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Why does lower gabbro contain more iron and magnesium than upper oceanic crust?
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oceanic crust
A cross section of Earth's outer layers, from the crust through the lower mantle.
The lower gabbro layer of the oceanic crust contains higher concentrations of iron and magnesium because it is a cumulate structure. This layered formation results from the sedimentation of minerals out of liquid magma at the base or sides of a magma chamber.
As these minerals settle, they create layers that are enriched in iron and magnesium—notably through the presence of olivine, an iron-magnesium silicate—while containing less silica than the upper portions of the crust.1