Download Lecture 10: Introduction to Earth Structure and Energetics

Petrology
Petrology: the study of rocks, especially aspects such as
physical,
p
y
, chemical,, spatial
p
and chronoligic.
g
Associated fields include:
– Petrography: study of description and classification of rocks
– Petrogenesis: study of the histories and origins of rocks
Classification:
– Igneous: crystallized from a melt or magma
– Metamorphic: changed in response to heat, pressure, directed
q
stress or chemicallyy active ggases or liquids
– Sedimentary: formed at Earth’s surface, largely observable
What do you want to learn from Petrology?
How did a specific rock originate?
– What is it? Descriptive (textural, minerals) e.g., granite
– Where did it come from? E.g.,
E g mantle or crust?
– What processes were involved? Partial melting? Fractional
crystallization?
– Under what conditions did it form? Pressure, temperature?
– When did it form? Radiogenic isotopes (4.567 Ga, Earth)
What does the rock tell us about Earth/planetary history?
– How does the planet work? E.g., plate tectonics
– How
H ddoes the
h planet
l
fform andd evolve?
l ? E.g.,
E Moon
M
There are still many controversies explaining the origins and
compositions of different types rocks! A lot of work yet
to be done!
This course will be divided into Igneous
and Metamorphic petrology
• 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.
Earth
• Metamorphic Rocks: formed by the reconstitution of preexisting
i i rocks
k at elevated
l
d temperatures well
ll beneath
b
h the
h
surface of the Earth. Lower bound of temperature range is
poorly defined, but usually > 200°C. Upper range
b
bounded
d d by
b melting
l i (~700°C),
( 700°C) above
b
which
hi h we are in
i the
h
igneous realm.
Review of Earth Basics
Is the Earth homogenous?
No, very heterogeneous!
–
–
–
–
–
–
Horizontally
Vertically
Petrologically
Mineralogically
Chemically
Isotopically
The Earth: Horizontally
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
The Earth: Vertically
Atmosphere
Biosphere
y
Hydrosphere
Solid Earth
• Crust
• Mantle
• Core
Figure 1.2 Major subdivisions of the
Earth. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology.
Prentice Hall.
The Solid Earth
C
Crust:
t
Oceanic crust
Thin: 10 km
Relatively uniform stratigraphy
= ophiolite
hi lit suite:
it
•
•
•
•
•
sediments
pillow basalt
sheeted dikes
more massive gabbro
ultramafic (mantle)
Continental Crust
Thicker: 20-90 km average ~40 km
Highly variable composition
‹ Average ~ granodiorite
The Solid Earth
Mantle:
Peridotite (ultramafic)
Upper to 410 km (olivine → spinel)
‹ Low Velocity Layer 60-220 km
(asthenosphere
Transition Zone as velocity increases ~
rapidly
‹ 660 spinel → perovskite
perovskite-type
type
)
SiIV → SiVI
Lower
o e Mantle
a te
Figure 1.2 Major subdivisions of the Earth. Winter (2001) An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Solid Earth
Core:
Fe-Ni metallic alloyy
Outer Core is liquid
‹
No S
S-waves
waves
Inner Core is solid
Figure 1.2 Major subdivisions of the
Earth. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology.
Prentice Hall.
Can you calculate the volume/mass percentage
of the crust, mantle and core in the bulk Earth?
By volume:
• Crust:
C t 0.6%
0 6%
• Mantle: 83%
• Core: 16.4%
V = 4/3 x (Pi) x r^3 where is the radius of
the ball
How do we know the Earth’s interior?
Seismic data
Crust: low density
2.8 to 3.3 g/cc
g
P-wave: 6.1 to 6.5 km/s
Mantle:
M
l higher
hi h density
d i
3-5 g/cc
P-wave
P
wave 55-13
13 km/s
Low velocity zone:
Asthenosphere
Core: high density
Fe Ni,
Fe,
Ni S,
S 10-13 g/cc
P-wave 8-10 km/s
Figure 1.3 Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left,
rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.
Distribution of pressure within the Earth
Rocks under high
g pressure
p
do not have high
g shear strength
g
and tend to flow like viscous liquid. Thus pressures
within the Earth can normally be calculated based on
lithostatic pressure i.e.,
i e the load pressure from above.
above
This is similar to calculating hydrostatic pressure:
Relation is: P = ρgh where P is pressure in Pascal, ρ is
density in kg/m3, g is the acceleration of gravity at the
d h considered,
depth
id d in
i m/s
/ 2 (not
(
the
h same at different
diff
depths, or for different planets), and h is the depth in m.
Distribution of pressure within the Earth
Example: what is the pressure at the base of 40-km
thick
hi k granitic
i i crust with
i h a density
d i off 2800 kg/m
k / 3?
