Hawaiian and other ocean island lava flows that reach the coastline can deposit significant volumes of lava in submarine deltas. The catastrophic collapse of these deltas represents one of the most significant, but least predictable, volcanic hazards at ocean islands. The volume of lava deposited below sea level in delta-forming eruptions and the mechanisms of delta construction and destruction are rarely documented. Here, we report on bathymetric surveys and ROV observations following the Kīlauea 2018 eruption that, along with a comparison to the deltas formed at Pu‘u ‘Ō‘ō over the past decade, provide new insight into delta formation. Bathymetric differencing reveals that the 2018 deltas contain more than half of the total volume of lava erupted. In addition, we find that the 2018 deltas are comprised largely of coarse-grained volcanic breccias and intact lava flows, which contrast with those at Pu‘u ‘Ō‘ō that contain a large fraction of fine-grained hyaloclastite. We attribute this difference to less efficient fragmentation of the 2018 ‘a‘ā flows leading to fragmentation by collapse rather than hydrovolcanic explosion. We suggest a mechanistic model where the characteristic grain size influences the form and stability of the delta with fine grain size deltas (Pu‘u ‘Ō‘ō) experiencing larger landslides with greater run-out supported by increased pore pressure and with coarse grain size deltas (Kīlauea 2018) experiencing smaller landslides that quickly stop as the pore pressure rapidly dissipates. This difference, if validated for other lava deltas, would provide a means to assess potential delta stability in future eruptions.

到达海岸线的夏威夷和其他海洋岛屿熔岩流会在海底三角洲沉积大量熔岩。这些三角洲的灾难性倒塌是海洋岛屿上最严重，但最不可预测的火山灾害之一。在形成三角洲的火山喷发中，沉积在海平面以下的熔岩的量以及三角洲构造和破坏的机制很少有文献记载。在这里，我们报告了2018年基拉韦厄火山喷发后的测深调查和ROV观测，并与过去十年在Pu'u'Ō'ō形成的三角洲进行了比较，从而提供了对三角洲形成的新见解。等深线的差异显示，2018年三角洲的火山喷发总量占火山喷发总量的一半以上。此外，我们发现，2018年的三角洲主要由粗粒火山角砾岩和完整的熔岩流组成，这与Pu'u'Ō'ō的三角洲包含大量细粒的透明质碎屑岩形成鲜明对比。我们将此差异归因于2018年的'a'ā流碎片化效率较低，导致坍塌而不是水力火山爆发碎片化。我们建议一个机械模型，其中特征晶粒尺寸会影响三角形的形状和稳定性，其中细晶粒尺寸delta（Pu'u'Ō'ō）经历较大的滑坡，而更大的跳动则受到孔隙压力增加和粗晶粒尺寸的支持三角洲（Kīlauea2018）经历了较小的滑坡，随着孔隙压力迅速消散，滑坡迅速停止。如果针对其他熔岩三角洲进行了验证，则该差异会有所不同，