Introduction to Geological Processes

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Introduction to Geological Processes - Earth Structure, Materials, Systems and Cycles

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University

Hazardous Processes
The Earth in the Solar System
The Planet Earth
Minerals
Formation of Minerals
Important Minerals
Rocks 
Igneous Rocks
Sedimentary Rocks 
Metamorphic Rocks
Geological Processes 
Energy
Forms of Energy
Sources of Energy
Heat Transfer
Time Scales
Plate Tectonics
Divergent Boundaries 
Convergent Boundaries
Transform Fault Boundaries
Why Does Plate Tectonics Occur?
Geologic Cycles
Hydrologic Cycle
Biochemical Cycles
Rock Cycle
Uniformitarianism and Catastrophism


Hazardous Processes


Geology is the study of the Earth and its history.
Geologic Processes effect every human on the Earth all of the time, but are most noticeable when they cause loss of life or property. Such life or property threatening processes are called natural disasters. Among them are:
All of these processes have existed throughout Earth history, but the processes have become hazardous only because they negatively affect us as human beings.  Important Point - There would be no natural disasters if it were not for humans.  Without humans these are only natural events.
Risk is characteristic of the relationship between humans and geologic processes.   We all take risks everyday. The risk from natural disasters, while it cannot be eliminated, can, in some cases be understood in a such a way that we can minimize the hazard to humans, and thus minimize the risk.  To do this, we need to understand something about the processes that operate, and understand the energy required for the process.  Then, we can develop an action to take to minimize the risk. Such minimization of risk is called hazard mitigation.
Although humans can sometimes influence natural disasters (for example when road construction sets off a landslide), other disasters that are directly generated by humans, such as oil and toxic material spills, pollution, massive automobile or train wrecks, airplane crashes, and human induced explosions, are considered technological disasters, and will not be considered in this course.

Some of the questions we hope to answer for each possible natural disaster are:

The Earth in the Solar System


The Solar System
The four bodies outside the orbit of Mars (Jupiter, Saturn, Uranus, Neptune, and the former planet  Pluto) have low densities because they mostly composed of gases, and are called the Jovian Planets.


solarsys.gif (17967 bytes)

Origin of the Solar System

The Planet Earth
The Planet Earth

Comparisons Between Earth and the Other Planets
Interior Structure of Earth
earthint.gif

All of the above is known from the way seismic waves (earthquake waves) pass through the Earth.

Minerals


Before we can begin to understand the causes and effects of natural disasters we need to have some understanding of the materials that make up the Earth, the processes that act on these materials, and the energy that controls the processes. We start with the basic building blocks of rocks - Minerals.
The Earth is composed of rocks. Rocks are aggregates of minerals. Minerals are composed of atoms. In order to understand rocks, we must first have an understanding of minerals.  We'll start with the definition of a Mineral. A Mineral is
Naturally formed  (it forms in nature on its own [some say without the aid of humans]) Solid ( it cannot be a liquid or a gas)
With a definite chemical composition (every time we see the same mineral it has the same chemical composition that  can be expressed by a chemical formula).
and a  characteristic crystalline structure (atoms are arranged within the mineral in a specific ordered manner).
Examples Glass - can be naturally formed (volcanic glass called obsidian), is a solid, its chemical composition, however, is not always the same, and it does not have a crystalline structure (individual atoms in a glass are arranged randomly similar to the arrangement in a liquid).  Thus, glass is not a mineral. Ice - Is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula H2O, and does have a definite crystalline structure when solid.  Thus, ice is a mineral. Liquid water is not since it is not solid and does not have a crystalline structure.
Halite (salt) - Is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula NaCl, and does have a definite crystalline structure, as shown below.  Thus halite is a mineral.

nacl.gif

Important Minerals in the Earth's Crust The variety of minerals we see depend on the chemical elements available to form them. In the Earth's crust the most abundant elements are as follows:
Note that Carbon (one of the most abundant elements in life) is not among the top 12.
 
sio4.gif

fig.1 Silicate tetrahedron

Because of the limited number of elements present in the Earth's crust there are only about 3000 minerals known. Only 20 to 30 of these minerals are common. The most common minerals are those based on Si and O:  the Silicates. Silicates are based on SiO4 tetrahedron. 4 Oxygens bonded to one silicon atom(fig1)

Formation of Minerals


Minerals are formed in nature by a variety of processes.  Among them are:

Since each process leads to different minerals, we can identify the process by which minerals form in nature. Each process has specific Temperature and pressure conditions that can be determined from laboratory experiments.

