Hydration of Minerals
When oceanic crust sits in seawater for millions of years, cracks form in the basalt. Seawater seeps into these cracks and reacts with the hot rock, creating new minerals like serpentine. Serpentine has a chemical structure that literally holds water molecules within its crystal lattice. This process is called hydration. The water is no longer free-flowing fluid; it is chemically bound.
The Sponge Analogy
Think of the oceanic crust like a sponge. When you pick up a wet sponge, the water doesn't drip out immediately if the sponge is compressed or held tightly. Similarly, as the plate subducts, the pressure and heat keep the water locked inside the 'rock sponge' (hydrated minerals), preventing it from leaking out into the mantle.