Mechanical weathering results from changes in temperature and pressure surrounding rocks. The expansion or contraction of air and pressure creates fractures in rock surfaces, ultimately making rocks break into smaller pieces. Mechanical weathering, unlike chemical weathering, changes only the size and shape of rocks rather than their chemical composition.
Mechanical weathering requires changes only to air temperature and pressure, but takes several forms. Common types of mechanical weathering include ice or frost wedging, exfoliation and abrasion. Frost wedging affects sedimentary rocks in colder climates.
This type of weathering takes place when rocks undergo a continual process of thermal heating and cooling. The continuous changes results in uneven growth of the minerals within rocks. Sedimentary rocks accrue and release water as they undergo repeating expansion and contraction phases. This change splits and fragments rock surfaces, causing them to crumble into smaller pieces.
Exfoliation, by contrast, primarily affects igneous rocks. This change involves rocks shedding their outermost layers, exposing new rock surfaces underneath. The inner surfaces expand, gradually exerting sufficient pressure for shedding sheets of outer rock. The outer sheets eventually fall off, relieving the pressure from rocks beneath.
Abrasion changes the shape of rock surfaces from physical pressures. Wind, water and other corrosive substances interact with rock surfaces, changing their shape and texture.