Weathering erosion and deposition are natural processes which help create stunning landscapes on Earth, but can become harmful when human activities accelerate them, such as deforestation or beach sand mining.
Water is often the preferred means for transporting rocks to new locations. Other methods may include frost wedging (rock cracking when frozen) and salt expansion, similar to those seen in limestone formation.
Weathering
Weathering refers to the process of breaking down and dissolving rock and minerals on Earth’s surface by water, ice, acids, salts, plants, animals and temperature changes. All these agents play an integral part in this process of weathering.
Physical weathering refers to the process of breaking apart rock without changing its chemical makeup, usually through freeze-thaw cycles, root wedging or abrasion.
Chemical weathering breaks down rocks by chemically converting them to different substances, for instance silicates in granite gradually deteriorate along their crystal boundaries to form clays exposed to air and water, which then transport dissolved cations back through rainwater runoff into rivers, eventually being dumped back in the ocean.
Thermal stress can also play an integral part of mechanical weathering. This process occurs when rocks expand when heated, then contract when cool again – further weakening them each time. Thermal stress weathering is particularly prevalent in rocky desert environments but can also be caused by human activities like burning coal and oil or producing acid rain.
Erosion
Erosion is the natural process of loosening and transporting rock fragments, soil particles, mud and dissolved materials (including nutrients or pollutants) from landscapes through wind, water, gravity waves or glaciers moving slowly across them to another location.
Weathering and erosion are often intertwined processes. However, it is important to remember that just because weathering breaks apart rocks does not necessarily translate to them being transported away by erosion; only pieces that can move will end up at other places.
Water erosion is the main mechanism by which rocks erode. Rainwater flowing down steep slopes cuts pathways through rocks and soil as it flows, known as erosion. Erosion also happens in streams, rivers and glaciers – it may even be accelerated by tectonic activity if an uplifted area experiences changes to its slope causing quicker rates of erosion.
Deposition
Weathering and erosion slowly change Earth’s surface. By dismantling rocks and materials, weathering and erosion break them down before pieces of them are transported by erosion and eventually deposited elsewhere – either locally, such as when wind carries away grains of sand to be dumped on beaches nearby; or further away such as when rainwater deposits salty debris from its atmosphere into raindrops on Earth’s surface.
Water erosion can erode riverbanks and floodplains, posing flood hazards such as flooding or debris flows; additionally, channel migration introduces risk into new areas.
Climate change and emissions together usually result in larger deposition than either process alone; for instance, when temperatures warm, wet NOy deposition increases, yet decreasing emissions largely mitigates its negative effect.
Impacts
Weathering erosion and deposition is an integral component of Earth’s rock cycle, gradually wearing away at and rebuilding Earth’s surface layer over billions of years.
Water is one of the primary agents of erosion. Rainstorms and rivers erode landscapes by washing away rocks and soil away from their original locations; rivers generally eroding more quickly on steeper terrain than flat land.
Wind and gravity both contribute to erosion, as evidenced by our daily experience of walking along a sandy beach or driving through cities with many cracks in sidewalks and roads. Ice also plays an integral part in this process by creating wedge-shaped ice wedges in cracks before melting away as part of its mass.
Human activities can increase erosion by altering how rocks and sediment are formed. Deforestation and plowing fields remove roots that protect sediments, increasing erosion rates 10-100 times faster than natural geologic processes.