The data consists of the raw data (time, load and displacement) from indirect tensile strength (IDTS) tests on ballistically ejected rock samples from 3 different volcanoes; Soufriere Hills Volcano, Montserrat, West Indies and Reventador and Sangay volcanoes, Ecuador, South America. The Brazil test method (ISRM, 1978) was used in a uniaxial hydraulic press in the Rock and Ice Physics Laboratory, University College London. ISRM, 1978. Suggested methods for determining tensile strength of rock materials. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 15 (3), 99–103. The data were collected, between June 2024 and November 2025, as part of the NERC-funded project NE/W008033/1 The explosivity of volcanic eruptions: Quantifying the critical role of permeability in magmas.