Magma in a sill that pushes up to form a rock dome is called a laccolith.

Here's why:

* Sill: A sill is a horizontal sheet of igneous rock that intrudes between layers of existing rock.

* Laccolith: A laccolith is a dome-shaped intrusion of magma that forces its way between layers of sedimentary rock, creating a bulge in the overlying strata.

Essentially, a laccolith is a type of sill that has been pushed upwards by the pressure of the magma.