Magmatic underplating of the crust is a common feature of major basalt provinces. The emplacement of magma within the lithosphere leads to surface uplift, with a magnitude of the order of 10% of the thickness of the underplated material. The composition of associated igneous rock samples suggests that much of the underplated material is made of gabbro. This gabbro is denser than the magma from which it crystallizes, so uplift of Earth's surface caused by the emplacement of magma at the crust-mantle boundary must be followed by subsidence as the magma solidifies. This subsidence is equal to approximately half the original uplift and takes place within ∼0.1 m.y. of injection of the magma. The association of magmatic underplating with surface uplift followed by subsidence provides a mechanism for changes of sea level, including previously unexplained episodic highstands of sea level.