Physical Geology

Prof. Stephen A. Nelson

Earth is covered by a thin “veneer” of sediment. The veneer caps igneous and metamorphic “basement.” This sediment cover varies in thickness from 0 to 20 km. It is hinner (or missing) where igneous and metamorphic rocks outcrolp, and is thicker in sedimentary basins.

In order to make this sediment and sedimentary rock, several steps are required:

• Weathering – Breaks pre-existing rock into small fragments or new minerals
• Transportation of the sediments to a sedimentary basin.
• Deposition of the sediment
• Burial and Lithification to make sedimentary rock.

Each Step in the process of forming sediment and sedimentary rocks leaves clues in the sediment.  These clues can be interpreted to determine the history of the sediment and thus the history of the Earth.

Weathering

Geologists recognize two categories of weathering processes 

1. Physical Weathering  - disintegration of rocks and minerals by a physical or mechanical process.
2. Chemical Weathering - chemical alteration or decomposition of rocks and minerals.

Although we separate these processes, as we will see, both work together to break down rocks and minerals to smaller fragments or to minerals more stable near the Earth's surface.Both types are a response to the low pressure, low temperature, and water and oxygen rich nature of the earth’s surface.

Physical Weathering

The mechanical breakup or disintegration of rock doesn’t change mineral makeup. It creates broken fragments or “detritus.” which are classified by size:

• Coarse-grained – Boulders, Cobbles, and Pebbles.
• Medium-grained – Sand
• Fine-grained – Silt and clay (mud).

Physical weathering takes place by a variety of processes.  Among them are: 

• Development of Joints - Joints are regularly spaced fractures or cracks  in rocks that show no offset across the fracture (fractures that show an offset are called faults).
  
  
• Joints form as a result of expansion due to cooling or relief of pressure as overlying rocks are removed by erosion.
  
  
• Igneous plutons crack in onionlike “exfoliation” layers.  These layers break off as sheets that slide off of a pluton. Over time, this process creates domed remnants. (See figure 7.4 in your text) Examples: Half-Dome (Ca.) (see figure 22.21a in your text) and Stone Mountain (Ga.).
  
  
• Joints form free space in rock by which other agents of chemical or physical weathering can enter.
  
  
• Crystal Growth - As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals.  As these crystals grow they may exert an outward force that can expand or weaken rocks.
  
  
• Thermal Expansion - Although daily heating and cooling of rocks  do not seem to have an effect, sudden exposure to high temperature, such as in a forest or grass fire may cause expansion and eventual breakage of rock.  Campfire example.
  
  
• Root Wedging - Plant roots can extend into fractures and grow, causing expansion of the fracture.  Growth of plants can break rock -  look at the sidewalks of  New Orleans for example.
  
  
• Animal Activity - Animals burrowing or moving through cracks can break rock.
  

• Frost Wedging - Upon freezing, there is an  increase in the volume of the water (that's why we use antifreeze in auto engines or why the pipes break in New Orleans during the rare freeze).  As the water freezes it expands and exerts a force on its surroundings.  Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles.

Chemical Weathering

Since many rocks and minerals are formed under conditions present deep within the Earth, when they arrive near the surface as a result of uplift and erosion, they encounter conditions very different from those under which they originally formed.  Among the conditions present near the Earth's surface that are different from those deep within the Earth are:
 

• Lower Temperature (Near the surface T = 0-50^oC)
  
  
• Lower Pressure (Near the surface P = 1 to several hundred atmospheres)
  
  
• Higher free water (there is a lot of liquid water near the surface, compared with deep in the Earth)
  
  
• Higher free oxygen (although O₂ is the most abundant element in the crust, most of it is tied up bonded into silicate and oxide minerals -  at the surface there is much more free oxygen, particularly in the atmosphere).
  

