earth structure
DESCRIPTION
Earth Structure. crust. obvious from space that Earth has two fundamentally different physiographic features: oceans (71%) and continents (29%). from: http://www.personal.umich.edu/~vdpluijm/gs205.html. global topography. Earth’s Plates. MORB Genesis. Submarine Pillow Basalt Formation. - PowerPoint PPT PresentationTRANSCRIPT
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Earth Structure
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obvious from space that Earth has two fundamentally differentphysiographic features: oceans (71%) and continents (29%)
global topography
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
crust
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Earth’s Plates
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MORB Genesis
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Submarine Pillow Basalt Formation
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Volumes of Igneous Rocks on Earth
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Convergent Margin Magma Genesis
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Forms of Energy
• Energy: commonly defined as the capacity to do work (i.e. by system on its surroundings); comes in many forms
• Work: defined as the product of a force (F) times times a displacement acting over a distance (d) in the direction parallel to the force
work = Force x distanceExample: Pressure-Volume work in volcanic systems.Pressure = Force/Area; Volume=Area x distance;
PV =( F/A)(A*d) = F*d = w
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Forms of Energy
• Kinetic energy: associated with the motion of a body; a body with mass (m) moving with velocity (v) has kinetic energy
» E (k) = 1/2 mass * velocity2
• Potential energy: energy of position; is considered potential in the sense that it can be converted or transformed into kinetic energy. Can be equated with the amount of work required to move a body from one position to another within a potential field (e.g. Earth’s gravitational field).
» E (p) = mass * g * Z
where g = acceleration of gravity at the surface (9.8 m/s2) and Z is the elevation measured from some reference datum
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Forms of Energy (con’t.)
• Chemical energy: energy bound up within chemical bonds; can be released through chemical reactions
• Thermal energy: related to the kinetic energy of the atomic particles within a body (solid, liquid, or gas). Motion of particles increases with higher temperature.
• Heat is transferred thermal energy that results because of a difference in temperature between bodies. Heat flows from higher T to lower T and will always result in the temperatures becoming equal at equilibrium.
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Heat Flow on Earth
An increment of heat, q, transferred into a body produces aProportional incremental rise in temperature, T, given by
q = Cp * T
where Cp is called the molar heat capacity of J/mol-degreeat constant pressure; similar to specific heat, which is basedon mass (J/g-degree).
1 calorie = 4.184 J and is equivalent to the energy necessaryto raise 1 gram of of water 1 degree centigrade. Specific heat of water is 1 cal/g°C, where rocks are ~0.3 cal/g°C.
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Heat Transfer Mechanisms
• Radiation: involves emission of EM energy from the surface of hot body into the transparent cooler surroundings. Not important in cool rocks, but increasingly important at T’s >1200°C
• Advection: involves flow of a liquid through openings in a rock whose T is different from the fluid (mass flux). Important near Earth’s surface due to fractured nature of crust.
• Conduction: transfer of kinetic energy by atomic vibration. Cannot occur in a vacuum. For a given volume, heat is conducted away faster if the enclosing surface area is larger.
• Convection: movement of material having contrasting T’s from one place to another. T differences give rise to density differences. In a gravitational field, higher density (generally colder) materials sink.
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Magmatic Examples of Heat Transfer
Thermal Gradient T betweenadjacent hotter and cooler masses
Heat Flux = rate at which heat isconducted over time from a unitsurface area
Heat Flux = Thermal Conductivity * T
Thermal Conductivity = K; rockshave very low values and thusdeep heat has been retained!
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Convection Examples
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Rayleigh-Bernard Convection
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Convection in the Mantle
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convection in the mantle
models
observed heat flowwarmer: near ridgescolder: over cratons
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
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From: "Dynamic models of Tectonic Plates and Convection" (1994) by S. Zhong and M. Gurnis
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note continuity of blue slab to depths on order of 670 km
blue is high velocity (fast) …interpreted as slab
from: http://www.pmel.noaa.gov/vents/coax/coax.html
examples from western Pacific
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example from western US
all from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
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Earth’s Geothermal GradientA
ppro
xim
ate
Pre
ssur
e (G
Pa=
10
kbar
)
Average Heat Flux is0.09 watt/meter2
Geothermal gradient = / z
C/km in orogenic belts;Cannot remain constant w/depthAt 200 km would be 4000°C
~7°C/km in trenches
Viscosity, which measuresresistance to flow, of mantlerocks is 1018 times tar at 24°C !
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Earth’s Energy Budget
• Solar radiation: 50,000 times greater than all other energy sources; primarily affects the atmosphere and oceans, but can cause changes in the solid earth through momentum transfer from the outer fluid envelope to the interior
• Radioactive decay: 238U, 235U, 232Th, 40K, and 87Rb all have t1/2 that >109 years and thus continue to produce significant heat in the interior; this may equal 50 to 100% of the total heat production for the Earth. Extinct short-lived radioactive elements such as 26Al were important during the very early Earth.
• Tidal Heating: Earth-Sun-Moon interaction; much smaller than radioactive decay
• Primordial Heat: Also known as accretionary heat; conversion of kinetic energy of accumulating planetismals to heat.
• Core Formation: Initial heating from short-lived radioisotopes and accretionary heat caused widespread interior melting (Magma Ocean) and additional heat was released when Fe sank toward the center and formed the core
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Rates of Heat Production and Half-lives
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Heat Production through Earth History
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Gravity, Pressure, and the Geobaric Gradient
• Geobaric gradient defined similarly to geothermal gradient: P/z; in the interior this is related to the overburden of the overlying rocks and is referred to as lithostatic pressure gradient.
• SI unit of pressure is the pascal, Pa and 1 bar (~1 atmosphere) = 105 Pa
Pressure = Force / Area and Force = mass * acceleration
P = F/A = (m*g)/A and (density) =mass/volume
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Earth Interior Pressures
P = Vg/A = gz, if we integrate from the surface to somedepth z and take positive downward we get
P/z = g
Rock densities range from 2.7 (crust) to 3.3 g/cm3 (mantle)270 bar/km for the crust and 330 bar/km for the mantle
At the base of the crust, say at 30 km depth, the lithostatic pressurewould be 8100 bars = 8.1 kbar = 0.81 GPa
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Changing States of Geologic Systems
• System: a part of the universe set aside for study or discussion
• Surroundings: the remainder of the universe
• State: particular conditions defining the energy state of the system
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Definitions of Equilibrium