Monthly Archives: October 2013

Born of Fire

To the surprise of many, New Mexico is sometimes called the Volcano State. It’s not that we have any erupting volcanoes – at present. But the sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. We are definitely an igneous state.

Back in fifth grade you probably heard about the three great groups of rocks on Earth: igneous, sedimentary, and metamorphic. Igneous rocks are rocks that crystallize from melts called magma. Magma is a mix of liquid silica-rich melt, suspended crystals, and dissolved gas like water vapor and carbon dioxide. It’s hot: 1300 to 2400 degrees Fahrenheit, hot enough to glow like fire. When it cools, it freezes in complex ways into igneous rocks – “born of fire”.

Most magma remains trapped in the Earth’s crust. But when it gets out, it forms volcanoes, named after Vulcan, the Roman god of fire, with his smoky forge Vulcano in the Mediterranean Sea. Magma extruded at the Earth’s surface is given an older Italian name, lava, which refers to both the flowing melt and the rock into which it cools. Lava flung into the atmosphere by the explosive expansion of dissolved gas forms a variety of fragmented and glassy materials called pyroclasts - “fire fragments”Lavas, loose pyroclasts, or tephra, and pyroclastic material consolidated into rock, called tuff, collectively form the volcanic rocks.

Since volcanic rocks are quenched at the Earth’s surface, they typically have fine-grained, glassy, or fragmental textures. It is usually easy to recognize a volcanic rock in the field, but assigning them to their specific family – basaltic, andesitic, trachytic, dacitic, or rhyolitic – can be frustratingly difficult. Much depends on finding and identifying small suspended mineral crystals to help out, which is something few of us do on a regular basis. Like algebra. Learning to simply recognize a lava, and to name variations based on texture, like pumice or obsidian, is an easy and rewarding undertaking for natural history buffs.

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Magmas trapped deep in the Earth’s crust belong to the plutonic realm, named after a darker god, Pluto, the Roman ruler of the underworld. These magmas are intimately associated with the underworld rocks which they intrude, rocks which have been changed by heat, confining pressure, and shearing stresses into metamorphic rocks.

Plutonic rocks, having cooled slowly deep in the crust, with all their juices sealed in, typically have coarse-grained, visibly crystalline textures – granitic textures. This makes it a little easier to assign them names in the field – granite, granodiorite, tonalite, diorite, gabbro, monzonite, syenite – but since magmatic rocks are mixes, not species, there is always some blurring and overlap. Learning to mentally gauge whether the rock is rich or poor in dark minerals, and rich or poor in visible quartz, helps out here. Light-colored, quartz-rich members of the granite and granodiorite family are much more common than the others.

Simply finding a plutonic or metamorphic rock in the field means something has transported that messenger from the underworld up to the surface. What could it be?

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There are any number of rock-identification guides and classification schemes both online and off, but one sweet site you might want to visit has been created by our Australian cousins: Igneous rock types. Go have a look!

 

 

 

Sand

Geologists adore the sedimentary rocks. These are the rocks that contain the Earth’s archives. These rocks outline areas of crustal subsidence and mountain uplifts. They reveal the distribution of ancient environments at the Earth’s surface. They record changes in climate and fluctuations in sea level. They preserve the record of life on our planet.

Of all the sediments, the sandy ones carry the most information about the widest variety of ancient environments: desert, piedmont, river valley, coastal plain, delta, beach, marine shelf and slope, in all their infinite variations.

Muddy sediments are much more abundant in the geological record, but since mud can accumulate anywhere there is slack water, onshore or off, shallow or deep, we depend on interbedded sand to help us understand their story.

Chemical concentrates, like limestone, dolomite, rock salt, gypsum, and coal are fascinating, and economically important, but they each form under very specific conditions. Their presence complements the story told by sand but cannot replace it.

On top of that, the layers of sandstone formed by sandy sediments are often the most eye-catching aspects of the landscape, forming resistant cliffs, rims, and benches when they are interbedded with more easily eroded mudstones and shale. And from an economic standpoint, the more porous sandy sediments are where much of our fresh water, oil, and natural gas is stored.

