33) Clastic Sedimentary Rocks - Part 1

A few points-to-remember:

Clastics are derived from the Mechanical Weathering (aka Physical Weathering) of pre-existing rocks.  (Chemical Weathering plays a subsidiary role in breaking down the rocks, too.  It will be partially addressed below.)

A shown, Physical Weathering is akin to smashing a rock with a sledgehammer.  The mineral makeup of the fragments (clasts) is not changed during the breakage, however, the increase in surface area (and creation of micro-fractures within the clasts) provides additional sites for subsequent Chemical Weathering in humid climates and in semi-arid/arid climates following their rare precipitation events.  

[In arid and semi-arid climates, the paucity of water restricts Chemical Weathering and soil formation.  In those settings, Physical Weathering dominates the natural "destruction" of rocks.] 

Modes of Physical Weathering include:

• Ice Wedging (water held within fractures and micro-fractures expands during freezing).
• Thermal Expansion and Contraction - Diurnal and Seasonal.  [There is (or has been) some disagreement over the relative importance of Diurnal Expansion and Contraction.]
• Exfoliation Jointing and Tectonic Jointing and Fracturing (aka Brittle Deformation).
• Root Wedging.
• Abrasion by wind or water.
• Impact Fracturing (aka Percussion) in streams or landslide events.  (May include human-caused impacts.)

Shown below are examples of Thermal Fracturing and Exfoliation Jointing. 

 Figure 1.

Figure 2.

 Figure 3.

Figure 4.

Figure 5.

In the early portion of the video, the clast-size scale discussed is known as the "Wentworth Scale".  

Clays, Claystones, Mudstones, and Shales (<0.004 mm on the Wentworth Scale) are the most common Clastic sediments/sedimentary rocks, though they owe part of their genesis to Chemical Weathering.  [The property of "fissility" (as shown above) and its origin is addressed first at about 3:06 in the video above.]

The tiny plates of the Clay Minerals are the residuum of Chemical Weathering of Silicate Minerals, largely by way of Hydrolysis and Oxidation.  (Remember that the Chemical Weathering susceptibility of the "rock-forming" Silicates is related to their position on the Bowen Reaction Series.  Olivine and Calcium Plagioclases are "first", while K Feldspar and Muscovite are "last".  See the "note" about Quartz in the next paragraph.)

Within the remainder of the Wentworth Scale realm (>0.004 mm), Quartz is the most common component and under surface conditions, Quartz is usually just "broken into smaller components" by Physical Weathering, while remaining chemically stable.

[For the sake of brevity, more related photos will be shown in another post, soon.]  

Modes of Chemical Weathering

This post was "dropped in" to help clarify the terms in past and upcoming posts in regard to chemical weathering of minerals.

To revisit Physical Weathering (aka Mechanical Weathering) briefly:

Modes of Physical Weathering include:

• Ice Wedging (water held within fractures and micro-fractures expands during freezing).
• Thermal Expansion and Contraction - Diurnal and Seasonal.  [There is (or has been) some disagreement over the relative importance of Diurnal Expansion and Contraction.]
• Exfoliation Jointing and Tectonic Jointing and Fracturing (aka Brittle Deformation).
• Root Wedging.
• Abrasion by wind or water.
• Impact Fracturing (aka Percussion) in streams or landslide events.  (May include human-caused impacts.)

A common "result" of these Physical Weathering Modes is that the rock's chemistry does not change, it is just broken into smaller pieces, yielding more surface area.  Chemical Weathering results in actual changes in chemistry due to Oxidation, Dissolution, and Hydrolysis.  In some cases, depending on the water chemistry, any two or all three of the processes may occur simultaneously. 

The stability of minerals - as solid, naturally-occurring chemicals - is largely related to the strength of their internal chemical bonds, whether Ionic, Covalent or a combination of the two.  Some minerals have both.  When a compound is composed of a metallic cation and a polyatomic anion, the compound is considered to be ionic, though there might be covalent bonds within the polyatomic anion.

