Chapter 13: Weathering, Karst Landscapes, and Mass Movement

Geomorphology – The study of the origin, evolution, form, and spatial distribution of landforms.

Denudation – Any process that wears away or rearranges landforms.

Dynamic Equilibrium Model – The balancing act between tectonic uplift and erosion, between the resistance of crust materials and the work of denudation processes. Landscapes show ongoing adaptation to rock structure, climate, local relief, and elevation.

Geomorphic Threshold – The point at which there is enough energy to overcome resistance against movement. At this threshold, the system breaks through to a new equilibrium as the landform adjusts.

Slow, continuous-change events, such as soil development and erosion demonstrate dynamic equilibriums. Dramatic events such as landslides or dam collapse demonstrate threshold events.

Geomorphic Processes

Slope Development – Materials that are loosened by weathering are susceptible to erosion and transportation. The pull of gravity must overcome the forces of friction, inertia, and cohesion.

Waxing slope – Concave surface near the top of the hillslope.

Free face – An area where rock outcrops and creates a cliff.

Debris slope – An area that receives the rock fragments form the overhanging free face.

Waning slope – A concave surface along the base of the slope.

Weathering Processes

Weathering – The disintegration of surfaces or dissolving of material into solution.

Regolith – Broken-up bedrock.

Bedrock – The parent rock that is weathered.

Physical Weathering Processes – The mechanical breaking-up of rock surfaces.

Frost Action – When water freezes it can expand up to 9% of its original volume, which can work to break apart rocks.

Talus Slope – Poorly sorted, cone-shaped deposit of angular debris at the base of a steep slope.

Crystallization – Salt crystal growth in desert areas can exert a force on rock layers and cause them to break apart.

Hydration – The wetting and drying of rocks make them expand a shrink, which physically breaks the rocks apart.

Pressure-Release Jointing – As the mass of material is weathered off the top of a pluton, the underlying rocks are released from the weight of the overburden. These igneous rocks then start to exfoliate or have material break off in sheets.

Chemical Weathering Processes – The actual decomposition of surfaces because the chemical reactions involving oxygen and water.

Hydrolysis – When minerals chemically combine with water. The elements in the rock go into solution and the rocks are broken down.

feldspars (K, Al, Si, O) + carbonic acid and water =

residual clays + dissolved minerals + silica

Oxidation – When oxygen combines with certain metals. The most common example is the rusting of iron.

iron (Fe) + oxygen (O₂) = iron oxide (hematite; Fe₂O₃)

Carbonation and Solution – When minerals dissolve into solutions of water. Water vapor combines with CO₂ in the atmosphere and creates carbonic acid. The then reacts with limestone in a process called Carbonation (which is a reacting in which carbon combines with other minerals).

calcium carbonate + carbonic acid and water =

calcium bicarbonate (Ca₂CO₂ + H₂O)

Karst Topography

The natural weathering features associated with limestone rocks. Approximately 15% of the Earth’s land area has some karst features.

Formation of Karst

The limestone formation must contain 80% or more calcium carbonate for solution processes to proceed effectively.

Complex patterns of joints in the otherwise impermeable limestone are needed for water to form routes to subsurface drainage channels.

There must be an aerated zone between the ground surface and the water table.

Vegetation cover supplies varying amounts of organic acids that enhance the solution process.

Sinkholes – When the limestone weathers away in a karst area, the ground can collapse forming a sinkhole. Sinkhole

Caves – Formed in limestone layers through carbonation and solution processes.

Mass Movement – A general term used to refer to any unit movement of a body of material propelled by gravity.

Falls and Avalanches – A volume of rock that falls through open air or quickly flows downslope and is fluidized by ice or water.

Landslides – A sudden movement of a cohesive mass that is not saturated with moisture.

Flows – Mass movement that involves a high concentration of water.

Creep – A persistent, gradual mass movement of surface soil. This causes telephone poles and fence posts to tilt. In creep, individual soil particles are lifted and moved due to the expansion of soil moisture from hydration and freezing.

Human Induced Mass Movement (Scarification)

Every human disturbance of a slope – highway, roadcut, surface -mining, or building of any structure – can hasten mass wasting because of destabilization of the surface.

Geomorphic Agent	Mass moved
Humans	40-45 Gt/year
Movement from Stream Meandering	39 Gt/yr
Sediment Movement from Mountain Building	34 Gt/yr
River Sediment Transfer	14 Gt/yr
Deep-ocean sedimentation	7 Gt/ yr
Glaciers	4.3 Gt/yr
Wave Action and Erosion	1.25 Gt/ yr
Wind Transport	1 Gt/yr

“Homo sapiens has become an impressive geomorphic agent. Coupling our earth-moving prowess with our inadvertent adding of sediment load to rivers and the visual impact of our activities on the landscape, one is compelled to acknowledge that , for better or for worse, this biogeomorphic agent may be the premier geomorphic agent of our time” (Hooke 1994).