Day 4: Erosion (Wind & Water) - by 3/13 (A) 3/12 (B)
Introduction (Water)
Streams – any running water from a rivulet to a raging river -- complete the hydrologic cycle by returning precipitation that falls on land to the oceans (Figure below). Some of this water moves over the surface and some moves through the ground as groundwater. Flowing water does the work of both erosion and deposition.
Streams – any running water from a rivulet to a raging river -- complete the hydrologic cycle by returning precipitation that falls on land to the oceans (Figure below). Some of this water moves over the surface and some moves through the ground as groundwater. Flowing water does the work of both erosion and deposition.
Erosion and Deposition by Streams
Erosion by Streams
Flowing streams pick up and transport weathered materials by eroding sediments from their banks. Streams also carry ions and ionic compounds that dissolve easily in the water. Sediments are carried as:
Erosion by Streams
Flowing streams pick up and transport weathered materials by eroding sediments from their banks. Streams also carry ions and ionic compounds that dissolve easily in the water. Sediments are carried as:
- Dissolved load: Dissolved load is composed of ions in solution. These ions are usually carried in the water all the way to the ocean.
- Suspended load: Sediments carried as solids as the stream flows are suspended load. The size of particles that can be carried is determined by the stream’s velocity (Figure below). Faster streams can carry larger particles. Streams with a steep gradient (slope) have a faster velocity.
- Bed load: Particles that are too large to be carried as suspended load are bumped and pushed along the stream bed as bed load. Streams with high velocities and steep gradients do a great deal of down cutting into the stream bed, which is primarily accomplished by movement of particles that make up the bed load.
Ground Water Erosion and Deposition
Rainwater absorbs carbon dioxide (CO2) as it falls. The CO2 combines with water to form carbonic acid. The slightly acidic water sinks into the ground and moves through pore spaces in soil and cracks and fractures in rock. The flow of water underground is ground water.
Ground water is a strong erosional force, as it works to dissolve away solid rock. Carbonic acid is especially good at dissolving the rock limestone.
Cave Formation
Working slowly over many years, ground water travels along small cracks. The water dissolves and carries away the solid rock gradually enlarging the cracks. Eventually a cave may form (Figure below).
Cave Formation
Working slowly over many years, ground water travels along small cracks. The water dissolves and carries away the solid rock gradually enlarging the cracks. Eventually a cave may form (Figure below).
Ground water carries the dissolved minerals in solution. The minerals may then be deposited, for example, as stalagmites or stalactites (Figure below).
If a stalactite and stalagmite join together, they form a column. One of the wonders of visiting a cave is to witness the beauty of these amazing and strangely captivating structures.
If the roof of a cave collapses, a sinkhole could form. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood (Figure below).
If the roof of a cave collapses, a sinkhole could form. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood (Figure below).
Wave Action and Erosion
Waves are important for building up and breaking down shorelines. Waves transport sand onto and off of beaches. They transport sand along beaches. Waves carve structures at the shore.
Some features of wave erosion are pictured and named in Figure below. A wave-cut platform is the level area formed by wave erosion as the waves undercut a cliff. An arch is produced when waves erode through a cliff. When a sea arch collapses, the isolated towers of rocks that remain are known as sea stacks.
Waves are important for building up and breaking down shorelines. Waves transport sand onto and off of beaches. They transport sand along beaches. Waves carve structures at the shore.
Some features of wave erosion are pictured and named in Figure below. A wave-cut platform is the level area formed by wave erosion as the waves undercut a cliff. An arch is produced when waves erode through a cliff. When a sea arch collapses, the isolated towers of rocks that remain are known as sea stacks.
Wave Deposition
Rivers carry sediments from the land to the sea. If wave action is high, a delta will not form. Waves will spread the sediments along the coastline to create a beach (Figure below). Waves also erode sediments from cliffs and shorelines and transport them onto beaches.
Rivers carry sediments from the land to the sea. If wave action is high, a delta will not form. Waves will spread the sediments along the coastline to create a beach (Figure below). Waves also erode sediments from cliffs and shorelines and transport them onto beaches.
Beaches can be made of mineral grains, like quartz, rock fragments, and also pieces of shell or coral (Figure below).
Waves continually move sand along the shore. Waves also move sand from the beaches on shore to bars of sand offshore as the seasons change. In the summer, waves have lower energy so they bring sand up onto the beach. In the winter, higher energy waves bring the sand back offshore.
Some of the features formed by wave-deposited sand are in Figure below. These features include barrier islands and spits. A spit is sand connected to land and extending into the water. A spit may hook to form a tombolo.
Some of the features formed by wave-deposited sand are in Figure below. These features include barrier islands and spits. A spit is sand connected to land and extending into the water. A spit may hook to form a tombolo.
Shores that are relatively flat and gently sloping may be lined with long narrow barrier islands (Figure below). Most barrier islands are a few kilometers wide and tens of kilometers long.
In its natural state, a barrier island acts as the first line of defense against storms such as hurricanes. When barrier islands are urbanized (Figure above), hurricanes damage houses and businesses rather than vegetated sandy areas in which sand can move. A large hurricane brings massive problems to the urbanized area.
Protecting Shorelines
Intact shore areas protect inland areas from storms that come off the ocean (Figure below).
Protecting Shorelines
Intact shore areas protect inland areas from storms that come off the ocean (Figure below).
Where the natural landscape is altered or the amount of development make damage from a storm too costly to consider, people use several types of structures to attempt to slow down wave erosion. A few are pictured in the Figure below. A groin is a long narrow pile of rocks built perpendicular to the shoreline to keep sand at that beach. A breakwater is a structure built in the water parallel to the shore in order to protect the shore from strong incoming waves. A seawall is also parallel to the shore, but it is built onshore.
