Earth Science: Chapter 5: Weathering, soil and mass movement
The Earth is a dynamic body. Some parts of the Earth’s surface are gradually elevated by mountains and volcanic activity. Meanwhile opposing processes are removing materials from the higher elevations and transporting the materials to lower elevations. This is done in three ways: weathering, erosion and mass wasting.
Weathering:
The Earth’s surface is always changing. Internal forces create volcanoes and mountains. External processes continually break rock apart and move debris to lower elevations. Weathering is a basic part of the rock cycle and a key process in the Earth System. There are two types of weathering: mechanical and chemical.
Mechanical Weathering: Physical forces break rock apart into smaller and smaller pieces without changing the rocks mineral composition. The breaking of rock into smaller pieces will increase the surface area to volume ratio and allows the rocks to be affected by chemical weathering.
There are 3 physical processes that are important in mechanical weathering: frost wedging, unloading and biological activity.
1) Frost Wedging: when liquid water freezes, it expands by about 9% and exerts a tremendous outward force. The water can work into the cracks of rocks and when it freezes, the cracks will be enlarged. After many freeze cycles, the rock will break. This is common in mountainous regions. Sections of rock that are wedged loose and may tumble into a large pile called a TALUS.
2) Uploading: Large masses of igneous rock may be exposed through uplift and erosion of the overlaying rocks. The pressure on the igneous rock is reduced with uploading. However, uploading causes the outer layers of rocks to expand more than the rock below. The slabs of the outer rock will separate and break loose…EXFOLIATION (a type of exfoliation is SHEETING. This occurs when layers of the mountain peels off, like layers from an onion.). This is common in rock masses made up of granite. The uploading forms large domed mountains. Mining reduces the pressure of igneous rocks and slabs can fall inside mines.
3) Biological activity: Plants, lichens, burrowing animals and humans can cause mechanical weathering. Lichens can grow on rock, roots from plants can grow into cracks and wedge rocks apart, and humans can deforest and blast rock to create new roads.
Chemical Weathering: A change in rock into one or more new compounds.
Water is the most important agent of chemical weathering. Water absorbs gases from the atmosphere and ground. These dissolved substances will chemically react with the minerals in the rocks.
1) Oxygen: dissolves in water to form oxides with certain minerals (e.g. iron + oxygen forms iron oxide).
2) Carbon dioxide: combines with water to form carbonic acid.
3) SOx: combines with water to form sulfuric acid. This is the major cause of acid rain. The SOx is usually formed by burning fossil fuels.
Let’s look at the chemical weathering of the following:
1) Granite: This is mostly feldspar and quartz. The carbonic acid will convert feldspar to clay minerals which will eventually wash away leaving the quartz. The clay will be transported by size. The clay will end up in the ocean. The quartz will be deposited. Potassium feldspar is a component of granite.
The K is now available to plants or the KHCO3 can enter the streams. Clay is left behind and is very stable. That’s why clay makes up a large percent of inorganic soil. The quartz, the other main component of granite, is resistant to chemical weathering. The feldspar crystals dull and slowly turn to clay. It gives the clay a glassy appearance.
2) Silicate Minerals: This mineral makes up most of the Earth’s crust: Na, Ca, K and Mg dissolve and are carried away by ground water. Fe will combine with oxygen (iron oxide). Al, SI and O will combine with water to produce clay minerals. The Fe gives soil its reddish-brown or yellowish color.
3) Spheroid: Weathering can change the physical shape of a rock. Water can enter joints…weathers the corners and edges (first). This type of weathering can form a spheroid shaped rock.
Rate of Weathering:
Mechanical weathering will affect the rate of chemical weathering. The broken rock will have a greater surface area to volume ratio. Two other factors are rock characteristic and climate.
Rock Characteristic:
Physical characteristics: cracks, mineral composition (marble will undergo rapid chemical change due to acid rain.) Marble is composed of calcite (CaCO3) and easily dissolves when exposed to weak acid. Silicates weather in the same sequence as the order of crystallization: olivine crystals then quartz. Climate: Temperate and moisture because of the freeze/thaw cycle. Climates will also encourage rate vegetation.
Differential weathering: different parts of the rock will undergo weathering at different rates. Why? There are differences in mineral composition in different parts of the rock.
