WEATHERING

 

Weathering is the decomposition and disintegration of rocks and minerals at the Earth’s surface. Weathering itself involves little or no movement of the decomposed rocks and minerals. This material accumulates where it forms and overlies unweathered bedrock.

Erosion is the removal of weathered rocks and minerals by moving water, wind, glaciers, and gravity. After a rock fragment has been eroded from its place of origin, it may be transported large distances by those same agents: flowing water, wind, ice, and gravity. When the wind or water slows down and loses energy or, in the case of glaciers, when the ice melts, transport stops and sediment is deposited. These four processes – weathering, erosion, transportation, and deposition – work together to modify the Earth’s surface.

 

MECHANICAL AND CHEMICAL WEATHERING

 

The environment at the Earth’s surface is corrosive to most materials. An iron tool left outside will rust. Even stone is vulnerable to corrosion. As a result, ancient stone cities have fallen to ruin. Over longer periods of time, rock outcrops and entire mountain ranges wear away. Weathering occurs by both mechanical and chemical processes. Mechanical weathering reduces solid rock to rubble but does not alter the chemical composition of rocks and minerals. In contrast, chemical weathering occurs when air and water chemically react with rock to alter its composition and mineral content. These chemical changes are analogous to rusting in that the final products differ both physically and chemically from the starting material.

 

MECHANICAL WEATHERING

 

Mechanical weathering breaks large rocks into smaller ones by does not alter the rock’s chemical nature or its minerals.

Five major processes cause mechanical weathering: pressure-release fracturing, frost wedging, abrasion, organic activity, and thermal expansion and contraction. Two additional processes – salt cracking and hydrolysis expansion – result from combinations of mechanical and chemical processes.

 

PRESSURE-RELEASE FRACTURING

 

Many igneous and metamorphic rocks form deep below the Earth’s surface. Over millennia, tectonic forces may raise the pluton to form a mountain range. The overlying rock erodes away as the pluton rises and the pressure on the buried rock decreases. As the pressure diminishes, the rock expands, but because the rock is now cool and brittle, it fractures as it expands. This process is called pressure-release fracturing. Many igneous and metamorphic rocks that formed at depth, but now lie at the Earth’s surface, have been fractured in this manner.

 

FROST WEDGING

 

Water expands when it freezes. If water accumulates in a crack and then freezes, its expansion pushes the rock apart in a process called frost wedging. In a temperate climate, water may freeze at night and thaw during the day. Ice cements the rock together temporarily, but when it melts, the rock fragments may tumble from a steep cliff. In mountains where the daily freeze-thaw cycle occurs, rockfall due to frost wedging is common.

Large piles of loose angular rocks, called talus slopes, lie beneath many cliffs. These rocks fell from the cliffs mainly as a result of frost wedging.

 

ABRASION

 

Many rocks along a stream or beach are rounded and smooth. They have been shaped by collisions with other rocks as they tumbled downstream and with silt and sand carried by moving water. As particles collide, their sharp edges and corners wear away. The mechanical weathering and grinding of rock surfaces by friction and impact is called abrasion. Note that pure water itself is not abrasive; the collisions among rock, sand, and silt cause the weathering.

Wind also hurls sand and other small particle against rocks. Glaciers also cause much abrasion as the drag particles ranging in size from clay to boulders across bedrock. In this case, both the rock fragments embedded in the ice and the bedrock beneath are abraded.

 

ORGANIC ACTIVITY

 

If soil collects in a crack in solid rock, a seed may fall there and sprout. The roots work their way down into the crack, expand, and may eventually push the rock apart. City dwellers often see the results of organic activity in sidewalks, where tree roots push from underneath, raising the concrete and frequently cracking it.

 

THERMAL EXPANSION AND CONTRACION

 

Rocks at the Earth’s are exposed to daily and yearly cycles of heating and cooling. They expand whey they are heated and contract whey they cool. When temperature changes rapidly, the surface of a rock heats or cools faster then its interior and, as a result, the surface expands or contracts faster than the interior, The resulting forces may fracture the rock.