Important Minerals


For the purposes of this course, three minerals that are most important (others may be introduced as needed) are:
Quartz - Chemical Formula SiO2. - Quartz is one of the primary minerals that originally forms by crystallization from a melt in igneous rocks. Although quartz is formed at relatively high temperatures it is stable (does not break down or alter) at conditions present near the Earth's surface. Thus quartz is a primary constituent of sand, soil, and sedimentary rocks called sandstones.

sheetsilicates.gif fig 2.

Clay Minerals - Clay minerals are sheet silicates, thus they have a crystalline structure that allows them to break easily along parallel sheets. Clay minerals form by alteration of other minerals during the process of chemical weathering (alteration under conditions present near the Earth's surface). Thus clay minerals are primary constituents of soils and are also found in the sedimentary rock known as shale or mudstone.(fig.2)
  fig.2 Sheet Silicates
  Calcite - chemical formula CaCO3 (calcium carbonate). Calcite is easily dissolved in water under slightly acidic conditions. Thus calcite can be precipitated directly from water. Organisms can extract the Calcium and Carbonate ions from water to precipitate their shells. Thus calcite is a primary constituent of chemical and biochemical sedimentary rocks.
 

Rocks

Rocks are aggregates of minerals that are held together to form a consolidated mass.

The three general types of rocks are:


Igneous Rocks

Igneous Rocks - rocks that result from crystallization from a melt - called a magma.
If the crystallization takes place deep beneath the surface of the Earth they are called Plutonic rocks.

Examples include:
Granite - coarse textured rock consisting mostly of quartz, and feldspar with small amounts of biotite and/or hornblende.
Gabbro - a coarse textured rock consisting mostly of pyroxenes, and plagioclase.
If the crystallization takes place on the surface of the Earth they are called Volcanic rocks.

Examples include:
Rhyolite - a fine grained to glassy rock containing crystals of quartz, feldspar, and biotite - chemically the same as a granite.
Andesite - a fined grained rock containing crystals of pyroxene, plagioclase, and sometimes hornblende.
Basalt - a fine grained rock containing crystals of olivine, pyroxene, and plagioclase - chemically the same as a gabbro.

Sedimentary Rocks


Sedimentary Rocks - rocks that form near the surface of the Earth through chemical precipitation from water or by cementation of loose fragments (called sediment).
Clastic Sedimentary Rocks - result from the cementation of loose fragments of pre-existing rock. The cementation occurs as a result of new minerals precipitating in the space between grains. Clastic sedimentary rocks are classified on the basis of the size of the fragments that makes up the rock

Name of Particle  Size Range  Loose Sediment  Consolidated Rock 
Boulder  >256 mm  Gravel  Conglomerate (if clasts are rounded)
      or Breccia (if clasts are angular) 
Cobble  64 - 256 mm  Gravel 
Pebble  2 - 64 mm  Gravel 
Sand  1/16 - 2mm  Sand  Sandstone 
Silt  1/256 - 1/16 mm  Silt  Siltstone 
Clay  <1/256 mm  Clay  Claystone, mudstone, and shale

Chemical Sedimentary Rocks - result from direct chemical precipitation from surface waters. This usually occurs as a result of evaporation which concentrates ions dissolved in the water and results in the precipitation of minerals.
Biochemical Sedimentary Rocks - result from the chemical precipitation by living organisms. The most common biochemical sedimentary rock is limestone, which is composed of the shells of organisms, which are in turn composed mostly of the mineral calcite.

Metamorphic Rocks


Metamorphic Rocks - result when any kind of pre-existing rock is buried deep in the Earth and subjected to high temperatures and pressures. Most metamorphic rocks show a texture that shows an alignment of sheet silicate minerals, minerals like biotite and muscovite, that gives them a layered appearance and allows them to break easily along nearly planar surfaces. Some common metamorphic rocks that we might encounter in this course are:
 
Slate - a fine grained metamorphic rock consisting mostly of clay minerals that breaks easily along smooth planar surfaces.
Schist - a coarser grained metamorphic rock consisting of quartz and micas that breaks along irregular wavy surfaces.

