Volcanic sands of Iceland - Diverse origins of aeolian sand deposits revealed at Dyngjusandur and Lambahraun
Volcanic sands of Iceland - Diverse origins of aeolian sand deposits revealed at Dyngjusandur and Lambahraun
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DOI：
10.1002/esp.2201
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发表时间：
2011-10-01
影响因子：
3.3
通讯作者：
Pinet, P.
中科院分区：
文献类型：
--
作者：
Baratoux, D.;Mangold, N.;Pinet, P.
火山砂的起源、形成和演化不如更常见的富含石英的砂层的形成那么为人所知。结合活跃的火山活动和寒冷的气候，冰岛约21%的表面被桑迪覆盖。在两个地点对沙子进行了详细分析，结果显示它们的来源不同。第一个地点是Dyngjusandur，位于Vatnajokull北部，第二个地点是Lambahraun地区，位于Langjokull南部。在这两个地点，沙子的来源是通过实地观察（风的风向）、岩石和沙子的化学和矿物学分析确定的。在Dyngjusandur，沙子主要由玻璃颗粒组成，这是冰岛砂平原的典型情况。目前埋在瓦特纳冰川下的透明碎屑岩脊是沙子的主要来源，沙子中只有大尺寸的斜长石晶体（0.5厘米）似乎来自熔岩流。玻璃碎屑岩山脊被冰川压碎，机械侵蚀的沉积物被融水冲刷到洪泛平原上。沙子的化学成分是空间均匀的，类似于邻近的亚空中熔岩流的平均组成，反映了有效的混合不同的来源下的冰川。丁久冰川北部存在的沙子可以作为一种探索冰川下未暴露的火山物质的平均化学成分的方法。在Lambahraun的情况下，盛行风表明在沙层北部有几个潜在的沙源。对沙子和岩石样本的化学和矿物学分析的比较有助于确定确切的起源。与第一个地点相比，沙子主要由晶体组成，化学成分与来自埃尔德博吉尔和斯凯尔斯利盾状火山熔岩流的物质混合物一致。对熔岩流和冰川之间接触的分析表明，玄武岩沙粒是由于冰川最近的前进磨损岩石而形成的。冰川和河流-冰川活动与玄武岩平原的直接相互作用似乎是在相对较短的时间内产生大量沙子的必要条件（
The origin, formation and evolution of volcanic sands are less well known than the formation of the much more common quartz-rich sand sheets. Combining active volcanism and a cold climate, Iceland is covered for about 21% of its surface by sandy areas. The sands were analyzed in detail at two sites and results reveal their diverse origins. The first site is Dyngjusandur, located north of Vatnajokull, and the second site is the Lambahraun area, located south of Langjokull. At both sites, the sand origin is determined from field observations (wind directions from ventifacts), chemical and mineralogical analyses of rocks and sands. At Dyngjusandur, the sand is dominated by glass grains, a situation typical of sand plains in Iceland. Hyaloclastite ridges presently buried beneath Vatnajokull are the dominant source of the sand, and only large size plagioclase crystals (0.5 cm) in sands seem to be derived from the lava flows. Hyaloclastite ridges were crushed by glaciers and mechanically eroded sediments were washed out by melt-water onto flood plains. The sand chemical composition is spatially homogeneous and similar to the average composition of neighboring sub-aerial lava flows, reflecting efficient mixing of distinct sources below the glacier. The presence of sand north of Dyngjujokull can be taken as a way to explore the average chemical composition of non-exposed volcanic material beneath the glacier. In the case of Lambahraun, prevailing winds indicate several potential sources of sand at the north of the sand sheet. Comparison of chemical and mineralogical analyses of sands and rock samples helped to refine the exact origin. In contrast with the first site, the sand is dominated by crystals and is chemically consistent with a mixture of material derived from the lava flows of Eldborgir and Skersli shield volcanoes. Analysis of the contact between the lava flows and the glacier reveals that basaltic sand grains formed as the result of recent advances of the glacier abrading the rocks. The direct interaction of glacial and fluvio-glacial activity with basaltic plains appears to be necessary to produce a large amount of sands in a relatively short period of time (