沿著池西海岸有豎立一排玄武岩，池西玄武岩獨特的地方在於它的形狀形似於“岩石瀑布”。這個地點將岩漿流動的流向給保留下來，站在玄武岩前方，你可以看到岩漿流向海洋與在此處冷卻的模樣。
玄武岩幾乎遍布整個澎湖區域，不同時期的岩漿入侵岩石或是在圍岩上流動，有些玄武岩柱坐落在另一個玄武岩柱上，有些玄武岩很大很厚且有著明顯的多角柱狀節理，而有些澎湖的玄武岩柱有不太一樣的柱狀節理，這些柱狀結構看起來像是中斷了，夾了一層不太一樣的柱狀節理，這個獨特的節理結構稱為Entablature(直柱構造/直柱節理)。池西岩瀑的形狀與結構相當特別，可以觀察到不同結構的柱狀節理，拜訪池西柱狀節理、瞭解更多跟柱狀節理結構有關的知識。
這個地質寶要做什麼?
觀察柱狀玄武岩的不同結構，拓展Entablature(直柱節理/直柱構造)的知識與辨識岩漿流動的方向。
Entablature(直柱構造/直柱節理)與柱狀節理
柱狀玄武岩是澎湖很常見的景色，事實上，整個島嶼群在1800萬年前至800萬年前的活山活動期間形成，熱岩漿快速冷卻時會向內收縮，會形成四角至七角形的柱狀玄武岩。柱狀玄武岩可以觀察到不同單位的構造，岩漿會在上方與下方表面開始冷卻，這些表面會因為相對低溫的物質而開始收縮，像是接觸空氣海水或是岩石。
單一單位的柱狀玄武岩可以觀察到三個基本構造，分別是上層角礫岩(upper breccia)、垂直熔岩柱(Colonnade)、和底層角礫岩(basal breccia)。不過，在多重單位的柱狀玄武岩，岩漿流的上層與下層區域開始冷卻形成垂直熔岩柱，但中間區域卻還是炙熱的，高溫讓岩漿在上層與底層之間區域中持續流動，所以中間區域的玄武岩冷卻成形時被岩漿流動干擾，中間區域沒辦法完美成形為垂直柱狀構造，這個區域稱為Entablature(直柱節理/直柱結構)，多重單位的柱狀玄武岩就包含上層角礫岩(上層垂直熔岩柱)、Entablature(直柱節理/直柱結構)、垂直熔岩柱、和底層角礫岩。
不過，Entablature沒有特定的厚度限制，許多岩漿流可形成發展良好的垂直熔岩柱與Entablature，有些岩漿流則只有發展不明顯的Entablature，這些岩漿流有明顯的上層與下層垂直熔岩柱，但只有發展不明顯的Entablature，有學者稱呼這個區域為柱狀接口(下圖左。Woodell, 2012)。關於Entablature的爭論還在持續中。
有些研究者將Entablature分類為兩個端點的類型，分別是塊狀節理與柱狀節理構造(Forbes et al., 2014)，他們定義塊狀節理構造的Entablature為：在這種節理中沒有明顯的"柱"，節理密集且大量破碎成塊狀(上圖右)。
等溫線與柱狀節理
整塊岩石露頭的柱狀節理不一定線性排列，這是因為節理會垂直於等溫線，若是等溫線在一塊岩石露頭中彎曲，那麼節理也會跟著彎曲，柱狀節理可能因為等溫線在下層中垂直排列，但是當岩漿流動到上層時開始彎曲接近水平地排列，等溫線是不同形狀之柱狀玄武岩的關鍵成因，區辨柱狀節理的形狀與構造可以透露岩漿在地球上的活動訊息。
Rock Waterfall #Chixi #Entablature?
At Chixi, there stands a columnar basalt along the coast. The uniqueness of Chixi columnar basalt is its shape resembling a “rock waterfall”. The direction of flowing lava was preserved at this site. Standing in front of the columnar basalt, you can tell how the lava moved forward to the sea and cooled down at the sea.
Columnar basalts cover almost all regions of Penghu. Lava activities intruded into rocks or flowed on the bedrock at different times. Some basaltic columns are covered with other columnar joints. Some basaltic columns are thick and are clear with polygonal joints. Some basaltic columns in Penghu show a slightly different form of columnar joint. Their columns seemed disconnected with a layer of basaltic rock between them. The unique structure inside the columnar joint is called entablature. The shape and structure of columnar joints at Chixi are very special. Different structures of columnar joints can be observed. It’s a good opportunity to visit the columnar joints and know more about the unit of column’s structure.
