Final answer:
Igneous intrusive rocks and metamorphic rocks are grouped as crystalline rocks in landscape evolution due to their formation under high temperatures and pressures, which produce interlocking crystals, and their commonality in originating from deep within the Earth's crust rather than from surface processes.
Step-by-step explanation:
It is reasonable to group igneous intrusive rocks and metamorphic rocks into a single category of crystalline rock during the discussion of landscape evolution for several reasons. Both of these rock types are formed under conditions involving high temperatures and, in the case of metamorphic rocks, high pressure as well. Due to these conditions, crystals within these rocks can grow larger and interlock, giving both intrusive igneous rocks (like granite) and metamorphic rocks (like gneiss) a similar crystalline texture. Additionally, these rocks often originate from deeper parts of the Earth's crust, and their formation does not involve direct accumulation from surface processes, as in the case of sedimentary rocks. Furthermore, invasive igneous rocks such as granite and metamorphic rocks can both undergo metamorphism, whereby solid rock is altered into a new rock type under heat and pressure without melting into magma or breaking up into sediment.
When evaluating geological provinces, plutonic and volcanic rocks are considered separately because of differences in their cooling rates and hence crystal size, leading to distinctive textures. The coarse-grained texture of plutonic rocks implies a slow cooling process deep within the Earth, whereas volcanic rocks, with finer grains, cool more quickly at or near the surface. When considering the youngest exposed crustal rocks, such as plutonic or metamorphic rocks, geologists focus on these because they provide information about seismic and erosive processes that brought these deeper rocks to the surface.