Magmatism in subduction zones fuels some of the most explosive volcanic eruptions and helps to produce new continental crust on Earth. Thermal, petrologic, and geophysical evidence indicate that the primary melting occurs across hundreds of kilometers (km) in both depth and distance within a mantle wedge. Such depths correspond to high pressures (P) ≤ 7 GPa. In contrast, the surficial volcanic arcs occur in narrow bands only tens of km wide. This discrepancy requires that the melt must be focused prior to reaching the arc during ascent from depth. The focusing of magma into narrow channels will be strongly influenced by the transport properties of the melt. In particular, the melt viscosity (η), which is inversely related to diffusivity of ions in the melt, will affect both the modes and timescales of melt migration. High-P experiments on silicate melt η constrain the mobility of magma at P and temperatures (T) relevant to subduction zones. These experiments show that the melt η is complexly influenced by P-T and silica contents. This is crucial as the magma in subduction zones often becomes dramatically enriched in silica from ≤ 52 to ≥ 63 weight % SiO2 during migration. Yet, we are lacking a thorough understanding of the combined P-T-composition effects on silicate melt η due to the technical challenges and high uncertainties of the high-P experiments. Geophysical soundings offer additional insights into the magmatic processes at depth but cannot directly determine the melt η. Notably, the ionic diffusivity also influences the electricity conductivity (σ) of silicate melt. Magnetotelluric (MT) soundings are also sensitive to the melt σ and could thereby constrain the melt mobility. In this work, we will present a summary of our ongoing work to investigate the relationship between η and σ of silicate melts at P-T relevant to subduction zones.