Fig. 5. Numerical model showing dyke arrest at a contact between mechanically dissimilar rocks. A stress barrier, that is, layer with a local stress field that is unfavourable for a fracture of a particular type (here a dyke) forms by principal-stress stress rotation at the contact. The only loading is magmatic excess pressure (5 MPa) in the magma chamber, located in a layer with a Young’s modulus of 40 GPa. The thick (blue) and thin (yellow) layers have Young’s moduli of 100 GPa and 1 GPa, respectively. The direction of the principal compressive stress, σ1, is indicated by ticks (short lines). (a) State of stress before dyke injection. (b) The dyke follows the σ1-ticks and, depending on its overpressure and the elastic mismatch across the contact, becomes arrested (as seen here) deflected into a sill (Fig. 3 above).