Structures that form when magma solidifies beneath the surface are called plutons.

Here's a breakdown:

* Magma: Molten rock found beneath the Earth's surface.

* Pluton: A body of intrusive igneous rock (formed from solidified magma) that is exposed at the surface by erosion.

* Intrusive: Describes igneous rocks that solidify beneath the Earth's surface.

Plutons come in various shapes and sizes, and they can be classified based on their geometry and size, such as:

* Batholith: A very large, irregularly shaped pluton.

* Stock: A smaller, more rounded pluton.

* Dike: A tabular, discordant pluton that cuts across existing rock layers.

* Sill: A tabular, concordant pluton that is parallel to existing rock layers.

* Laccolith: A dome-shaped pluton that forces overlying rock layers upwards.