2.3.1 - The Lithosphere (KQ3): Weathering and Accumulation of Debris on Slopes

Rock Weathering and the Accumulation of Debris on Slopes

KQ3 focuses on mass-movements on slopes (ex. landslides).  These can be devastating and are sometimes preventable where human tragedy is concerned.  We will be looking at the processes behind the mass-movements, how we can manage the land to prevent them affecting us, and how we can best respond.

Rock Weathering

We already had a basic introduction to rock weathering in our rock cycle unit here.  It is important to revisit and expand on this topic, because in order to truly understand how to deal with a problem, we need to know how and why it occurs.  We're going to do this with a little help from this site.  Many of the weathering ideas presented here are influenced from the information there, but there is more information there than you need at this point, so I will summarize below.

Weathering  is a process which acts at the earth's surface to decompose and break down rocks.

There are 2 types of weathering we are mainly concerned with:  Mechanical (Physical) and Chemical

Mechanical Weathering:  Basically, breaking down rock from bigger to smaller pieces without changing the composition.

There are 4 basic types of mechanical weathering:

1. Expansion and Contraction - the thermal heating and cooling of rocks causing expansion and
   
   contraction.
2. Frost Action - Water freezes at night and expands because the solid occupies greater volume. Action wedges the rocks apart. Requires adequate supply of moisture; moisture must be able to enter rock or soil; and temperature must move back and forth over freezing point.  WARNING...the following is a graphic!!!
3. Exfoliation - process in which curved plates of rock are stripped from a larger rock mass.
4. Other types - Cracking of rocks by plant roots and burrowing animals.

Chemical Weathering:  Breakdown of rocks by chemical processes/reactions.  Water is the main agent, carrying dissolved chemicals (including acids) that react with the rocks and deteriorate them.  The major acid that is responsible for chemical weathering is carbonic acid...which is produced with carbon dioxide dissolves in water.  The carbonic acid will break down rock into smaller pieces...and smaller pieces are much more unstable.

The Factors which effect the rate of chemical weathering are:

• Particle size - Smaller the particle size the greater the surface area and hence the more rapid the weathering
• Composition
• Climate (See Figure)
• Type and amount of vegetation

As you can see, with differing average annual temperature and rainfall amounts, the types of rock weathering change per region. The dryer and colder areas seem to have more mechanical weathering (think...frost action), while the wetter, hotter areas get more water, and therefore more carbonic acid, causing an increase in chemical weathering.

Accumulation of Debris on Slopes

Mechanical and Chemical weathering can both result in the accumulation of rocks on a slope.  The rock is broken down by the weathering processes and then will roll down a slope.  The terms used for this rock are talus or scree (they mean the same thing).  The accumulated rock are sometimes called talus piles or scree slopes.

Because their mass is greater, larger rocks will have momentum due to gravity and fall down towards the bottom of the slope, while smaller fragments may stop and rest at higher areas on a slope.

Rocks sitting on a slope, large or small, may become unstable and fall to the bottom because of increased lubrication (water falling down the hill during rain), increased weathering and erosion forcing more rocks to fall and collide with the ones in place, or frost melting forcing a water flow down the slope.