Soil:
Characteristics of soil: Soil is the bridge between life and nonlife. Soil is the interface between
different systems. Soil is the combination of minerals, organic matter, water and air. It will support plant growth. Weathering produces a layer of rock and mineral fragments called REGOLITH. Soil is part of the regolith that supports the growth of plants. There are 3 important characteristics
1) Composition: There are 4 components
a. 45% is Broken down rock (mineral matter) b. 5% humus: organic matter
c. 25% water d. 25% air.
The amount of organic matter in soil varies. It is an important source of plant nutrients and increases the ability for soil to retain water.
2) Soil texture: Most soils have 3 categories of particle size: clay, silt and sand. The texture strongly influences a soils ability to support plant life. Sand: largest particle size, it is usually nutrient poor, drains and dries out too quickly. Clay: smallest particle size, it is rich in nutrients (has a negative charge, so it holds positive ions), but doesn’t drain and the roots might have a problem growing in clay. Silt: in between sand and clay. Loam is made up of all three soil types and is the best.
3) Soil structure: determines how easily a soil can be cultivated and the susceptibility it is to erosion. Soil structure will also affect the ease roots can grow and the movement of nutrients.
Soil particles are seldom completely independent of one another. They usually form clumps, called PEDS that give soil its particular structure: Platy, prismatic, blocky and spheroidal. These all have to deal with how water moves through the soil. Platy and Sperhoidal are tough to move through. Prismatic and blocky are easy to move through (if you’re water).
1) Parent material: source of mineral matter in soil. Parent material can be either bedrock (residual soil) or unconsolidated deposits (transported soil). The soil can be moved from place to place by gravity, wind, water and ice. The parent material can affect the rate of soil
formation—transported soil is already partly weathered. This increases the surface area to volume ratio for weathering. Chemicals/minerals that make up the parent material can affect soil fertility.
2) Time: The longer the soil has been forming, the thicker the soil layer.
3) Climate: The greatest effect on soil formation. Variations in temperature and precipitation will affect the rate, depth, and type of weathering. Hot, wet climates will produce a thick layer of chemically weathered soil. Precipitation can influence how nutrients spread. The climate affects the type of organisms that live on and in the soil.
4) Organisms: the types and numbers of organisms will have a major impact on the physical and chemical properties of soil. Soils are classified by the vegetation (prairie soil, forest soil, tundra soil…). Plants and fungus are a major source of organic material for the soil. Animals and Plants die, the bacteria and fungus are decomposers and they return the nutrients back to the soil. Lichens can also play a role in soil formation by mechanically breaking down rock. Burrowing animals mix the soil and allow air and water to penetrate.
5) Slope: This will affect soil by affecting erosion and the water content. Steep slopes will increase erosion and decrease water content. The direction of the slope will also affect soil formation. In the northern hemisphere, south facing slopes get more sunlight—warmer and drier…more plants. Soil Profile: Soil varies in composition, texture, structure and color at different depths. These zones are known as soil horizons. A vertical section through soil horizons is called the soil profile. In some profiles, the horizons blend from one zone to another. In other profiles, the zones are very distinct.
1) A Horizon: Topsoil. The upper part consists mostly of organic matter (loose leaves, partly decomposed plants, roots…). Topsoil is full of insects, fungi and bacteria. Lower parts or A is a mixture of minerals and organic matter.
2) B Horizon: Subsoil. This layer contains fine clay particles that filter from the A horizon through pores. The clay can form a layer that is very hard called HARDPAN this is impenetrable. Horizon B is usually the lower limit for a burrowing animal and plant roots.
3) C Horizon: This layer is made up of partly weathered parent material. 4) Unweathered Parent material:
Soil Types: There are 3 types of soil: pedalfer, pedocal, and laterite.
1) Pedalfer: This soil is formed in temperate biomes (63 cm of rain/year) and found in much of the eastern ½ of the US. Most often this soil is found in forested areas. The B horizon contains a large amount of iron oxide and aluminum rich clays (brown to red-brown in color).
2) Peducal: This type of soil is found in the drier western US with grasses and brush vegetation. Chemical weathering is slower, so there is less clay. Instead, it contains an abundant amount of calcite and calcium carbonate which is light gray to brown in color.
