Erosion is the process by which sediment, weathered rock, and soil are moved from one place to another. Unlike weathering, which breaks rock apart, erosion transports those materials. Moving water is one of the most powerful agents of erosion. It shapes the land along streams and rivers, at beaches, and even underground.

Imagine standing at the top of a mountain and following a drop of rain as it falls from the sky, travels through the river’s source, and reaches the crashing waves of the ocean. Because of gravity, rainwater becomes runoff and flows downhill. As water moves, it carries sediment and rock particles. Over time, this movement shapes the land, creating many different landforms.

A stream is an active channel of flowing water that erodes land and transports sediment. As a river flows, it creates features such as waterfalls, flood plains, meanders, oxbow lakes, and deltas.  Waterfalls form when a river flows over layers of rock that erode at different rates. Often, a flat layer of harder rock lies on top of a softer layer. The softer rock erodes more quickly, while the harder rock resists erosion. Eventually, pieces of the harder rock break off as the softer rock beneath it wears away, creating a steep drop known as a waterfall.

As rivers continue to flow across flatter land, they begin to curve and form loop-like bends called meanders. Over time, these bends can become more curved. If sediment is deposited and blocks off the ends of a meander, the bend can be cut off from the main river. When this happens, an oxbow lake forms.

Floodplains are wide, flat areas of land along a river. These areas are often covered with water when a river overflows during a flood. During flooding, rivers deposit nutrient-rich sediment across the flood plain, which can make the soil very fertile.

As a river approaches the ocean or a lake, it slows down and deposits sediment. This buildup of sediment forms a delta, a landform created where a river flows into a larger body of water.

Not all water becomes runoff. Some water soaks into the ground and becomes groundwater. Groundwater can cause erosion through chemical weathering, especially in areas with limestone. Over time, groundwater can dissolve rock and form caves.  Inside limestone caves, mineral deposits create formations called stalactites and stalagmites. A stalactite forms from mineral deposits in dripping groundwater and hangs from the ceiling of a cave. A stalagmite forms when minerals build up on the cave floor in a cone-shaped structure.

Along coastlines, waves are the primary force of erosion. As they crash against the shore, they break down rocks and transport sand and other sediment. The energy carried by waves can cause rocks to crack and break apart, while abrasion—the grinding action of sand and rock fragments—further wears away the shoreline. Over time, this continuous process reshapes coastal areas, forming features such as beaches, sea arches, and sea stacks.

One landform created by wave erosion is a sea arch. A sea arch forms when waves erode a softer layer of rock beneath a harder layer. Over time, the softer rock wears away, leaving an opening through the harder rock.  Waves also shape coastlines by depositing sediment. A beach is an area where sediment carried by waves is deposited along the shore. Most beaches are made of sand, but some are made of shells or fragments of coral. The constant movement of waves continues to reshape beaches over time.

While wind is the weakest agent of erosion compared to water and ice, it can still significantly shape the land. Wind causes erosion through a process called deflation. Deflation occurs when wind removes loose surface materials. Fine particles can be carried through the air, while larger particles may slide or roll along the ground. The stronger the wind, the larger the particles it can move.

Wind erosion also creates distinct landforms. Two common types of windblown deposits are dunes and loess. A dune is a pile of windblown sand that varies in shape and size. Dunes can slowly shift and change position as the wind continues to move sand. Loess is a crumbly, windblown deposit made of silt and clay. Because loess contains fertile soil, it is often valuable for farming.  Through both erosion and deposition, waves and wind continuously shape and reshape Earth’s surface.

Review:

1. What features are formed by rivers?
2. What is the difference between a stalactite and a stalagmite?
3. Explain deflation.