Physical Weathering

Physical weathering produces regolith from massive rock by the action of forces strong enough to fracture rock. Some physical weathering processes include frost action, salt-crystal growth, unloading, expansion-contraction, and wedging by plant roots.

Joint-block separation and granular disintegration are two common forms of bedrock disintegration.

Physical Weathering

Physical Weathering

Frost-shattered blocks: Ice crystal growth within the joint planes of rock can cause the rock to split apart. This split boulder, on the high country of the Sierra Nevada of California, is an example.

Niche Formation

Zones of rock lying close to the base of a cliff are susceptible to erosion by the growth of salt crystals. Where water evaporates to leave salt, the crystals exert a force to break rocks apart. In niche formation, it is the seepage of groundwater and the subsequent growth of salt crystals that create a niche which identifies a change in lithologies.

Niche Formation

Chemical Weathering

Chemical weathering involves the reaction of minerals within rock to water in the environment. For example, water becomes acidic as carbon dioxide dissolves in it to create a weak carbonic acid. Carbonate rocks such as limestone react with carbonic acid, to create a weaker form that is removed by solution.

Chemical Weathering

These weathered tombstones are from a burying ground in Boston, Massachusetts. The marker on the left, carved in marble, has been strongly weathered, weakening the lettering. The marker on the right, made of slate, is much more resistant to erosion.

Unloading

As a rock mass is revealed as overlying material is removed, the pressures on underlying layers of rock lessen. Consequently, rocks exposed at the surface may break apart through the weathering process known as unloading.

Unloading

This granite outcrop in Yosemite National Park, California, displays sheetlike joints, giving a stepped appearance to the mountain slope.

Biological Weathering

Organisms also have a role to play in the breakup of surface materials. Burrowing animals and plant roots are just examples of where animals and plants can exert forces to break rocks apart.

Biological Weathering

A Ponderosa pine tree that began growing in a crack on this outcrop of bedrock has caused a large flake of rock to break away, exposing the tree's expanding root system. The rock is granite.

Slopes and Mass Wasting

Mass wasting is the spontaneous downhill movement of soil, rock, and regolith under the influence of gravity.

Slopes are mantled with regolith and which downwards towards bedrock. Soil may also develop on the regolith and support vegetation.

Slopes and Mass Wasting

Alluvium, a form of transported regolith, lies in the floor of an adjacent stream valley.

Mass Wasting

Mass wasting is the spontaneous downhill movement of soil, rock, and regolith under the influence of gravity.

Examples of mass wasting include soil creep, mudflows, landslides debris falls and slumps.

Mass Wasting

Examples of slurry flows and granular flows.

Mass Wasting

Examples of slope failures.

Soil Creep

Alternate wetting and drying of the soil and the formation of needle ice produces tiny displacement of particles which eventually result in their movement downslope. Consequently, soil creep is an extremely slow and gradual movement of slope materials. It causes the gradual tilting of objects such as fence posts and monuments.

Soil Creep

Indicators of soil creep.

Soil Creep

Needle ice growth.

Earth flow

Slope materials may retain their stability as long as no changes are made to the mass of the slope or alters the cohesion of the slope materials. However, if an earthflow begins to form, a scarp will indicate where slope materials are beginning to move downslope. Materials flow down as well as forward to create a rotational slump zone. Materials flow outward at the flowage zone. This debris then protrudes at the base of the earth flow to create a toe.

*See animation on earth flow in the geodiscoveries section of your text’s website.

Periglacial Landscapes

Periglacial landscapes account for 25% of the land surface, and form over areas of permafrost. Permafrost is permanently frozen ground, ranging from the deep, continuous permafrost of very cold regions, to the thinner, discontinuous permafrost of less cold regions.

Above the permafrost is the active layer which thaws in spring and summer to create a mobile and water logged volume of surface materials.

Periglacial Landscapes

Distribution of permafrost in the northern hemisphere.

Diagrammatic transect across northeastern Siberia showing distribution and thickness of permafrost and thickness of the active layer.

Periglacial Landscapes

Periglacial Ice Wedges

In periglacial regions, the upper layer of the ground, or active layer, is strongly influenced by extremely low temperatures. Where surface materials are fine, as the ground freezes and contracts, it creates a vertical crack.

*See animation on periglacial ice wedges in the geodiscoveries section of your text’s website.