THE DYNAMIC EARTH (continued): Composition of the Earth
First, a quick review of what seismic data told us:
a) the Earth is layered
> crust (oceanic and continental)
> mantle
> core (inner and outer)
> the asthenosphere and the lithosphere
b) one layer is liquid (the outer core)
> the other layers are solid
Let's look at how seismic velocity and density vary in the earth:
> P-and S-wave velocities increase in going from crust to mantle
> in the outer part of the mantle, the velocities drop slightly in the low-velocity zone, and
then increase steadily towards the mantle core boundary
> there is a density jump going from the crust to the mantle, and then a steady density
increase going through the mantle to the mantle-core boundary
>> although you might expect seismic velocities to DECREASE deeper in the mantle as
density increases (since velocity is inversely proportional to density), the opposite
happens. This is because the elasticity increases more rapidly than the density as you go
deeper in the mantle; in the low-velocity zone, there IS a drop in velocity, because there
is a little bit of partial melting at that depth, which reduces the elasticity (i.e. the rocks are
more plastic there)
> in going from the bottom of the mantle to the top of the outer core, the S-waves stop
(because the outer core is liquid), and the P-wave velocity drops by almost half (from 13
km per second to 8 km per second)
>> the drop in P-wave velocity is because the outer core is less elastic (as it is a liquid),
and because the density in the outer core is about double that of the lower mantle (10 g/cc
versus 5 g/cc). The density and P-wave velocity then increase smoothly through the outer
core, jump slightly in going into the solid inner core, and then increase smoothly to the
center of the earth (elasticity goes up faster than density goes up)
Also, we need a brief comment on Seismic Waves and Engineering:
> we can carry out seismic studies to examine the upper part of the crust to solve
engineering problems
>> we do this by setting up a series of small, sensitive seismometers and setting off a
small dynamite blast, an air-gun blast, or a sudden episode of ground shaking (e.g., with a
hammer). The seismometers receive the energy, and we use the arrival-time data to
model the upper part of the earth
This approach help engineers in 2 broad areas:
(1) it is used to find boundaries that are important for civil engineers and geological
engineers:
e.g., (a) the soil/rock interface; (b) the interface between weathered rock and unweathered
rock; (c) faults
(2) it is used to find rock "structures" that might contain oil and gas (in particulr, up-arch
folds in sedimentary rocks)
O.K.!! Time now for the next step in developing our understanding the nature of the
dynamic earth. The last step was Step 7 (Seismic X-Raying of the Earth). Next is:
Step 8: Composition of the Earth Layers
How do we determine the composition of the layers?
(a) by direct sampling of earth rocks
> we can directly sample rocks at earth's surface, and drill down and get rock samples
from as deep as 15 km in the continental crust
>> volcanoes sometimes bring up chunks of rocks from the deep crust, and from the
upper part of the mantle
>> also, rocks from the deep crust and upper mantle have, rarely, been brought up from
depth along major faults
(b) from moment of inertia studies
> such studies place some limits on the density distribution in the earth, and we can use
that to suggest, and put limits on, possible materials for the different layers
(c) seismic studies combined with high-Pressure and high-Temperature laboratory
experiments
> the P-and S-wave velocities that we use in our models of the earth (which come from
the x-raying of the earth) place some limits on possible earth materials in the layers
>> we can couple this with laboratory experiments using a diamond-anvil press to
recreate deep mantle and outer core temperature and pressure conditions, whereby we can
measure seismic waves in materials in the lab to see if they match the results in the earth
>> these experiments support an earth-model of an iron core with a "silicate" mantle
(d) presence of earth's magnetic field
> this supports the existence of a liquid iron outer core (which can act as a self-generating
dynamo)
(e) study of meteorites
>perhaps the most important tool for modeling the composition of the earth's layers
>> we need to amplify on this:
METEORITES
> most meteorites are:
(a) around 4550 million years (Ma) old
(b) derived from the asteroid belt (between Mars and Jupiter)
>> when asteroids collide in this belt, they can knock pieces out of orbit and they can
cross the earth orbit and hit us
There are three general kinds of meteorites:
(1) IRON metorites: comprised mainly of iron (Fe) with 5 to 20% Ni
(2) STONY meteorites" made mainly of silicate minerals like those that we find in rocks
from the upper mantle (e.g. pyroxene; olivine)
(3) STONY-IRON meteorites: mixtures of (1) and (2)
Let's look more closely at the Stony Meteorites:
> there are 2 main kinds of stony meteorites
a. CHONDRITES:
>> these contain spherical chondrules (up to 1 mm in diameter) composed of silicate
minerals and iron crystals
>> chondrites have solar abundances of elements (i.e. the same ratio of elements as the
sun, excluding the volatile elements such as H, He, etc.)
b. ACHONDRITES:
>> they contain no chondrules, and less iron than the chondrites
What is the significance of these various types of meteorites??