Geologic Processes


A variety of processes act on and within the Earth - here we consider those responsible for Natural Disasters
 
  • Melting - responsible for creating magmas that result in volcanism.
  • Deformation - responsible for earthquakes, volcanism, landslides, subsidence.
  • Isostatic Adjustment due to buoyancy - responsible for earthquakes, landslides, subsidence.
  • Weathering - responsible for landslides, subsidence.
  • Erosion - responsible for landslides, subsidence, flooding.
  • Atmospheric Circulation - responsible for hurricanes, tornadoes, flooding.

    • Energy

    All processes that occur on or within the Earth require energy. Energy can exist in many different forms, and comes from a variety of sources. Natural disasters occur when there is a sudden release of the energy near the surface of the Earth.

    Forms of Energy

    Forms of Energy
    Energy may exist in many different forms, but can be converted between each of these forms

  • Gravitational Energy -- Energy released when an object falls from higher elevations to lower elevations. As the object falls the energy can be converted to kinetic energy (energy of motion) or heat energy.
  • Heat Energy -- Energy exhibited by moving atoms, the more heat energy an object has, the higher its temperature. Heat energy can be converted to kinetic energy, as it is when fuel is burned in an engine and sets the car in motion.
  • Chemical Energy -- Energy released by breaking or forming chemical bonds. This type of energy usually is converted to heat.
  • Radiant Energy -- Energy carried by electromagnetic waves (light).  Most of the Sun's energy reaches the Earth in this form, and is converted to heat energy.
  • Nuclear Energy -- Energy stored or released in binding of atoms together. Most of the energy generated within the Earth comes from this source, and most is converted to heat when it is released.
  • Elastic Energy (also called strain energy)- By deforming an elastic material (like rubber bands, wood, and rocks) energy can be stored in the material. When this energy is released it can be converted to kinetic energy and heat.
  • Electrical Energy -- Energy produced by moving electrons through matter. Most of this energy is generated by humans and is converted into heat energy to heat homes or water or is converted to kinetic energy to drive air conditioners, vacuum cleaners, can openers, etc.

    • Sources of Energy

    The Earth has two basic sources of energy - that reaching the Earth from the Sun (Solar Energy) and that reaching the surface of the Earth the Earth itself (Internal or Geothermal Energy).

    Solar Energy - reaches the Earth in the form of radiant energy, and makes up 99.987% of the energy received by the Earth.
    About 40% is immediately reflected back into space by the atmosphere and oceans.
    Some is converted to heat and is absorbed by the atmosphere, hydrosphere, and lithosphere, but even this eventually escapes into space.
    Some is absorbed by plants during photosynthesis and is stored in plants, used by other organisms, or is stored in fossil fuels like coal and petroleum.
    Solar Energy drives the water cycle, causing evaporation of the oceans and circulation of the atmosphere, which allows rain to fall on the land and run downhill. Thus solar energy is responsible for such natural disasters as severe weather, and floods.

    Internal Energy - is generated within or because of the Earth. It only amounts to about 0.013% of the total energy reaching the Earth's surface, but is responsible for deformational events that build mountains and cause earthquakes, for melting in the Earth to create magmas that result in volcanism.
    Two source of internal energy are:
    Radioactive Decay
    Conversion of Gravitational Energy

    Some of the heat flowing out of the Earth is heat that has been produced by gravitational compaction of the Earth which has caused matter to move closer to Earth's center.

    Heat Transfer

    Heat Transfer

    Since much of the energy that reaches the Earth's surface eventually is converted to heat, it is important to understand how heat can move through materials. Three basic modes of heat transfer are possible

    Time Scales


    As discussed before, the Solar System began to form about 6 billion years ago and the Earth and other planets about 4.5 billion years ago. Geologic processes have operated on the Earth ever since. Some of these processes, like mountain building events expend energy on time scales of several hundred million years, whereas others, like earthquakes, expend energy on time scales of a few seconds (although the storage of energy for such an event may take hundreds or thousands of years). If we examine the time scales of various geologic and other processes, we see that those processes that affect humans and that may be responsible for natural disasters occur on time scales less than a few years.

    timescales.gif

    Plate Tectonics

    Much of what occurs near the surface of the Earth is due to interactions of the lithosphere with the underlying asthenosphere. Most of these interactions are caused by plate tectonics. Plate Tectonics is a theory developed in the late 1960s, to explain how the outer layers of the Earth move and deform. The theory has caused a revolution in the way we think about the Earth. Since the development of the plate tectonics theory, geologists have had to reexamine almost every aspect of Geology. Plate tectonics has proven to be so useful that it can predict geologic events and explain almost all aspects of what we see on the Earth.
    The theory states that the Earth's lithosphere is divided into plates (about 100 km thick) that move around on top of the asthenosphere. Continental crust is embedded within the lithospheric plates. The Plates move in different directions, and meet each other at plate boundaries.

