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.

The rise of the Mountains of the Moon is perhaps the best-documented geologic event on Westeros, and is directly responsible for the tremendous wealth of the House Lannister.  Similar to the Black Mountains to the north, the Mountains of the Moon are jagged, rocky, and snow capped.  Based on morphology alone, we cannot rule out that the Black Orogeny and the Moon Orogeny occurred simultaneously.  However, analysis of the most-likely faulting geometry reveals that the Mountains of the Moon exist to the south of a microplate (yes, microplates exist on Earth too).  Based on this geometry, it is more likely that the Moon Orogeny occurred slightly earlier than the Black Orogeny (similar to the microplate tectonics responsible for the Mariana Trench and volcanic island arc of Japan).  Knowing that the Black Mountains are 60-80 million years old, we therefore surmise that the Mountains of the Moon are 80-100 million years old, comparable to the Canadian Rocky Mountains in North America.  The Moon Orogeny is more complex than the Black Orogeny, and we propose that the Mountains of the Moon formed in two stages: 1) early subduction of the microplate beneath southern Westeros, and 2) later continental collision between northern and southern Westeros.

The more recent continental collision between northern and southern Westeros is evident in the width of the Mountains of the Moon, far wider than the Black Mountains, and nearly twice the width of the Himalaya on Earth.  While most mountain ranges on Earth formed as a result of subduction (think: Rocky Mountains, Alps, and Andes), the much larger and wider mountains of the Himalaya formed as a result of two continents (India and Asia) colliding with little to no subduction.  This is the most striking evidence of a collision between northern and southern Westeros.  Yet these mountains are also home to Casterly Rock and the gold deposits from which the House Lannister has so benefited.  Gold is typically deposited on the ocean floor near mid-ocean vents.  The gold mixed with the basalt (extrusive igneous rock) until it reached a subduction zone.  As the oceanic crust that previously separated northern and southern Westeros subducted beneath the southern part of Westeros, layers of ocean rock were scrapped off (like a bulldozer), accreted onto the continent, and uplifted.  The heat and pressure of subduction dissolved the gold, which then intruded into the overlying rock and solidified within quartz veins (this is where the gold from the California Gold Rush came from).  The presence of the Lannister gold (and silver found at Silverhill to the south) supports the notion that subduction tectonics played an equally important early role in the Moon Orogeny.

The final piece of evidence that northern and southern Westeros were previously separate continents is the Iron Islands.  On Earth, iron ore is most widely extracted from rocks known as banded-iron formations (more on these later).  These rocks formed in shallow oceans.  During the initial subduction phase, the ocean floor that wasn’t directly scraped off onto southern Westeros (as Casterly Rock had) was uplifted, pushing the banded iron formations that make the Iron Islands to the surface.  Millions of years of exposure, erosion, and weathering have left behind only the strongest of rocks.  But their very presence owes to uplift caused by the subduction of the microplate.

Long ago, the territory surrounding Winterfell was not prowled by direwolves, but rather by corals, fish, and perhaps the occasional reef shark.  While we know that Winterfell’s protective walls are made of granite, the grey hue of the majority of the fortress suggests a different building material, which we interpret as limestone (a common building material during the middle ages on Earth).  And limestone, if you have not already surmised, is formed by large “carbonate factories” typically associated with shallow seas and reefs.  The evidence for limestone is indirect but compelling, and requires a step back to survey the territory surrounding Winterfell more broadly.  To the southwest of Winterfell lie the Flint Cliffs (which we choose to interpret literally).  Flint is a form of chert, a siliceous rock that forms on the shallow ocean floor.  The proximity of shallow ocean rocks to Winterfell suggests an oceanic origin to the stones used to build the castle.  The presence of caves used as crypts by the family Stark suggests karst topography, which is indicative of limestone and the “swiss cheese” geology associated with subsurface water eroding complex cave systems (think, Carlsbad Caverns in New Mexico, USA, or Waitomo Caves in New Zealand).  The rapidly flowing subsurface water would also help explain the Green Fork River, originating near The Twins.  The rivers that pass through The Twins are somewhat problematic given that the vast majority (~99.9%) of rivers on Earth begin in areas of higher elevation (like mountain ranges).  Yet these rivers seem to simply appear, much like the Warlocks.  We propose that these rivers originate from complex subsurface water pathways making their way to the surface through the chert and limestone, emerging as springs.  Again, while indirect, the combined observations of grey building stones, flint, karst topography, and the Green Fork River strongly suggest that Winterfell sits near large limestone deposits.

Dating the Winterfell limestone is more complicated, but not impossible.  Reefs on Earth only appear within a narrow range of latitudes, approximately 30° north to 30° south (at least in the modern climate – during warmer greenhouse conditions, the latitudinal range is much broader).  Currently, Winterfell sits at near 60° north, far too cold for coral reefs, even during greenhouse conditions.  This suggests that Winterfell, and indeed the entire continent, has moved over the eons, further supporting our early assertion of active tectonics.  Earlier, we concluded that the limestone was likely uplifted during the Moon Orogeny, 80-100 Mya.  Therefore, we know that the Winterfell limestone is older than 100 million years. More exact dating requires comparison to similar rocks on Earth: the Carboniferous Limestone of Great Britain and Ireland.  The Carboniferous Limestone currently sits at similar (within 5°) latitude as Winterfell, and like the Winterfell Limestone, formed as ocean sediments much closer to the equator.  The Carboniferous Limestone is approximately 350 million years old, and required an elongate and curvilinear journey to move from the tropics to its current location.  We assume a more direct path for the Winterfell Limestone, and therefore place an approximate age of 280-300 million years.

The scandalous wedding of young Robb Stark to Jeyne Westerling isn’t the only thing to have been stained red in the history of Westeros (spoiler!).  The Red Keep, home to the Iron Throne in King’s Landing, appears to be made of red-hued sandstone.  We again assume that the quarry is near King’s Landing to facilitate construction, and that the sand was likely eroded from the aptly named Red Mountains to the south.  The crimson coloration might come from a variety of sources, including ferric iron oxide hematite (an iron mineral) that also binds together the New Red Sandstone found in the UK on Earth.  This mineral is indicative of terrestrial origin, suggesting formation within a desert environment.  From our analysis of the size of the Game of Thrones planet, we know that deserts form within a narrow latitude range near 30°.  And our analysis of the Winterfell Limestone suggests that Winterfell was located near 25° north 300 Mya.  We therefore conclude that the Red Keep Sandstone originated in a terrestrial desert environment on southern Westeros around 450 Mya, when King’s Landing, and the entirety of southern Westeros, was within the southern hemisphere.  This approximate age is constrained by the eons required for tectonic forces to translate southern Westeros to its eventual collision with northern Westeros 350 million years later.

The Red Orogeny is the earliest piece of Westeros’ geologic history that we can infer with the available data.  From our analysis of the Red Keep Sandstone and the Winterfell Limestone, we know that Westeros has moved gradually north throughout its history.  From this, we can infer that the Red Mountains formed as oceanic crust to the south subducted beneath continental crust to the north.  An oceanic spreading ridge (like the mid-Atlantic ridge) located south of the map likely drives this process.  When the Red Mountains formed, they likely extended into southern Essos as the oceanic plate subducted beneath the entire continent.  Due to their age, and perhaps more dominant weathering and erosion on the eastern flank of the range due to climatic conditions, the Red Mountains in Essos have been lost to time.  The Red Mountains are notably lower in elevation and far easier to traverse, betraying their true age.  The morphology of such weathered mountains is similar to the Appalachian Mountains in the eastern US, which formed around 500 Mya.  We infer a similar age for the Red Mountains.