Chapter 6 - Weathering and Soils

• 4. Biological Activity

  • Burrowing animals: break down by moving fresh material to surface where weathering process can attack it.
  • Decaying organisms: produce acid that contribute to chemical weathering.
• Weathering - physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth's surface.

  2 categories:

  • Mechanical - physical forces that break rock into smaller pieces without a change in mineral composition.
  • Chemical - chemical trasformation of rock into one or more compounds.

  Rock weather as a response to changing environments.
• Mechanical Weathering

  1. Frost Wedging - when water works its way into the crack of the rocks, the freezing water enlarges the cracks and angular fragments break off.

  • Frost Heave - term used when moist soils freeze and expand.
• Mechanical Weathering

  
  

  2. Salt Crystal Growth - common settings are rocky shorelines and arid regions.

  • happens when sea spray from waves or salty groundwater penetrates crevices and pore spaces in rocks. As this water evaporates, salt crystals form.
  • when these crystals grow larger, the rock weakens by pushing apart the surrounding grains on enlarging tiny cracks.
  • e.g. crumbling of road ways when salt is spread to melt ice and snow in water > salt dissolves in water > seeps into cracks > water evaporates > salt crystals grow and breaks pavement.
• Mechanical Weathering

  
  

  3. Sheeting - happens when igneous rocks are exposed by erosion and conentric slabs begin to break loose, generating onion-like layers.

  • takes place due to reduction in pressure that occurs as overlying rock is eroded away or a process called unloading.
  • Unloading > sheeting > exfoliation dome
• Mechanical Weathering

  • Exfoliation dome - created due to continued weathering that causes the slabs to break loose.
  • Joints - fractures produced by unloading, contraction during crystallization of magma and tectonic forces generally form definite patterns. These are structures that allow water to penetrate to depth and begin weathering before rock is exposed.
• Mechanical Weathering

  4. Biological Activity

  • Burrowing animals: break down by moving fresh material to surface where weathering can attack it.
  • Decaying organisms: produce acid that contribute to chemical weathering.
• Chemical Weathering

  1. Water - pure water is nonreactive but a small amount of dissolved materials can activate it.

  • Oxidation - occurs when electrons are lost from 1 element during the reaction.
  • Iron was oxidized because it lost 1 element to oxygen.
  • Oxidation of iron progresses slowly in a dry environment but with water, reaction is with great speed.
  • Important in decomposing ferromagnesian minerals.
  • e.g. oxygen combines with iron = hematite (Fe2O3), limonite [FeO(OH)]
• Chemical Weathering

  • Carbonic Acid - occurs when CO2 is dissolved in water.
  • Rain dissolves CO2 as it falls thru atmosphere.
  • Ionizes to form the reactive hydrogen ion and bicarbonate ion.
  • Readily decompose rocks and produce products that are water soluble.
  • e.g. Calcite - attacked by a weakly acidic solution.
• • When chemically weathered, the common silicate minerals yield sodium, calcium, potassium, and magnesium ions.
  • These form soluble products which may be removed by groundwater.
  • Iron combines with oxygen > producing insoluble oxides.
  • Aluminum, silicon, oxygen join with water > residual clay minerals.
• Differential Weathering

  • masses of rock do not weather uniformly
  • responsible for many unusual rock formations and landforms.
  • e.g. Shiprock, Mexico - volcanic neck protrudes high above the terrain.
• Soil

  • bridge between life and inanimate world.
  • an interface or common boundary when different parts of a system interact.
  • Earth's land surface is covered by regolith.
  • > layer of rock and mineral fragments produced by weathering.
  • > not soil.
  • Soil - combination of mineral, organic matter, water and air.
  • Soil is a portion of regolith that supports the growth of plants.
  • About 1/2 total volume of soil is a mixture of disintegrated and decomposed rock.
  • Humus - decayed remains of plant and animal (organic matter)
• Controls of Soil Formation

  1. Parent Material - source of weathered mineral matter from which soils develop

  • Major factor influencing newly forming soil.
  • Can either be underlying bedrock or a layer of unconsolidated deposits.
  • Parent material (bedrock): soils termed as residual soils
  • Parent material (unconsolidated sediments): soil termed as transported soils
  • Transported soils - form in place on parent materials that have been carried from elsewhere and deposited by gravity, water, wind or ice.
• Controls of Soil Formation