Because of these differing conditions, minerals in rocks react with their new environment to produce new minerals that are stable under conditions near the surface. Minerals that are stable under P, T, H₂O, and O₂ conditions near the surface are, in order of most stable to least stable:

•  Iron oxides, Aluminum oxides - such as hematite Fe₂O₃, and gibbsite Al(OH)₃.
• Quartz*
• Clay Minerals
• Muscovite*
• Alkali Feldspar*
• Biotite*
• Amphiboles*
• Pyroxenes*
• Ca-rich plagioclase*
• Olivine*
  

Note the minerals with *.  These are igneous minerals that crystallize from a liquid.  Note the minerals that occur low on this list are the minerals that crystallize at high temperature from magma.  The higher the temperature of crystallization, the less stable are these minerals at the low temperature found near the Earth's surface (see Bowen's reaction series in the igneous rocks chapter).

• An acid is solution that has abundant free H⁺ ions.
  
  
• The most common weak acid that occurs in surface waters is carbonic acid.
  
  
• Carbonic acid is produced in rainwater by reaction of the water with carbon dioxide (CO₂) gas in the atmosphere.
  

   

  H⁺ is a small ion and can easily enter crystal structures, releasing other ions into the water.  

Types of Chemical Weathering Reactions

• Hydrolysis - H⁺ or OH^- replaces an ion in the mineral.  Example:

• Leaching - ions are removed by dissolution into water.  In the example above we say that the K⁺ ion was leached.
  
  
• Oxidation - Since free oxygen (O₂) is more common near the Earth's surface, it may react with minerals to change the oxidation state of an ion.  This is more common in Fe (iron) bearing minerals, since Fe can have several oxidation states, Fe, Fe⁺², Fe⁺³.  Deep in the Earth the most common oxidation state of Fe is Fe⁺².


   
 

• Dehydration - removal of H₂O or OH^- ion from a mineral.


 
 

• Complete Dissolution - all of the mineral is completely dissolved by the water.

 


• Living Organisms - Organisms like plants, fungi, lichen, and bacteria can secrete organiic acids that can cause dissolution of minerals to extract nutrients.  The role of microorganisms like bacteria has only recent been discovered.

Interaction of Physical and Chemical Weathering
 
Since chemical weathering occurs on the surface of minerals, the water and acids that control chemical weathering require access to the surface.  Physical weathering breaks the rock to provide that surface.   Fracturing the rocks, as occurs during jointing, increases the surface area that can be exposed to weathering and also provides pathways for water to enter the rock.  (See figure 7.8 in your text).  As chemical weathering proceeds, new softer minerals, like oxides or clay minerals, will create zones of weakness in rock that will allow for further physical weathering.  Dissoluiton of minerals will remove material that holds the rock together, thus making it weaker. 

When rock weathers, it usually does so by working inward from a surface that is exposed to the weathering process. If joints and fractures in rock beneath the surface form a 3-dimensional network, the rock will be broken into cube like pieces separated by the fractures.  Water can penetrate more easily along these fractures, and each of the cube-like pieces will begin to weather inward. The rate of weathering will be greatest along the corners of each cube, followed by the edges, and finally the faces of the cubes.  As a result the cube will weather into a  spherical shape, with unweathered rock in the center and weathered rock toward the outside.  Such progression of weathering is referred to as spheroidal weathering (See figures 7.10a and 7.10b in your text).

Factors that Influence Weathering

• Rock Type & Structure
  
• Different rocks are composed of different minerals, and each mineral has a different susceptibility to weathering.  For example a granite consisting mostly of quartz is already composed of a mineral that is very stable on the Earth's surface, and will not weather much in comparison to limestone, composed entirely of calcite, which will eventually dissolve completely in a wet climate.
  
  
• Bedding planes, joints, and fractures, all provide pathways for the entry of water.  A rock with lots of these features will weather more rapidly than a massive rock containing no bedding planes, joints, or fractures.

• Slope - On steep slopes weathering products may be quickly washed away by rains. On gentle slopes the weathering products accumulate.  On gentle slopes water may stay in contact with rock for longer periods of time, and thus result in higher weathering rates.
  
  
• Climate- High amounts of water and higher temperatures generally cause chemical reactions to run faster.  Thus warm humid climates generally have more highly weathered rock, and rates of weathering are higher than in cold dry climates.  Example:  limestones in a dry desert climate are very resistant to weathering, but limestones in a tropical climate weather very rapidly.
  
