警告 ：請注意潮汐。
Warning：Note the tide. Dashibi Maar only exposed at the lower tide.
大石鼻西衛火山口位於馬公北方，此處低平火山口的放射狀節理有獨特構造，讓此處有別於澎湖其他火山口。這個地質寶要帶地理尋寶玩家瞭解岩脈(dyke)與礦脈(vein)，運用這些知識來分辨這些獨特構造是岩脈還是礦脈。
Dashibi Maar in Xiwei village is located at the north of Magong. This maar has a unique feature that is different from other maars in Penghu. Some special formations are in the radial fissure joints of the maar. This earthcache requires geocachers to distinguish these formations as dykes or veins.
登錄任務Logging task
1. 在地質寶點可以找到哪些？Which ones can be discovered at the posted coordination?
2. 你認為節理之間的構造是岩脈還是礦脈？它是如何在放射狀節理之間形成的呢？根據現場的觀察解釋你的答案。Do you think these formations among joints are dykes or veins? How did it form among the radial joints? Explain your answers based on the observation at the posted coordination.
3. 與大石鼻低平火山口合照，內容包含大石鼻火山口與你、你的代表物、或你的GPS裝置。Photo the Dashibi Maar with you, your identifying item, or your GPS device.
除了任務3，請不要公布你的答案。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。希望你喜歡這個地質寶，歡迎給予FP，有任何問題請寄送訊息。p.s. 入口請看參照點與Spoiler。 Don't post your answers except task 3. 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 some favorite points. p.s. the entrance was posted in the waypoint and spoiler.
柱狀節理與低平火山口 Columnar Basalt Joint and Maar
柱狀玄武岩是澎湖很常見的景色，熱岩漿碰觸到溫度較低的物質(空氣、海水、或岩石)時，快速冷卻過程中會向內收縮，會形成四角至七角形的柱狀玄武岩。
低平火山口是由岩漿和水相互作用發生爆炸而形成，在地表下形成了深切到圍岩的圓形火山口。低平火山口的節理與典型柱狀節理形狀不同，它的節理環繞火山口呈放射狀，岩漿流與噴發活動可能造成這種不同的形狀。大石鼻低平火山口位在海濱，退潮時露出，有平坦的凹陷、明顯的放射狀節理、與外環。在節理之間，特殊的構造讓這個火山口與眾不同。
岩脈與礦脈 Dyke and Vein
岩脈(Dyke)是指形成於岩石裂縫之間的一片新的岩石，岩脈可以是岩漿岩脈或是沉積岩脈。岩漿岩脈(Magmatic Dyke)的成因為岩漿侵入圍岩後冷卻形成，當岩漿侵入的液壓大於或等於最小正應力，液壓將垂直最小正應力的平面撐開，形成裂隙後，岩漿沿此裂隙灌入後冷卻形成岩脈。岩脈的寬度遠小於深度、長度，其長度可延伸幾釐米至數百公尺，岩脈的組成包含玄武岩、輝綠岩、花崗岩、與流紋岩，不過最常見的類型仍是玄武岩。沉積岩脈(Clastic Dyke)存於沉積岩中，垂直於其他水平岩層的沉積岩體。
礦脈(Vein)通常是指透過沉積作用填滿岩石裂隙的礦物結晶，廣義的礦脈定義可以區分為三種類型，包含火成岩礦脈、沉積礦脈、與礦物礦脈。
火成岩礦脈實際上相同於岩漿岩脈，不過兩者之間仍有所不同。岩脈外型上狹窄、筆直如牆，且可延伸很遠的距離，火成岩礦脈則外型上不規則、不連續出現，只存在很有限的距離之間。火成岩礦脈形成原因可能與岩漿入侵、凝固有關，岩漿入侵時不一定一次就全部進入圍岩，入侵的岩漿流有可能是持續性地逐步入侵，待最晚入侵岩漿進入圍岩時，最早入侵的部分岩漿已經凝固了。沉積岩礦脈相對來說比較少見，這是成形於火成岩裂隙之間的灰燼、沙或黏土。礦物礦脈則是最常見、也是較具有重要性的礦脈類型。其外型上是岩石之中的一片岩石、成分為礦物結晶，通常涉及熱液系統的作用，溶於液體或氣體的礦物透過沉積作用逐漸形成礦脈，礦脈的組成通常有別於周遭的岩石，像是安山岩、頁岩中的金礦、石英礦。澎湖常見出產的礦物包含褐鐵礦、泥煤、黏土礦、文石(霰石)。
Columnar basalt joint and Maar
Columnar Joints are a common scene in Penghu. Columnar basalt was formed by the contraction while the lava was cooling down while contacting relatively lower temperature substances(fresh air, sea, or rocks). It would be contracted into four- to seven-sided polygonal columns of basaltic rocks.
A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption. The shape of the joint is slightly different from classic columnar basalt joints. It is radially centering the crater. The lava flow and eruption might cause this different shape. Dashibi maar is at the seashore and exposed at the low tide. The maar has a flat depression in the centre, clear radial fissure joints, and tephra ring. Among the joints, some special formations are there.
Dykes and Vein
A dyke in geology is a sheet of rocks that is formed in a fracture of a pre-existing rock body. It can be a magmatic or clastic dyke. Magmatic dykes formed when magma flows intruded into the adjacent rock and then solidified. When the fluid pressure caused by magma was equal to or greater than the minimum principal stress of the adjacent rock, it caused a crack perpendicular to the plane of the minimum principal stress. The thickness of the dyke is much smaller than its other two dimensions. It can range from a few millimeters to hundreds of meters. Dykes range in composition from basaltic, diabase, granite, to rhyolite, but mostly are basaltic rock. Clastic dykes are vertical bodies of sedimentary rock that cut off other rock layers.
Vein in geology is normally indicating a fracture filled with mineral crystals that precipitate from water or vapors. A broad definition of vein can be classified in three groups: vein of igneous rock, vein of sedimentary rock, and vein of mineral deposited by water.
Veins of igneous rock are practically the same as dykes. However, there are some distinctive differences. Dykes are narrow, straight-walled and extended for a long distance. And the veins of igneous rock are irregular, discontinuous, and of a limited distance. Veins of igneous rock were formed for the reason that the great intrusive masses have not as a rule been injected in on stage but have been slowly enlarged by gradual or repeated inflows. And the earliest portions had consolidated before the last were introduced. Veins of sedimentary rock are few and less important. It forms in the crevices of igneous rocks with ashes, sand and clay. Veins of minerals are the most common and with the greatest importance. It is a distinct sheet-like body of crystalized minerals within a rock. It might be involved in hydrothermal circulation, and is formed when mineral constitutes, carried by solutions within the rock, are deposited through precipitation. The veins of minerals are usually in different compositions from wall rocks, like veins of gold or quartz in andesite or shale. Common minerals mining in Penghu are limonite, peat, clay, and Wenstone (unique Aragonite in Penghu).
Reference
澎湖地景多樣性資料庫-地圖協作平臺
Veins(Geology) Vein- Wiki Dyke- Wiki