The mountains we call the Ortiz are the stub-end remnants of a volcano that 29 million years ago, when it was at its maximum size, was probably 4,700 feet higher* than it is today – almost a mile taller! The original mountain was more than twice its present size. One Santa Fe geologist exclaims, “Think Mount Fuji!”
Long ago below the earth’s surface the hot magma pushed up into the existing, mostly sedimentary rocks, and in a process called contact metamorphosis it baked the older rocks while it distorted and tilted them. Little or no lava flowed from the Ortiz volcano, but it is likely some pyroclastic material (such as volcanic ash) was produced. Eventually, different minerals were concentrated in the cracks and vents that were created.
The top portion of the old Ortiz volcano is gone today, the metamorphosed, uplifted native rock and the pyroclastic material mostly eroded away. Much of the durable intrusive magmatic material that went into the creation of the mountain remained beneath the surface where it hardened into a porphyry rock. After 29 million years of erosion what we see today of the old Ortiz volcano is a lot of altered porphyry rock. What we see today are the remnants of the volcano’s interior plumbing. Left to nature, in another 15 million years or so the Ortiz Mountains should be completely gone.
The Ortiz Mountains are part of a 30-mile long range of Tertiary-intrusive mountains called the Ortiz Porphyry Belt that extends from north of Edgewood (South Mountain, Monte Largo and the San Pedros) northward to La Ciénega (Cerro Seguro and the Cerrillos Hills). The Ortiz Mountains are located in the middle of this range. The main mass of the Ortiz Porphyry Belt is composed of a fine-grained granite-like rock, usually augite monzonite porphyry, which was formed when the molten magma was injected into the softer layers of the subsurface sedimentary strata, where it cooled and hardened. Near the earth’s surface, the thick magma cooled too quickly for the magma to differentiate and form large crystals, resulting in the hard, fine-grained stone we call porphyry.
Such volcanic activity as there was most likely was a series of hot gas blow-outs, known as diatremes, similar to the eruption a few decades ago of Mt. St. Helens in Oregon.
Intrusive, subsurface magma is properly called plutonic (as extrusive, blow-out magma is volcanic). The mass of magma that hardened inside the original mountain is the Ortiz pluton. The Ortiz Mountains were certainly created by plutonic processes and probably also by volcanic.
The wind and rain of the last 28 million-plus years have worn away most of the bulk of the Ortiz Mountains. The gravels produced have accumulated around the bases of all of the Ortiz-belt mountains in wide, gently sloping gravelly shelves known as the Espinaso Formation. In turning off Highway 14 onto Goldmine Road and ascending the mesa toward the Preserve, you are traveling upon the Espinaso Formation. From the valley of the Galisteo River to the gate of the Ortiz Mountains Educational Preserve you are traversing the erosional crumbs of the original Mount Fuji-like Ortiz Mountain.
Once you arrive in the OMEP you will be in the heart of the Ortiz Mountains, in a near-circle of peaks around a valley that opens to the northeast. This valley, the central core of the original mountain, was the part most ‘softened’ by the hot magma, and the part least resistant to erosion
Until the Galisteo River and the Rio Grande Rift appeared about 20 million years ago all that ‘soft’ Ortiz gravel helped to fill the Galisteo Basin. Since then much of the material has gone as fill into the Rio Grande Rift.
* Geophysical rule of thumb for the American Southwest: 1 million years of erosion = 50 meters.
More information on the Geology of the Ortiz Mountains - "The Gold"