Internal structure and occurrence of accretionary lapilli — a case study at Laacher See Volcano
Internal structure and occurrence of accretionary lapilli — a case study at Laacher See Volcano
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增生火山岩的内部结构和产状——以拉彻湖火山为例
DOI：
10.1007/bf00493689
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发表时间：
1991
影响因子：
3.5
通讯作者：
H. Schmincke
中科院分区：
文献类型：
--
作者：
R. Schumacher;H. Schmincke
在Laacher See火山的细粒火山碎屑流和浪涌沉积物以及相关的共熔凝灰岩/共浪涌灰层中，增生火山砾很常见。火山碎屑可分为两种不同的形态类型：（1）边缘型火山碎屑由粗粒火山岩核和细粒火山岩边缘组成。边缘是内部分级的或由几层交替的细粒和极细粒灰组成。(2)核心型火山砾缺乏细粒边缘。场的关系，内部和粒度特征是特定的增生火山砾从不同类型的火山灰矿床。因此，增生火山砾可能是一个有用的工具，以推断不同起源火山灰的起源。在共熔凝灰岩火山灰中，增生火山砾一般集中在底部，而火山碎屑流和浪涌沉积物则在个别薄层的上部含有火山砾。火山碎屑流和涌浪沉积物中发现了边缘型火山砾，距离源头4公里。核心型火山砾出现在更远的距离或与泡状凝灰岩，他们是在1公里内的喷口。增生火山砾从共同的熔结凝灰岩/共同浪涌灰显示开放的框架纹理和边缘到面接触的个别灰颗粒。囊泡度通常较低，但40%至50%的总孔隙率导致平均密度为1200 kg/m3。火山碎屑流和涌浪沉积物中的增生火山砾堆积得更密集，片状颗粒经常面对面接触。火山碎屑流沉积物的泡状度明显较高;总孔隙率约为30%至40%，平均密度为1600 kg/m3。粒度分析表明，共熔凝灰岩/共涌火山灰堆积物中的增生火山砾粒度最细，中值（Md）为20 ~ 30 μm，最大粒度为250 ~ 350 μm。火山碎屑流沉积物的增生火山砾具有30 ~ 50 μm的中等Md值和350 ~ 500 μm的最大粒度。涌浪沉积物的颗粒较粗，M_d值为30 ~>63 μm，最大粒径可达2 mm。
Accretionary lapilli are common in fine-grained pyroclastic flow and surge deposits and related co-ignimbrite/co-surge ash layers of Laacher See volcano. Two morphologically different types are distin-guished: (1) Rim-type lapilli are composed of a coarse-grained core surrounded by a fine-grained rim. Rims are internally graded or made up of several layers of alternating fine and very-fine grained ash. (2) Core-type lapilli lack fine-grained rims. Field relationships, internal, and grain-size characteristics are specific to accretionary lapilli from different types of tephra deposits. Accretionary lapilli may therefore be a helpful tool to infer the origin of tephra of different origin. In co-ignimbrite ashfall, accretionary lapilli are generally concentrated at the base, whereas pyroclastic flow and surge deposits contain lapilli in the upper parts of individual, thin-bedded layers. Rim-type lapilli are found in pyroclastic flow and surge deposits up to 4 km from the source. Core-type lapilli occur at greater distances or are associated with vesiculated tuffs where they are within 1 km from the vent. Accretionary lapilli from co-ignimbrite/co-surge ash show open framework textures and edge-to-face contacts of individual ash particles. Vesicularity is generally low but the overall porosity of 40% to 50% results in an average density of 1200 kg/m3. Accretionary lapilli in pyroclastic flow and surge deposits are more densely packed and platy particles are often in face-to-face contacts. Vesicularity of those from pyroclastic flow deposits is significantly higher; the overall porosity is about 30% to 40% and the average density 1600 kg/m3. Grain-size analyses show that the accretionary lapilli in co-ignimbrite/co-surge ashfall deposits are the most fine-grained with a median (Md) of 20 to 30 μm and a maximum grain size of 250 to 350 μm. Accretionary lapilli from pyroclastic flow deposits have intermediate Md-values of 30 to 50 μm and a maximum grain size of 350 to 500 μm. Those of surge deposits are the coarsest grained with Md-values of 30 to >63 μm and a maximum grain size up to 2 mm.