>we believe that they formed in the early stages of the formation of the Solar
System, and thus they give insight into how the early Solar System formed
>> consider how we think the solar system formed between 4600 Ma ago and
4500 Ma ago:
(1) it started out as a nebula of gas and dust that started to collapse and spin, with the Sun
forming at the center to make a solar nebula
(2) dust starting to condense out of the gas as the temperature dropped (at those low
pressures, gas went directly to solid without intervening liquid), generating a dust cloud
(3) dust particles were attracted gravitationally to each other, and larger clumps
(embryos) of solid material formed
(4) further clumping together formed bigger bodies called planetesimals (lumps the size
of asteroids: i.e., up to tens or even a couple of hundred kms across)
(5) further clumping led to 9 proto-planets forming, which then continued to do a good
job of sweeping up material in their orbit, until the 9 planets formed by ca. 4500 Ma ago
(the asteroid belt is a huge concentration of planetesimals that were prevented from
amalgamating to form a planet by the strong gravitational effect of Jupiter)
> let's think about the planetesimals in the asteroid belt (the source of most
meteorites):
>> we believe that they formed initially as (essentially) homogeneous bodies of the
composition of chondrite meteorites (i.e., solar abundances with the exception of
volatiles). Some of the asteroids stayed that way, and when pieces of these ones are
broken off they can reach the earth as chondritic meteorites
>> some of the bigger asteroids (tens of km in diameter or more), however, were heated
sufficiently in the early history of the solar system to DIFFERENTIATE ( i.e., they partly
melted so that much of the iron sank to the center to form a CORE, leaving behind a
MANTLE depleted in iron, with the chondrule shapes destroyed by the melting process)
When such a DIFFERENTIATED asteroid gets broken up by collision, then pieces of the
interior can be knocked into earth orbit. The pieces from the CORE correspond to iron
meteorites; pieces from the MANTLE correspond to achondrite meteorites; and pieces
from the CORE-MANTLE boundary correspond to stony-iron meteorites
What does this mean for the composition of the Earth????
> well, we think the earth first formed ca. 4600 Ma ago as essentially a homogeneous
body just like a chondritic meteorite
>> within 100 Ma it had heated up sufficiently to partly melt and to differentiate to form
the iron core and the silicate mantle
Thus, we think that the AVERAGE composition of the earth is close to that of a
chondrite meteorite ( i.e., chondrite meteorites gives us a good measure of the earth's
average composition:
So, earth's average composition is (in terms of the top 4 elements):
> by weight % Fe (33%), O (31%) Si (15.5%), Mg (14.2%)
> by number of atoms O (50%) Mg (15.3%) Fe (15.2%) Si (14.4%)
SO!! In the Chondritic Model of the Earth, 90% of the Earth is made up of 4 elements
(Fe, O, Si, Mg)
But the Earth is DIFFERENTIATED, just like differentiated meteorites:
> so we conclude that the core of the earth is mostly iron (just like iron meteorites), that
the mantle is mostly "silicates" (like in achondrite meteorites)
So, to summarize the Chemical Composition of the different parts of the earth:
> the continental crust is FELSIC
> the oceanic crust is MAFIC
> the mantle is ULTRAMAFIC
> the core is mainly IRON with some Ni
>> the above list is in order, from top to bottom, of increasing density
In a little more detail:
(a) Continental Crust (felsic):
> average thickness of 30 km (ranges from 20 up to 70 km thick)
>> it is quite inhomogeneous, with rocks of all compositions
>> has the average composition of GRANODIORITE
(i.e., approximately 60% feldspar, 20% quartz, and 20% Fe-bearing silicates such as
biotite mica, hornblende and pyroxene)
(b) Oceanic Crust (mafic):
> thickness of 5 km (and covered with, on average, 5 km of seawater)
> it is quite homogeneous in composition
>> has the average composition of GABBRO
(i.e., approximately 50% pyroxene and 50% Ca-rich plagioclase)
NOTE: in order of decreasing abundance (by weight) of elements, the composition of an
average crust (combined oceanic and continental) is O, Si, Al, Fe, Ca, ...
(c) Mantle (ultramafic):
> extends from the crust-mantle boundary (the MOHO) down to ca. 3000 km deep
> average composition of PERIDOTITE
(i.e., olivine plus pyroxene)
NOTE: in order of decreasing abundance (by weight) of the elements, the mantle is O, Si,
Fe, Mg, ....
(d) Core (Fe mainly):
> the liquid outer core extends from mantle-core boundary down to ca. 5000 km
>> it consists of 86% Fe, 12% S, and 2% Ni
> the solid inner core consists of 80% Fe and 20% Ni
Last revision: 4 October 2000
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