鎖港海蝕洞位在鎖港村東側。鎖港海蝕洞其實不是洞穴而是海蝕拱門，退潮時拱門才會露出來，此處獨特景色是通往海洋的柱狀玄武岩通道，存在且承受大自然持續改變的力量。鎖港海蝕拱門是典型的海蝕地景，流出地表的岩漿在砂質泥岩上凝固成柱狀玄武岩，然後海水侵蝕柱狀玄武岩的脆弱線/裂縫、後續形成海蝕洞、演變為海蝕拱門。
W.E.D.的力量
大自然不斷透過各種力量持續改變地球表面樣貌，W.E.D.是風化作用(Weathering)、侵蝕作用(Erosion)、沉積作用(Deposition)的縮寫，這些是自然常見於影響岩石、土壤的改變力量。首先，風化作用會先發生。風化作用讓地球表面岩石碎裂開來成為較小的岩塊，水、冰、酸、塩類、植物、動物、和溫度變化都是風化作用的影響因子。接著，侵蝕作用接續發生。侵蝕作用為較小岩塊被搬動的過程，前述那些較小的岩塊被侵蝕作用搬動至新的地點。最後，接續風化與侵蝕作用之後，沉積作用會發生。沉積作用的運輸介質通常為水或風，指較小的岩塊被移到新的地點、且堆積在新地點，像是沖積扇。
這三種力量彼此之間會緊接著發生在其中一種之後，當W.E.D.作用於山脈時，山頂風化作用較旺盛，山腰侵蝕作用與沉積作用較多，出海口則較常觀察到沉積作用。
這個循環會年復一年持續重複發生，造就眼前我們在自然中每天見到的地景樣貌。
W.E.D.作用於鎖港海蝕拱門
現在，深入檢視看看鎖港海蝕拱門，風化作用、侵蝕作用如何影響與造成什麼樣的結果。一般的柱狀玄武岩為四到七角形的柱狀體，柱狀玄武岩上層表面如果沒有覆蓋角礫岩碎屑，通常會是平坦的表面，不過鎖港海蝕拱門的柱狀玄武岩上層表面多有裂縫，且顏色也不同於玄武岩典型的黑色，這明顯是強力風化作用的結果，另外，部分柱狀玄武岩上層表面出現蜂窩岩、風化窗，這是鹽類結晶或酸性物質的風化作用影響，讓岩石的形狀變化。
節理為岩石的脆弱線，柱狀玄武岩充滿節理，很容易受到侵蝕作用的影響形成海蝕洞，尤其又是位在海邊受海浪影響。鎖港海蝕拱門形似突出海岸的海岬，海岸與海岬連接處易受到強力海浪影響，還會形成海水漩渦，這種情況特別容易受到侵蝕作用強力影響進而形成海蝕拱門、海蝕柱。此外，當侵蝕作用影響的區域同時有兩種不同物質時，會發生差異侵蝕，鎖港海蝕拱門的柱狀玄武岩表面、靠近海平面的位置，遭到藤壺攀爬佔據，有趣的是這些有機生物其實提供了生物保護力量，進而縮減岩石的溫度變化，所以風化作用、侵蝕作用的效果也就減少了。
典型的海蝕拱門存有許多地質細節，這個地質寶的任務便是找出作用於鎖港海蝕拱門的這些改變力量。
Suo-gang hai-shi-dong, located in the east side of Suogang village, is actually an erosional arch. The arch is exposed at lower tide. The unique sight here looks like a columnar basaltic causeway to the sea. It stands well and endures constant changing forces from nature. Suogang Erosional Arch is a classic coastal erosion landform. Suogang Erosional Arch was formed from the weak lines/cracks of the consolidated lava on the sandy mudstone layer. It was continuingly affected by weathering and erosion, forming a cave first and then an arch.
Power of W.E.D.
