Mhlatuzana River at GZ
Definitions
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Soil: The portion of the Earth's surface consisting of disintegrated rock & humus.
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Gravel: Small stones and pebbles, or a mixture of these with sand (in roadmaking).
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Sand: The fine debris of rocks, consisting of small loose grains, often of quartz.
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Silt: Earthy matter, very fine sand, or the like carried by moving water and deposited as a sediment.
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Clay: A natural earthy material that is plastic when wet, consisting essentially of hydrated silicates of aluminium.
Different types of Soil
Soil Formation & Weathering
Formation
Soil forms from a complex interaction between earth materials, climate, and organisms acting over time. The brightly colored soils of the humid tropics reflect the intense chemical reactions occurring in warm climates. Fertile prairie/savannah soils evolved from the nutrient-rich organic matter left by decaying grasses. Regardless of soil characteristics, the whole process starts with the breakdown of earth material.
Weathering
Weathering refers to processes that physically break down and chemically alter earth material. Physical weathering, also known as mechanical weathering, is the breakdown of large pieces of earth material into smaller ones. Think of physical weathering as the disintegration of rock without changing its chemical composition. There are many ways earth material can be physically weathered. When water freezes in rock crevices it expands creating stress in the crevice. As the stress increases, the crevice widens ultimately breaking the rock. Plant roots wedge rocks apart as they grow into rock crevices too. The shrinking and swelling by alternating heating and cooling weakens mineral bonds causing the rock to disintegrate.
A very important result of physical weathering is its impact on the surface area of weathered material. When a block of earth material is broken into several smaller pieces, the amount of exposed surface increases. Examine the diagram below. A block with a width, depth, and height of 1 cm has a total surface area of 6 square centimeters. If we break the block in half in all directions it yields eight smaller pieces all with width, height, and depth of 0.5 cm. Breaking the block apart creates additional exposed surfaces such that the total surface area is now 12 square centimeters.
Weathering of Soil
Having more total exposed surface provides more area upon which chemical reactions can take place to further weather the material. The shape of the pieces also affects the the amount of exposed surface area. Plate-like pieces have more exposed surface area than do block-like pieces.
Chemical weathering breaks down earth material by chemical alteration. This usually means adding a substance like water or air to the material. For instance, when oxygen is added to iron bearing minerals, oxidation takes places and a loose mantle of iron oxide is created (rust). Hydrolysis is an exchange reaction involving minerals and water. Free hydrogen (H+) and hydroxide (OH-) ions in water replace mineral ions and drive them into solution. As a result, the mineral's structure is changed into a new form. Hydrolysis is a common process whereby silicate minerals are weathered into a clay mineral. Think of chemical weathering as the decomposition of earth material.
Soil Characteristics
Texture
Soil textures are classified by the fractions of each soil separate (sand, silt, and clay) present in a soil. Classifications are typically named for the primary constituent particle size or a combination of the most abundant particles sizes, e.g. "sandy clay" or "silty clay." A fourth term, loam, is used to describe a roughly equal concentration of sand, silt, and clay, and lends to the naming of even more classifications, e.g. "clay loam" or "silt loam."
Determining the soil textures is often aided with the use of a soil texture triangle.
Soil Classification Triangle
Particle Size
Soil separates are specific ranges of particle sizes. The smallest particles are clay particles and are classified as having diameters of less than 0.002 mm. The next smallest particles are silt particles and have diameters between 0.002 mm and 0.05 mm. The largest particles are sand particles and are larger than 0.05 mm in diameter. Furthermore, large sand particles can be described as coarse, intermediate as medium, and the smaller as fine. Many countries have their own particle size classifications.
| Name of Soil Separate |
Diameter Limits (mm) |
| Clay |
less than 0.002 |
| Silt |
0.002 - 0.05 |
| Very Fine Sand |
0.05 - 0.10 |
| Fine Sand |
0.10 - 0.25 |
| Medium Sand |
0.25 - 0.50 |
| Coarse Sand |
0.50 - 1.00 |
| Very Coarse Sand |
1.00 - 2.00 |
| Fine Gravel |
2.00 - 10.0 |
| Medium Gravel |
10.0 - 25.0 |
| Coarse Gravel |
greater than 25 |
Graphical Representation of Particle Size
Particle Shape
Soils can also be classified by shape. Shapes commonly used are granular, platy, blocky and prismatic.
Soil Particle Shape
The Earthcache
In order to log a find on this Earthcache, please perform the following tasks and email me your answers via the geocaching.com website.
Answers not received within 7 days will result in log deletion.
Task 1
Describe in your own words the characteristics and colours of the soils on the river bank and in the river bed.
Give an opinion on why the colours are different.
Task 2
Take a handful of the material from the centre of the river bed.
Form an opinion of particle size and where your handful of material would fit in the soil triangle diagram above.
Hint: Use the soil triangle and the particle size diagram to formulate your answer.
Task 3
What shape would you say the majority of the material is?
Hint: Refer to the particle shape diagram above.
Task 4 (Optional)
Photographs of you / your team and your GPS at the co-ordinates, with your sample of river soil will be appreciated.
References
http://www.dictionary.com
http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/soil_systems/soil_development_weathering.html
http://en.wikipedia.org/wiki/Soil_texture