When magma does not have enough pressure to break completely through the neck and into a central vent, it forms a laccolith.

Here's how it works:

* Magma rises: Magma, molten rock beneath the Earth's surface, rises due to its buoyancy.

* Pressure builds: As the magma pushes upward, pressure builds within the surrounding rock.

* Incomplete breakthrough: If the pressure is not enough to break through the overlying rock completely, the magma will spread out horizontally, creating a lens-shaped intrusion.

* Laccolith formation: This horizontal intrusion, which pushes the overlying rock upward, forms a laccolith.

Think of it like a pancake batter pushing up a thin layer of dough!