Santa Fe River carrying sand to the Rio Grande

Santa Fe River carrying sand to the Rio Grande

Continental Environments

Onshore, in continental settings, streams cut and and then fill channels with sand, carrying the finer mud on downstream, or depositing it over the banks during floods. Sediments deposited onshore by running water are called alluvium, and large amounts of sand can accumulate in the alluvial plains built up by streams. Where a stream enter a body of standing water, like the sea or a lake, much of its load of sediment is quickly dropped to form a delta, and sand can be deposited in large river mouth bars. As a delta builds outward, it extends the alluvial plain seaward, or fills the lake.

Coarse sand, mixed with pebbles and cobbles, frequently accumulates as alluvial fans at the margins of basins that flank mountain ranges, filling stacks of ill-defined and overlapping channels.

Fine sand can be remobilized by wind in arid climates, where it often collects in vast dune fields. These dune fields are a kind of large onshore sand bar. Since this sand is windblown, and not carried by running water, it is not considered an alluvial, but rather an aeolian sediment.

Permian red beds, with fossil dunes forming the steep cliff

Permian red beds, with fossil dunes forming the steep, poorly-bedded sandstone cliff

Marine Environments

Offshore, in marine settings, waves and currents rapidly redistribute the sand brought in by streams into beaches, barrier islands, and other kinds of shallow marine bars. Estuaries and lagoons also trap sand in their shallow waters. These environments are collectively called marginal marine or shallow marine environments.

Further offshore, fine sand can  be remobilized by undersea currents flowing in submarine channels cut into the continental shelves, and carried deep on to the ocean floor, where it accumulates as channel fill and thin overbank turbidites in submarine fans.

Shallow marine sand bars

Shallow marine sand bars forming ledges of sandstone

So you can see, sandy sediments truly are the great archivists of the sedimentary record. Learning to read the stories written in shifting sand will greatly enrich your appreciation of the natural world.

 

 

 

What on Earth are you doing with Howard Bannister’s rocks?

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The classification of the igneous rocks is a morass from which you would be well advised to steer clear. Even Judy Maxwell said, “I can take your igneous rocks or leave them. I relate primarily to micas, quartz, feldspar. You can keep your pyroxenes, magnetites, and coarse-grained plutonics as far as I’m concerned.”

Personally, I love the igneous rocks. Nevertheless, there is one coarse-grained plutonic up in the mountains above Santa Fe which has given me fits in trying to classify. And it points perfectly to the sort of look-alike confusion which plagues the field identification of these rocks.

Here’s the rock:

The speckled rock along Tesuque Creek

The speckled rock along Tesuque Creek

Ideal countertop material, you might say. I asked a hiking companion what he thought it was and was told “it looks just like the granite back home up in the Sierra” – the Sierra being the Sierra Nevada Mountains in California. And it does look just like those granites, except for the caveat going though my head – the curse of an education – that, as the geologist P.B. King relates, in the Sierra Nevada, “true granites in the technical sense are rather minor, most of them being the somewhat more mafic quartz monzonites, granodiorites, and quartz diorites”.

I thought it might be diorite. Diorite is an interesting construction, a French name built from the Greek root dior izein, ‘to distinguish’. Diorite is a granular igneous rock made up of bright white feldspar and dull black hornblende, with a classic “salt and pepper” appearance that every first year geology student learns to identify on sight.

Unfortunately, diorite is very difficult to distinguish from gabbro, another dark speckled igneous rock, which is what another hiking companion (understandably) always thought it was.

There’s a reason field geologists carry around that little ten-power hand lens, and when you look at this rock up close, you discover that most of the dark minerals are the black mica called biotite, and that there is an awful lot of quartz mixed in with the white feldspar. This throws the ball back into granite’s court, petrologically speaking, and there is a surprising name for the common hybrid between granite and diorite: granodiorite. So that’s where I finally decided to pigeonhole the rock. A very dark granodiorite.

Except that I found out its real name is tonalite.

The point is, the point is… oh god, I’ve forgotten my point.

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