  1. Parent Material influences soil in two ways:

  • A. type of parent material influences the rate of weathering and soil formation.
  • since unsconsolidated deposits are partly weathered, soil development on this material will rapidly progress than the bedrock as the parent material.
  • B. chemical makeup of parent material will affect soil's fertility
  • this influences the character of natural vegetation the soil can support.
• Controls of Soil Formation

  2. Climate

  • most influential
  • temperature and precipitation
  • hot, wet climate > produce thick layer of chemically weathered soil
  • cold, dry climate > produce a thin mantle of mechanically weathered debris.
  • amount of precipitation influences the degree to which various materials are removed from the soil by percolating water (or leaching).
• Controls of Soil Formation

  3. Plants and Animals

  • types and abundance of organisms influenced the physical and chemical properties of a soil.
  • for well-developed soils > significance of natural vegetation is implied in the names used by soil scientists (prairie soil, forest soil, and tundra soil)
  • certain bog soils are composed almost entirely of organic matter while desert soils contain as little as 1%
  • plants: primary source of organic matter
  • decomposed organic matter supplies important nutrients to plants
  • microorganisms play an active role in the decay of plant and animal remains.
• Controls of Soil Formation

  4. Time

  • nature of soil is strongly influenced by length of time processes have been operating.
  • the longer a soil has been forming, the thicker it becomes and the less it resembles the parent material.
• Controls of Soil Formation

  3. Plants and Animals

  • the end product of it is humus.
• Controls of Soil Formation

  5. Topography

  > on steep slopes, soils are often poorly developed.

  • due to rapid runoff, the quantity of water soaking in is slight; thus moisture content of soil may not be sufficient for vigorous plant growth.
  • due to accelerated erosion, soils are thin or nonexistent.
• Controls of Soil Formation

  5. Topography

  > poorly drained and waterlogged soils in bottomlands have different character.

  • soils are thick and dark
  • dark color results from the large quantity of organic matter that accumulated because saturated condition retard the decay of vegetation.
  • optimum terrain for soil development is a flat to undulating upland surface. (has good drainage, minimum erosion, and sufficient infiltration of water into soil.)
• Controls of Soil Formation

  5. Topography

  > slope orientation - direction a slope is facing

  • south facing slopes receives more sunlight than a north-facing slope.
  • this causes differences in soil temperature and moisture content.
• Classifying Soils

  1. Soil Profile

  • soil forming processes operate from surface downward.
  • soil composition, texture, structure, and color evolve at varying depths.
  • these vertical differences divide the soil into zones called horizons.
  • soil profile: a vertical section thru all of this soil horizon.
  • O, A, E, B and C - common to soils in temperate regions.
• Classifying Soils

  1. Soil Profile

  • O soil horizon
  • consist of organic material
  • upper portion is primarily plant litter (loose leaves & organic debris)
  • lower portion is made up of partly decomposed matter (humus) in which plant structures can no longer be identified
  • filled with microscopic life
• Classifying Soils

  1. Soil Profile

  • A horizon
  • largely mineral matter, yet biological activity is high and humus is present (up to 30%)
  • O and A horizons make up the "topsoil"

  
  

  • E horizon
  • light colored layer that contains little organic material.
  • as water percolates downward here, finer particles are carried away. this is called eluviation.
  • the water also dissolves soluble inorganic soil components and carries them to deeper zones.
  • this depletion of soluble materials from upper soil is called leeching.
• Classifying Soils

  1. Soil Profile

  • B-horizon
  • "subsoil"
  • where material removed from E-horizon by eluviation is deposited.
  • thus, b-horizon is often referred as the zone of accumulation.
  • accumulation of fine clay particles enhances water retention in this zone.
  • hardpan: clay accumulation can form this compact and impermeable layer.
  • O, A, E, B horizons together constitute the solum (tree soil).
  • In the solum, soil forming processes are active and living roots, plant and animal life are confined.
• Classifying Soils

  1. Soil Profile

  • C-horizon
  • characterized by partially altered parent material
  • parent material is easily identifiable here.
  • it has not yet crossed the threshold that separates regolith from soil.

  
  

  • Mature soil - a well developed soil that have been stable over an extended time span.
  • Immature soil - soil building have been going for only a short time. steep slopes where erosion strips sway the soil, preventing full development.