Monthly Archives: April 2014

It’s Sedimentary, my dear Watson

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Sometimes you can’t see the forest for the trees. Take, for example, the sedimentary rocks: a complex family of rocks that reflects the taking-apart, the rotting, the sorting, the dispersion, and the reconstituting of bedrock that operates so vibrantly on the lively surface of our planet. As one of my students wrote, after puzzling sadly over a tray of sedimentary rock samples in his lab, “the sedimentary rocks vary wildly from one another”.

And it’s true:  the most recalcitrant, homogenous, chemically self-satisfied lavas and schists become sands of silica pure enough to make glass, clays fine enough to fire into porcelain, salt you can eat, carbon you can burn, lime you can cast into concrete, and pigments with which you can color your paints. Not to mention some less civilized – we prefer to say ‘immature” – members practically indistinguishable from the rocks that broke down to make them.

Working out a scheme to bring some order to this chemical fiesta is challenging. But in a fundamental way, the sedimentary rocks fall into two camps: those that were transported into place and those that formed in place.

Bedrock breaks down into rubble, sand, clay, and a variety of dissolved minerals here at the Earth’s surface, often mixed with organic material where soils form. This weathered material can be mobilized by slumping and sliding, running water, or wind, and transported elsewhere, exposing fresh bedrock to weathering. The mobilized material is called sediment, and the visible component made up of broken fragments of bedrock, resistant grains of quartz, and particles of clay is called clastic or detrital sediment.

The vast majority of this detrital sediment is transported by streams of running water, where it is  efficiently sorted into various size fractions, and finally dropped, possibly to be remobilized by wind,  waves, or turbidity flows. From this activity comes all the mudstone, shale, sandstone, conglomerate, and breccia that makes up the bulk of the sedimentary record. What all these detrital sediments have in common is the fact that they were transported into place.

Red mudstone and lenses of sandstone in the Abo Formation, NM

Red mudstone and lenses of sandstone in the Abo Formation, near Sena, NM

The invisible component of dissolved minerals mixes freely and anonymously in lakes and lagoons, swamps and seas, where a variety of chemical and biological processes operate to precipitate out salts and silica of remarkable purity. Carbon, carried by invisible carbon dioxide, plays its part, trapped in vegetation, which can be preserved as coal, or bound up in the seas with calcium and magnesium to form limestone and dolomite. Depending upon the role played by inorganic vs organic processes, these sediments are called the chemical or organic sediment. But what they all have in common is the fact that they were formed in place. As geologist Noel James famously quipped, “carbonates are born, not made.” The same could be said about all the chemical and organic sediments.

White, chemically-precipitated gypsum of the Todilto Formation, covering beds of red shale near San Ysidro, NM

White, chemically-precipitated gypsum of the Todilto Formation, covering beds of red shale near San Ysidro, NM

Asking yourself whether the layers of sedimentary rock you’re seeing were transported there, or simply formed where they lie, should trigger a chain of questions that will have you thinking like a geologist in no time!

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Strain

One of my absolutely favorite outcroppings in the mountains above Santa Fe is this little stream-polished window into the depths of the Earth’s crust, along Little Tesuque Creek, not far above the Bishop’s Lodge Resort:

Hornblende schist - a kind of amphibolite - along Little Tesuque Creek, Santa Fe, New Mexico

Hornblende schist – a kind of amphibolite – along Little Tesuque Creek, above Santa Fe, New Mexico

The strain displayed by this dark schist is hard to miss. Outcroppings of metamorphic rocks are abundant in the ancient crystalline core of the Santa Fe Range, but few of them exhibit such dramatic stretching as this example. Here’s a close up of a structure known as boudinage:

Boudins of more quartz-rich layers in the schist

Boudins of more quartz-rich layers in the schist

These stony strings of sausages are the result of differing competencies among the mineral components of the schist. The smooth borders attest to the overall ductility of the rock as it was being slowly sheared, deep in the plutonic realm of the middle crust.

These schists are sturdy, and rounded cobbles of this rock are common throughout the foothills and stream beds west of Santa Fe. Most of them glitter in a dark way, as tiny prismatic crystals of hornblende, aligned by shearing strain, send back a little light. Harder to see is their matrix of white plagioclase feldspar, which probably makes up over half the rock’s mineral content.

Volcanism

One of the joys of teaching is, oddly, learning. Teaching a basic class in physical geology to a wide range of students at the local community college brings me weekly opportunities to hone my communication skills – to try to find concise ways to summarize complex topics in an accurate, yet simple, manner. The struggle to do this teaches me far more, I suspect, than I teach my students.

Volcanism is a case in point. It’s a big topic. Is there a way to capture its essence in a blog-sized post? Here’s a try:

As I’ve mentioned before, New Mexico is the Volcano State. The sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. And yet all of this volcanic activity can be encompassed between two poles: effusive eruptions and explosive eruptions.

Effusive eruptions

Effusive behavior refers to relatively quiet outpourings of molten lava from a volcanic vent. Effusive eruptions of fluid basaltic lavas are exceedingly common, forming stacks of thin flows that pile gently into broad shield volcanoes, or spread out across the countryside in lava plateaus.

Valley of Fires, a vast flow of basaltic lava, in southern New Mexico

Valley of Fires, a vast flow of basaltic lava, near Carrizozo, New Mexico. Click to enlarge.

Effusive extrusions of viscous silicic lavas, like dacite or rhyolite, are much less common. These eruptions typically form short, thick, glassy flows, piling into rubble-covered lava domes that usually plug their own vent.

An arc of rhyolitic lava domes in the Valles Caldera

An arc of rhyolitic lava domes in the Valles Caldera, west of Los Alamos, New Mexico. Click to enlarge.

Explosive Eruptions

Explosive eruptions occur when violently expanding gases fragment molten lava into clouds of pumice, scoria and volcanic ash. The first phase of many otherwise effusive eruptions of basalt is a gassy discharge that builds a one-shot, steep-sided cone of scoria, commonly called a cinder cone.

A cinder cone you can walk into, near Grants, New Mexico

A cinder cone you can walk into, near Grants, New Mexico. Click to enlarge.

Eruptions of somewhat more silicic lavas, like andesite, dacite, and trachyte often alternate between explosive and effusive phases, over tens of thousands of years, building “composite” cones of lava and ash that can grow into large – but unstable – mountains.

Mt. Taylor, near Grants, New Mexico, is a classic composite volcano

Mt. Taylor, near Grants, New Mexico, is a classic composite volcano. Click to enlarge.

Although thankfully uncommon, very large eruptions of silicic rhyolite lava can be catastrophically explosive, burying thousands of square miles of countryside under  incandescent blankets of ash flow tuffs up to a thousand feet thick. Events of this magnitude probably alter the global climate. These eruptions leave circular zones of collapsed crust, miles across, called calderas, so large that they are difficult to recognize from the ground.

The Valles Caldera from space

The Valles Caldera from space. Click to enlarge.

What lies below

Magmas trapped and crystallized as shallow intrusives in the feeder systems beneath volcanoes – dikes, sills, and shallow stocks – are often grouped together with the other volcanic rocks.  Where erosion exposes these formerly buried structures, fascinating landforms result, complementing the flows, cones, craters, and calderas of modern New Mexico.

Cabezon Peak, a basaltic volcanic neck, west of San Ysidro, New Mexico

Cabezon Peak, a basaltic volcanic neck, west of San Ysidro, New Mexico. Click to enlarge.