    The plates and their boundaries are shown below:

    tectonicplates.gif

    Plate boundaries are important because plates interact at the boundaries and these are zones where deformation of the Earth's lithosphere is taking place. Thus, plate boundaries are important areas in understanding geologic hazards. Three types of plate boundaries occur:

    Divergent Plate Boundaries

    divergingpltbndy.gif

    Divergent Plate Boundaries - These are boundaries where plates move away from each other, and where new oceanic crust and lithosphere are created. Magmas rising from the underlying asthenosphere intrude and erupt beneath and at an oceanic ridge to create new seafloor. This pushes the plates on either side away from each other in opposite directions.
    The margin itself becomes uplifted to form oceanic ridges, which are also called spreading centers, because oceanic lithosphere spreads away on each side of the boundary. While most diverging plate boundaries occur at the oceanic ridges, sometimes continents are split apart along zones called rift zones, where new oceanic lithosphere may eventually form. Volcanism and earthquakes are common along diverging plate boundaries



    Convergent Plate Boundaries

    oceanocean.gif

    Convergent Plate Boundaries - These are boundaries where two plates move toward each other. Atsuch boundaries one of the plates must sink below the other in a process called subduction. Two types of convergent boundaries are known.
    Subduction Boundaries - These occur where either oceanic lithosphere subducts beneath oceanic lithosphere (ocean-ocean convergence), or where oceanic lithosphere subducts beneath continental lithosphere (ocean-continent convergence). Where the two plates meet, an oceanic trench is formed on the seafloor, and this trench marks the plate boundary.
    When two plates of oceanic lithosphere run into one another the subducting plate is pushed to depths where it causes melting to occur. These melts (magmas) rise to the surface to produce chains of islands known as island arcs. A good example of an island arc is the Caribbean islands.
    When an plate made of oceanic lithosphere runs into a plate with continental lithosphere, the plate with oceanic lithosphere subducts because it has a higher density than continental lithosphere.

    oceancont.gif
     
    Again, the subducted lithosphere is pushed to depths where magmas are generated, and these magmas rise to the surface to produce, in this case, a volcanic arc, on the continental margin. Good examples of this type of volcanic arc are the Cascade mountains of the northwestern U.S. and the Andes mountains of South America.  squeezing together and uplifting the continental crust on both plates. The Himalayan mountains between India and China where formed in this way, as were the Appalachian Mountains about 300 million years ago
    All convergent boundaries are zones of frequent and powerful earthquakes.

    Transform Fault Boundaries


    westustect.jpg

    Transform Fault Boundaries- When two plates slide past one another, the type of boundary occurs along a transform fault. These are also zones of frequent and powerful earthquakes, but generally not zones of volcanism. The famous San Andreas Fault of California is an example of a transform fault, forming one part of the boundary between the Pacific Plate and the North American Plate.

    Why Does Plate Tectonics Occur?


    Thus, if the asthenosphere moves by convection, with rising currents carrying heat toward the surface at the oceanic ridges, and, descending currents sinking at subduction zones after loosing heat to the surface, then the brittle plates riding on top of the convection cell will be forced to move over the surface, being in a sense, dragged along by the moving asthenosphere.

    mantleconvect.gif

    Geologic Cycles


    Although we have discussed various parts of the Earth as separate entities, in reality each of the entities, atmosphere, hydrosphere, lithosphere, etc, interact with each other continuously exchanging both matter and energy. This exchange of matter and energy occurs on a cyclical basis, with both matter and energy cycling between various storage reservoirs on various time scales. Because matter and energy is thus cycled, the various geologic cycles play a large role in the development of natural disasters. We here look at a few of these geologic cycles.

    Hydrologic Cycle


    Perhaps the easiest of the cycles to understand is the hydrologic cycle that involves the movement of water throughout Earth systems. Water moves between 7 main reservoirs:
    The ocean is by far the largest of these reservoirs with 97% of all water.

