In this paper, the stress-strain response of two extrusive volcanic igneous rocks, e.g., compact basalt (CB) and volcanic breccia (VB), under a high rate of loading is studied experimentally and numerically. The dynamic characterization of rocks is done for five different specimen sizes using a split Hopkinson pressure bar (SHPB) device with a
diameter. Based on the test results, the response of the two rocks is compared, and suitable specimen dimensions to be used in SHPB testing are decided for both the rocks. Furthermore, simulations of SHPB tests with a strain rate dependent Johnson-Holmquist (JH-2) model are performed, and numerical simulation results are compared with experimental data to determine the parameters of the JH-2 model for the two rocks. Thus, the parameters obtained are used in the analysis of a tunnel subjected to a 20 kg trinitrotoluene (TNT) explosion in CB and VB, and the simulation results are compared. The suitable specimen sizes for characterizing CB specimens are proposed to be 54 mm in diameter, with a 0.5 slenderness ratio, and 76 mm in diameter, with a 0.3 slenderness ratio. For VB rock, the suitable specimen sizes are 54 mm in diameter, with a 0.2 slenderness ratio, and 76 mm in diameter, with a 0.2 slenderness ratio. The dynamic increase factor varies from 1.69 to 6.77 for CB and from 1.09 to 4.83 for VB. In the blast analysis for tunnels in the two rocks, nearly 4 times higher stress and 4.4 times lower displacement are observed at the tunnel crown in CB as compared to that in VB.