Download Igneous and Metamorphic Petrology: Overview of Fundamental

Igneous and Metamorphic Petrology:
Overview of Fundamental Concepts
• What role is played by energy in its various forms to create
magmatic and metamorphic rocks?
• What is the source of internal thermal energy in the Earth?
How does this drive rock-forming processes?
• What is the role of the Earth’s mantle?
• How does mantle convection focus rock forming processes
in specific tectonic settings?
• What are the most significant properties of rocks and does
each tell us about rock-forming processes?
• How does a petrologist study rocks to determine their
nature and origin?
Temperature ranges of Igneous and
Metamorphic Rocks
• Igneous Rocks: formed by the cooling and solidification of
magma, defined as mobile molten rock whose temperature
is generally in the range of 700-1200°C (1300-2200°F).
Most magmas are dominated by silicate melts on Earth.
• Metamorphic Rocks: formed by the reconstitution of preexisting rocks at elevated temperatures well beneath the
surface of the Earth. Lower bound of temperature range is
poorly defined, but usually > 200°C. Upper range
bounded by melting (~700°C), above which we are in the
igneous realm.
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
Volumes of Igneous Rocks on Earth
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
• 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
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
• 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
Heat Conduction
Definitions
Thermal conductivity is a property of materials that
expresses the heat flux f (W/m2) that will flow through the
material if a certain temperature gradient DT (K/m) exists over
the material.
The thermal conductivity is usually expressed in W/m.K. and
called l. The usual formula is:
f = l * DT
It should be noted that thermal conductivity is a property that
is describes the semi static situation; the temperature gradient
is assumed to be constant. As soon as the temperature starts
changing, other parameters enter the equation.
More Definitions
In case of changing thermal parameters, also the heat capacity C
(J/K.m3) starts playing a role. The heat capacity is again a material
property. It expresses the fact that for changing the temperature DT (K) of
a certain volume V (m3) of material energy E (J) must flow in or out. The
heat capacity is usually linked to the density (kg/m3) of the material.
The heat capacity is usually found in the textbooks a specific heat capacity
Cp (J/K.kg), which must be multiplied by the density to get the full
picture.
C =  * Cp
When dynamic processes are involved, the change of temperature versus
time, at known boundary conditions is determined by both thermal
conductivity and heat capacity.
a = l /  * Cp , where l is the thermal conductivity.
The thermal diffusivity a ( m2/s) is always encountered in the
equations multiplied by the time t (s).
models
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
convection in the mantle
observed heat flow
warm: near ridges
cold: over cratons
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
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 !
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
Cartoon of Earth’s Interior
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
Gravity, Pressure, and the Geobaric Gradient
• Geobaric gradient defined similarly to geothermal gradient: DP/D; in
the interior this is related to the overburden of the overlying rocks and
is referred to as lithostatic pressure gradient.
• SI unit of force is the Newton
• SI unit of pressure is the Pascal, Pa and 1 bar (~1 atmosphere) = 105 Pa
Force = mass * acceleration = kg*(m/s2) = kg m s-2 = N
Pressure = Force / Area
P = F/A = (m*g)/A and (density) = mass/volume (kg/m3)
P (in Pa) = (kg * m/s2)/m2 = kg/m1s2 = kg m-1 s-2 = Nm-2
Earth Interior Pressures
P = Vg/A = gz, if we integrate from the surface to some
depth z and take positive downward we get
DP/Dz = 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 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