  
• Animals- burrowing organisms like rodents, earthworms, & ants, bring material to the surface were it can be exposed to the agents of weathering.
  

“Soil consists of rock and sediment that has been modified by physical and chemical interaction with organic material and rainwater, over time, to produce a substrate that can support the growth of plants.” Soils are an important natural resource.  They represent the  interface between the lithosphere and the biosphere - as soils provide nutrients for plants.  Soils consist of weathered rock plus organic material that comes from decaying plants and animals.  The same factors that control weathering control soil formation with the exception, that soils also requires the input of organic material as some form of Carbon.

Although you will not be expected to know all of the soil terminology discussed on pages 162 through 164 in your text, the following terms are important.  

•  Caliche - Calcium Carbonate (Calcite) that forms in arid soils in the K-horizon by chemical precipitation of calcite. The Ca and Carbonate ions are dissolved from the upper soil horizons and precipitated at the K-horizon.  In arid climates the amount of water passing through the soil horizons is not enough to completely dissolve this caliche, and as result the thickness of the layer may increase with time.
  
  
• Laterites - In humid tropical climates intense weathering involving leaching occurs, leaving behind a soil rich in Fe and Al oxides, and giving the soil a deep red color.  This extremely leached soil is called a laterite.
  
  
• Paleosols - If a soil is buried rapidly, for example by a volcanic eruption, the soil may be preserved in the geologic record as an ancient soil called a paleosol.

Soil Erosion
     In most climates it takes between 80 and 400 years to form about one centimeter of topsoil (an organic and nutrient rich soil suitable for agriculture).  Thus soil that is eroded by poor farming practices is essentially lost and cannot be replaced in a reasonable amount of time.  This could become a critical factor in controlling world population.

Sedimentary Rocks

Rivers, oceans, winds, and rain runoff all have the ability to carry the particles washed off of eroding rocks. Such material, called detritus, consists of fragments of rocks and minerals. When the energy of the transporting current is not strong enough to carry these particles, the particles drop out in the process of sedimentation. This type of sedimentary deposition is referred to as clastic sedimentation. Another type of sedimentary deposition occurs when material is dissolved in water, and chemically precipitates from the water. This type of sedimentation is referred to as chemical sedimentation. A third process can occur, wherein living organisms extract  ions dissolved in water to make such things as shells and bones.  This type of sedimentation is called biogenic sedimentation.  Thus, there are three major types of sedimentary rocks: Clastic Sedimentary Rocks, Chemical Sedimentary Rocks, and Biogenic Sedimentary Rocks.

Clastic Sediments and Sedimentary Rocks

Classification - Clastic sedimentary particles are classified in terms of size

Name of Particle 

Size Range 

Loose Sediment 

Consolidated Rock 

The formation of a clastic sedimentary rock involves three processes:  

1. Transportation -  Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers, or ocean currents. The distance the sediment is transported and the energy of the transporting medium all leave clues in the final sediment that tell us something about the mode of transportation.

2. Deposition -  Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium.
   
   
3. Diagenesis - Diagenesis is the process that turns sediment into rock. The first stage of the process is compaction. Compaction occurs as the weight of the overlying material increases. Compaction forces the grains closer together, reducing pore space and eliminating some of the contained water. Some of this water may carry mineral components in solution, and these constituents may later precipitate as new minerals in the pore spaces. This causes cementation, which will then start to bind the individual particles together. Further compaction and burial may cause recrystallization of the minerals to make the rock even harder.  
   
   Other conditions present during diagenesis, such as the presence of absence of free oxygen may cause other alterations to the original sediment. In an environment where there is excess oxygen (Oxidizing Environment) organic remains will be converted to carbon dioxide and water. Iron will change from Fe²⁺ to Fe³⁺, and will change the color of the sediment to a deep red (rust) color. In an environment where there is a depletion of oxygen (Reducing Environment), organic material may be transformed to solid carbon in the form of coal, or may be converted to hydrocarbons, the source of petroleum.

