Saturday, December 11, 2010
What a Geologist Sees - Part 9 [Original Post Date 9/26/07]
The photo at top is of an embankment of saprolite, which is colloquially called "rotten rock". This embankment was exposed during a short period of local construction. Saprolite is the term used to describe exposures which display some original rock structures, but because of intense chemical weathering, the internal structures of most of the minerals have been broken down to the point where the "rock" can be crumbled in your hand - thus the term "rotten rock". The processes that yield saprolite are what produce the inorganic components of soils.
If the exposure has been reduced to a structureless clay, e.g., the famous Georgia red clay, we call this "residuum".
The lower photo is of the original "country rock", a fresh, hard biotite gneiss, a metamorphic rock that is similar to granite. This particular gneiss (silent "g", rhymes with "rice"), probably part of the widespread Lithonia Gneiss, is composed of potassium (orthoclase) feldspar, plagioclase feldspar, quartz, biotite mica and probably muscovite mica, along with some accessory and trace minerals.
The progression from gneiss to saprolite takes place through chemical reactions involving water and natural acids over long periods of time. In the process of hydrolysis, hydrogen cations replace the metallic ions (K, Na, Ca) that occur in the mineral structures of feldspars, micas, and other silicate minerals. The removed metallic ions then become part of the dissolved minerals in ground water (and surface waters) and are flushed out of the system. Any iron bearing minerals, including silicates are altered by oxidation, in addition to hydrolysis. There are other chemical weathering processes, but these two are probably most prevalent.
At or near the surface, quartz is generally fairly stable, so probably most of the original quartz remains, while the feldspars and micas have broken down into various clays.
In addition to the chemical weathering, whereby the actual chemistry of the minerals change, there is also physical weathering, where the rock is broken into smaller pieces by natural processes, yielding more surface area upon which the chemical processes can occur, i.e., both types can occur simultaneously and tend to enhance one another.
It is a little unusual, in this area, to see the fresh parent rock and the resulting saprolite in the small area of a stand-alone drug store construction site.
On a side issue, if there is enough quartz present (as opposed to clays) and there is enough porosity and permeability (hydraulic conductivity), saprolites can sustain small, residential drinking water wells, in this area.