, coastal, wind and glacial processes. The endogenous processes mainly occurred around 17-7 million years ago due to continental plate spreading. Magma emerged at the surface of the crust and the rocks then became eroded by exogenous processes such as weathering and coastal erosion. The weathering elements include seawater, air and animals.
Marine processes on the cliffs
The main basaltic landscapes of the archipelago
The main basaltic landscapes of the archipelago include:
They are islands with a flat plateau surface.
Due to shrinking during the cooling of magma, the basalt joints form a systematic pattern. The hexagonal form is iconic for the archipelago.
These are intrusions of magma into earlier rocks. The dikes, due to their better resistance to weathering, show their geological environments and shapes well. Many dikes at the archipelago have a width of 50 cm and various length. For example, a long dike on the flat tidal area between Kueibishan and Chiyu has a length of 300 meters.
The volcanic domes of the archipelago had experienced serious weather ing and erosion. It is thus not easy to recognize their unique characteristics. Several sites appear to be the domes of volcanoes. For example, the Lotus platform of Tongpan.
The Lotus platform from above at Tongpan Islet
Joints and columnar joints
Basalt landscapes often have a compact and fine texture and many columnar joints. The joints of basalts easily become rupture surfaces due to continuing weathering and erosion. Therefore, columnar basalt may present their forms differently. Some are hexagonal and some are rounded or even onion shaped due to weathering. There are many onion-ring shaped rocks on the islets of Tong-pan and Hu-jin. The onion ring form develops as the rock peels and exfoliates due to the perfect combination of moist, air, sunlight and heat effecting the rock joints. Starting at certain point of the rock, the thermal expansion continues and exfoliates the rock formation to form an onion ring on the surface. This is called onion skin weathering.
The flat mesa-like basalts of Penghu appears to have many hexagonal joints. The columnar joints are the results of contractions of the magma during its cooling process. However with weathering, some eroded joints detach, collapse, and form colluvium deposits on the foot of slope.
Sea level change
About 10000 years ago, during to the last glacial period, the sea level was 100 meters lower than the present sea level. At that time, the present PengCheu archipelago was part of the Eurasian continent When the last glacial periods ended about 8000 years ago, with the sea level rise, the archipelago appeared. Weathering, crust compression and marine processes (including tidal and current erosion) then became important elements for the coastal landscape today
Inter-tidal zone will disappear during high tide at Kueibishan
Weathering processes and landforms
The weathering processes start when a rock is born, or when a rock is exposed on the earth surface. The main processes can be caused by physical, chemical or biological processes. Physical weathering will cause rock to dissemble or detach by temperature and moisture changes. These processes, repeatedly shrinking and expanding the rock, are known as physical processes. Chemical weathering can often cause oxidation of the rocks and makes them crumble. The rocks dissolute gradually into various different materials, and at the end become part of soil. Oxidation of rocks with iron content turn them into brown and red colors on the surface of the rocks. The colors and patterns may form weathering rings as a result of repeated dissolving and condensation processes of iron or staining along the joints.
Onion weathering is a common phenomenon around basaltic rocks, because:
(1) The rock has well developed joints to allow moist to get into rock.
(2) The texture of the rock is fine and homogeneous.
(3) The rock layer is thick.
Coastal erosional landforms
Marine processes may produce sea notches. As sea notches continued to be eroded, they retreated and even formed colluvium deposits on the foot of slopes. The resistant part of the rock became sea cliffs. Through time(or With time), the sea cliffs retreated and made space for a wide marine platform.
Sea cliffs and gullies:
These are created by marine processes, mainly ocean waves, tidal flows and sea currents. The waves delivered ocean energy, transmitted it to the coast, and caused the coast to retreat and to form various coastal landforms. The most attacked area on the coast by marine processes includes capes or headland areas.
The bottom of sea cliff often could be attacked by the tidal flow as they deliver ocean energy and transmit it onto the soft part or joints of the rock to form depression or holes. In time, the erosion continues and sea caves could be formed.
Maokong rock is a local name for the rock with many small holes on its surface. Hu-Jin island is famous for its many Maokong rocks. The Maokong rocks are outcomes of various weathering and erosion processes. They include mainly biological and salt processes. The acid emitted from sea urchins and barnacles enhances the weathering processes. In addition, the salt from seawater through repeated physical dry-and-wet processes and the crystallization of salt chemical process becomes agent of weathering and erosion to make the holes or cracks on the surface to become bigger. Boring organisms also penetrate the rock. The local name of Maokong rock comes from the holes which have a similar name in the local dialect.
Deposition landscapes occur due to the deposition of the deposits at different places. After weathering and erosional processes, the rock debris was detached from the rock mass via wind, river, sea water, glacial processes. When there is not much energy to carry away these sediments deposition takes place to form landforms such as the alluvial plains and beaches. As those deposition landscapes are not so steep, they are used by most of the people for different types of landuse.
Around Chiyu, there are gravel or boulder deposits. The smaller particles, such as sand, silt particles have been removed and left the gravel boulders on beach.
The inter-tidal zone
“Moses Parts the Sea”
On the way to Chiyu, the wide intertidal zone is flooded during high tide and a special landscape is exposed during the ebb tide. The micro-topography of this wide intertidal zone shows the result of subtle differences of tidal variation. The sea water and waves wash the coastal area. They scour and carry away smaller rocks and sand, and leave the bigger rocks and boulders on the beach to form a typical boulder coast. It is often a good place for intertidal biological and ecological observation.
Stone pagoda: The livelihoods of most islanders depend highly on fisheries. In order to survive in this difficult environment, a special talisman – stone pagoda (one type of protective deities) is situated in the surroundings of the village. It is believed to quiet down the wind and ward off the ghost. Stone pagoda is one of the largest sized talismans in Taiwan and one of the most powerful talismans believed by the villagers.
Stone weirs: The stone weirs were made of coral, basalt stones and rocks show the traditional wisdom of capitalizing on the shadow tidal flats to harvest fish from the sea. Stone weirs must be built in the broad intertidal zone with strong waves and large tidal range, and in the area where the stone material can be easily obtained. Nowsaday, those fishing stone weir architectures become unique and valuable heritages attracting tourists. The famous Double-Heart stone weir in Cimei islet, Couple stone weir in Erkan village are all typical examples.
Submerged Double-heart stone weir and its islet at low tide