Chemical Weathering
(some material on this page borrowed from USGS)

Chemical Weathering

The two main types of weathering are mechanical (physical) weathering and chemical weathering.  This page describes several types of chemical weathering.  Also, this section explains the strange but common weathering process called "grusification" and the resultant weathering form called (you guessed it....) GRUS!  (some pronounce this word "gruss" as in "Russ", while others pronounce it "grew-ss" as in "truce").  You will see a LOT of grusification when you venture to the Landslide Module! 

Rainwater is always slightly acidic (and in some places, very acidic!). Carbon dioxide dissolves in water and becomes carbonic acid, which is the stuff that makes sodas acidic (the concentration of carbon dioxide is much lower in regular water, which is why it doesn't feel or taste acidic). This acid reacts chemically with minerals and either dissolves them or turns them into other minerals.

Here are three types of chemical weathering which I want you to know about:

These rocks are rusting... (oxidizing)
Water has dissolved the limestone that once occupied this void.

Grus and grusification & spheroidal weathering

If you have ever walked the trails of South Mountain (the mountain just south of Phoenix with all the radio towers on it), you have probably seen (or slipped on) grus.  Grus is a loose (unconsolidated) collection of weathered "grains".  It is granular disintegration of rock...  It looks like little angular ball bearings, or very sharp little grains of granite.

The image here shows a retaining wall (in California) designed to hold back the grus...  it is so loose that it tends to drift onto the road!  Photo source is here.  

How does grus form?  I thought you'd never ask!  Basically, it is the granular disintegration (or grusification) of granite due to the hydration of feldspar and biotite minerals which break down into clays.  The swelling of these clay minerals as they abosorb water further weathers the granite and biotite minerals in particular also undergo oxidiation, leading to the reddish color of grus deposits.  Because hydration is the main chemical process breaking granite down, it follows that water plays a big role.  It turns out that granite weathers best in the subsurface where soil moisture is in constant contact with bedrock granite.  This why you often find in desert regions piles of rounded boulders sitting in random piles on the landscape.  Below surface, the granites joints and edges (more surface area) are chemically attacked and the result following exhumation is a pile of random boulders.  This is known as spheroidal weathering and we will see a lot of these on the next module.  In wetter regions you develop really weathered granite outcrops that border on soils and known as sapprolites.


What happens when granite is weathered? (A nice summation from here)


Here's a shot of spheroidal weathered granite producing grus.  Image source is here.

Here's a shot of granite exposed at the surface (out at the McDowell Landslide site).  The sunlight reveals a "rough" sharp (ouch!) profile of "grusified" rock.  The little "chunks" of granite are weathering and accumulating in topographic lows.