Chapter 12 – Unstable Land
What is unstable land?
- ground that moves
- steep shoreline bluffs, slowly slumping hillsides are examples
- unstable land on seafloor can cause tsunamis
- over time, all land is unstable. But the focus here is land that moves and changes in
ways that are disruptive to people’s lives and properties – these include landslides, mudflows,
debris flow, soil creeps, sinkholes, subsidence.
Landslides: the downslope movement of masses of rock, soil or debris. AKA mass movement
- can be large or small and occur repeatedly in the same area
- reactivated by storms or human activity
- new landslides triggered by earthquakes, rain, rapid snowmelt, volcanic eruptions,
Mudflows and Debris Flows: landslides that are saturated with water and are accompanied by
heavy rains or volcanic eruptions,
- form rivers of mud, rock, soil (create a consistency of liquid cement)
- destroy anything in its path. (cause more damage than a volcano)
- example, Casita Volcano in Nicaragua on oct 30 1998
Soil Creep: less dramatic than landslides and mudflows, but still destructive
- are the very slow, downhill movements of soil, happens too slowly for people to see
- will gradually tilt trees or fence posts and can tilt and break building foundations
- takes place of upper soil (several feet), causes by differences in soil expansion and
Sinkholes: crater-like holes that form when the land surface collapses,
- form on land that is underlain by a network of under ground channels and caves (karst)
- karst regions are caves, sinkholes and underground streams that are formed by
- these rocks create cracks but then gradually enlarge and produce caves and channels.
- sinkholes commonly form in what seem to be gentle and stable landscapes. They form
when the roofs of rock caves within karst networks become weak and collapse, creating a large
Subsidence: the gradual lowering of the land surface over large areas.
- Are caused by withdrawal of ground water
- once water has been removed from compressible unconsolidated material, It compacts
and the ground surface sinks. Causes of Unstable Land
Gravity: The Driving Force
- on slopes, gravity’s pull is split into 2 parts; one pulling down and one pulling in a
direction parallel to the slope.
- The steeper the slope, the greater the gravitational component pulling surface materials
downhill and the more likely it is that they will collapse, fall, or slide.
Friction, Cohesion – The Resisting Forces
- a steep slope isn’t enough to cause unstable land, there’s forces that resist gravity’s pull
- Friction: the force resisting movement along a surface or boundary between 2 objects.
- The more friction there is the harder it is to occur along the surface
- Cohesion: the force that causes individual particles in a material to stick to one another.
- sand has little cohesion, clay-rich soil has a lot of cohesion
- Friction and cohesion combine to resist gravities pull on slope materials
- the stability of slopes depends on angle and the properties of the material covering the
- rocks without fractions: high internal friction and cohesion and are stable at high slope
- weathered (fractured) rocks has low friction and cohesion and is unstable
Talus: the base of a steep or slope
Alluvium: unconsolidated material deposited by rivers or streams
Angle of response: the maximum angle at which unconsolidated material is stable. This angle
differs epending on the size and shape of material- there is more friction between angular
particles than rounded so they will remain on a steeper slope
Role of the Water Table
- land subsidence is caused by changes underground
- (lowering of the water table, underground mining, thawing of permafrost)
- most common land subsidence in Canada is lowering of the water table:
- (excessive withdrawal of ground water)
- Below the water table, water fills the open spaces in fractured bedrock and
unconsolidated material. When the water is removed from the sand and gravel, most of the
open spaces remains.
- over pumping of ground water lowers the water table and decreases the buoyancy in
unconsolidated material - can create sinkhole!
Precipitation - rain or snowmelt and its weight is an added factor of landslides
Earthquakes - shaking decreases friction and cohesion of the surface materials
Volcanic Eruptions - heat can melt snow, same effect as precipitation.
Steepened Slopes - erosion at the base of a slope causes unstable land: erosion makes
slopes steeper and increases gravity pull
Vegetation - Root can prevent erosions of by soaking up water and by anchoring materials
- Steepness increases when the loss of vegetation occurs (fire,
People - land development can affect ground water levels as well as cause
changes to slopes, loss of vegetation, and other factors.
Chapter 14 – Using Earths Soil
Soil can be measured in acidity, electrical conductivity, and buffering capacity and differs
between the climate, farming, and underlying rocks
How Soil Forms
- Weathering: the interactions of air and water with the geospheres surface that causes
surface to decompose (through physical, chemical and biological processes)
- Physical Weathering: causing rocks to disintegrate or break down into smaller and
(water freezing in rock fracture, expansion forces rock to break)
- Chemical Weathering: changes minerals on Earth’s surface
- water + minerals = clay
- O2 in air + water + metal bearing minerals = oxides
- Chemical with physical weathering results in mineral build-up rich in clay, quartz, and
- Biological Processes: aid in physical and chemical weathering.
- Example burring animals can mix loose materials and add organic debris such as
plants and evolve carbon dioxide that reacts to form weak acids
- Humus: organic matter that imparts a dark color to upper parts of soil, consists of both
and animal debris, facilitates many physical and chemical processes
- Humic acids: humus material that is acidic and insoluble to water
General functions of soil organic matter:
- supply nutrients (nitrogen and phosphorus)
- help develop structure that assists aeration
- water-holding capacity
The Soil Profile
- Soil profiles characterize soils and their differences and contain three to four master
- O horizon: layer of organic material, decaying vegetation
- A horizon: organics (humus-dark colour) and minerals are mixed
- B horizon: clay that is moved downwards by saturating water accumulates
- C horizon: weathered remnants of the underlying parent material
- Internal characteristics like colour, texture and structure help describe how the soil was
- organic material makes the upper horizon look darker
- light gray or white indicates zones that have been leached
- yellow, brown, or red in B is where clay and oxidized material is - red indicated that soil is being oxidized, yellow indicate soil is being poorly aerated
- soil texture is determined by the size of mineral particles, the 3 sizes are sand,
clay and silt
- soil structure reflects how soil particles are aggregated together. 4 common soil
1) Granular – the soil breaks up into subsequent pieces 1 to 10 mm across
2) blocky – the soil breaks u