The oceanic crust is solid. It is composed primarily of basalt, a dense volcanic rock that forms from the cooling and solidification of molten material from the Earth’s mantle. This solid crust lies under the ocean and is typically around 5 to 10 kilometers thick.
Although the oceanic crust itself is solid, it rests atop the semi-fluid asthenosphere, which is part of the upper mantle. This layer can flow slowly over geological timescales, allowing the rigid plates of the lithosphere, which includes both the oceanic crust and continental crust, to move and shift.
In conclusion, while the oceanic crust is indeed solid, it interacts with the underlying mantle, which behaves like a viscous fluid in certain conditions, enabling tectonic activity such as earthquakes and the formation of new crust at mid-ocean ridges.