The Polarity of Water molecules (one side positively-charged, the other side negatively-charged) is responsible for many of Water's interesting properties, including water acting as a "carrier" of ions (positive and negative).  

Under natural conditions, water can combine with atmospheric gases producing natural acids such as Carbonic, Nitrous, and Sulfurous (some of these may be enhanced by gases within soils as rainwater percolates downward).  Human activities can add additional acids to the mix.

Figure 1.

Figure 2.

The most ubiquitous form of Oxidation that we see is Iron Oxidation, whether it is of iron-bearing Oxides and Silicates in Diabase (Basalt), of the iron-bearing steel of old cars or old beer cans, or of iron-bearing Sufides, e.g., Pyrite.

Dissolution is the "dissolving" of certain minerals, most easily seen with the halides which have weak Ionic Bonds (NaCl - halite, table salt easily dissolves in water).  In the case of calcium carbonate (Calcite in Limestone) and calcium sulfate (Gypsum in Evaporites), the ionic bond between the Calcium and the polyatomic anion is fairly-easily broken by the combination of water and naturally-occuring acids. 

 Figure 3.

Figure 4.

When we think of caverns, we often think about structures such as stalagmites, stalactites, columns, and such.  These are actually secondary depositional structures, not Dissolution Features.  As pictured above, Sculpted Ceilings are a good subsurface example of Dissolution.  

In the early stages of Karst development, the caverns begin as exposed joints, fractures, and bedding planes in limestone and marble (and rarely gypsum) - below the Water Table.  Dissolution slowly takes place as the water replenished by more-acidic percolated groundwater, following surface precipitation events.  

Where the Water Table is "tied" to exposed surface features, e.g., nearby creek and river valleys, as down-cutting occurs, the underground Water Table is "dragged down", exposing the cavern ceiling, while the cavern floor dissolution continues.  

(It is after this subterranean exposure that the stalactites begin to grow.  But that's another story.)

Figure 5.

The hand-sized sample of Potassium Feldspar above shows the effects of Hydrolysis (here termed Kaolinization), as seen by the chalky appearance.  As you can see traces of the original Feldspar Cleavage, the Kaolinization is not yet complete.

Except for Quartz, every Silicate Mineral in the Bowen Reaction Series is susceptible to Hydrolysis.  As related previously, the minerals near the "top" of the Bowen Reaction Series (as with the presumed aphanitic Olivine, Pyroxene, and Plagiolcase in the Diabase) are more susceptible to the effects of Chemical Weathering than is the K Feldspar (near the bottom). 

[Below the surface, Quartz is soluble in certain circumstances, but "that's another story", too.]

Bowen's Reaction Series

In addition to the "3 Utilizations" of the Bowen Reaction Series listed and described in the videos of the three previous days, understanding the Bowen Reaction Series helps you understand why - in igneous rocks - certain minerals "associate" with others, while other minerals don't usually "associate" with each other.

For instance, because of the higher temperatures associated with Ultramafic and Mafic rocks, Quartz and Muscovite are not usually found with those rock types, as they are still molten while the ultramafic/mafic minerals are crystallizing and "squeezing out" the remaining liquids.  [Or if the mafics were erupted in a setting with a paucity of "free quartz", e.g., the Mid-Atlantic Ridge or the Hawaiian Hot Spot" or where the crust is thin, allowing the mafic magmas to "quickly" reach the surface, e.g., a Continental Rift Zone.]

Likewise, it is not "normal" for there to be Olivine in a Granite (or similar Felsic rock).  The Geology Department chair at my undergraduate college had a sizeable chunk of Olivine Granite that he had collected from the Big Bend region of Texas.  Most likely, the semi-molten granite had "plucked" the olivine granites from an adjacent peridotite, preserving them as "micro-xenoliths" or xenocrysts.

Geology: Bowen's Reaction Series

To follow up on the posts of the last two days, there are 3 "utilizations" of the Bowen Reaction Series.  They are:

1.) The classic prediction of the order-of-crystallization of major silicate minerals in a cooling magma (or lava).

2.) Order-of-melting during partial melting (increasing temperatures).