3) Laterite: In tropical climates. Intense weathering causes the soils to be deeper. The soil is prone to constant leeching. The silica and oxides (aluminum and iron) will concentrate in the soil and sink. The soil isn’t very fertile, that’s why they shouldn’t be farmed.
Soil Erosion: Removal of the thin layer of soil. This is a big problem.
How Soil is Eroded: Erosion is a natural process. Once soil is formed, water, wind and ice will move it from one spot to another. When water lands, via raindrops, it strikes the soil like tiny bombs that blast movable soil particles. The water flowing across the surface will move the course soil (SHEET
Rates of Erosion: In the past, more land was covered with trees and vegetation. Human activities removed the vegetation and exposed the soil. Prior to human appearance, studies have shown that here was just over 9 billion of MT /year of soil has been eroded. After humans appeared, we are at 24 billion MT/year of soil eroded.
Sedimentation and Chemical Pollution: Another problem is sedimentation. All of this sediment has been depositing in reservoirs, lakes, and streams. In some cases, the soil has been contaminated with pesticides and chemical. This can threaten aquatic life.
Weathering Creates Ore Deposits: Weathering creates many important mineral deposits. The minerals are scattered and this is called secondary enrichment. Chemical weathering results in a downward precipitation of water and minerals. Then if there is a high concentration of minerals near the surface, it is carried to a lower zone.
Bauxite: formation of bauxite, the principle aluminum ore, is an example of an ore created as a result of enrichment by weathering. Formed in rainy tropical climates in association with LATERITES. When all of the other minerals are washed away, Aluminum is left behind and concentrates.
Other deposits: Cu and Ag can result when weathering concentrates the metals. This usually forms low grade primary ore. Chemically washed metals (with H2SO4) will gradually migrate downward and collect.
Mass Wasting: The work of gravity:
The Earth’s surface is not flat, it consists of many slopes (different types of slopes). Slopes are not static, gravity will work to move the material down the slope. The movement can be gradual or extreme. Rapid forms of mass wasting can be hazardous (landslides which kill 600 people/year worldwide).
Mass Wasting and Landform Development: landslides are spectacular examples of a basic geological process called MASS WASTING, the downslope movement of rock, regolith, and soil under the direct influence of gravity. Mass wasting can follow weathering. Once weathering weakens or breaks rocks, the process of wasting can occur.
Controls and Triggers of Mass Wasting: Gravity is the controlling force, others are factors. Water: water can enter the soil and act as a lubricant that allows for the movement of the soil.
Water can also initiate rock breaking.
Oversteeped Slopes: This brings up the ANGLE of REPOSE: the steepest angle at which the soil is stable. It usually varies between 25 and 40o.
Vegetation: Plants protect against erosion – contribute to the stabilizing of the slope. Earthquakes: Earthquakes and aftershocks can trigger movements in the soil.
Falls, Slides and Flows:
Fall: Movement of material that involves a free fall of any piece of any size.
Slides: Material moves along on a well defined surface, parallel to the slope (landslide). Flow: Materials move downslope as a viscous fluid.
Slump: downward sliding of a mass of rock or unconsolidated material moving as a unit along a curved surface. The material doesn’t travel fast or far. A crescent shaped SCARP (cliff ) is created.
Rockslide: frequent in high mountain areas. Rain/snow can lubricate the rocks.
Debris Flow: rapid type of mass wasting that involves the flow of soil and regolith containing large amounts of water…mud flow.
EarthFlow: debris flows that are frequently confined to channels in a semiarid region. The water saturates the soil and regolith on a hillside. The material may break away, leaving a scar on a slope and forming a tough or teardrop shaped mass that flows downslope. The materials are usually rich is clay and silt. Earth flows are very viscous and move slowly (1 mm to several m/day). LIQUIFICATION: a type of earthflow that is associated with earthquakes. When shaken the particles lose cohesion and the ground flows.
Slow Movements:
Creep: gradual downhill movement of soil and regolith. The primary cause is an alternation of expansion and contraction of the soil. This can be caused by freezing/thawing or wetting/drying. The freezing/wetting lifts the material (as the soil expands). Thawing and drying will cause the soil to contract and the material lowers. Each cycle will move the material downhill.
