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

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.