Answer: P = 2800 kg/m3 x 9.80 m/s2 x 35,000 m
= 0.96 x 109 kg/m2/s2 or Pascal (Pa)
= 0.96 GPa
= 9.6 kbar
For the Earth’s crust, the relation between pressure and
depth is roughly 1 GPa or 10 kbar per 35-40 km.
The Pressure Gradient
in the Earth: P vs. depth
• P = ρgh
• Nearly linear through mantle
~ 30 MPa/km
~ 1 GPa at base of ave crust
p y
• Core: r incr. more rapidly
since alloy more dense
p
• Densityy (ρ) increases with depth
but pressure gradients decreases,
due to decreasing of g.
Figure 1.8 Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356.
Distribution of temperature within the Earth
In comparison to pressure, temperature calculations for a
given depth are not so easy! E.g., on Earth’s surface
Heat sources in the Earth
• Heat from the early accretion and differentiation of the Earth
– Accretion: conversion of kinetic energy to thermal
– Core formation: conversion of g
gravitational potential
p
energy
gy to heat
• Heat released by radioactive decay
– Long-live radioactive elements (40K, 235U, 238U, 232Th)
– Short-live
Short live radioactive elements (26Al)
• Solar energy (minor)
Earth is cooling as a consequence of mantle convection
In the Earth’s crust, heat is generated mostly from
radioactive decay.
decay This can be calculated
calculated.
The rate of heat production per unit volume of a rock,
rock A,
A
is the sum of the products of the decay energies of each
radioactive isotope
p present
p
ei, and the concentration of
the isotope in the rock, ci (ppm), and the density of the
rock (ρ) such that: A = ρ Σei ci in µW/m3
Heat Transfer from Regions of High-Temperature to
Regions of Low-Temperature (the surface)
• Radiation: involves emission of EM energy from the surface of
hot body into the transparent cooler surroundings. Only
important at T’s >1200°C, e.g., deep mantle. Vacuum OK.
• Advection: involves flow of a liquid through openings in a rock
whose T is different from the fluid (mass flux).
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
T’ss from
one place to another. T differences give rise to density
differences. In a gravitational field, higher density (generally
colder) materials sink.
Geothermal gradient in the Earth: T vs. depth
To obtain thermal gradient must account for:
• Heat production from radioactive element
• Convective cooling at depth
• Radiative cooling
The Geothermal Gradient
Figure 1.9 Diagrammatic
cross-section through the
upper 200-300 km of the
Earth showing geothermal
gradients reflecting more
efficient
ffi i t adiabatic
di b ti ((constant
t t
heat content) convection of
heat in the mobile
asthenosphere (steeper
gradient in blue)) ) and less
g
efficient conductive heat
transfer through the more
rigid lithosphere (shallower
gradient in red). The
boundary layer is a zone
across which the transition
in rheology and heat transfer
mechanism occurs (in
green). The thickness of the
boundary layer is
exaggerated here for clarity:
it is probably less than half
the thickness of the
lithosphere.
lithosphere
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Geothermal Gradient
Figure 1.9 A similar
example for thick
(continental) lithosphere.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Geothermal Gradient
Figure 1.9 Notice that
thinner lithosphere allows
convective heat transfer to
shallower depths, resulting
in a higher geothermal
gradient
di t across th
the
boundary layer and
lithosphere.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The
Geothermal
Gradient
Figure 1.11 Estimates of
oceanic (blue curves) and
continental shield (red curves)
geotherms to a depth of 300 km.
The thickness of mature (>
100Ma) oceanic lithosphere is
hatched and that of continental
shield lithosphere is yellow. Data
from Green and Falloon ((1998),
Green & Ringwood (1963),
Jaupart and Mareschal (1999),
McKenzie et al. (2005 and
personal communication),
Ringwood (1966), Rudnick and
Nyblade (1999), Turcotte and
Schubert (2002).
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
How to calculate temperature within the
Earth with a given geothermal gradient?
Example: what is the temperature at the base of 40-km
thick granitic crust with a geothermal gradient 15
oC/km?
Answer: T = 40 km x 15 oC/km
= 600 oC
This temperature is not high enough to melt crustal
rocks.
Very
y important
p
figure!
g
Oceanic and continental
thermal gradients,
gradients with
the melting curves for
peridotite and granite
i l d d N
included.
Note
t th
thatt
melting should not occur
within the continental
crust given these
gradients. Somehow,
these gradients must be
perturbed. i.e., plate
tectonics
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Summary
• Three types of rocks
• The interiors of the Earth (by seismic waves)
– Crust-mantle-core
– Know
ow how
ow too calculate
c cu e vol/mass
vo / ss percentage
pe ce ge of
o
crust/mantle/core in Earth
• Pressure distribution in the Earth
– Know how to calculate pressure in the Earth: P = ρgh
• Temperature distribution in the Earth
– Heat sources
– Know how to read/interpret thermal gradient figures (TGFs)
– Know how
h to calculate
l l temperature in
i the
h Earthh with
i h TGFs
G