Macquarie Island: A window into the oceanic crust and upper mantle

Because of the inaccessibility of the deep-ocean floor, our knowledge about the composition and structure of the oceanic crust is very limited. Macquarie Island is the only fragment of ocean crust exposed above sea-level in the world, providing a unique opportunity to study the ocean crust directly in unprecedented detail.

From the abstract of the referenced paper:

Macquarie Island preserves largely in-situ Miocene oceanic crust and mantle formed at a slow-spreading ridge. The crustal section on the island does not conform to a simple 'layer cake pseudo-stratigraphy', but is the result of multiple magmatic episodes. Macquarie Island crust did not grow by top-down cooling, but rather from the base up. Peridotites cooled first and formed the basement into which gabbro plutons were intruded. This was followed by cooling and deformation, and by intrusion of dykes that fed a sheeted dyke-basalt complex. Finally, lava filled grabens were formed. These relative age relations rule out simple co-genetic relations between rock units.

由于深海海底难以抵达，人类对洋壳（oceanic crust）的组成与结构认知极为有限。麦夸里岛（Macquarie Island）是全球唯一一处露出海平面的洋壳残片，为我们以空前精细的尺度直接开展洋壳研究提供了独一无二的契机。

摘自所引用论文的摘要：
麦夸里岛大体保留了在慢速扩张洋脊（slow-spreading ridge）形成的原位中新世洋壳与地幔。该岛上的地壳剖面并非简单的"层状假地层"结构，而是多期岩浆作用的产物。麦夸里岛洋壳并非自上而下冷却形成，而是自下而上生长：橄榄岩（peridotites）率先冷却形成基底，辉长岩岩体（gabbro plutons）随后侵入该基底；随后发生冷却与变形作用，侵入岩墙为席状岩墙-玄武岩杂岩（sheeted dyke-basalt complex）提供了补给；最终形成充填熔岩的地堑。这些相对年代关系排除了各岩石单元间存在简单同成因关系的可能。