Abstract
<title>Abstract</title> <p>The viscosity of the lower mantle controls material and heat transport through a large part of the Earth and is determined by a two-phase system consisting of the minerals bridgmanite and ferropericlase. Estimates of the lower mantle viscosity vary by up to 5 orders of magnitude depending on whether it is bridgmanite or ferropericlase controlled. The stronger bridgmanite could control the effective viscosity through its higher volume fraction, while the weaker ferropericlase could significantly reduce the effective viscosity by forming interconnected layers through grain elongation. Here we experimentally determined the rounding rate of elongated ferropericlase grains in a bridgmanite-ferropericlase system under lower mantle pressure and temperature conditions and found that the rounding rate driven by surface energy minimisation is significantly higher than the elongation rate expected from mantle flow. Our results thus show that the effective viscosity of the lower mantle is close to that of bridgmanite and orders of magnitude higher than previously hypothesized.</p>