Transcript Internal Structure of the Earth

Internal Structure of the Earth: geology's most wildly speculative topic
•Stony meteorites have a composition like that of the sun with volatile elements removed. (Ratios of
refractory elements is similar to the sun's). Earth is believed to have a bulk composition similar to that of a
stony chondrite meteorite with most of the volatile elements boiled off.
•The earth is a bunch of concentric shells, with inner shells denser than outer shells.
–The crust is rigid and comes in two flavors:
• Oceanic is about 7 km thick, is basaltic (pyroxene + plagioclase), and has a density around 3.0 g/cc
fresh, increasing as it cools. Oceanic crust is elastic-brittle all the way through.
• Continental is about 35 km thick, is granodioritic and has a density around 2.7 g/cc. (Granodiorite has
intermediate-to-sodic plagioclase + K-spar +assorted mafics [mainly amphibole]+ minor quartz)
Continental crust below 15km is plastic. Under mountains, crust can be much thicker.
• Crustal columns usually have the same total mass: they float like blocks of wood in the liquid-like
mantle. Mountain chains have low-density roots (they're like icebergs). Trenches have complex
density structure.
–The mantle is a thick section that has a peridotite (olivine + pyroxene) composition. Part of it is squishy
and flows plastically (the asthenosphere) and the outer 100 km is rather rigid and bound to the crust. The
mantle is 2900 km thick and makes up most of the earth's volume, and has density ranging from 3.3 to
5.5 at the bottom due to compression and phase changes.
–The core is made largely of iron with nickel, sulfur, and possibly other elements. The outer part is liquid,
the inner part is solid. The density is around 10 to 13 g/cc. It is 2250 km thick, but accounts for much
mass.
v 0015 of 'Internal Structure of the Earth' by Greg Pouch
at 2011-03-25 13:48:14 LastSavedBeforeThis 2011-03-25 13:48:03 18InternalStructure
Internal Structure of the Earth
3 Conclusions Figures
4 How do we know that?
5 Seismic Observations
6 Geophysics: a Quick Introduction
7 Layers
8 Isostasy
9 Isostasy (table)
10 How did the earth get this way?
Conclusions Figures
Fig. 17.5
How do we know that?
• Geophysics
–Seismology, mainly
• By studying arrival times of seismic waves , we can determine the velocity structure of the earth. It is
consistent with a radial organization, except for the crust, which varies from place to place. The text
discusses how seismology has been used to probe the interior of the earth.
• Seismic waves come in several flavors. Among the important ones here are P (Primary,
compressional) and S (Secondary, shear) waves. Only solids can transmit S-waves. Solids and liquids
transmit P waves. We know the earth has a liquid outer core and a solid inner core because it transmits
P-waves but not S-waves; we know the inner core is solid based on P-wave to S-wave conversions.
• The velocity of a seismic wave depends on the density and elastic properties of the medium through
which it travels. Velocities can vary sharply (easy to detect, usually at a compositional interface) or
gradually (hard to detect, often due to phase changes or a gradual changes in composition).
–Gravity: by using very sensitive measuring scales, geophysicists measure the strength of gravity, usually
for exploration or missile-lobbing. These measurements have shown that most areas have nearly the
same mass below them (roots under mountains and holes under basins).
–Magnetics Earth, for reasons that are very poorly understood, has a magnetic field. It is a dipole, kind of
parallel to the spin axis. Measurements on orientation of magnetic field frozen into volcanic rocks
indicates that the earth's magnetic field flips from time to time, which we understand even less.
–Moment of Inertia The rate at which the earth wobbles on its axis can be used to estimate its moment of
inertia. The values found indicate that the core must be very dense.
• Meteorites Analysis of meteorites and various rock samples suggests that the earth has a bulk composition
similar to carbonaceous chondrites, one of the more commonly found types of meteorites, except that most
of the light elements (carbon, hydrogen) have boiled off. Carbonaceous chondrites also have composition
similar to the sun (based on ratios of heavy elements)
• Heat flow can be measured, and indicates that the continents have rather high heat flow, trenches and ocean
floor have very low heat flow, and mid-ocean ridges have high heat flow rates that are rather spotty.
• Speculation and Extrapolation are the main tools in most discussions of earth’s interior.
