EENS 3050 | Natural Disasters |
Tulane University | Prof. Stephen A. Nelson |
Earth Structure, Materials, Systems, and Cycles |
Objectives
Since this course is about how Earth processes can adversely affect us as human beings, we need to first discuss some of the basic principles of the science of Earth - that is geology. The objectives here will be to gain an understanding of :
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The Solar System
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Origin of the Solar System
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The Planet Earth |
Interior Structure of Earth
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All of the above is known from the way seismic waves (earthquake waves) pass through
the Earth. |
Before we can begin to understand the causes and effects of natural
hazards and 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. 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
Examples
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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: |
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Note that Carbon (one of the most abundant elements in life) is not among the top 12.
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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. |
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 This Course 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. |
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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 |
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Geologic Processes A variety of processes act on and within the Earth - here we consider those responsible for Natural Disasters
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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
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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).
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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
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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 release energy on time scales less than a few years, but build over timescale's much larger than human history. |
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. |
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:
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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 |
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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. |
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Why Does Plate Tectonics Occur?
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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:
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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:
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Residence time in each of the reservoirs is generally proportional to the size of the
reservoir
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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:
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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 |
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The Rock Cycle
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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.
Examples of questions on this material that could be asked on an exam
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