Gradation

Tectonic forces shape the earth from within., but there are also forces outside of the lithosphere which shape the land. These forces are called "Gradational Forces" and they include gravity, rivers, waves, glaciers and wind.

While tectonic forces uplift the earth, Gradational forces wear it down.

Erosion: The weathering and transportation of the materials while gradational forces are acting on the earth.

Weathering refers to the break down of rock material, not it's transportation. There are 2 types of weathering, Mechanical and Chemical.

1. Mechanical Weathering
   
   Refers to the physical breaking down of rock. It dominates cool and dry climates, of the high and temperate latitudes. (e.g. Canada)
   
   

   a) Frost shattering: Occurs as water freezes in the cracks of rocks. Water expands by 10% when frozen, and it is this property which forces the crack apart, thus breaking the rock open. Frost shattering is most effective in temperate latitudes where it is common to have daily freeze & thaw cycles, which speed up the process.
   
   b) Exfoliation: Occurs when rocks, formed under great temperature and pressure, end up on the surface of the earth. Once the pressure (which helped the rook together) is released, the rock literally falls apart, and flakes apart.
   
   c) Thermal expansion: Refers to the shattering of rocks due to rapid temperature change. The temperature change necessary to break most rocks rarely occurs, even in the most extreme climates such as deserts.

   
   
2.  Chemical Weathering
   
   Involves the breaking down of parent material through the chemical processes of solution, hydrolysis, and oxidation. Dissolved CO₂ and O₂ play a key role in these processes. CO₂ dissolves in water and forms, weak, Carbonic acids. Carbonic acid is responsible for weathering by solution and hydrolysis. Oxygen weathers rock through oxidation.
   
   

   a) Solution: Direct removal of readily dissolvable minerals (e.g. Calcium)
   
   b) Hydrolysis: The breaking down of silicate minerals when exposed to carbonic acid. In this process, H2O replaces ions and clay is left behind. This process dominates where there is excess precipitation
   
   c) Oxidation: Removes iron and other ions from rock via highly reactive oxygen.

Mass Wasting

Landslides, rock falls, soil creep, solifluction, and avalanches all move unconsolidated material in a process called mass wasting. several factors are at place which influence the amount of mass wasting that occurs.

1. Slope:
   
   The steeper the slope, the greater the chance of movement. Slope angle can change as a result of other gradational processes or through human alteration.
   
   
2. Unconsolidated Material:
   
   Rock shape and size will affect the angle of repose, (the minimum angle at which mass wasting will occur.)
   
   
3. Vegetation:
   
   The roots of plants help hold unconsolidated material together. Under gravity, soil creep results as soil gradually moves downhill. This is visible in the curved trunks of trees and titled poles.
   
   
4. Tectonic Instability:
   
   Earth Tremors can trigger rapid mass wasting events. Explosions are used to trigger avalanches before dangerous levels of snow can accumulate on slops about highways or ski-runs.
   
   
5. Moisture Content:
   
   Moisture is important in lubricating unconsolidated material. Water fills pore spaces and reduces friction between particles resulting in mudslides and debris flows. Solifluction is similar to soil creep, but occurs when a saturated surface layer thaws on top of a permafrost layer, The top layer begins to slide over the permanently frozen base.

The following is a generalization of how all of the factors will determine the angle of repose. Several factors will ultimately determine the slope angle. Mass wasting is a dynamic process with periods of stability, followed by movement. All of the factors can be compounded to create rapid or sudden mass movements with, sometimes, catastrophic consequences.

Human Causes of mass wasting:

1. Road Building increases slope angle.
2. Water line breaks, or water leaks.
3. Dams can create tremors, due to the weight of their lakes.
4. Logging removes vegetation that holds unconsolidated material together.

To minimize mass wasting in an area, people can

1. Plant trees (After disturbing area.)
2. Build Retaining walls.
3. Put up Rock Curtains.
4. Bolt rock faces.