Seismic Observations
Fig. 17.7
Geophysics: a Quick Introduction
•Geophysics is the use of physical measurements to deduce the distribution and identity of earth
features. It is a lot like radiology in medicine. Geophysics is divided into specialties, largely
along the lines of the physical phenomena used, such as seismology, grav-mag, electrical…
•In most geophysical techniques, there is a model of how the property varies, and deviations
from this are called anomalies. For example, in gravity, the earth can be treated as a rotating
ellipsoid, so the modeled gravity at a point can be calculated based on
–distance from the center of the earth and speed of rotation, which both depend on latitude
–elevation above sea level moving the point away from center of mass (free air)
–a correction for the mass between the observer and sea level, treated as an infinite slab
(Bouguer correction)
–a correction to the last correction accounting for hills above and valleys below, both
reducing gravity (terrain correction)
–correction for tides and instrumental drift (usually done by measuring at a fixed location)
•In conducting a gravity survey, you would measure gravity (with a fixed mass on a very
accurate scale) recording (for the corrections) at each station the latitude, elevation, time, and
maybe local topography and any other information deemed relevant. For each station, you
calculate modeled/expected gravity (involving whatever level of detail), and observed gravity.
The difference is the anomaly. From the anomaly, you might look for structures like folds and
faults, figure out whether a mountain chain is has a root, find buried stream valleys, etc. …
•In addition to the effects from the sources of interest, there are effects due to larger features,
which we call regional variations, and smaller features and instrumental errors, which we call
noise (e.g., if I am looking for stream valleys that cut into bedrock and are covered by later
sediments, variations due to crustal thickness associated with ancient mountain-building are the
regional [and I'll correct for them] and variations due to individual boulders are noise, and I'll
ignore them)
•The figure at right illustrates the important point that the interpretations of the data are nonunique: there are many ways to get the same data, so geologic knowledge comes into play.
This would look similar for
gravity, in these cases.
Layers
Compositional (Density) Structure
Crustal composition is based on direct
observations, mantle is based on
seismic velocities, xenoliths, and
indirect observations, and the core is
based on seismology, indirect
observations, and moment of inertia.
•Crust
–Continental: granodiorite
– Oceanic: Basaltic
•Mantle: Peridotite
Mechanical Layers
This is based on our observation of density structures, laboratory experiments determining mechanical
properties of certain rocks at various pressure-temperature conditions, and on estimates of the variation of
temperature with depth
•Lithosphere is solid and includes the crust and the rigid, outermost part of the mantle.
–Crust has P-wave velocities <8 km/sec BY DEFINITION (usually 5.5-7.2 km/sec)
•Continental crust is granodiorite-like and usually about 35 km thick, going up to 70-100 in collision
zones. In areas of extension, it can be thinner. The lower part of continental crust is plastic.
•Oceanic crust is basaltic and usually about 0-7 km thick. At ocean ridges it is thinner. It is less dense
than the mantle when hot (recently-extruded=young) and slightly denser when cold (old). This may be
one of the main driving forces behind plate tectonics.
The strange density situation is due to partial melting of peridotite. Generally, partial melts are more
iron-rich than the source rock: the restite is typically more magnesium-rich. The liquids are less dense
than the solids, but as they cool, they contract and the denser, more easily melted iron-rich product
becomes denser. When the overlying solid is denser, it tends to sink.
•The Moho is the boundary between fast rock and slower rock above, and is taken as crust-mantle boundary
•Mantle has P-wave velocities >8 km/sec
–Uppermost mantle has fast P-waves and S-waves
–Asthenosphere has fast P-waves and slow, attenuated S-waves, indicating partial melting or plastic state
–Lower mantle has fast P-wave and S-waves, indicating solid behavior.
–There are at least two important phase changes in the mantle, one where olivine goes to a denser spinel
structure and one where it goes to an even denser perovskite structure.
•Core: Iron-Nickel-Sulfur, some other •Core
elements
–Outer Core has lower P-wave speeds than the mantle and results in a shadow zone of P-waves and loss
of direct S-waves. No S-waves indicates definitely liquid behavior.
–Inner Core transmits both P and S waves, and is solid.
Isostasy
• Large areas (>100 km) are in isostatic (equal-force)
balance, because they are all floating in equilibrium on
some dense fluid (mid- to lower mantle). Small areas
(<10 km) are not in isostatic equilibrium: they are
supported by rock strength. These numbers appear to
have increased from the Archean to now.
