Weathering refers to the gradual breakdown of rocks and minerals on Earth’s surface, while erosion refers to their transport from one location to the next.
Weathering and erosion are ubiquitous. Everyday we observe cracks in roads, sand on beaches and even letters eroded from gravestones as evidence of weathering. Weathering may occur gradually like the Grand Canyon or quickly like mudslides.
Water is one of the main elements that affect weathering erosion and deposition. Rainwater seeps into cracks in rocks, slowly wearing away at their foundation. Sometimes this water freezes solidifying into wedges that split rock apart; hence you see road crews filling potholes caused by this water erosion. Water also can transport pieces of eroded material away to new locations where they can further be deposition.
Water can play an essential part in chemical weathering. It dissolves minerals present in rocks at varying speeds; for instance, calcite disintegrates faster in water than does feldspar.
Physical erosion changes the shape of rocks without altering their chemical composition, typically over time and is the main cause of landslides. Physical erosion often takes place in mountainous regions with steep terrain slopes and river banks where its material falls off, creating clastic sediments composed of both rock fragments and organic matter.
Wind can influence weathering erosion and deposition in several ways. It can direct dust particles against rocks and cliffs, wearing away at them until they crumble away completely; or carry these particles away and deposit them elsewhere.
Deposition refers to when particles settle onto surfaces – either nearby like within feet, or further away like along a riverbank.
Physical weathering alters the shape of rocks without altering their chemical makeup; the resultant material, known as clastic sediments, includes sand gravel and fragments of rock worn down through mechanical weathering.
Physical weathering can also be caused by plant roots that become lodged into cracks in rocks. Over time, pressure from these roots may progressively widen an existing crack until eventually it ruptures and splits apart the rock itself.
Ice has a profound effect on weathering processes. It can fill cracks in rocks, weakening them as it moves. Furthermore, ice wedges itself between rock grains to cause further weathering effects – this type of mechanical weathering is commonly seen at Scotts Bluff National Monument.
Water can cause chemical erosion of rocks as it flows over them, leading to cave formation and sinkhole formation. This form of weathering takes place when acidic rainwater or groundwater interacts with calcium carbonate in limestone rocks, creating caves or sinkholes.
Moving water, ice and wind can transport pieces of rock to new places where they can be deposited as deposition. Deposition can happen within feet, such as when waves crash onto rocks at seashore beaches, or further away, such as when glaciers transport rocks by picking them up and depositing them far away into valleys – in either case these deposits might contain different rock types from their original beds – known as glacial erratics.
Once weathering has dislodged rock fragments, gravity can carry them away from their source of erosion and into deposition – most sediments ultimately find their way to oceans (Figure 9-6).
Gravity can play an indirect role in erosion by forcing large masses of rocks and soil downhill at once, such as through landslides, avalanches or mudflows. Slow-onset erosion can be reduced through planting vegetation on hillsides, building retaining walls or providing good drainage systems.
Ice can also cause erosion by freezing and expanding in cracks in rocks, creating wedges that split them and can split away chunks of rock, such as Bryce Canyon in Utah’s Bryce Canyon National Park, with this technique known as frost wedging as its name implies. Hoodoos in Bryce Canyon were formed as a result of this kind of erosion; also responsible for many mountainous landscape changes and contributing most of the sediment that forms beaches – it’s also responsible for changing mountainous terrain.