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.]