What to do with this earthcache?
Observe different structures of columnar basalt. Expand knowledge of entablature and distinguish the direction of lava flow.
Entablature and Columnar Joint
Columnar Joints are a common scene in Penghu. In fact, the whole groups of islands were formed by volcanic activity mostly around 18 MA to 8MA before. Columnar basalt was formed by the contraction while the lava was cooling down. It would be contracted into four- to seven-sided polygonal columns of basaltic rocks. Some different units of columnar basalt can be distinguished. Lava flow would be cooled down at the upper and basal surface. Those surfaces were contacting relatively lower temperature substances, such as fresh air, sea, or rocks.
In the single unit columnar basalt, lava flow cooled down and contracted. Three basic structures were composed in the single unit as upper breccia, colonnade, and basal breccia. However, in multiple unit columnar basalt, the upper and basal area of the lava flow were started to cool down into colonnades. But the middle area was still in heat. High temperature kept the lava flowing between the upper and basal area. Thus, basalts in the middle area were interfered by the flowing movement while cooling down. The middle area was not perfectly contracted as a straight column. The middle area was called entablature. A multiple-unit lava flow contained four parts of structure, as upper breccia (upper colonnade), entablature, colonnade, and basal breccia.
However, there is no particular limitation on the thickness of entablature. Many lava flows have well organized colonnades and entablature. Some flows have poorly organized entablature. These flows have clear upper and lower colonnades but with a poorly organized entablature, only forming a zone called column interface (Woodell, 2012). Debates about entablature are still ongoing.
Some research categorized entablature into two main end members, cube-jointing and column-jointing form (Forbes et al., 2014). They defined Cube-jointing style of entablature as no clear ‘columns’ can be seen in this type of entablature jointing when blocks are too densely fractured.
Isotherm and Columnar Joint
Columnar Joints do not necessarily propagate linearly throughout an outcrop. Because the joints perpendicular to isotherms. If these isotherms curve in an outcrop, the joints will curve as well. Columnar joints may be vertical at lower column but curve towards horizontal as the isotherm propagates upwards. The isotherms is the key to different shapes of columnar joints. Distinguishing the shape and structure of columnar joints can reveal the activities of lava on the earth.
登錄任務
觀察池西岩瀑
1. 池西岩瀑是單一單位或多重單位?解釋你的答案。
2. 為你的答案找尋支持的地質證據。
3. 請留意安全，站在廢棄魚塭處正面看著池西岩瀑，根據垂直熔岩柱的構造來繪製池西岩瀑的等溫線。按照下圖的作法來進行、或手繪草圖、或照相後文字說明。
4. 透過池西岩瀑的等溫線來說明岩漿流動的方向，嘗試著描述岩漿流動的變化。再試著想像一下，過去岩漿流動的時候，原來的地形可能會是如何，平坦還是有凹洞或有小丘呢?
5. 請你、你的辨識物、或是你的GPS裝置與垂直熔岩柱(Colonnade)拍照。
6. (自主任務)請你與塊狀節理構造的直柱節理(Cube-jointing Entablature)拍照。
線上登錄這個地質寶，請將答案寄到Geocaching.com的信箱，請不要在線上登錄公佈詳細答案，若有什麼問題，會再回覆給你。
Logging Task
Observed whole Chixi columnar basalt.
1. Single unit or Multiple units at Chixi columnar basalt? Explain the answer.
2. Find geological evidence to support your answer.
3. Depict the isotherm from the structure of colonnade at the spot in front of the Chixi Columnar basalt. Do as in the picture below or draw a sketch on papers or picture with descriptions. Remember the joints perpendicular to isotherms.
4. Can you tell the direction of lava flow through the isotherm at Chixi Columnar basalt? Describe the change of lava flow's movement. How do you think what the original landform would be? e.g. flat ground, a hill, or a drop?
5. Photo the Colonnade with you, your identifying item, or your GPS device.
6.(Voluntary task) Photo with the joint which you suspect as the cube-jointing entablature.
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. Please, do not post detailed answers on your log.
Reference
Forbes et al. (2014)
Hetényi et al. (2012)
Woodell (2012)
澎湖火山頸中玄武岩柱狀節理多樣性之成因探討
感謝拜訪地質寶。Thanks for your visiting.