Using ultra-high-resolution, 2D-thermomechanical modeling, we explore the evolution of the mantle of a terrestrial planet with a stagnant lid like Venus or the early Earth. Without plate tectonics, the mantle will heat over time from the decay of radiogenic isotopes. The convecting upper mantle will undergo partial melting and thicken a basaltic crust. When the crust reaches a critical thickness, the base of it transforms into eclogite. As eclogite is considerably denser than the underlying mantle it will ultimately delaminate. While ringwoodite in the peridotitic ambient mantle turns into perovskite and periclase at around 24 GPa and becomes about 10% denser, the similar phase reaction for garnet into perovskite in the basaltic crust occurs deeper, at around 27 GPa. The delaminated crust therefore accumulates in the 24-27 GPa range, where it has an intermediate density of the material above and below, and therefore suppresses convection across. Over time, this causes a significant temperature offset, as the upper mantle can continuously vent radiogenic heat, while the lower mantle cannot. Eventually, the crust-rich layer is pushed beyond 27 GPa and becomes denser than the underlying lower mantle. This triggers a run-away global mantle overturn. Superheated lower mantle streams upwards into the melting zone, increasing magmatic production by orders of magnitude for ≈50 Myr. After overturn, the mantle is highly heterogeneous with preserved domains of primitive mantle in a fooliated mélange of depleted mantle and basaltic crust.