It's made up of rocks, which are solid materials. However, it's important to note that the crust isn't a single, uniform solid. It's actually made up of different types of rocks with varying properties, and it's broken into large pieces called tectonic plates. These plates are constantly moving, which is why we experience earthquakes and volcanic activity.

While the crust itself is solid, it can be influenced by the underlying mantle, which is semi-solid. This means that the crust can be deformed and even flow very slowly over time, leading to the movement of tectonic plates.