Volcanic eruptions shape Earth’s surface and provide a window into deep Earth processes. How the primary asthenospheric melts form, pond and ascend through the lithosphere is, however, still poorly understood. Since 10 May 2018, magmatic activity has occurred offshore eastern Mayotte (North Mozambique channel), associated with large surface displacements, very-low-frequency earthquakes and exceptionally deep earthquake swarms. Here we present geophysical and marine data from the MAYOBS1 cruise, which reveal that by May 2019, this activity formed an 820-m-tall, ~5 km³ volcanic edifice on the seafloor. This is the largest active submarine eruption ever documented. Seismic and deformation data indicate that deep (>55 km depth) magma reservoirs were rapidly drained through dykes that intruded the entire lithosphere and that pre-existing subvertical faults in the mantle were reactivated beneath an ancient caldera structure. We locate the new volcanic edifice at the tip of a 50-km-long ridge composed of many other recent edifices and lava flows. This volcanic ridge is an extensional feature inside a wide transtensional boundary that transfers strain between the East African and Madagascar rifts. We propose that the massive eruption originated from hot asthenosphere at the base of a thick, old, damaged lithosphere.

火山喷发塑造了地球表面，并为深入地球过程提供了一个窗口。然而，主要的软流圈熔体是如何形成、沉淀和上升穿过岩石圈的，人们仍然知之甚少。自 2018 年 5 月 10 日以来，马约特岛东部（北莫桑比克海峡）近海发生了岩浆活动，与大的地表位移、极低频地震和异常深的地震群有关。在这里，我们展示了 MAYOBS1 巡航的地球物理和海洋数据，这些数据显示，到 2019 年 5 月，该活动在海底形成了一座 820 米高、约 5 平方公里的火山大厦。这是有史以来最大的活跃潜艇喷发。地震和变形数据表明深（> 55 km 深）岩浆储层通过侵入整个岩石圈的岩脉迅速排出，地幔中预先存在的近垂直断层在古老的破火山口结构下重新激活。我们将新的火山大厦定位在一个 50 公里长的山脊的顶端，该山脊由许多其他近期的建筑和熔岩流组成。这个火山脊是一个在东非和马达加斯加裂谷之间传递张力的宽阔的横贯边界内的伸展特征。我们提出，大规模喷发起源于一个厚厚的、古老的、受损的岩石圈底部的热软流圈。这个火山脊是一个在东非和马达加斯加裂谷之间传递张力的宽阔的横贯边界内的伸展特征。我们提出，大规模喷发起源于一个厚厚的、古老的、受损的岩石圈底部的热软流圈。这个火山脊是一个在东非和马达加斯加裂谷之间传递张力的宽阔的横贯边界内的伸展特征。我们提出，大规模喷发起源于一个厚厚的、古老的、受损的岩石圈底部的热软流圈。