    Reservoir % Input Output Residence Time
    Oceans  97.5 Precipitation from Atmosphere
    Melting of Glaciers
    Flowage from Streams
    Flowage from Groundwater     		Evaporation into Atmosphere
    Subduction into Lithosphere     		Thousands of years
    Atmosphere  <0.01 Evaporation from Oceans Evaporation from Surface waters
    Transpiration from Biosphere
    Volcanism from Lithosphere     		Precipitation as snow and rain on land and in Oceans
    Uptake by Biosphere     		A few days
    Glaciers  1.85 Precipitation from Atmosphere Melting into Surface Waters  Melting into Oceans
    Evaporation into Atmosphere
    		 Thousands of years
    Surface Lakes & Streams  <0.01 Precipitation from Atmosphere
    Melting from Glaciers
    Flowage from Groundwater     		Seepage into Groundwater
    Flowage into Oceans
    Evaporation into Atmosphere     		A few weeks
    Groundwater  0.64 Seepage from Surface Lakes & Streams
    Seepage from Oceans
    Precipitation from Atmosphere     		Flowage into Surface Lakes & Streams
    Flowage into Oceans
    Uptake by Biosphere     		Hundreds of years
    Biosphere  <0.01 Uptake from Surface Waters, Atmosphere, Oceans, and Groundwater Transpiration into Atmosphere
    Burial into Lithosphere     		A few days
    Lithosphere ? From Groundwater into Hydrous Minerals
    From Biosphere by burial into Sediments
    From Oceans by Subduction     		Weathering into Groundwater & Oceans
    Volcanism into Atmosphere     		Millions of years

    The main pathway by which water moves is through the atmosphere. Two main sources of energy drive the cycle:

    Water may reside in the lithosphere for millions of years.

    Biogeochemical Cycles

    Although the hydrologic cycle involves the biosphere, only a small amount of the total water in the system at any given time is in the biosphere. Other materials, for example Carbon and Nitrogen have a much higher proportion of the total residing in the biosphere at any given time. Cycles that involve the interactions between other reservoirs and the biosphere are often considered differently because they involve biological processes like respiration, photosynthesis, and decomposition (decay). These are referred to as biogeochemical cycles.
    A good example is the Carbon Cycle, as it involves the cycling of Carbon between 4 major reservoirs:

    Reservoir Input Output
    Biosphere From Lithosphere by plant uptake
    From Oceans by chemical precipitation 
    From Atmosphere by photosynthesis     		Into Lithosphere by burial
    Into Oceans by decay
    Into Atmosphere by decay, respiration, & burning
    Lithosphere From Biosphere by Burial
    From Oceans by chemical precipitation
    From Atmosphere by precipitation & groundwater flow     		Into Biosphere by uptake of organisms
    Into Oceans by dissolution (weathering)
    Oceans From Atmosphere by precipitation
    From Lithosphere by dissolution
    From Biosphere by decay & respiration     		Into Biosphere by uptake of organisms
    Into Lithosphere by chemical precipitation
    Into Atmosphere by evaporation
    Atmosphere From Biosphere by respiration, burning, & decay
    From lithosphere by seepage of and burning fossil fuels and volcanism
    From the Oceans by evaporation      		Into Biosphere by photosynthesis Into Oceans by precipitation

    In all reservoirs except the lithosphere, residence time is generally short, on the order of a few years. Human burning of fossil fuels adds Carbon back to the atmosphere at a higher rate than normal, and thus the concern for greenhouse gas warming induced by humans.

    The Rock Cycle


    RockCycle.GIF

    Uniformitarianism and Catastrophism


    Prior to about 1850 most humans thought of the Earth as being a relatively young feature and that processes and landforms that occur on the Earth were the result of catastrophic events (like creation and the flood) that occurred very rapidly. But, careful observation of Earth process led some, like James Hutton and Charles Lyell) to hypothesize that processes that one could observe taking place at the present time had operated throughout the history of the planet. This led to the development of the concept of uniformitarianism, often stated as "the present is the key to the past". A more modern way of stating this principle is that since the laws of nature have operated the same way throughout time, and all Earth processes must obey the laws of nature (i.e the laws of physics and chemistry. Initially one of the most difficult problems in applying this principle to the Earth, was that an assumption was made that the rates of all geologic processes had been the same throughout time. We know that the Earth is very old (4.5 billion years) and that it was hotter near its birth than it is now. Thus, it is likely that the rates of some geologic processes has changed throughout time. We also now recognize that there can in fact be catastrophic events that occur infrequently that can cause very rapid changes in the Earth. Because these catastrophic events occur infrequently, it is difficult to observe their effects, but if we can recognize them, we still can see that even these infrequent catastrophic events follow the laws of nature.