Transcript Composition of the Earth - FAU

Composition of the Earth
GLY 4200
Fall, 2012
1
Interior of the Earth
• Earth’s interior is
divided into zones,
with differing
properties and
compositions
• Since we live on the
crust, it is the most
studied
• The core and mantle
are very important in
understanding the
behavior of the earth
2
Composition of the Crust –
Major Elements
• Earth’s crust is
composed
predominantly of eight
elements
• Figure for Si here is
correct – figure 5.2 in
text has a misprint
• Numbers are in weight
percent
3
Abundances Measurements
• We can specify abundances using differ
methods
• The most common are:
 Weight per cent
 Atom per cent
 Volume percent
4
Comparison of Methods
Element
O
Si
Al
Fe
Ca
Na
K
Mg
Weight %
46.60
27.72
8.13
5.00
3.63
2.83
2.59
2.09
Atom %
62.55
21.22
6.47
1.92
1.94
2.64
1.42
1.82
5
Minor and Trace Element Definition
• Minor elements have abundances between 0.1
to 1.0 weight percent
• Elements with abundances less than 0.1% are
called trace elements
• Their abundance is usually given in parts per
million (ppm) or parts per billion (ppb)
6
Minor and Trace Elements in Crust
• Only 17 elements occur with abundances of at least
200 parts per million (ppm) – in addition to those on
the major element slide, these include:
Element
Weight %
Element
Weight ppm
Ti
0.44%
F
625
H
0.14%
Sr
375
P
0.10%
S
260
Mn
0.09%
C
200
Ba
0.04%
7
Ores
• Many valuable elements are in the trace
element range, including the gold group (Au,
Ag, and Cu) and the platinum group (Pt, Pd, Ir,
Os), mercury, lead, and others
• Useage does not always reflect abundance –
copper (55 ppm) is used more than zirconium
(165 ppm) or cerium (60 ppm)
8
Effect of Pressure
• As pressure increases, minerals transform to
denser structures, with atoms packed more
closely together
• This is seen in the mantle
• The upper mantle is dominated by the mineral
olivine, Mg2SiO4
• Magnesium is in VI, and Si in IV
9
Transition Zone
• In the transition zone, from about 400 to 660
kilometers below the surface, olivine
transforms to denser structures
 olivine (ρ = 3.22 gm/cm3) →
wadsleyite (ρ = 3.47 gm/cm3) →
ringwoodite (ρ = 3.55 gm/cm3)
10
Lower Mantle
• Pressures are so great that silicon becomes six
coordinated (CN = VI), and some magnesium
becomes eight-coordinated (perovskite
structure)
 Ringwoodite (ρ = 3.55 gm/cm3) → MgSiO3
(perovskite structure) and (Mg, Fe)O
(magnesiowűstite - halite structure)
11
Core
• The core is divided into two regions, the liquid outer
core and the solid inner core
• There is a definite chemical discontinuity between the
lower mantle and the outer core
• The main elements in the core are an iron and nickel
alloy
• Increasing temperature first melts the alloy to make
the outer core
• Increasing pressure freezes the alloy to produce the
inner core
12
Outer Core
• Ranges from 2900 to 5100 kilometers below
the earth
• Composition is iron with about 2% nickel
• Density of 9.9 gm/cm3 is too low to be pure
metal
• Best estimates are that silica makes up 9-12%
of the outer core
13
Inner Core
• From 5100 to 6371 kilometers below surface
• 80% iron, 20% nickel alloy
• Pressures reach about 3 megabars, or 300,000
megapascals
• Temperature at the center is about 7600ºC
14