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.

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