AGENTS OF EROSION AND DEPOSITION Forces of weathering and erosion are constantly reshaping Earth’s surface. Erosion includes the chemical and physical breakdown of rocks and their transport from their point of origin to another location. Blowing wind, running water, flowing ice and gravity are the forces that erode rock and sculpt the landscape. The Cycle of Erosion and Deposition The first step of erosion is weathering. Mechanical weathering refers to physical processes that break down rock, like repeated heating and cooling, frost wedging, abrasion from rocks hitting each other, glaciers grinding rock and sediment against each other, and waves pounding against rocks at the seashore. Chemical weathering refers to the chemical processes that break down rock, such as acid precipitation, naturally occurring carbonic acid dissolving limestone, chemical decay of sulfide minerals like pyrite, and even natural acids in root tips that allow plants to cling to and eat through bedrock. The next step of erosion is transportation. Technically erosion refers to the moving of rock from its point of origin to another locality. It takes energy to move rock material. The energy is applied by wind, water, and moving ice (glaciers). This energy not only moves the © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
rock but also wears it down into smaller and smaller particles. When the amount of energy decreases to the point that it cannot move the particles anymore, the rocks are deposited. Over time the broken sediments accumulate, often into immense layers. These sediments can be in place for millions of years during which time they can even lithify, that is, become stone. The cycle of erosion and deposition, however, doesn’t stop once sediments are deposited. After deposition, they can be further weathered and eroded and moved again and then deposited in yet another location. Since sea level is the lowest physical point of all the continents, sediments eventually make their way from the higher points of the continents to be transported by rivers and deposited in the oceans. Lesson Checkpoint: Name the two steps in the erosion process. Water Erosion Streams and rivers are constantly eroding and shaping the landscape. For example, the Mississippi River erodes and moves 436,000 tons of sediment every single day. All the materials that a river or stream carries is called its load. Load includes dissolved load which is salts dissolved in the water, suspended load which is smaller particles held in suspension by the energy of the moving water, and bed load which is larger particles that are bounced along the bottom or bed of the river. When the moving water from a river empties into a standing body of water, like a lake or the ocean, the sediments fall out of suspension and are deposited. The result is a triangular-shaped deposit known as a delta. The sediments fall out of suspension because the river water stops moving and therefore loses its energy. When the energy of the flowing water stops, it can no longer hold the sediments in suspension and the sediments then settle to the ocean or lake floor. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
When moving water transports sediments from a mountainous or hilly area into a flat, low-lying plain, the water suddenly slows down and the sediment is deposited in a triangular-shaped formation. In this situation it is called an alluvial fan (see image below). There are different kinds of rivers. Each represents different stages of erosion and what is called the river’s maturity. Mountain rivers are considered youthful rivers. They are narrower and, due to the mountain slope, run rapidly. They are high-energy and cut down into the mountain rock. Meandering rivers erode their channels wider rather than deeper and do not have a steep gradient. Such rivers are described as mature. Very broad, relatively shallow rivers that wander back and forth across the landscape are called meandering rivers. They flow slowly and therefore are low-energy rivers. They meander because the landscape on which they flow is very flat with very little change in elevation from one end to another. These are considered old rivers. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
As we have seen, the Earth is a dynamic planet. Given the right geologic forces, a region that has been eroded to the point of being an old river running across a well-eroded landscape can be rapidly uplifted. This uplift causes the meandering river to dramatically carve down into the sediments. This situation is known as a rejuvenated river. Lesson Checkpoint: What is an alluvial fan? Waves Erode the Coastline The energy of waves pounding against coastal bedrock turns this rock into sand and dramatically reshapes the coastline. Waves produce two different features: depositional features and erosional features. Depositional features include beaches, sand bars, and barrier spits. Beaches are large deposits of grains that have been eroded from rock. This process takes thousands and millions of years. Most people are familiar with common tan, sandy beaches. This sand is quartz sand. The type of sand on a beach, however, depends on the source rock of the sand. Granite is the source of the tan beaches of places like Cape Cod and the Jersey Shore. The black sand beaches of Hawaii are derived from the black basaltic lava from which the Hawaiian islands were formed. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
When waves crash on a shoreline, they come in at an angle. This creates currents of water that run parallel to the beach. These currents are called long shore currents and they are responsible for moving sand down a beach and producing formations like barrier spits. Cape Cod, Massachusetts is a dramatic example of a barrier spit. A spit is a stretch of land that sticks out from the mainland into the open ocean. It is described as a barrier because it is a barrier that protects the mainland from ocean storms. Erosional features are rock formations that have been cut and sculpted by waves and material thrown against the rocks by waves. Hard rock cliffs that stick out from the mainland into the ocean are called a headland. Waves can cut into a headland and create sea caves. When the waves cut completely through the rock a sea arch is formed. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
