Layer 3: Deep Gabbro | AI Learning Guide

Finally, beneath the basalt, we find gabbro. This is the foundation of the ocean floor. While basalt cooled quickly, gabbro cooled very slowly deep underground. This slow cooling allows large crystals to form, making it a coarse-grained rock. This layer is much thicker than the basalt above it and makes up most of the oceanic crust. It is incredibly solid and acts as a massive, nearly impermeable barrier, preventing water from sinking deeper into the Earth.

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The Foundation: Gabbro

Below the thin sheet of basalt lies a much thicker layer of gabbro. Unlike basalt, which cooled quickly on the surface, gabbro forms when magma cools very slowly deep underground. Because it cools slowly, the crystals inside have time to grow large, making gabbro a coarse-grained rock. This layer makes up the bulk of the oceanic crust, sitting right on top of the Earth's mantle. It is extremely solid and impermeable, acting as a massive barrier to water flow.

Basalt vs. Gabbro

Basalt (Upper Crust)

Cools rapidly at the surface
Fine-grained texture
Thinner layer
Forms pillow lavas

Gabbro (Lower Crust)

Cools slowly underground
Coarse-grained texture
Thicker layer
Massive crystalline structure