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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?

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igneous and metamorphic petrology overview of fundamental concepts
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
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 pre-existing 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.
slide3

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

forms of energy
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 energy6
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
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
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
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
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

definitions
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 T (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 * T

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
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 T (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).

slide14

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

earth s geothermal gradient
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 !

Approximate Pressure (GPa=10 kbar)

slide16

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

slide17

example from western US

all from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

earth s energy budget
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
gravity pressure and the geobaric gradient
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 r (density) =mass/volume (kg/m3)

P (in Pa) = (kg * m/s2)/m2 = kg/m1s2 = kg m-1 s-2 = Nm-2

earth interior pressures
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
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