In time the bridge across a sea arch can collapse, leaving a smaller remnant of the headland and an isolated rock sticking up out of the water. This isolated, individual rock now separated from the headland is called a sea stack. Lesson Checkpoint: How is a sea arch created? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Wind Causes Erosion Erosion by wind is a different process than wave erosion. The wind itself does not really cause erosion. The particles that are moved by the wind, like sand on beaches and deserts and ice in arctic environments, actually cause the erosion. Geologists use a number of different terms to describe and define the erosional caused by wind energy. Wind moves sand particles by bouncing them across the surface. They bounce into each other and then are moved forward in the direction of the blowing wind. This process of moving sand particles is called saltation. The process of wind blowing particles against rocks, thereby grinding them down and, in some cases, polishing them, is called abrasion. A rock that has been abraded by wind-blown particles is known as a ventifact. A ventifact is pictured here. When the circumstances are right the wind can blow the sand cover completely away, revealing the hard underlying bedrock. This is a process called deflation. When blowing wind encounters an unmovable object, like a rock or a plant, it slows down. When it slows down, it loses energy and the sand particles it carries will fall to the ground. Eventually these particles build up. A large deposit of sand particles created by wind is called a sand dune. The wind blows sand particles up the windward side of the dune. When the particles come to the crest of the dune, they slide down the steep slip face of the dune. Dunes migrate in the direction of the blowing wind as sand is pushed up the windward face and slides down the slip face. Different types of dunes are identified based on their shapes. Barchan dunes are crescent-shaped with their “horns” pointing in the direction of the blowing wind. Transverse dunes form a wavelike ridge that is transverse to the wind. U-shaped dunes form a U-shape in which the open end of the U faces the oncoming wind. Longitudinal dunes (also known as linear dunes) are long, straight, dunes that form parallel to the direction of the blowing wind. Network dunes are complex groups of dunes that are created by wind that blows into a region from a number of different directions. Lesson Checkpoint: Which way do dunes migrate? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Glaciers There are two different types of glaciers. Continental glaciers are massive sheets of ice that literally cover large portions of continents. Alpine glaciers (also known as mountain glaciers) form in the high altitudes of mountains. In fact, most alpine glaciers form in valleys that were first carved by streams. It is not surprising, then, that glaciers have been described as “rivers of ice.” Glaciers move as ice is added to the glacier. Gravity pulls alpine glaciers downhill. In both types, the ice flows, like a mass of very thick putty or plastic. Continental glaciers create flattened, relatively smooth landscapes. Alpine glaciers create rugged landscapes with dramatically sculptured mountain features. Both types of glaciers cause erosion due to their sheer weight pushing against rock as they move. They break off large chunks of rock and grind them down. If a glacier has enough time to work on a particular geographic area, it can grind rock down to a powder as fine as flour (collections of windblown glacial “dust” are called loess). As with waves, glaciers create both erosional and depositional features. The erosional features are the more dramatic of the two. River valleys are typically V-shaped. When glaciers move through river valleys, they are carved by the glacier and become U-shaped. When three alpine glaciers erode a single mountaintop, a rugged, pointed peak is formed which is called a horn. An arête is a ridge created by two adjacent glaciers carving their way through two adjacent valleys. Both types of glaciers create similar depositional features. Moraines are ribbons of glacial till which are deposited at the sides and the end of glaciers. “Till” is unsorted silt and rock created by glaciers. Drumlins are spoon-shaped hills of till that form underneath continental glaciers. The long tail end of the drumlin points in the direction of the glacier’s movement. Kames and eskers are deposits of sorted glacial sediment that are deposited by running water that flows either on top of or through glaciers as they melt. Lesson Checkpoint: How do glaciers erode and what features do they form? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Mass Wasting Large masses of rock material can move suddenly when they become unstable. The force causing the movement is gravity. This is called mass wasting. There are two basic types of mass wasting. Rapid mass wasting refers to the sudden and highly destructive movement of large masses of soil and strata. Slow mass wasting refers to the slow, persistent movement of soil and dirt over long periods of time. A number of factors are involved in a mass wasting event, including the type of rock and soil involved, the angle at which it rests on the landscape, and the amount of water in the rock and soil. Loose soil that sits on a steep slope that is saturated with water is highly likely to suddenly move. The sudden and motion of large volumes of rock and dirt is called a landslide. This is an example of rapid mass wasting. landslide Sediment or soil that sits on a gently sloping hill can also move, but its movement is so slight year by year that it is difficult to notice. The evidence of this movement can be seen in fence posts that are tipping in the same direction. This slow mass wasting is known as creep. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.