The story of Mt. Taylor begins with the slow but powerful movements squeezing and uplifting the region in the Pliocene times, five to seven million years ago. The North American continental plate was moving westward and colliding with the Pacific plate along the California coast. One of the effects of the compressing collision was to slowly warp the continental crust into large wave-like folds. In places the surface pushed upward, and in between the ground was drawn downward. At the same time, the folding stresses cracked and fractured the crust deeply to its base. Mt. Taylor lies above and along one of the down folds that occurred in the region, and is called the McCartys syncline. Thousands of feet of sediments and the underlying granites were pushed deeply underground by the compressive forces and soon became heated close to their melting point. The composition of the continental sediments allowed for quick melting, but formed a thick viscous melt that moved like honey. The fracturing of the crust allowed magma from much deeper to begin a slow migration upward. The deeper basaltic magma was of a different composition, hotter and much more fluid than the shallow melted sediments. The compressive and uplifting forces affecting the crust continued to accelerate the melting and migration of the magmas. About four million years ago, the first appearance of the shallower magmas began the construction of Mt. Taylor upon the surface of faulted and folded sandstone sediments. Trachyte lava was the stuff that oozed out of the ground, belching gas, and piled itself into a sticky grayish mound. The hidden hazard of this mass was its reluctance to flow, which held back the tremendous forces that pushed it upward. Almost 250,000 years after the trachyte’s arrival, the pressures below exploded through the surface ejecting huge clouds of vaporized crystal rock. The ash fell many times over the next million years, building up layers that coated the countryside all the way to Kansas, as well as building a mountain of considerable size on top of the main vents. This rock’s composition and character gives it the name rhyolite tuff. A tuff is a material that falls from the air rather than flows along the ground. It is a little like foamed glass fragments, and forms a deposit as the falling fragments are melted together by their retained heat. Charles Hunt (1936) estimated from the remains that we see today that 5.1 cubic miles of rhyolite tuff and flows covered the area. Together with the rain of ash, the area near the vents was covered by many thick fast moving mudflows. The mudflows left their mark with thick slope covering layers of broken rock, soil, and muddied ash called pyroclastic debris. In fact, each eruption of the vents left some mudflow debris. Within a million years of the first rhyolite explosions, the hot fluid basalt magmas that had been moving upward reached the surface among the rhyolitic cones. The appearance of the first basaltic eruptions followed the pathway of the deepest part of the downfolded McCartys syncline in the crustal sediments. Most of the basalt however did not reach the surface, but mixed with the melted crustal rocks still remaining below the surface. The final eruptions of the relatively pure rhyolite crustal rocks must have blown the mountain apart producing many interconnected fractures and a large caldera. Again, the debris from the explosions produced large amounts of pyroclastic ash-fall deposits and mud flows. The eruptions that followed differed from previous eruptions. A new type of lava was produced that came from mixing the shallow crustal melt and deeper basaltic melt. The resulting rock is called latite or andesite depending on the amount of basalt mixed with the crustal rhyolite. Starting about 2.8 million years ago, the latites and andesites flowed out of the mountain through cracks produced during the rhyolite caldera explosion. The hardening of the lava between flows produced a series of narrow vertical sheets of rock that radiate outward from the center of the mountain. Today, these sheets called dikes, stand out as the back bones of the ridges seen in the caldera. About 7.6 miles of latite and andesite were erupted to form the majority of the mountain seen today (Hunt 1936). While the andesites and the laltites were building the bulk of Mt. Taylor, more and more relatively pure basalt was leaking out around the flanks of the mountain. The basalt that reached the surface uncontaminated, flowed easily, and covered the surrounding slopes with many thin, mostly horizontal flows. A continuation of the uplifting forces that had been causing the entire region to rise, renewed the supply of basalt from the depths. The supply of crustal melt diminished while the basalt supply increased. Mt. Taylor’s eruptions slowly became more basaltic and less andesitic. The hardening flows served less to build the mountain, and more to surround the mountain with a pediment of dark black layers which stretched miles in all directions forming among others, La Jara mesa, Horace mesa, Frog mesa and Mesa Chivato. About one million years ago, the number of eruptions decreased and stopped, following a decrease in the movement of the underlying plates. Just to the east, the Rio Grande valley was being opened by the stretching of the crust. While activity was increasing there, eruption events were decreasing on and around Mt. Taylor. The mountain’s new height increased the rate of erosion and following the cessation of volcanic activity, the washing away of the mountain accelerated. Many of the surrounding basalt capped mesas were cut back and some were cut off from the mountain by streams which downcut into much softer sediments. The Grants valley was cut from between Horace and Grants mesas by Lobo creek and today is 1300 feet below Horace mesa. The removal of supporting sediments around the mountain caused faulting and slumping of the fringing mesas that can be seen in the western slope of Horace mesa. The many feeder pipes that brought the lava to the surface and produced the surrounding mesa flows were eroded and exposed as necks separated from the flanks of the mesa flows. Erosion has worn down the main mountain and its caldera, giving observers a chance to examine the interior of an interesting and unusual volcano.