Erosion and Deposition

1. Erosion and Deposition Earth Science

2. Why do we care about Erosion & Deposition? • Both processes are responsible for creating many landforms on Earth. • They help to form rivers, streams, hills, valleys, etc.

3. What happens with Erosion? • Erosion is the transport of sediments. • This means that erosion breaks things down (like rocks or soils) and carries the small pieces (sediments) to other locations. • Causes of erosion include flowing water, waves, wind, ice, or gravity

4. What happens with Deposition? • After erosion carries away sediments, those sediments have to be dropped somewhere else. • Deposition is when the sediments are dropped and pile up. • Just think: Deposition means Deposit!

5. How is H2O important? • Flowing water is a VERY important agent of erosion. • Flowing water can erode rocks and soil bydissolving minerals from the rocks and carrying the ions away. • This process happens really slowly. • But over millions of years, flowing water dissolves massive amounts of rock. • The ability to erode is affected by the velocity, or speed, of the water.

7. Water Speed & Erosion • Faster-moving water has more energy. Therefore, it can carry larger particles. • The steeper the slope, the faster the water flows. • Streams with a lot of water flow faster than streams that are nearly dry.

9. Particle Size & Erosion • The size of particles determines how they are carried by flowing water. • Minerals that dissolve in water form salts. • Small particles, such as clay and silt, are carried in suspension. These particles are not dissolved in the water. • Somewhat bigger particles, such as sand, are moved by saltation. The particles move in little jumps near the stream bottom. They are nudged along by water and other particles. • The biggest particles, including gravel and pebbles, are moved by traction. In this process, the particles roll or drag along the bottom of the water.

10. Deposition by Water • Flowing water slows down when it reaches flatter land or flows into a body of still water. • The water starts dropping the particles it was carrying. • As the water slows, it drops the largest particles first. • The smallest particles settle out last.

11. Streams get involved too! • Streams – any running water from a rivulet to a raging river • Flowing water does the work of both erosion and deposition. • 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.

12. How are sediments carried? • Dissolved load: is composed of ions in solution. These ions are usually carried in the water all the way to the ocean. • Bed load: Particles that are too large to be carried as suspended load are bumped and pushed along the stream bed as bed load. • Bed load sediments do not move continuously. • Streams with high velocities and steep gradients do a great deal of down cutting into the stream bed

13. How are sediments carried? • Suspended load: Sediments carried as solids as the stream flows. • The size of particles that can be carried is determined by the stream’s velocity • Faster streams can carry larger particles. • Streams that carry larger particles have greater competence. • Streams with a steep gradient (slope) have a faster velocity and greater competence.

14. Stages of Streams • As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. • At a stream’s headwaters (where the stream starts), often high in the mountains, gradients are steep. • The stream moves fast and does lots of work eroding the stream bed. • As a stream moves into lower areas, the gradient is not as steep. • Now the stream does more work eroding the edges of its banks.

15. Other stuff streams do… • As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain • Floodplain: a flat level area surrounding the stream channel & can flood easily • Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. • Streams work to down cut in their stream beds until they reach base level. • The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do.

17. Stream Deposition • As a stream gets closer to base level, its gradient lowers and it deposits more material than it erodes. • On flatter ground, streams deposit material on the inside of meanders. • A stream’s floodplain is much broader and shallower than the stream’s channel. • When a stream flows onto its floodplain, its velocity slows and it deposits much of its load.

18. What happens when it floods? • A stream at flood stage carries lots of sediments. • When its gradient decreases, the stream overflows its banks and broadens its channel. • The decrease in gradient causes the stream to deposit its sediments, the largest first. • These large sediments build a higher area around the edges of the stream channel, creating natural levees.

20. Ground Water Erosion & 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.

21. Ground Water Continued • Ground water is a strong erosional force, as it works to dissolve away solid rock

22. Where do caves come from? • 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!

23. Caves, Caves, Caves • Ground water carries the dissolved minerals in solution. • The minerals may then be deposited, for example, as stalagmites or stalactites.