 
Textures of Clastic Sedimentary Rocks

When sediment is transported and deposited, it leaves clues to the mode of transport and deposition. For example, if the mode of transport is by sliding down a slope, the deposits that result are generally chaotic in nature, and show a wide variety of particle sizes. Grain size and the interrelationship between grains gives the resulting sediment texture. Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and deposition.

If the particles have the same density, then the heavier particles will also be larger, so the sorting will take place on the basis of size.  We can classify this size sorting on a relative basis -  well sorted to poorly sorted. Sorting gives clues to the energy conditions of the transporting medium from which the sediment was deposited.

• Beach deposits and wind blown deposits generally show good sorting because the energy of the transporting medium is usually constant.
  
  
• Stream  deposits are usually poorly sorted because the energy (velocity) in a stream varies with position in the stream.

Chemical Sediments and Sedimentary Rocks

Cherts - chemically precipitated SiO₂

Evaporites - formed by evaporation of sea water or lake water.  Produces halite (salt) and gypsum deposits by chemical precipitation as concentration of solids increases due to water loss by evaporation.

Biogenic Sediments and Sedimentary Rocks

Limestone - calcite (CaCO₃) is precipitated by organisms usually to form a shell or other skeletal structure.  Accumulation of these skeletal remains results in a limestone.

Diatomite - Siliceous ooze consisting of the remains of radiolarian or diatoms can form a light colored soft rock called diatomite.

Coal - accumulation of dead plant matter in large abundance in a reducing environment (lack of oxygen).

Oil Shale - actually a clastic sedimentary rock that contains a high abundance of organic material that is converted to petroleum during diagenesis.

Features of Sedimentary Rocks That Give Clues to the Environment of Deposition

Stratification and Bedding

• Rhythmic Layering - Alternating parallel layers having different properties.  Sometimes caused by seasonal changes in deposition (Varves). i.e. lake deposits wherein coarse sediment is deposited in summer months and fine sediment is deposited in the winter when the surface of the lake is frozen.

• Cross Bedding - Sets of beds that are inclined relative to one another.  The beds are inclined in the direction that the wind or water was moving at the time of deposition.  Boundaries between sets of cross beds usually represent an erosional surface. Very common in beach deposits, sand dunes, and river deposited sediment.  

• Graded Bedding  - As current velocity decreases, first the larger or more dense particles are deposited followed by smaller particles.  This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed.

• Non-sorted Sediment - Sediment showing a mixture of grain sizes results from such things as rockfalls, debris flows, mudflows, and deposition from melting ice.

• Ripple Marks  - Characteristic of shallow water deposition.  Caused by waves or winds.

• Mudcracks - result from the drying out of wet sediment at the surface of the Earth.  The cracks form due to shrinkage of the sediment as it dries.
  
   

• Raindrop Marks - pits (or tiny craters) created by falling rain. If present, this suggests that the sediment was exposed to the surface of the Earth.
  
  
• Fossils - Remains of once living organisms.  Probably the most important indicator of the environment of deposition.
  
  • Different species usually inhabit specific environments.
    
    
  • Because life has evolved - fossils give clues to relative age of the sediment.
    
    
  • Can also be important indicators of past climates.
    

Color

• Iron oxides and sulfides along with buried organic matter give rocks a dark color.  Indicates deposition in a reducing environment.
  
  
• Deposition in oxidizing environment produces red colored iron oxides.
  

Sedimentary Facies

A sedimentary facies is a group of characteristics which reflect a sedimentary environment different from those elsewhere in the same deposit. Thus, facies may change vertically through a sequence as a result of changing environments through time. Also, facies may change laterally through a deposit as a result of changing environments with distance at the same time.

Common Sedimentary Environments

• Non-marine environments
  
  
• Mountain streams
  
  
• Alluvial fans
  
  
• Lakes
  
  
• Glacial (ice deposited)
  
  
• Sand Dunes
  
  
• Rivers
  
  
• Marine environments
  
  
• Estuarine sediments
  
  
• Deltaic sediments
  
  
• Beach sediments
  
  
• Carbonate shelf sediments
  
  
• Marine evaporite sediments
  
  
• Deep Sea Sediments

Sedimntary Basins

Diagenisis

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