Rocks of different origins and ages occur in three fundamentally different geological provinces
Rift systems
- Consist of linked collections of deep valleys and moderately high flanking mountain ranges
- Steeply-inclined faults mark the boundaries between deep valleys and flanking ranges
- Faults accommodate crustal elongation
- Valleys and flanking ranges are often the locus of extensive igneous activity
- Active volcanoes in rift systems are rarely explosive; volcanism is quiescent