Monthly Archives: September 2013

What to take away

My last post on the variety of natural features in Northern New Mexico led me to consider just what geological insights a traveler to our beautiful country could take away with them. It’s easy to be overwhelmed with loads of scenery, lots of facts and examples, and a few really unfamiliar words.

Looking out into the Espanola Basin from the Sangre de Cristo Mountains

Looking out into the Espanola Basin from the Sangre de Cristo Mountains

There are, however,┬ásome basic concepts that a geologist would love to feel you’ve carried home:

  • Northern New Mexico has a spectacular landscape which conceals a rich and eventful geologic history.
  • This history can be pieced together from the rock record, some of which you have just encountered.
  • Geologists attempt to understand the rock record in terms of processes which we can see operating today. This works well for rocks that form at or near the Earth’s surface, less well for rocks that form deep in the crust.
Santa Fe River carrying sand to the Rio Grande

Santa Fe River carrying sand to the Rio Grande

  • Sedimentary and volcanic rocks form at or near the Earth’s surface. On the continents, these rocks form an extensive cover that rests upon an older crystalline basement. The crystalline basement is made up of metamorphic and plutonic rocks that originally formed far below the surface.
Mississippian dolomite resting nonconformably on Precambrian crystalline basement

Paleozoic sedimentary strata resting unconformably on Precambrian crystalline basement

  • Because sedimentary strata and many volcanic lavas and tuffs were originally laid down in horizontal layers, we can infer subsequent crustal movements by their displacements. This includes deformation by flexure, offset by faulting, and elevation above or below sea level.
  • Sedimentary rocks contain the Earth’s archives. They outline areas of uplift and subsidence, record the distribution of ancient environments, and track changes in sea level and climate over the ages. They also preserve the record of life on Earth.
  • Volcanic rocks contain the Earth’s clocks and compasses. They freeze in radioactive elements that act like an hourglass, which we can use to measure the age of the rock. They freeze in magnetic minerals aligned with ancient magnetic fields, which we can use to measure continental drift and plate movements.
  • The metamorphic and plutonic rocks, many of which formed from deeply buried volcanic and sedimentary rocks, contain a blurred, but fascinating, record of conditions and movements deep in the Earth’s crust.
  • The leveling process – weathering and erosion at the Earth’s surface – works swiftly and ceaselessly to wear down the high places and fill in the low ones. Landscapes of great relief and drama like New Mexico’s point to ongoing tectonics and volcanism.
Slot canyon in rhyolite tuff

Slot canyon in rhyolite tuff at Kasha Katuwe Tent Rocks

 

Santa Fe and Northern New Mexico

Rowe Mesa above the Glorieta Pass into Santa Fe

Rowe Mesa above the Glorieta Pass into Santa Fe

I’ve been leading a few guided hikes lately, in the countryside around Santa Fe, and while I’ve lived and hiked here for many years, I’m still amazed at the diversity of natural features we enjoy in this corner of the Southwest. Finding a suitable walk for guests with geological interests is never a problem. And when you add in the rich overlay of human cultures in New Mexico, almost any walk becomes a dream-like journey though times past, from symbols with which we can resonate, to artifacts of an almost alien world.

Inscriptions on El Morro

Inscriptions on El Morro, a cliff made up of the ancient dunes of a Jurassic desert

Four great provinces of the American West come together near Santa Fe, to account for this diversity. We sit at the foot of the southernmost range of the Southern Rockies, a group of mountains bordered on the east by the Great Plains, and buttressed on the west by the Colorado Plateau. A rift valley bisects these regions from north to south, bringing a prong of the fourth province, the Basin and Range, into our mountain setting.

All of these regions stand far above sea level, basking in the sharp light and dry air of their high altitude settings. The Colorado Plateau averages 2 km above sea level, and a few peaks in the Sangre de Cristo Mountains reach 4 km. Even Albuquerque, in its basin along the Rio Grande, is 1.6 km above the sea. Rocks are well exposed in this high and dry country, and getting out to see them is always a pleasure.

And the variety! All four provinces host young Cenozoic volcanic features: lava flows, ash-flow tuffs, volcanic cones and domes, as well as good exposures of sub-volcanic structures such as dikes, laccoliths, necks, and stocks.

A Pliocene basalt flow on La Bajada Mesa

A Pliocene basalt flow on La Bajada Mesa

Ancient Precambrian metamorphic and plutonic rocks are extensively exposed in the cores of our mountain uplifts.

A boulder of migmatite high in the Sangre de Cristo Mountains

A boulder of migmatite high in the Sangre de Cristo

In each province sedimentary rocks form a colorful blanket, carrying a record of environmental change that ultimately spans the late Paleozoic, Mesozoic, and Cenozoic Eras.

Permian red beds

Permian red beds

A visit to Santa Fe and Northern New Mexico is an invitation to explore a vast and varied natural history with only a little time and effort. Immerse yourself in Deep Time and you will find your travels here enriched in ways you never expected.