Structure, Composition, and Rheology of the Earth

The Earth is not a homogenous ball of stuff--it has layers.  Actually, it is layered in two important ways and fundamentally different ways: in composition, and in rheology.   Rheology refers to the way in which material deforms--whether it stretches, snaps, or flows.

 

Principal Compositional Layers

The three principal compositional layers of the Earth are the core, the mantle/crust, and the atmosphere:

These layers were formed early in Earth history when the Earth--which started out as a homogeneous ball of stuff--grew very hot and partially melted.  The partial melting released gases which streamed to the surface and produced liquid iron metal which drained into the core. The rest of the solid material in the Earth floated above the iron core and became rock.  Thus the three principal layers of the Earth--metal, rock, and gas--were formed.


Originally, there was only one type of rocky layer in the Earth--the mantle. However, over millions of years the rocky mantle has partially melted and re-melted many times, producing a thin cap layer of different rock, the crust, above it.  Thus the solid portion of the Earth can be subdivided into three compositional layers: the core, the mantle, and the crust.

 


Practice Quiz: 2.1 Which of the following compositional layer(s) of the Earth is composed of metal?  

 

Practice Quiz 2.2: Which of the Earth's compositional layers formed latest? 

 

Discovery of Earth's  Layers

Before we go further, it is worth pausing at this point to ask the very important question: how do we know that the Earth has layers, and what the layers are made of?  The answer doesn't come from direct observation.  In fact, except for the uppermost crust, almost the entire Earth is inaccessible to humans.  A few pieces of the uppermost mantle are occasionally carried up by volcanic eruptions, but these are uncommon and hardly representative of the entire mantle.  The deepest exploratory wells only penetrate 10-12 kilometers, not even into the lower crust.  So how can geologists say with confidence that the Earth has a core, mantle, and crust, and can even state the composition of these layers, if they have never been able to put their hands on anything but a few rocks from near the surface? 

Since direct inspection is out of the question, geologists have had to use inference to determine the composition of the Earth.  The three main lines of inference involve: (a) the density of the whole Earth; (b) the composition of certain meteorites called carbonaceous chondrites; and (c) the speed of earthquake waves passing through the Earth.

  1. The average density of the Earth can be calculated by observing how the Earth perturbs the orbits of other planets and the moon.  This calculation was first completed by Isaac Newton in 1686 and produced the average density of the Earth accepted today: 5,515 kg/m3 or 5.515 g/cm3.  Since this value is about twice as large as the density of typical rocks found on the Earth's surface, it implies that there must be heavier material at depth to compensate for the light rocks on top.

  2. In addition to the average density of the Earth, its average composition can be inferred from carbonaceous chondrite meteorites.  Carbonaceous chondrites are relatively rare meteorites that formed about the same time as the Earth did, and are thought to represent fragments of a small Earth-like planetary body that collided with something and broke up before it had a chance to melt and separate into different layers.  Thus the composition of the carbonaceous chondrites is similar to the composition of the 'bulk Earth'--the composition of the whole Earth if you were to re-homogenize it.

  3. The composition of the bulk Earth provides useful constraints on the composition of the individual layers, because they must "sum up" to the bulk earth-- but it doesn't in itself tell us what any of the layers is made of.  For this, earthquake waves are used.  Every time an earthquake occurs it sends out seismic waves in all directions that travel through the Earth and ultimately reach the surface, where they can be recorded by seismographs, or sensitive "shake-sensors" used by geologists.  Earthquake waves do not travel at the same speed  throughout the Earth; they generally travel faster through denser, more rigid rocks.  By observing thousands of waves from thousands of earthquakes, scientists have more or less determined the distribution of density within the Earth.  These density variations correspond to the layers of the Earth.
This diagram is a little complicated.  First note that in this diagram the thickness of the crust is so small that it cannot be shown to-scale (it is thinner than the 0-depth line!), so the properties of the crust are shown in a separate box.  Second, note that there are actually two types of seismic waves produced by earthquakes shown on this diagram: P-waves and S-waves.  You will learn much more about these later in this class.  The P-wave always travels faster than the S-wave, but is conspicuously absent in the outer part of the core (P waves don't occur here).  Finally, notice that there are distinct 'discontinuities' at several places in the Earth--places where the seismic wave velocity (and thus the density) jumps abruptly.  One of these is where the crust meets the mantle, and cannot really be seen as a discontinuity on the diagram though it is one since the S-wave velocity jumps from 4 to 4.5 km/s and the density jumps from about 3 to about 3.5 g/m3.  The other important ones are at the boundary between the mantel and core, and within the core.  This latter discontinuity corresponds to a change in the state of the Fe metal in the core (i.e., its rheology).