• At some compensation depth, the weight of the
overlying rock column (downward) is balanced by the
pressure of the fluid (upward)
• The mass excess of a mountain chain is balanced by a
mass deficiency at depth
• An example: Suppose the mantle (l) has a density of
3.3, the crust (s) a density of 2.8. A crustal thickness of
35km implies a depth below Moho of 29.7km, and an
elevation above Moho of 5.3 km, or 0.8km above sea Physics of Isostasy
A
level. (See isostasy.xls) Changing the crustal thickness The block at right is subject to two forces: its weight W and
the buoyant force B on its bottom (pressure)
to 34km (1000m of actual erosion), the elevation
d
W=A solid T g =Area*weight_density_solid*thickness
W
changes to 650m (150m of elevation decrease)
B=A liquid d g =Area*weight_density_liquid *depth
Similarly, adding a 1000m of sediments of density 2.8 When they are equal (W=B), the block is in isostatic
equilibrium or
would only raise the elevation by 150m.
B
solid T=liquid d or solid/liquid = d/T
• This is how you can get really thick piles of sediments If there is a big density contrast, you get lots of relief for given
variation in thickness.
that all accumulate in shallow water, and why
If there is a big difference in thickness, you get a big difference
continents are surprisingly flat, given the variation in in relief
crustal thickness.
T
Isostasy
Isostasy and Continental Crust
45.00
Depth
40.00
Elevation from "top of mantle"
35.00
Elevation from Sealevel
Length in kilometers
Thickness
30.00
25.00
20.00
15.00
10.00
5.00
0.00
-5.00
20
25
30
35
40
Thickness in Kilometers
• A big change in thickness results in a small change in elevation. This is a good example of a negative
feedback loop.
• The ratio of elevation change to thickness change is (T-d)/T=1-solid/liquid
The graph above is an Excel Chart object. Double-clicking it while in edit mode should open it in Excel, and let you see the formulas and values
and play with the values.
How did the earth get this way?
• It appears the earth coalesced from solid, stony meteorites (carbonaceous chondrites) in the solar nebula, at
fairly low temperatures. I favor homogeneous accretion followed by separation, but inhomogenous
accretion is certainly a possibility.
• The present structure of core-mantle-crust seems to be due to melting/partial melting event involving
– heat from coalescing the planet (mechanical energy from meteorites falling in),
– short-lived isotopes that decayed and released heat early on (The solar system formed shortly after and,
at least in part, from two super-novas. There were a lot more unstable isotopes in the first few hundred
million years than now.), and
–settling of iron into the core.
• Once the earth got plastic enough for the iron to start settling, the release of heat due to the release of
potential energy would have continued the process. The iron catastrophe may have resulted in the formation
of the crust.
• Continental crust, especially that younger than 2.5Ga, contains most of the radioactive isotopes and so has a
much a steeper geothermal gradient than the rest of the planet. True granites containing K-spar don't really
occur much in Archean terrains.
• The 2.5Ga granite forming event may be related to ex-solution of a water and potassium rich aqueous phase
from the mantle.
Internal
Structure
of
the
Earth
• Stony meteorites have a composition like that of the sun with volatile
elements removed. (Ratios of refractory elements is similar to the
sun's). Earth is believed to have a bulk composition similar to that of
a stony chondrite meteorite with most of the volatile elements boiled
off.
• The earth is a bunch of concentric shells, with inner shells denser than
outer shells.
• The crust is rigid and comes in two flavors:
–Oceanic is about 7 km thick, is basaltic (pyroxene + plagioclase),
and has a density around 3.0 g/cc fresh, increasing as it cools.
Oceanic crust is elastic-brittle all the way through.
–Continental is about 35 km thick, is granodioritic and has a density
around 2.7 g/cc. (Granodiorite has intermediate-to-sodic
plagioclase + K-spar +assorted mafics [mainly amphibole]+ minor
quartz) Continental crust below 15km is plastic. Under mountains,
crust can be much thicker.
Crustal columns usually have the same total mass: they float like
blocks of wood in the liquid-like mantle. Mountain chains have
low-density roots (they're like icebergs). Trenches have complex
density structure.
• The mantle is a thick section that has a peridotite (olivine + pyroxene)
composition. Part of it is squishy and oozes (the asthenosphere) and
the outer 100 km is rather rigid and bound to the crust. It is 2900 km
thick and makes up most of the earth's volume, and has density
ranging from 3.3 to 5.5 at the bottom due to compression and phase
changes.
• The core is made largely of iron with nickel, sulfur, and possibly other
elements. The outer part is liquid, the inner part is solid. The density
is around 10 to 13 g/cc. 2250 km thick, but accounts for much mass.