Determining the age of the various Westeros mountain ranges is problematic without geochemistry; the wildlings make sample collection difficult. However, we can infer ages based on the current shape, or morphology, of the mountains. As Jon Snow and the men of the Night’s Watch have observed, the northern mountains, which we will call the Black Mountains, are jagged, rocky, snowcapped and quite treacherous to cross. This jagged morphology suggests that the Black Mountains are relatively young, and the morphological similarity to the Rocky Mountains in the western US suggests that the Black Mountains are 60-80 million years old. During this time of mountain building – called orogeny – an oceanic plate began to subduct beneath the northern territories of Westeros. As the plate slid deeper into the planet, the heat of the mantle boiled away the water, which in turn melted the rock above, which then erupted forth as lava at the surface and created the volcanic mountain range (this form of volcanism is common on Earth, and is primarily responsible for the “ring of fire” around the margins of the Pacific plate). Again, we are somewhat troubled by the lack of documented volcanism in the Black Mountains. Our only explanation is that subduction of the plate ceased for some unknown reason, thereby killing the volcanic engine.
The evidence of plate subduction is heavily inferred from geologic features found in Winterfell, specifically the hot springs that keep the town warm during winter, and the protective granite walls that secure the town’s interior. While hot springs have multiple geologic origins, we propose that the Winterfell hot springs are the result of faulting – cracks in the crust that allow water to flow deep within the earth, get heated, then return to the surface via other faults (hot springs of this sort can be found all over the world, including coastal California). The tremendous pressures involved as the plate subducted beneath the northern territories were surely enough to have caused faulting of this type, and the inferred volcanism would provide more than ample heat. Thus, we interpret the Winterfell hot springs as fault-caused, consistent with plate subduction. The protective granite walls offer more evidence of plate subduction. Granite is an intrusive igneous rock, meaning that a magma chamber never erupted and the large body of molten rock cooled to granite within the earth. Intrusive granites are common in subduction zones (think, Sierra Nevada mountains in California). Given Winterfell’s proximity to the Black Mountains, we propose that a granite quarry must be nearby, easing the distance traversed by the giants of legend who hauled the granite slabs to Winterfell.