Assessing the potential and likelihood of explosive eruptions is vital for almost 800 million people living near active volcanoes. Magma reservoirs are not directly accessible, limiting our capability to understand the prerequisites for an explosive eruption and our ability to perform experiments that simulate magmatic processes at depth. Therefore, deciphering volcanic precursors is key to forecasting volcanic eruptions. In this study, we used synchrotron-based X-ray tomographic microscopy to observe felsic magma vesiculation at high temperature (1,100°C) to establish the pre-eruptive conditions that favor explosive activity. Specifically, we studied the increase of seismic attenuation with increasing gas content during magma vesiculation. These results are applied to the natural range of volcanic tremors (0.2–5 Hz) to detect the proximity of gas-charged magma to the surface prior to volcanic eruption: gas-rich, crystal-free felsic magmas are detected at larger depth (40 MPa) than gas-rich, crystal-bearing systems (10 MPa).