 As the continental crust stretches, the near-surface rocks fracture along steeply dipping cracks parallel to the rift known as faults.Hot basaltic magma rises along these fractures to create passive lava eruptions.The compositions of lavas in the Northern Cordilleran Volcanic Province are mantle-derived alkali olivine basalt, lesser hawaiite and basanite, which form the large shield volcanoes and small cinder cones throughout the volcanic province.