Earth System Science

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
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.
    • 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.
     

    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

     - 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:

    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

    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.

    • 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.

    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

    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 - 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
    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.

    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.

     

    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:

  • Solar energy causes evaporation of the surface waters and atmospheric circulation, and
    • gravitational energy causes the water to flow back to oceans. Residence time in each of the reservoirs is generally proportional to the size of the reservoir
  • with water residing in the oceans and glaciers for many thousands of years,
  • in groundwater for hundreds of years,
  • in surface waters for months,
  • in the atmosphere and biosphere for days.

    • 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:

  • Biosphere, where it is the major building block of life,
  • Lithosphere, where it is a component in carbonate minerals and rocks and fossil fuels such as coal and petroleum,
  • Oceans, where it occurs as a dissolved ion in seawater, and
  • Atmosphere, where it occurs as Carbon Dioxide (CO2) gas.

    • 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
  • The rock cycle involves cycling of elements between various types of rocks, and thus mostly involves the lithosphere.
  • But, because materials such as water and Carbon cycle through the lithosphere, the rock cycle overlaps with these other cycles.
  • The rock cycle involves the three types of rocks as reservoirs (1) igneous, (2) sedimentary, and (3) metamorphic.
  • Chemical elements can reside in each type of rock, and geologic processes move these elements into another type of rock.
  • The rock cycle can be divided into two main circuits, one through continental crust, and one through the mantle.
  • Energy for the parts of the crustal cycle near the Earth's surface is solar and gravitational energy (which control erosion and weathering), whereas

    • energy that drives processes beneath the surface is geothermal and gravitational energy (which control uplift, subsidence, melting, and metamorphism).


    RockCycle.GIF (23044 bytes)

    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.


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