Mechanical, also known as physical weathering, can be divided into two main categories: fracturing and abrasion. Meanwhile, it's often related to other kinds of weathering: Biological weathering – which includes the wedging-apart of rocks by plant roots and lichen – broadly overlaps with mechanical weathering, which by exposing more rock surface to the elements can also enhance chemical weathering.
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Earth scientists often divide mechanical weathering into two major categories: fracturing, which includes frost- and salt-wedging, and abrasion, such as sandblasting.
Frost Wedging or Freeze-Thaw
Water expands by 9 percent when it freezes into ice. As it expands, it exerts up to 4.3 million pounds per square foot of pressure, enough to open cracks and fissures in rocks. Repeated freezing and thawing allows water to seep deeper into these crevices and enlarge them. Cracks may also allow entry of roots, agents of biological weathering that can also pry apart rock.
Crystal Formation or Salt Wedging
Crystal formation cracks rock in a similar way. Most water contains dissolved salts. When water in rock fissures evaporates, salt crystals form that, like ice, can force open fissures. This “salt wedging” tends to be most pronounced in arid regions given the high evaporation rates; it also occurs along seacoasts.
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Unloading and Exfoliation
Granitic rocks formed by cooling magma underground and later exposed by uplift and erosion may “exfoliate”: The release of pressure causes strips or sheets of rock to peel away. Rock once compressed under the weight of glaciers may also exfoliate due to unloading: When the glacier finally melts – for example, at the start of an interglacial period – the rock expands from the reduction of pressure. This causes fracturing between the layers parallel to the Earth’s surface. The top layer breaks apart in sheets, having no load above it at all. As the rock below is exposed, it too exfoliates.
Thermal Expansion and Contraction
Heating causes rock to expand. Cooling causes it to contract. The resulting cracking looks similar to frost wedging, though it tends to take a much longer time. Areas with extreme swings in daily temperature may see higher rates of this kind of wear. The moon has almost no atmosphere and no tectonic activity to weather the rock, and the temperature variation between day and night is 536 degrees F (280 degrees C). Thermal expansion and contraction may therefore be the only form of weathering that occurs.
In dry regions, wind-driven sand abrades exposed rock in a natural form of sandblasting. In streams, rivers and ocean surf, water turbulence causes particles of rock to collide with one another and grind against larger bodies of rock: abrasion that ultimately whittles them into smaller particles. Boulders, stones and grit embedded in glaciers also abrade the rock surfaces over which the ice flows.
Rocks toppling off cliffs or steep slopes due to the tug of gravity or swept up in landslides split into smaller pieces, another form of physical weathering by abrasion and impact. The actual gravitational transport of rocks and sediments is known as mass wasting, which isn’t itself a form of weathering but rather a process by which weathered material moves from one place to another.