Weathering and erosion are slow processes that gradually alter Earth’s surface, producing astonishing landscapes such as Grand Canyon or mushroom rocks.
Water is an immensely powerful force that causes weathering and erosion. Rainstorms can dislodge soil particles and break apart rock layers through mechanical weathering; fast-flowing rivers erode faster than slow moving ones, leaving behind meandering rivers or even oxbow lakes as features in its wake.
Sedimentation
Erosion in nature can be defined as any mechanical process carried out by wind, flowing water, glacial ice, plant and animal activity or downhill rock movement under gravity (known as mass wasting; see Chapter 10, Sedimentation and Deposition for more). Erosion creates sediment: rock particles or fragments which are then transported and deposited by flowing waterways.
Water can transport sediment of all sizes – boulders, cobbles, gravel, sand, silt and clays. Particle size determines its rate of transport: extremely fine-grained particles such as clay or silt tend to stick together and require lower water velocities than larger grains like sand for transport.
Material that has eroded from cliffs and shorelines often ends up as beach sand or in river basins, creating colluvium deposits. Organic remains from dead plants can also contribute to these sedimentary formations in swamps. According to geologists, sedimentary deposits fall into four major groups depending on how they were created: clastic, chemical, biochemical and organic deposits.
Lithification
Lithification, the process by which unconsolidated sediment becomes rock, is an intricate one involving various physical and chemical reactions such as compaction and cementation; recrystallization may also take place within pore spaces within sediment layers.
Freshly deposited sediment typically appears as loose material with ample pore space, such as clays or muds. Coarser grains like sand or gravel often settle quickly with no air pockets because their weight exceeds density; with time, however, the sediments become compacted through rearrangement under pressure which reduces their pore space further while forcing out extra interstitial water from within their matrix.
Once this process is completed, sediment has been transformed into a clastic sedimentary rock such as sandstone or conglomerate. Clasts that make up these rocks can often be traced back through minerals contained within them – particularly zircon – to determine when exactly it formed, helping geologists date the lithified rock.
Compaction
Compaction refers to the process of reducing pore space, which reduces water infiltration and gas exchange. When building HMAs, compaction is determined by factors like soil type, mix design and structural design as well as contractor and agency control during construction – with optimal moisture content and relative compaction being of primary concern.
Signs of compaction may help identify potential issues. Areas without vegetation or water ponds on the ground often indicate poor compaction; this is likely because roots cannot penetrate compacted soil to grow downward through it.
Another symptom is when compaction causes a lack of organic matter in the soil, as organisms cannot access essential minerals through compacted layers. Mechanical weathering includes erosional grinding by fast-moving stream waters or waves on coastlines and the expansion and splitting of previously deeply-buried compressed rocks due to erosion unloading; expansion/splitting; splitting through expansion/unloading mechanisms, as well as cementation with new minerals like clay and silica binding sediment grains together, cementation.
Cementation
Cementation is the process in which coarse clastic sediments become cemented together into hard, dense rock over time, either at the same time as sedimentation or later. Cementation may occur simultaneously or separately.
Diagenesis creates the characteristically round boulder piles seen on landscapes by compressing sediment grains more tightly together and cementing them with minerals like calcite or silica that bind them.
Erosion is a mechanical process caused by wind, water, gravity or ice that moves weathered rock and soil to new locations. Examples include waves crashing against cliffs; gravel and boulders moving along streams; or glaciers shattering and grinding rocks (see Chapter 10, Mass Wasting).
Erosion is generally caused by precipitation such as rain or hail. This may also contribute to sedimentation by depositing materials eroded from higher elevations and transported downhill by running waters, as well as chemical weathering such as the degradation of granite by means of weathering agents such as water.