Download Plate Tectonic Overview and Introduction to Energy, Work, and Heat

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
QuickTi me™ and a V ideo decompressor are needed t o see thi s pi ct ure.
Volumes of Igneous Rocks on Earth
Convergent Margin Magma Genesis
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 distance
Example: Pressure-Volume work in volcanic systems.
Pressure = Force/Area; Volume=Area x distance;
PV =( F/A)(A*d) = F*d = w
Forms of Energy
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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
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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
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.
Heat Flow on Earth
An increment of heat, Dq, transferred into a body produces a
Proportional incremental rise in temperature, DT, given by
Dq = Cp * DT
where Cp is called the molar heat capacity of J/mol-degree
at constant pressure; similar to specific heat, which is based
on mass (J/g-degree).
1 calorie = 4.184 J and is equivalent to the energy necessary
to 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.
Heat Transfer Mechanisms
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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.
Magmatic Examples of Heat Transfer
Thermal Gradient = DT between
adjacent hotter and cooler masses
Heat Flux = rate at which heat is
conducted over time from a unit
surface area
Thermal Conductivity = K; rocks
have very low values and thus
deep heat has been retained!
Heat Flux = Thermal Conductivity * DT
Convection Examples
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Rayleigh-Bernard Convection
Convection in the Mantle
models
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
convection in the mantle
observed heat flow
warmer: near ridges
colder: over cratons
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
QuickTime™ and a Animation decompressor are needed to see this picture.
From: "Dynamic models of Tectonic Plates and Convection" (1994) by S. Zhong and M. Gurnis
examples from western Pacific
blue is high velocity (fast)
…interpreted as slab
note continuity of blue slab
to depths on order of 670 km
from: http://www.pmel.noaa.gov/vents/coax/coax.html
example from western US
all from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
Approximate Pressure (GPa=10 kbar)
Earth’s Geothermal Gradient
Average Heat Flux is
0.09 watt/meter2
Geothermal gradient = DT/ Dz
20-30C/km in orogenic belts;
Cannot remain constant w/depth
At 200 km would be 4000°C
~7°C/km in trenches
Viscosity, which measures
resistance to flow, of mantle
rocks is 1018 times tar at 24°C !
Earth’s Energy Budget
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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
Rates of Heat Production and Half-lives
Heat Production through Earth History
Gravity, Pressure, and the Geobaric Gradient
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Geobaric gradient defined similarly to geothermal gradient: DP/Dz; in the
interior this is related to the overburden of the overlying rocks and is referred
to as lithostatic pressure gradient.
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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 r (density) = mass/volume
Earth Interior Pressures
P = rVg/A = rgz, if we integrate from the surface to some
depth z and take positive downward we get
DP/Dz = rg
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 pressure
would be 8100 bars = 8.1 kbar = 0.81 GPa
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
Definitions of Equilibrium