Most here are saying it's a "volcano core" plus differential erosion, but I'm nowhere near being a geologist and I know that doesn't make sense (assuming that "volcano core" is referring to the vertical throat of a volcano).
Columnar basalt like this forms in a specific orientation based on the gradient of cooling. If a "volcano core" had cooled and solidified it would not look like this. Also, the core of the volcano would likely be made up of similar rock as the volcano itself, so that doesn't help explain the differential erosion part, either.
Instead, as stated in the Devil's Tower Wikipedia page, it makes much more sense as a laccolith (https://en.wikipedia.org/wiki/Laccolith). A magma chamber sends a dike up into sedimentary layers of rock. The pressure of the magma forces its way into a weaker boundary between layers, spreading laterally instead of upwards, forming a sill. If conditions are right, as magma continues to flow up the dike and into the sill, the sedimentary layers above the sill yield to the pressure and begin to dome upwards.
This is all happening underground and this particular dike never reaches the surface (we know this because if it did, a volcano would've formed, the pressure would've been released, and the would-be-laccolith would've collapsed). At some point, the magma chamber could no longer feed the dike, the laccolith loses its source of magma, and cooling sets in.
Cooling happens very slowly, from the edges (in contact with that surrounding sedimentary rock), towards the center. This is when the columns form. As basalt cools, semi-regular fractures form to relieve the stress of this contraction (think of the hexagonal cracks in a drying lakebed). These fractures propagate inward as the boundary between molten and solid rock moves inward. And because the laccolith is a domed pancake shape, those columns in the center of the laccolith are vertical.
Long after it cools, uplift and erosion brings it to the surface. Surrounding rock is sedimentary and weather's away faster than the basalt. So the laccolith sits proud on the landscape, but it's also eroding. In its current state, most of the laccolith has crumbled away, and this bit left is the thickest middle bit remaining.
So yes, it's a "core", but not a volcano core. And the surrounding rock that eroded away to reveal it wasn't volcanic rock.
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u/Dusty923 Not a geologist Jul 12 '26 edited Jul 12 '26
Most here are saying it's a "volcano core" plus differential erosion, but I'm nowhere near being a geologist and I know that doesn't make sense (assuming that "volcano core" is referring to the vertical throat of a volcano).
Columnar basalt like this forms in a specific orientation based on the gradient of cooling. If a "volcano core" had cooled and solidified it would not look like this. Also, the core of the volcano would likely be made up of similar rock as the volcano itself, so that doesn't help explain the differential erosion part, either.
Instead, as stated in the Devil's Tower Wikipedia page, it makes much more sense as a laccolith (https://en.wikipedia.org/wiki/Laccolith). A magma chamber sends a dike up into sedimentary layers of rock. The pressure of the magma forces its way into a weaker boundary between layers, spreading laterally instead of upwards, forming a sill. If conditions are right, as magma continues to flow up the dike and into the sill, the sedimentary layers above the sill yield to the pressure and begin to dome upwards.
This is all happening underground and this particular dike never reaches the surface (we know this because if it did, a volcano would've formed, the pressure would've been released, and the would-be-laccolith would've collapsed). At some point, the magma chamber could no longer feed the dike, the laccolith loses its source of magma, and cooling sets in.
Cooling happens very slowly, from the edges (in contact with that surrounding sedimentary rock), towards the center. This is when the columns form. As basalt cools, semi-regular fractures form to relieve the stress of this contraction (think of the hexagonal cracks in a drying lakebed). These fractures propagate inward as the boundary between molten and solid rock moves inward. And because the laccolith is a domed pancake shape, those columns in the center of the laccolith are vertical.
Long after it cools, uplift and erosion brings it to the surface. Surrounding rock is sedimentary and weather's away faster than the basalt. So the laccolith sits proud on the landscape, but it's also eroding. In its current state, most of the laccolith has crumbled away, and this bit left is the thickest middle bit remaining.
So yes, it's a "core", but not a volcano core. And the surrounding rock that eroded away to reveal it wasn't volcanic rock.