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.
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。
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.
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)
2.2 哪種礦物最可能是顏色的主因? (鐵或鋁)
4.2 觀察Waypoint I 或是節理的深處，風化比較少的岩石顏色是?
Hencher, S.R. and Martin, R.P., (1982), The Description and Classification of Weathered Rocks