Geology Cafe

Weathering and Erosion:
Processes That Break Down Rocks and Landscapes

Weathering is the natural process of breaking down materials. Weathering processes fall in two categories: mechanical weathering and chemical weathering. Weathering processes are essential components of erosion and deposition of sediments.
  • Mechanical weathering includes all processes that simply "break big pieces into little pieces."
  • Chemical weathering are processes that gradually dissolve or disassemble rocks through electrochemical activity.
  • In nature, the two types of weathering processes may occur simultaneously, but not always at the same rates.
  • Erosion is simply moving material in some fluid (such as air, water, ice).
  • Deposition involves processes that stop the surface transport of materials. This is followed by burial or further erosion.
hammer with broken rock soup pot gravel sand ripples dust devil
Mechanical weathering is breaking big pieces into smaller fragments, producing sediments prone to erosion. A boiling pot of soup illustrates the chemical breakdown of materials. In nature the process is slower, but similar. Erosion, the transport of sediment, continues the processes of both chemical and mechanical weathering. Erosional transport not only breaks down sediments,but can results in the natural sorting and separation of particles. Sediments may endure a long history of weathering and erosional transport before finally being deposited.

Mechanical Weathering: From Start to Finish, Forces That Break Rocks

Mechanical weathering is typically associated with processes at or near the surface, and may take place with processes associated with erosion. However, rocks are also broken in the subsurface by tectonic forces, such as by fracturing and shattering of rocks from earthquakes, asteroid impacts, volcanic activity, and gravitational forces. Erosion involves all natural processes that move earth materials, including movement by a fluid in motion (wind, water or ice), downslope movement by the force of gravity, or by biological activity. Many factors influence weathering processes, including bedrock characteristics, climate and precipitation, plant cover, and other landscape characteristics. Examples of mechanical processes include:
  • Tectonics - Breaking and Moving Rocks, (jointing, sheeting from release of pressure).
  • Freeze and Thaw - Ice formation, daily to seasonally, can fracture and move rocks
  • Heating and Cooling - surficial expansion can cause materials to expand and contract, fracturing them.
  • Wetting and Drying - Wetting and drying causes clay minerals to expand and contract, and salts may dissolve and re precipitate.
  • Biological Activity
  • Mass Wasting (force of gravity pulling earth materials down hill).

Click on thumbnail images for a larger view.
Grand Canyon Tectonic forces, such as earthquakes, break and shatter rocks in the earth crust. Many of these fractures formed long ago, but are reactivated and enhanced by the ongoing forces raising the land, and the gravitational pull and release of pressure as erosion removes the confining overburden of rock over time.   Some rocks, such as this granite exposed in Yosemite National Park, formed under great pressure at great depth. As erosion removed the confining overburden, these rocks fracture into sheets of rock, allowing for the passage of water and roots into the rock, allowing weathering to start.
The battering of waves on a shoreline is perhaps a most dramatic illustration of mechanical weathering. In this view along the Santa Cruz coast, wave action is carving sea cliffs from sedimentary rocks exposed by the uplift of the Coastal Ranges (Santa Cruz Mountains).   In this view of Devils Postpile National Monument (east-central California), gravitational collapse of basalt columns is probably enhanced by the daily and seasonal freezing and thawing of ice in the fractures between pillars. Expanding plant roots and occasional ground shaking during earthquakes probably also contribute to the collapse of the basalt columns.
Debris flows, landslides, floods, and avalanches can pluck rock material from the bedrock and transport it quickly, sometimes over long distances. Note that large boulders in valleys and along streams possibly reveal catastrophic forces in the past, and serve as a warning for future events and preparedness.   The roots exposed by a fallen tree is an example of the forces that biological action (plants and animals) can have on weathering processes, both mechanical and chemical. Biological action can be a dominant force in wet climates, also seasonally.

