Component parallel to slope - acts to drag/pull material downslope

Component perpendicular to slope - acts to keep material in place

If the force parallel to slope exceeds frictional resistance, then the material will move downhill

(1) Increasing the slope angle, increasing forces parallel to slope & reducing forces perpendicular to slope

(2) Adding to the mass of material, e.g. saturating it with water, increases forces parallel to slope

(3) Reducing coefficient of friction on the plane of slip, or reducing internal strength (cohesiveness) of the rock

(1) Clays are platy "sheet silicate" minerals composed of layers of “sheet silicates” that are common in near-surface rocks

• expand when wet, as water molecules are absorbed between mineral layers, causing clay expand
• increase separation & lubrication allow layers to slide past each other more easily, greatly reduce rock strength
• Clays are by-products of grinding action of ice in bedrock; glacial till = sediments left behind by glaciers
  

(2) Soils & sediments rocks contain pore spaces between grains

• The porosity (volume of pores + total volume) is 10-30% of the volume of most sedimentary rocks, & up to 50% of the volume in soils
• If replace air with water in pore spaces (making it water-saturated), it greatly increasing its weight
• reduce stability of sediments and internal strength (as water is incompressible)

(3) Groundwater (water flowing through rocks) can greatly reduce rock strength by dissolving minerals that normally bind the rock together (cement) or by physically eroding material, forming caves

• Cement help glue grains together, water can dissolve the cement chemically & reduce rock strength

(4) Water is unusual in expanding when it freezes (which is why ice floats on water - it is less dense); Repeating melting & freezing cycle leads to freeze-thaw weathering or "frost shattering"

• When water gets into cracks in a rock, & freezes, this expansion exerts pressure on the rock & forces the crack apart

It determine slope instability, since different rock types have different strengths

Hillslope determined by pre-existing geological conditions

Sedimentary rocks contain bedding planes along which sliding may preferentially occur

The orientation of bedding planes within a hillside may create a strong stable condition, where weakness are angled into the slope, or a weak unstable condition, where weaknesses parallel the slope

Tectonics generates topography & steepens slopes, & ground shaking in large earthquakes that dislodges materials from hillslopes

Preconditions = bring a slope to the brink of failure

Triggers = push it over the edge

E.g. 2008 Wenchuan earthquake, China
o Preconditions? Tectonic uplift & erosion by rivers that steepened the slopes
o Triggers? Shaking of the earthquake

E.g. 2007 Pe Ell landslide, WA
o Preconditions? Deforestation/logging activities, rivers
• River incision steepened slopes, geology (glacial till) & logging activity weakened them
o Triggers? Heavy rainfall preceded landslide