The pyroclastic deposits of the 1875 eruption of Askja, Iceland
The pyroclastic deposits of the 1875 eruption of Askja, Iceland
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1875 年冰岛阿斯贾喷发的火山碎屑沉积物
DOI：
10.1098/rsta.1981.0023
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发表时间：
1981
期刊：
影响因子：
--
通讯作者：
H. Sigurdsson
中科院分区：
文献类型：
--
作者：
R. Sparks;L. Wilson;H. Sigurdsson
1875年冰岛Askja的爆发是1874年和1875年发生在北方裂谷带的一系列区域性火山和构造事件的一部分。这些事件的特点是区域地震活动，地堑形成和Sveinagja的玄武岩裂缝喷发，以及3月28日至29日Askja的plinian喷发。地壳裂陷导致玄武质岩浆与流纹质岩浆混合，触发了海侵喷发。在Askja形成了一个破火山口，Oskjuvatn，大小为3x 4公里，深267米。可以识别出六个可区分的火山碎屑层。主喷发开始于形成层B的一个小的亚普利尼亚浮石喷发。下一阶段产生了细粒、分选不良的浮石和灰存款，具有发育良好的分层（C层），其中包含靠近源的底涌床，被解释为起源于蒸汽岩浆作用。主plinian阶段的喷发持续了6小时，形成了一个粗粒，不良层状浮石落存款（D层），其中包含75%的总喷出物。后期爆炸形成了一层岩屑碎屑（E层）。等厚线和粒度等值线图显示，喷口在Oskjuvatn现在占据的地区沿沿着一条1.5公里长的线从南向北迁移。随着时间的推移，喷发的强度和柱高增加，如所示的反向分级和连续层的分散指数增加。喷出物主要由白色流纹岩浮石和火山灰组成。岩性包括流纹岩黑曜石，部分熔融trondhjeimite，玄武岩碎片：层D含有2.1质量%的岩性。所有层均含有丰富的灰色浮石碎屑，由深棕色玄武质和棕色流纹质玻璃的紧密混合物组成。D层中混合浮石的质量百分比为4.7，其中40%为玄武玻璃。这些混合的浮石碎屑通过风成分选集中在D层30-80 km处。灰色、富含晶体的安山质浮石以包裹体形式出现在白色浮石中。层D显示中值粒径的系统性减小，但随着距源的距离，Cr^没有变化。C层的中值粒径没有变化，但随着距离源的增加而减小。在步行者（1973）的Md$对Cr^图、a对a*（偏斜度）图和F对D图上，可以很容易地将C层等深成岩浆矿床与普林岩矿床区分开来。顺风，在C层的粗尾分级是由于落细灰作为团块和聚集体。D层和C层的总粒度分布呈双峰型。在D层中，一个次要的模式，在灰的大小类反映了二次破碎的碰撞过程中的通风口和列，而主要的模式是由于破坏岩浆的气体膨胀。在层C的罚款模式是占主导地位的，并代表广泛的破碎与水的爆炸相互作用。D层的野外和粒度研究表明，冲击破碎在近震源区起主要作用。
The 1875 explosive eruption of Askja, Iceland was part of a series of regional volcanic and tectonic events which took place in the northern rift zone in 1874 and 1875. These events were marked by regional seismicity, graben formation and a basaltic fissure eruption at Sveinagja, and the plinian eruption of Askja on 28-29 March. Crustal rifting caused basaltic magma to be mixed with rhyolitic magma, triggering the plinian eruption. A caldera, Oskjuvatn, was formed in Askja measuring 3 x 4 km and 267 m deep. Six distinguishable pyroclastic layers can be recognized. The main eruption began with a small sub-plinian pumice eruption forming layer B. The next phase produced a fine-grained, poorly sorted pumice and ash deposit with well developed stratification (layer C), which contains base surge beds near source and is interpreted as phreatomagmatic in origin. The main plinian phase of the eruption lasted 6 h and formed a coarse-grained, poorly bedded pumice-fall deposit (layer D) which contains 75% of the total ejecta. Late-stage explosions formed a layer of lithic clasts (layer E). Isopach and grain-size isopleth maps show that the vents migrated from south to north along a line 1.5 km long in the area now occupied by Oskjuvatn. The intensity and column height of the eruption increased with time as shown by reverse grading and an increasing dispersal index in successive layers. Most of the ejecta is composed of white rhyolitic pumice and ash. Lithics consist of rhyolitic obsidian, partially fused trondhjeimite, and basalt fragments: layer D contains 2.1 mass % lithics. All layers contain abundant grey pumice clasts consisting of intimate mixtures of dark brown basaltic and brown rhyolitic glasses. The mass percentage of mixed pumice in layer D is 4.7, of which 40 % is basaltic glass. These mixed pumice clasts are concentrated at distances of 30-80 km in layer D by aeolian sorting. A grey, crystal-rich, andesitic pumice occurs as inclusions in the white pumice. Layer D shows a systematic decrease in median grain diameter, but no change in cr^ with distance from source. Layer C shows no change in median grain diameter, but a decrease in with distance from source. Phreatomagmatic deposits such as layer C can be readily distinguished from plinian deposits on a Md$ against cr^ diagram, on a against a* (skewness) diagram and on the F against D plot of Walker (1973). The downwind, coarse-tail grading in layer C is attributed to fall-out of fine ash as clumps and aggregates. The total grain-size distributions of both layers D and C show bimodality. In layer D a minor mode in the ash size classes reflects secondary processes of fragmentation by collisions in the vent and column, whereas the major mode is due to disruption of magma by expanding gases. In layer C the fine mode is dominant and represents extensive fragmentation by explosive interaction with water. Field and grain-size studies of layer D show that impact breakage is of major importance near source.