Columnar Basalt to Soil?! Daguoye #WeatheringProfile
What to do with this earthcache?
Knowing weathering and pedogenesis on Daguoye Columnar Basalt. Observe weathering profile at Daguoye Columnar Basalt to expand knowledge and experience in geology.
Columnar Joints in Penghu
Columnar Joints are common scene in Penghu. In fact, the whole groups of island are formed by volcanic activity mostly around 18 MA to 8MA before. Lava projected from cracks near the Earth’s surface and formed into basalts. Outcrops at Penghu sometimes show more than one layer of basalt, which mean more than one volcanic activity. Heat lava will be contracted while it is rapidly cooling down, which resulting in four- to seven-sided polygonal columns of basaltic rocks.
Daguoye Columnar Basalt
Daguoye Columnar Basalt is located at Daguoye, Penghu. These columnar joints was discovered unexpectedly when people explored the area and decided to build a port nearby.
Daguoye Columnar Basalt is a classic vertical type columnar joints, the most common one. Daguoye Columnar Basalt is unique in geology not only with obvious hexagon columnar jointing but also with a clear demonstration of weathering profile and pedogenesis.
Weathering Profile at Daguoye Columnar Basalt
A clear weathering profile can be observed at the spot. A thin layer of soils is on the upper area of Daguoye Columnar Basalt. Under the soil layer, Daguoye Columnar Basalt is deeply affected by chemical and physical weathering, causing a weathering profile.
Soil on Daguoye Columnar Basalt
Daguoye Columnar Basalt is under the effect of weathering and pedogenesis. Weathering is the breaking down process on rocks, soils, and minerals. Pedogenesis is the process of soil formation affected by parent material, climate, organism, topography, and time.
Weathering can break rocks into blocks, stones, or even into small particles. These particles are also regulated by influencing factors mentioned in pedogenesis. Then, weathering keeps on working and organisms like microbes, fungi, animals, or plants also play important roles in the formation of soils on the basalt.
Abundant weathering on Daguoye Columnar Basalt
Penghu is located at subtropical zone with high temperature and raining season. Chemical weathering, such as hydrolysis and carbonation, occurs at area with more raining and high temperature. Some soluble materials were carried deeper and left the more insoluble minerals ions on the surface, predominantly iron and aluminium.
Temperature at Penghu is very high at daytime. But, it can be very cool or cold at night. Hence, physical weathering based on temperature is also an important influencing factor at the spot.
These processes produce a weathering profile with different level of weathering from up to low area of columnar basalt.
Classification system for Weathering Profile
A weathering classification system for granite and volcanic rocks was proposed to distinguish different layers of rocks affected by weathering (Hencher and Martin, 1982). Pity that this classification is based on granite. The discussion of weathering profile on basalt is few. However, it’s still a good example to understand the weathering in depth. The classification system shows that weathering affects more on the upper Grade than lower Grade and shows typical distinctive characteristic of each Grade.
這個地質寶要做什麼?
認識大菓葉玄武岩的風化作用與成土作用，觀察大菓葉玄武岩的風化剖面，藉此拓展地質的知識與經驗。
澎湖柱狀玄武岩
柱狀玄武岩是澎湖很普遍的風景。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，岩漿從地表裂縫噴發後快速冷卻形成玄武岩，澎湖地區露出的岩層經常不只一層，表示這些地區不只一次的火山活動，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。
大菓葉玄武岩
大菓葉玄武岩位在澎湖池東大菓葉，這些柱狀玄武岩是人們為了建造港口，探索該區域時的意外發現。
大菓葉玄武岩在地質上的獨特性不只有明顯的六角柱狀節理，更能清楚地呈現風化作用與成土作用。
大菓葉柱狀玄武岩的風化剖面
在這裡可以觀察到清晰的風化剖面，在大菓葉玄武岩上方區域有一層薄土壤層，下方玄武岩則深深受到化學與物理風化作用的影響。
大菓葉柱狀玄武岩上方的土壤
大菓葉柱狀玄武岩受到風化作用與成土作用影響。風化作用讓岩石、土壤、礦物從堅硬變得鬆軟；成土作用則是土壤的形成過程，受到母質、大氣、有機生物、地形、與時間的影響。
風化作用能將岩石變為石塊、石頭、甚至小粒子，這些粒子受到前述成土作用中影響因素的調節，接著，在玄武岩上方形成土壤的過程中，風化繼續發揮作用，有機物也發揮作用，像是微生物、黴菌、動物、或是植物。
大菓葉玄武岩豐富的風化作用
澎湖位在高溫、雨季的亞熱帶區域，化學風化特別容易發生於高溫多雨的環境，像是水解和碳酸化作用。水溶性比較高的礦物離子會被帶往更深的地方，讓水溶性較低的礦物離子留在表層，這些表層礦物主要是鐵和鋁。
澎湖白天氣溫很高，到了晚上很涼、甚至會冷，溫度為主的物理風化也是該區域重要的影響因子。
這些風化過程讓柱狀玄武岩在上層與下層區域，形成不同的風化程度，造就出風化剖面的樣子。
風化剖面的分類系統
學者(Hencher and Martin, 1982)提出一個風化剖面的分類系統，透過觀察花崗岩與火山岩石的風化程度，來區分不同的風化層，可惜的是過去少有研究玄武岩的風化剖面，這個分類系統是根據花崗岩的觀察為基礎所提出。不過，這依然是一個很好的例子來理解隨著深度變化的風化程度。從下方分類系統中可發現風化作用對上層的影響程度大於下層，每層各有典型的區辨性特徵。
Question
Classification system for weathering profile on volcanic rocks.
Observe at posted coordination. Answer and interpret.
1.1 The layer A is Grade VI. (True/False)
1.2 The layer C is Grade IV. (True/False)
1.3 The layer F is Grade I. (True/False)
1.4 Interpret the answer of 1.1-1.3
Weathering on Daguoye Columnar Basalt
2.1 Chemical weathering is the key role for the soil’s color on the top of columnar basalt. (True/False)
2.2 Which is the possible mineral for the color? (iron or aluminium)
3.1 How thick is the layer of soil?
3.2 Observe the environment at the spot, what influencing factors may hinder the accumulation of soil?
4.1 Observe rock’s color at Daguoye Columnar Basalt. What is the color of the weathered rock?
4.2 Observe rock’s color at waypoint I or the deep side of joints. What is the color of the less weathered rock?
5. Photo the soil layer on Daguoye Columnar Basalt with you, your identifying item, or your travel bug.
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.
問題
火山岩風化剖面的分類系統
觀察標示座標處，回答和解釋答案
1.1 區域A是Grade VI。(是/否)
1.2 區域C是Grade IV。(是/否)
1.3 區域F是Grade I。(是/否)
1.4 解釋1.1-1.3的觀察。
大菓葉柱狀玄武的風化作用
2.1 化學風化是柱狀玄武岩上方土壤顏色的主因。(是/否)
2.2 哪種礦物最可能是顏色的主因? (鐵或鋁)
3.1 土壤層大約多厚?
3.2 觀察該區域的環境，哪些因子可能會阻礙土壤的堆積?
4.1 觀察大菓葉柱狀玄武岩的顏色，此處風化岩石的顏色是?
4.2 觀察Waypoint I 或是節理的深處，風化比較少的岩石顏色是?
5. 拍照大菓葉上的土壤層與你、你的代表物、你的旅行蟲，歡迎拍照。
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。
Reference
Weathering @wiki
Pedogenesis @wiki
Laterite @wiki
Hencher, S.R. and Martin, R.P., (1982), The Description and Classification of Weathered Rocks