Physical Geography 102
Weathering and Erosion
Weathering - the destructive processes by which materials at or near the Earth's surface are changed in color, texture, composition, firmness or form, with little or no transportation of the altered material.
Erosion - the mechanical or chemical destruction of the land and removal of material by moving water, ice or wind.
Produces finer particles by fracturing rock.
The forces involved must be strong enough to exceed the compressional, tensional or shear strength of the rock.
6 main processes: Impact & Abrasion, Ice Wedging, Salt-Crystal Growth, Exfoliation, Thermal Expansion, Root Wedging.
Impact and Abrasion
Moving material can impact or abrade on a surface.
The surface can fracture, flake or loose small particles.
Frost Action - Ice Wedging
The repeated growth and melting of ice in pore space or fractures.
As water freezes it expands.
Water which freezes in a fracture can force the fracture to grow.
The initial size of the fracture is not important - over time the fracture will expand as the freeze/thaw cycle continues.
Can result in the formation of a talus slope, or talus cone.
Frost Heave occurs in horizontal layers or lens shaped bodies.
Roadbeds can have water accumulate in the roadbed - when it freezes it expands lifting and cracking the road surface.
Surface feature in tundra areas formed by frost action include stone circles, stone rings, and ice polygons.
Similar to ice wedging.
Both result from forces created during crystallization.
Ice wedging is most prevalent in areas which experience cyclic freezing and thawing.
Salt-crystal growth is most common in arid environment.
Three minerals are involved in this process: Halite (salt), Calcite, Gypsum.
As water evaporates, minerals are left behind to grow between grains in the rock.
Can result in cavities or alcoves forming in vertical faces.
Exfoliation - Unloading
Rocks which form under the surface form under pressure - confining pressure.
When that pressure is released, the rock will expand.
Pressure is released as the rock is brought to the surface - tectonics, isostasy, erosion.
As it expands the outer layers break free in sheets - exfoliation.
• Exfoliation example 1
• Exfoliation example 2
• Exfoliation example 3
• Exfoliation example 4
Rock has a low thermal conductivity - tends to make a good insulator.
When rock is heated on a surface that heat does not travel inward very rapidly.
The heated exterior of the rock will expand more rapidly than the cool interior, causing the exterior to fracture - spalling.
Root wedging occurs when a plant, especially trees, sink root systems into existing joints and fractures.
As the root grows it forces the fracture to expand.
Relatively minor weathering force in rocks, but is very important for soil development.
• Root Wedging example 1
• Root Wedging example 2
• Root Wedging example 3
• Root Wedging example 4
• Root Wedging example 5
• Root Wedging example 6