Chemical Weathering - Dissolution In Depth

Chemical breakdown of earth materials takes place mostly below the lands surface involving interactions of rock, water, and biological agents. Soil that covers the bedrock across the land surface is a residue of weathering processes (both chemical & mechanical). Water is a universal solution, and nearly all earth materials can be chemically altered, dissolved, transported, and deposited by it depending on conditions. The content and volume of materials dissolved or suspended in water can have effects on pH (acid/base), eH (available oxygen), and chemical reactivity of the water and its dissolved or suspend load. Likewise, climatic factor strongly influence the rate of chemical weathering. Chemical weathering rates change with changing weather conditions, seasonally, and over long period climate cycles associated with continental glaciation cycles and other forces driving climate change.

Chemical weathering of bedrock follows fractures and boundaries of materials of composition and hardness. Reactive minerals, such as many salts or carbonate minerals will dissolve in water. iron-bearing silicates common in most rocks and are prone to alteration into clays, iron oxide and hydroxide minerals (like rust on an old car). Only hard and stable minerals, like quartz, and physically isolated materials (such as material in the core of a large boulder) survive long periods. Rock material that looses cohesion by exposure to weathering will break down to form a regolith. At the surface, biological action and ongoing weathering convert regolith to soil and exposure to erosional forces.
 
Chemical weathering associated with fracture in bedrock are evident in these sedimentary rocks exposed in a road cut in the Santa Cruz Mountains. Freshwater from precipitation seeps into the fractures and then into the porous rock, dissolving and redepositing iron minerals (note the brown color banding inside the cores of the bedrock blocks). Chemical weathering along fractures is visible in this shale outcrop along a road cut in the Loma Prieta area of the Santa Cruz Mountains.
Spherically weathered boulders of granite in Joshua Tree National Park. The original fractures between boulders are still visible in many parts of this desert outcrop. Finer sediments derived from the breakdown of the granite has been removed by erosion. Spheroidally weathered boulders in Joshua Tree National Park. Large boulders on the landscape are remnants of bedrock that escaped weathering while the landscape around it wore away.
Cavern formation highlights chemical weathering underground. Limestone, made up of mineral calcite, easily dissolves in freshwater. Water seeping into fractures dissolve the calcite, creating passages. Karst: Areas where limestone outcrops are exposed at the surface can become eroded into unusual shapes. The name, karst, applies to landscapes underlain by limestone that has cavern systems that drain away runoff underground. Calcite (calcium carbonate) is a ma
Wetting and Drying: Episodic periods of wetting and drying over time may cause an unusual surface texture called tafoni. Tafoni forms on porous rocks (like sandstone) where case-hardened surfaces form when minerals in porous rocks are soaked, then drying causes some mineral to precipitate on the surface. The rock inside (below the surface) looses it mineral cements during wet periods. Tafoni weathering gives Ghost Rock in Castle Rock State Park its honeycomb appearance. When the case-hardened surface falls away, the sand below the surface that has lost its mineral cement easily erodes away, leaving the unusual pattern of holes in the rock.
Some rocks are more durable to weathering and erosion. These outcrops near Tomales Bay are made up of silica-rich blue schist. The composition, hardness, degree of fracturing, hill slope, and climatic setting are all factors that impact weathering. The chemical nature of bedrock influences the composition of soils that form from the bedrock. California is famous for its unique fauna and flora adapted the areas with soils derived from serpentinite (the State Rock). The serpentinite has high concentrations of magnesium and other metals that can be toxic to many other species.

Formation of Soil: Soil is basically the accumulation of weathering products and organic matter along the surface of the earth between the atmosphere and bedrock. The top few inches of soil is where most organic remains accumulate in association with biological activity. This picture shows a thick sub-topsoil profile consisting of a regolith of chemically weathered bedrock material at the base, overlain by material further degraded by leaching and hardened by infiltration with clay minerals, and precipitation of calcium carbonate and and other mineral cements. On gently rolling landscape, the thickness of the soil cover can vary considerable, from near shallow bedrock along stream beds to thick profiles, dozens of feet thick on gentle hill slopes and hilltops. On steeper landscapes, erosional processes tend to dominate over the formation of regolith and soil (also called colluvium).
 
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8/28/2012