The 1996 eruption at Gjálp, Vatnajökull ice cap, Iceland: efficiency of heat transfer, ice deformation and subglacial water pressure
The 1996 eruption at Gjálp, Vatnajökull ice cap, Iceland: efficiency of heat transfer, ice deformation and subglacial water pressure
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1996 年冰岛瓦特纳冰川冰盖 Gjálp 喷发：传热效率、冰变形和冰下水压
DOI：
10.1007/s00445-003-0295-9
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发表时间：
2004
影响因子：
3.5
通讯作者：
Thórdís Högnadóttir
中科院分区：
文献类型：
--
作者：
M. Gudmundsson;F. Sigmundsson;H. Björnsson;Thórdís Högnadóttir
1996年10月Vatnajökull冰盖内持续13天的Gjálp喷发，为研究温带厚冰川中冰-火山相互作用提供了重要数据。这次喷发产生了0.8 km3的主要火山玻璃，其中含有玄武岩冰岛石成分（相当于0.45 km3的岩浆）。6公里长的火山裂缝上方的冰层厚度最初为550-750米。喷发以冰下喷发为主，形成150 ~ 500 m高脊；只有2-4%的火山物质是从地下喷发出来的。对喷口上方冰釜形成的监测提供了冰融化、热通量和间接喷发速率的数据。前4天的热流密度为5-6×105 W m-2。这种高热流只能用岩浆破碎成火山玻璃来解释。火山喷发期间和喷发后的冰融化模式表明，火山喷发部位岩浆与冰之间的瞬时热交换效率为50-60%。如果这是冰下喷发中岩浆破碎的特征，那么在大多数情况下，火山物质和融水将比喷发中融化的冰占据更多的空间。因此，水的积累会导致基础水压的增加，并导致融水的迅速释放。因此，在温带冰川的冰下喷发中，融水的持续排放是最有可能发生的情况。冰的变形和破裂对这次喷发起了重要作用，并改变了冰下水压。研究发现，在下沉的大锅下的通风口处的水压要比在上覆冰的静态载荷下的水压小得多，因为这种载荷部分地被快速变形的冰的剪切力所补偿。除了在Gjálp由于下沉而形成的密集裂缝外，在火山喷发的第一天，火山裂缝的最南端形成了一条又长又直的裂缝。这表明，给料堤坝可能越过基岩-冰界面，造成了高变形速率，并将冰破坏到地表。裂缝后来改变了融水的流动，解释了水流过大厦最高部分的表面流动。Gjálp火山喷发中岩浆碎裂的优势表明，如果要以枕头熔岩喷涌喷发为主要活动方式，至少在类似的高初始岩浆流量喷发中，初始冰厚度必须大于600-700 m。
The 13-day-long Gjálp eruption within the Vatnajökull ice cap in October 1996 provided important data on ice–volcano interaction in a thick temperate glacier. The eruption produced 0.8 km3 of mainly volcanic glass with a basaltic icelandite composition (equivalent to 0.45 km3 of magma). Ice thickness above the 6-km-long volcanic fissure was initially 550–750 m. The eruption was mainly subglacial forming a 150–500 m high ridge; only 2–4% of the volcanic material was erupted subaerially. Monitoring of the formation of ice cauldrons above the vents provided data on ice melting, heat flux and indirectly on eruption rate. The heat flux was 5–6×105 W m-2 in the first 4 days. This high heat flux can only be explained by fragmentation of magma into volcanic glass. The pattern of ice melting during and after the eruption indicates that the efficiency of instantaneous heat exchange between magma and ice at the eruption site was 50–60%. If this is characteristic for magma fragmentation in subglacial eruptions, volcanic material and meltwater will in most cases take up more space than the ice melted in the eruption. Water accumulation would therefore cause buildup of basal water pressure and lead to rapid release of the meltwater. Continuous drainage of meltwater is therefore the most likely scenario in subglacial eruptions under temperate glaciers. Deformation and fracturing of ice played a significant role in the eruption and modified the subglacial water pressure. It is found that water pressure at a vent under a subsiding cauldron is substantially less than it would be during static loading by the overlying ice, since the load is partly compensated for by shear forces in the rapidly deforming ice. In addition to intensive crevassing due to subsidence at Gjálp, a long and straight crevasse formed over the southernmost part of the volcanic fissure on the first day of the eruption. It is suggested that the feeder dyke may have overshot the bedrock–ice interface, caused high deformation rates and fractured the ice up to the surface. The crevasse later modified the flow of meltwater, explaining surface flow of water past the highest part of the edifice. The dominance of magma fragmentation in the Gjálp eruption suggests that initial ice thickness greater than 600–700 m is required if effusive eruption of pillow lava is to be the main style of activity, at least in similar eruptions of high initial magma discharge.