3.) During the "downward" progression-of-solidification as shown in the diagram, the internal structures of the silica minerals are becoming more "organized" and more stable.  In other words, at the "top" of the diagram, Olivine, Pyroxene, and Calcium Plagioclase, the "lesser" internal organization makes these minerals more-susceptible to Chemical Weathering than Muscovite Mica and Quartz (at the "bottom").

This is cited as one reason for the propensity of exposed granites over gabbros and peridotites and the primary reason for quartz being the dominant type of sands on the beaches of the world.

14) Bowen's Reaction Series

Yesterday's post was an informal Part 1 of this short series of videos on the Bowen Reaction Series. 

The Bowen Reaction Series has three "utilizations". 

1.)  Predicting the order-of-crystallization of minerals in a cooling magma or lava. 

2.) When the order-of-crystallization is reversed, it becomes an order-of-melting (or partial melting) of a solidified magma.  [This was hinted at in yesterday's video, though not explicitly explained.]  That being the case, as Quartz is the "last" mineral to solidify, it becomes the "first" mineral to melt as the temperature increases.

This is what occurs during episodes of partial melting - if the "proper minerals" are present.  As the temperature increases, and as time duration, pressure and fluid contents "permit", Quartz, Muscovite, K Feldspar, Biotite, Sodium Plagioclase, ... progressively melt.  [Pressure and Fluid Content affect the Melting Point.]

Figure 1.

In the Figure 1 "scenario", assuming that a Gabbro is the "starting point", the "end product" is a function of the conditions listed above (time-duration, temperature, pressure, and fluid content).

[The third "utilization" of the Bowen Reaction Series will be presented tomorrow.]

Earth Parts #29 - Bowen's Reaction Series

The image below is used for educational purposes.  (As I don't have a good hand sample of a Diorite - as an example of an Intermediate Igneous Rock - I had to use a Porphyritic Andesite, from the "Campus Andesite", a shallow Eocene intrusion underlying the University of Texas-El Paso campus.)

Figure 1.

Below (Figures 2 and 3) are descriptions of the Mafic and Felsic "End Members" of this range. 

 Figure 2.  Gabbro - unknown source.

Figure 3.  Granite - Pennsylvanian Elberton Granite.

Figure 4.  Ultramafic Mantle Xenolith - Quaternary Maar eruption.

At least two more Bowen Reaction Series posts are planned for the coming days.  Repetition helps enforce memorization, while different articulations may induce different insights.

A "Twofer"

An example of an image that serves two purposes...

Differential Weathering and the "universal melding of joy and grief" for field geologists.

This is a scanned 35mm slide from 40+ years ago, somewhere on the Georgia Piedmont (revisiting that past error of not labeling slides).

Prior to eons of Chemical Weathering, (based upon texture observations and a knowledge of prevailing local rock types), this was probably an Amphibolite, with some quartz-rich intervals (including the ledge upon which the Estwing prybar is perched).

"Differential Weathering" is due to the relative difference of susceptibility to Chemical Weathering between Mafic (Fe-rich silicate minerals) vs. Quartz.  Generally speaking, the minerals "higher" on the Bowen Reaction Series (including the Fe-Mg rock-forming minerals, e.g., Pyroxenes, Amphiboles, and Biotite) are more susceptible to chemical weathering at (or near) the surface.  In other words, at a quick glance, the ledges are probably Quartz.

As for the "universal melding of joy and grief", it is reference to finding (and then losing) tools in the field.  It was the subject of a past post, from 2011.  In other words, the post included the regrets of lost hammers, prybars, chisels, handlenses,...and other items.  We usually leave these things because we are tired.  Sometimes we are so obsessed with our newfound "toys" (that we may have carried several hundred yards to the vehicles), that we don't want to make one more trip back to the area to check for tools.

The Estwing prybar in the photo had such an "experience".  I found it at a now long-forgotten locality, kept and used it for a few years, then left it somewhere else.  (The "great circle of life"?)  I hope another Geologist found it and took it home.  So, if you do find a tool in the field, say a little "prayer of empathy" for the fellow scientist that lost it.