The earth’s surface is constantly changing through forces in nature. W.E.D stands for weathering, erosion, and deposition. These are three common forces affecting rocks and soils. First, weathering occurs. Weathering breaks down the Earth’s surface into smaller pieces. Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. Weathering, either physical or chemical, changes the shape of the land or makes it look damaged or different. Next, erosion takes place. The smaller pieces move in a process called erosion. This means that those pieces of the land are moved to a new location. Last, after weathering and erosion have taken place, deposition will occur. This means that those smaller pieces, normally transported by water or wind, are deposited to a new place, like an alluvial fan.
These three forces may occur immediately following the other one. When W.E.D. occur in a mountain, weathering may be vigorous at the peak, erosion or deposition occur more in the middle, and depositions are easy to observe at the exit of the river.
The cycle repeats over and over again, constantly year by years. And it forms the landform we see every day around us in nature.
Power of W.E.D. on Suogang Erosional Arch
Back to Suogang Erosional Arch, let’s look deeper into the effect and outcome of weathering and erosion here. Normal columnar basaltic joints would be four- to seven-sided polygonal columns. The upper area in the column might be flat if not covered with breccia. However, the upper areas in the column are full of cracks and the color is different from normal black of the basalt. It clearly was the outcome of a strong weathering effect. There's something else, some developing honeycomb weathering on the upper area of columnar basalt. This kind of form changing is the weathering effect of acid or salt crystals.
Joints are the weak line of rocks. Columnar basalt is full of joints which are easily affected by erosion forming erosional caves(when it is at the side of the sea). Suogang erosional arch is a tiny cape out to the sea. Waves are strong and cause vortexes at the connection area of seashore and cape. Strong erosion occurs within this situation forming erosional arch or stacks. Besides, when erosion occurs at a spot with different kinds of material, differential erosion will occur. The area of columnar basalt’s surface near the sea level is occupied with barnacles. Interestingly, these organisms offer some bio-protections by lowering changes of temperature in rocks. Hence, the power of weathering and erosion are lowered.
A classic erosional arch stands here with many details. Tasks for this earthcache are to figure out how changing forces affect the Suogang Erosional Arch.
登錄任務
探索地質寶點與參照點。
1.1 請辨識公布點有哪些類型W.E.D.?請說明你的觀察。 1.2 請辨識參照點有哪些類型W.E.D.?請說明你的觀察。
2. 比較公布點與參照點的風化/侵蝕作用。哪個地點受到比較多W.E.D.作用?請根據現場觀察解釋你的答案。
3. 預測看看鎖港海蝕拱門未來會如何改變?解釋你的預測。
4. 與鎖港海蝕拱門拍照，照片內容包含海蝕拱門與你、你的代表物、或你的GPS裝置。
線上登錄這個地質寶，請寄到Geocaching.com的信箱，若有什麼問題，會再回覆給你。除了登錄任務4，請不要在線上log中公佈你的答案，感謝。希望你希望這個地質寶。
Logging task
Explore post coordination and waypoint.
1.1 Which type of W.E.D. do you distinguish at the posted coordination? What’s your observation for your answer?
1.2 Which type of W.E.D. do you distinguish at the waypoint? What’s your observation for your answer?
2. Compare weathering/erosion at the posted coordination and waypoint. Which spot endures more the power of W.E.D.? Elaborate your answer based on the observation of the outcome of W.E.D.
3. How would you predict the change of Suogang Erosional Arch in the future? Explain your prediction.
4. Photo the Suogang Erosional Arch with you, your identifying item, or your GPS device.
Don't post your answers except task 4. Log this cache "Found it" and send me your suggested answers either via my profile, or via geocaching.com (Message Center) messaging, and I will contact you if there is a problem. Hope you like this earthcache and welcome favorite points.
Reference
Cool barnacles (M.A. Coombes et al., 2016)
Experimental Removal of Mussels (J.A. Gonzalez et al., 2021)
weathering and coastal erosion