  Pressure is a key parameter in the physics and chemistry of planet formation
and evolution. Previous studies have erroneously assumed that internal
pressures monotonically increase with the mass of a body. Using smoothed
particle hydrodynamics and potential field method calculations, we demonstrate
that the hot, rapidly-rotating bodies produced by giant impacts can have much
lower internal pressures than cool, slowly-rotating planets of the same mass.
Pressures subsequently increase due to thermal and rotational evolution of the
body. Using the Moon-forming impact as an example, we show that the internal
pressures after the collision could have been less than half that in
present-day Earth. The current pressure profile was not established until Earth
cooled and the Moon receded, a process that may take up to 10s Myr after the
last giant impact. Our work defines a new paradigm for pressure evolution
during accretion of terrestrial planets: stochastic changes driven by impacts.
