EarthCache: Niuxin Mountain - Basalt eruption
Niuxin Mountain is located in Xiyu, Penghu. At the posted coordination stands a basaltic rock that shows volcanic events by columnar joints, baked zone, weathering or even laterite.
This EarthCache will guide you through the geological processes involved in lava eruption, how to identify them, and how to determine the number of volcanic events at this location.
What was formed during a lava eruption event?
Lava Flows: Look for solidified lava flows, which can be identified by their rough, jagged surfaces or smooth, rope-like textures. These flows indicate the direction and extent of past eruptions.
Pyroclastic Layers: These are deposits of volcanic ash, pumice, and other fragments ejected during explosive eruptions. They often form distinct layers that can be seen in exposed rock faces.
Columnar Jointing: This feature occurs when thick lava flows cool and contract, forming hexagonal columns. It is commonly observed in basalt formations and can indicate the cooling rate and structure of the lava.
What happens when lava from different events contact?
Baked Zone: This is the area surrounding the intrusion that has been thermally altered by the heat from the magma. Rocks in the baked zone often appear darker and more crystalline due to the high temperatures they were exposed to. Lava eruption would affect the contacting area at the bottom of lava flow.
Cross-cutting Relationships: Intrusive events can be distinguished by examining the cross-cutting relationships between different rock layers. Younger intrusions will cut through older rock formations, providing a timeline of geological events.
What happens after lava solidifies without any other event?
Weathering Profile: The weathering profile of an intrusion can provide clues about its history. Look for zones where the rock has undergone significant chemical alteration. This can help in identifying multiple intrusion events, as each event will have a distinct weathering profile. Spheroidal weathering, a type of weathering that leads to the formation of rounded boulders, can often be observed at the upper area within these profiles.
Laterite: laterite is a soil type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. At this point, the Law of Superposition should be explained. The Law of Superposition states that beds of rock on top are usually younger than those deposited below. These features mean laterite probably formed on the top area of the youngest layer by heavy raining. If there is another layer upon the laterite, the later events or changes of climate occurred after the period of laterite.
How to determine the number of eruption events?
Determining the number of eruption events involves careful observation of the rock layers and features. Each eruption typically creates a distinct set of deposits and landforms. By studying these features, you can count the number of eruption events and determine it as a single or multiple unit. And you can distinguish the sequence of volcanic events by reading unique volcanic features, such as baked zones or cross-cutting relationships.
If the eruption event occurred not at the same period, they would have a distinct layer following the basic principle of Superposition and weathering results. Read some examples.
Logging tasks
1. How many times of eruption events would you determine at posted coordination(Niuxin Mountain)? Find supporting geological evidence to elaborate your answer.
2. Photo a solidified lava flow with you, your identifying item, or your GPS device.
Log this cache "Found it" and send me your suggested answers via geocaching.com messaging. Please, do not post your answers in the log, except task 2. Hope you like this earthcache. Favorite Point Appreciated.
地質寶：牛心山 - 玄武岩噴發事件
牛心山位於澎湖西嶼。在此地標座標點處，可以觀察到一塊玄武岩，其顯示出火山活動的證據，包括柱狀節理、烘烤區、風化、甚至紅土。
本地質寶將引導您了解火山噴發的地質過程，如何識別它們，以及如何確定此地發生火山活動的次數。
火山噴發事件中形成了什麼？
熔岩流(lava flow)：尋找固化的熔岩流，這些熔岩流可以通過其粗糙、鋸齒狀的表面、或光滑的繩狀紋理來識別。這些流動顯示了過去噴發的方向和範圍。
火山碎屑層(Pyroclastic Layers)：這些是由爆炸性噴發時噴出的火山灰、浮石和其他碎屑組成的沉積物。它們通常在裸露的岩石表面形成明顯的層次。
柱狀節理(Columnar Jointing)：當厚熔岩流冷卻和收縮時，形成六邊形柱狀結構。這種特徵常見於玄武岩層，並可指示熔岩的冷卻速率和結構。
當來自不同事件的熔岩接觸時會發生什麼？
烘烤區(Baked zone)：這是受熔岩熱量影響而發生熱變質的區域。燒結區的岩石通常因高溫暴露而顯得更暗且更結晶化。火山噴發會影響熔岩流底部的接觸區域。
交切關係(Cross-cutting Relationships)：通過檢查不同岩層之間的交切關係，可以區分出侵事件。年輕的侵入會切割較老的岩層，提供一個地質事件的時間線。
當熔岩固化後沒有其他事件發生時會發生什麼？
風化剖面(Weathering Profile)：風化剖面可以提供關於其歷史的線索。尋找岩石中經歷了顯著化學變質的區域。這可以幫助識別多次侵入事件，因為每次事件都會有不同的風化剖面。球狀風化，一種導致圓形巨石形成的風化類型，通常可以在這些剖面的上部區域觀察到。
紅土(Laterite)：紅土是一種富含鐵和鋁的土壤類型，通常被認為是在炎熱和潮濕的熱帶地區形成的。幾乎所有的紅土都呈鏽紅色，因為它們含有大量的氧化鐵。在這一點上應該解釋超疊律(Law of Superposition)。超疊律表明，上層的岩層通常比下層的年輕。這些特徵意味著紅土可能是在最年輕層的頂部由強降雨形成的。如果在紅土之上還有另一層，則在紅土形成之後發生了後來的事件或氣候變化。
如何確定噴發事件的次數？
確定噴發事件的次數需要仔細觀察岩層和特徵。每次噴發通常會創建一組獨特的沉積物和地貌。通過研究這些特徵，可以計算噴發事件的次數並將其確定為單一或多重事件。您可以通過識別獨特的火山特徵（如烘烤區或交切關係）來區分火山事件的順序。
如果噴發事件不是在同一時期發生的，它們將遵循超疊律(Law of Superposition)和風化結果，形成明顯的層次。請參閱以下例子。
登錄任務
1. 你認為牛心山有多少次噴發事件？找到支持的地質證據來詳述你的答案。 2. 拍攝一張與你、你的標識物或GPS裝置一起的固化熔岩流的照片。
將此地質寶登錄為“ found it”並通過geocaching.com消息系統發送您的建議答案給我。請不要在日誌中發布您的答案，除了任務2。希望您喜歡這個地質寶。非常感謝您的FP。