The Commencement of Continental Denudation: Mechanical Weathering

Abstract

Although weathering processes are increasingly perceived as of a synergistic nature, mechanical (or physical) weathering can be viewed as the most frequent commencement of continental denudation. The action of temperature, humidity, and pressure interact to breakdown minerals and rocks, thus increasing the specific surface and facilitating the action (subsequent or concurrent) of biological and chemical agents. In order to understand the processes involved in mechanical weathering, field observations are necessary but not sufficient to fully appreciate the mechanisms of rock mechanical breakdown. In the past decade, laboratory experimentation has provided a wealth of information on the physics involved to reduce the size of outcropping rock blocks, so as to finally allow the transportation of continental rock debris to its deposition on the ocean floor, which is ultimately the final stage of the Earth’s exogenous cycle.

Glossary

Albedo:

The reflectivity coefficient of a surface to short-wave radiation, i.e., the fraction of incoming solar radiation that is reflected rather than absorbed, from the Earth back into space. A value of 0 means the surface is a “perfect absorber” and a value of 1 means the surface is a “perfect reflector.”

Anhydrous:

A substance free from water, especially water of crystallization.

Biosphere:

Part of the Earth’s crust, waters, and atmosphere that supports life; the global sum of all ecosystem that interact within the elements of the lithosphere, hydrosphere, and atmosphere.

Electro-osmosis:

a.k.a. electro osmotic flow, i.e., the motion of water or any other liquid induced by a potential across a porous material, capillary tube, or any other fluid conduit.

Exfoliation:

Surface-parallel fracture systems in rock often leading to erosion of concentric slabs.

Haloclasty:

A type of physical weathering caused by the growth of salt crystals. The process is first started when saline water seeps into cracks and evaporates depositing salt crystals, when the rocks are then heated, the crystals will expand putting pressure on the surrounding rock which will over time splinter the stone into fragments.

Hydrate:

A substance that contains water of crystallization or water of hydration, in a definite ratio as an integral part of the crystal.

Lithosphere:

Rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle. It continually interacts with the atmosphere and hydrosphere. It is broken into about a dozen separate, rigid blocks, or plates.

Regolith:

Layer of loose heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestrial planets and moons.

Tafoni:

Cave-like features found in granular rock with rounded entrances and smooth concave walls.

Thermal fatigue:

The progressive and localized structural damage that occurs when a material is subjected to repeated heating and cooling. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands, and grain interfaces. Eventually a crack will reach a critical size, and the structure will suddenly fracture; materials do not recover when rested.