Whale Cave 鯨魚洞--located on Xiaomen Islet in the northern part of Penghu's Xiyu Island--is a natural landform that resembles the head of a whale. Local legend is that long ago a baby whale became beached at this location. The mother whale tried to rescue her baby, but was unable. The mother whale, stricken with grief, crashed against the cliff wall, where her figure remains to this day.
Whale Cave is a spectacular example of sea-eroded topography. It is primarily composed of columnar basalt and sandstone, which has coal seams running through it.
Columnar basalt is formed after lava is erupted onto Earth’s surface and cools. However, lava may take awhile to cool; most commonly, the top of the lava flow will be cooler than the bottom of the lava flow.
When the lava cools, it contracts and forms joints or fractures. If a flow cools relatively rapidly, significant contraction forces build up. The fracture pattern that forms at the cooling surface will tend to form geometric columns.
Because hexagons fit together efficiently with no vacant space ("tessellation"), this is the most common pattern that develops. Pentagonal, heptagonal or octagonal joint patterns are also known, but are less common.
Sandstone contains sand-sized grains of rock fragments and individual minerals broken down by weathering and erosion from other, older rocks (“source rocks”). Because quartz is such a hardy mineral, unlikely to undergo chemical changes during erosion, it is the mineral found in the greatest amounts in many types of sandstone.
After a source rock is weathered and eroded, the resulting sand grains might fill a bowl-shaped basin on land or underwater. With the pressure from more sediments landing above, and the movement of water through the grains, the sand becomes sandstone. Finer grains of rock and new mineral growth fill the spaces between the grains, cementing the new rock.
The sandstone portion of the Whale Cave has coal seams running through it. The formation of these seams change generation after generation, and contain vegetation that is not native to the area.
In order to log this Earth Cache, please answer the following questions, based on your observations and the information above:
- 1. Columnar Basalt is categorized as what basic type of rock?
- 2. What kind of erosional landform is Whale Cave?
- 3. Based on your observation of the area, how do you suppose non-native vegetation ended up in the coal seams?
- 4. Opposite Whale Cave, on the other side of the path, is a fine example of a columnar basalt wall. The lower portion of this wall has a striking reddish-brown seam running horizontally across the gray and black stones. Which mineral do you suppose gives this seam its rust color?
- 5. [Optional] Post a photo of yourself at Whale Cave or the surrounding area.
You may post your find as soon as you send me your answers; you don't need to wait for my approval. If there is a problem with your answers I will contact you. Please do not post the answers in your log.
More information about the geological formation of Xiaomen Islet, as well as all of Penghu, may be found at the nearby Xiaomen Geological Gallery (but you don't need to visit the gallery to be able to answer the questions).