People do not always want to choose safe building practices, and instead choose to build a beach house right on the beach. Protecting development from wave erosion is difficult and expensive.
Protection does not always work. The northeastern coast of Japan was protected by anti-tsunami seawalls. Yet waves from the 2011 tsunami that resulted from the Tohoku earthquake washed over the top of some seawalls and caused others to collapse. Japan is now planning to build even higher seawalls to prepare for any future (and inevitable) tsunami.
Introduction (Wind)
The power of wind to erode depends on particle size, wind strength, and whether the particles are able to be picked up. Wind is a more important erosional force in arid than humid regions.
Transport of Particles by Wind
Wind transports small particles, such as silt and clay, over great distances, even halfway across a continent or an entire ocean basin. Particles may be suspended for days. Wind more easily picks up particles on ground that have been disturbed, such as a construction site or a sand dune. Just like flowing water, wind transports particles as both bed load and suspended load. For wind, bed load is made of sand-sized particles (Figure below). The suspended load is very small particles of silt and clay.
Protection does not always work. The northeastern coast of Japan was protected by anti-tsunami seawalls. Yet waves from the 2011 tsunami that resulted from the Tohoku earthquake washed over the top of some seawalls and caused others to collapse. Japan is now planning to build even higher seawalls to prepare for any future (and inevitable) tsunami.
Introduction (Wind)
The power of wind to erode depends on particle size, wind strength, and whether the particles are able to be picked up. Wind is a more important erosional force in arid than humid regions.
Transport of Particles by Wind
Wind transports small particles, such as silt and clay, over great distances, even halfway across a continent or an entire ocean basin. Particles may be suspended for days. Wind more easily picks up particles on ground that have been disturbed, such as a construction site or a sand dune. Just like flowing water, wind transports particles as both bed load and suspended load. For wind, bed load is made of sand-sized particles (Figure below). The suspended load is very small particles of silt and clay.
Wind Erosion
Wind is a stronger erosional force in arid regions than it is in humid regions because winds are stronger. In humid areas, water and vegetation bind the soil so it is harder to pick up. In arid regions, small particles are selectively picked up and transported. As they are removed, the ground surface gets lower and rockier, causing deflation. What is left is desert pavement (Figure below), a surface covered by gravel sized particles that are not easily moved by wind.
Wind is a stronger erosional force in arid regions than it is in humid regions because winds are stronger. In humid areas, water and vegetation bind the soil so it is harder to pick up. In arid regions, small particles are selectively picked up and transported. As they are removed, the ground surface gets lower and rockier, causing deflation. What is left is desert pavement (Figure below), a surface covered by gravel sized particles that are not easily moved by wind.
Particles moved by wind do the work of abrasion. As a grain strikes another grain or surface it erodes that surface. Abrasion by wind may polish natural or human-made surfaces, such as buildings. Stones that have become polished and faceted due to abrasion by sand particles are called ventifacts (Figure below).
Exposed rocks in desert areas often develop a dark brown to black coating called desert varnish. Wind transports clay-sized particles that chemically react with other substances at high temperatures. The coating is formed of iron and manganese oxides (Figure below).
Wind Deposition
Deserts and seashores sometimes have sand dunes (Figure below). Beach dunes have different compositions depending on their location. Beach dunes are usually quartz because in humid areas other minerals weather into clays. In the tropics, sand dunes may be composed of calcium carbonate, which is common. In deserts, sand dunes may be composed of a variety of minerals. There is little weathering and so less stable minerals are left behind.
Dune sands are usually very uniform in size and shape. Particles are sand-sized, because larger particles are too heavy for the wind to transport by suspension. Particles are rounded, since rounded grains roll more easily than angular grains.
Deserts and seashores sometimes have sand dunes (Figure below). Beach dunes have different compositions depending on their location. Beach dunes are usually quartz because in humid areas other minerals weather into clays. In the tropics, sand dunes may be composed of calcium carbonate, which is common. In deserts, sand dunes may be composed of a variety of minerals. There is little weathering and so less stable minerals are left behind.
Dune sands are usually very uniform in size and shape. Particles are sand-sized, because larger particles are too heavy for the wind to transport by suspension. Particles are rounded, since rounded grains roll more easily than angular grains.
For sand dunes to form there must be an abundant supply of sand and steady winds. A strong wind slows down, often over some type of obstacle, such as a rock or some vegetation and drops its sand. As the wind moves up and over the obstacle, it increases in speed. It carries the sand grains up the gently sloping, upwind side of the dune. As the wind passes over the dune, its speed decreases. Sand cascades down the crest, forming the slip face of the dune. The slip face is steep because it is at the angle of repose for dry sand, about 34° (Figure below).
Wind deposits dune sands layer by layer. If the wind changes directions, cross beds form. Cross beds are named for the way each layer is formed at an angle to the ground (Figure below).
Loess
Windblown silt and clay deposited layer on layer over a large area are loess, which comes from the German word loose (Figure below). Loess deposits form downwind of glacial outwash or desert, where fine particles are available. Loess deposits make very fertile soils in many regions of the world.
Windblown silt and clay deposited layer on layer over a large area are loess, which comes from the German word loose (Figure below). Loess deposits form downwind of glacial outwash or desert, where fine particles are available. Loess deposits make very fertile soils in many regions of the world.
Questions:
- Define the three kinds of load that make up the particles a stream carries.
- What are two formations that form inside caves?
- What erosional feature formed by ground water could swallow up your house?
- Name three structures that people build to try to prevent wave erosion. How well do they work?
- Name three natural landforms that are produced by wave erosion.
- Discuss the factors necessary for sand dunes to form.
IN CLASS