Influence of pre-eruptive storage conditions and volatile contents on explosive Plinian style eruptions of basic magma
Influence of pre-eruptive storage conditions and volatile contents on explosive Plinian style eruptions of basic magma
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喷发前储存条件和挥发分含量对基性岩浆爆发性普林尼式喷发的影响
DOI：
10.1007/s00445-010-0343-1
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发表时间：
2010
影响因子：
3.5
通讯作者：
J. Gardner
中科院分区：
文献类型：
--
作者：
K. Goepfert;J. Gardner
次普林尼期到普林尼期的基性岩浆喷发对火山行为的建模提出了挑战，因为许多模型依赖于岩浆在上升过程中变得足够粘稠，从而表现出脆性并导致碎裂。然而，这样的模型无法使低粘度的岩浆变形得足够快，使其表现出脆性。这是假设这些岩浆实际上具有低粘度，但事实上，普林尼纪罕见的基性岩浆喷发可能是由于它们具有极高的粘度。在这里，我们研究两个这样的爆发，公元前122年。夏威夷玄武岩喷发埃特纳火山和更新世晚期马萨亚火山口玄武岩安山岩的喷发，以测试它们是否粘稠。我们对这两种岩浆进行了热液实验，并分析了斜长石斑晶中的玻璃包裹体，以确定它们最可能的喷发前温度和含水量。我们发现夏威夷岩最后储存在1，000 - 1，020 ° C，而玄武安山岩最后储存在1，010 - 1，060 ° C，并且两者都是水饱和的，重量百分比为103.0%。水溶解在其中。这样的含水量还不足以引发普林尼式的爆炸行为，因为含水量更高的基性岩浆喷发得不那么猛烈。此外，尽管相对较冷，但两种岩浆的粘度范围从喷发前的102.2 - 2.5 Pa·s到基本脱气后的104.4 Pa·s，所有这些都是流体，不会导致脆性破裂。在不援引特殊外力来解释所有这些喷发的情况下，一个更合理的解释是，当气泡含量达到某个临界值时，脆弱的泡沫状岩浆就会破裂。普林尼期基性岩浆喷发的罕见性可能是因为这种岩浆必须上升得足够快才能保留它们的气泡。
Sub-Plinian to Plinian eruptions of basic magma present a challenge to modeling volcanic behavior because many models rely on magma becoming viscous enough during ascent to behave brittlely and cause fragmentation. Such models are unable, however, to strain low viscosity magma fast enough for it to behave brittlely. That assumes that such magmas actually have low viscosities, but the rare Plinian eruptions of basic magma may in fact result from them being anomalously viscous. Here, we examine two such eruptions, the 122 B.C. eruption of hawaiitic basalt from Mt. Etna and the late Pleistocene eruption of basaltic andesite from Masaya Caldera, to test whether they were anomalously viscous. We carried out hydrothermal experiments on both magmas and analyzed glass inclusions in plagioclase phenocrysts from each to determine their most likely pre-eruptive temperatures and water contents. We find that the hawaiite was last stored at 1,000–1,020°C, whereas the basaltic andesite was last stored at 1,010–1,060°C, and that both were water saturated with ∼3.0 wt.% water dissolved in them. Such water contents are not high enough to trigger Plinian explosive behavior, as much more hydrous basic magmas erupt less violently. In addition, despite being relatively cool, the viscosities of both magmas would range from ∼102.2–2.5 Pa s before erupting to ∼104 Pa s when essentially degassed, all of which are too fluid to cause brittle disruption. Without invoking special external forces to explain all such eruptions, one of the more plausible explanations is that when the bubble content reaches some critical value the fragile foam-like magma disrupts. The rarity of Plinian eruptions of basic magma may be because such magmas must ascend fast enough to retain their bubbles.