Erosion and soil formation

So a rock undergoes weathering by any of the three types described in the last page. This creates small particles of rock which will normally undergo erosion. This is a very simple process really, by which the particles are transported from one place to another.

The particles are moved by wind and water to a new location where they are deposited. They will be deposited when conditions are right, in a river this is when the water flow is slowed down. As the condition will remain the same in most cases for some time, layers of particles are deposited which form a soil. Lets look in depth at a good example of erosion and deposition in a river.

Rivers often start in mountains or hills because this is where most rain falls. The rain falls there due to a process know as relief (Orographic) rainfall. This is where warm moist air (therefore containing water vapour) is forced to go over a mountain or hill. As the air rises it is cooled by the atmosphere, at a certain point known as the dew point the water vapour condenses to form water droplets and clouds. This results in heavy rainfall on the top of the mountain.

The water flows down the mountain in small streams. These are known as river tributaries. The tributaries will meet with other tributaries on their way down the mountain and join together. As more and more join, a river trunk is formed (trunk just refers to the main river channel). The water in the newly formed river contains large amounts of rock. Some of these particles may be as big as boulders! But these are soon dropped as other tributaries joining the trunk cause the water to spread out and slow down. Smaller particles remain in suspension and flow downstream towards the sea. As the river reaches the flat land around the base of the mountain the river will begin to meander (sort of zig zag through the fields etc.). This slows the progress of certain areas of the river and speeds up others, you find one side is often eroded while the other is deposited. These areas are often called flood plains as the river will frequently burst it's banks in times of high rainfall, leading to a sudden fall in the water speed and turbidity (a measure of the speed of the water and the particles suspended in it) of the water. Therefore sediment is deposited on the flood plain during times of flood, which enriches the soil. This makes flood plains very fertile land. However when the river is not in flood the sediment often remains in suspension until the river joins the sea. At this point the turbidity decreases and the sediment is deposited to form a delta. This forms a island over time and river water will form channels between the delta plains.

This is one way eroded material may form a soil, however the material may not be eroded. If this is the case, the soil forms on top of the parent rock.

So that is how a soil is formed but what about the soil itself, click here to find out more about the soil...