 

Practice Quiz 2.3: How do scientists know the average density of the Earth? 

 

Practice Quiz 2.4: If the Earth were to collide with another planet and disintegrate, it would produce millions of meteors with the same composition as carbonaceous chondrites. 

 

 

Chemical Composition of Earth: Gas, Rocks, Metal

One way to show the composition of the different Earth layers is in a table, as shown below.  In this table an omitted entry means the concentration is much less than 1 wt% (i.e., Na and K in the Bulk Earth; S and Ni in the crust).  The table is color-coded to the the different layers shown in the figures above.

Weight Percent of Elements in Earth and its Layers

Element

Bulk Earth Crust Mantle Core
Fe (iron) 32% 5% 6% 85%
O (oxygen) 30% 47% 44% 6%
Si (silicon) 15% 28% 22%  
Mg (magnesium) 14% 2.1% 23%  
S (sulfur) 2.9%     2%
Ni (nickel) 1.8%     5%
Ca (calcium) 1.5% 3.6% 2.3%  
Al (aluminum) 1.4% 8% 2.2%  
Na (sodium)   2.8% 0.2%  
K (potassium)   2.6%    

From this table we can see that the core consists almost entirely of iron (and contains most of the iron in the Earth), while oxygen, silicon, and magnesium are strongly concentrated into the rocky mantle and crust. 

So far this discussion has proceeded without actually defining 'rock' and 'rocky'.  In a sense, 'rocky' is self-defining--similar to the rocks and stones we see around us on the surface of the Earth.  Chemically what distinguishes these rocks is the abundance of oxygen.  This shouldn't be surprising, as oxygen is the most abundant element in the rocky part of the Earth (mantle and crust).  In addition to oxygen, the great majority of rocks in the crust and essentially all of those in the mantle also contain abundant silicon (Si) and are termed silicate rocks.  The only abundant exceptions are certain sedimentary rocks like limestone that contain oxygen but no silicon, and are found near the surface of the crust in places like Florida. 

 

Practice Quiz 2.5: Which of the following statements about the rocks in the mantle and crust is false

 

 

Chemical Compositional Variations in Rocks of the Crust and Mantle

Although both are broadly 'rocky', the crust has a different chemical composition than the mantle.  This difference is somewhat subtle, but is very important to understanding the way the Earth works and also to understanding the properties of certain geologic hazards like volcanoes.  In general, the chemistry of rocks is complex because there are at least 8 elements that are relatively abundant in rocks of the mantle and crust: O, Si, Fe, Mg, Ca, Na, K, and Al.  However, since O is always present we can combine it with the others by forming 'oxides' which somewhat simplifies the chemical description.  Fortunately, we can go a lot further in simplifying the chemical description of these rocks.

The following table shows the chemical composition of four important rock types found in the mantle and crust in terms weight percent of the 7 principal oxides:

Oxide (compound with O) A B C D
SiO2 45% 50% 60% 70%
Al2O3 4% 15% 15% 15%
MgO 38% 7% 3.7% 1%
FeO 8% 12% 5% 3%
CaO 3% 11% 4.7% 2%
Na2O 0.3% 2% 3% 3.5%
K2O <0.1% 0.3% 2% 4%

As the color coding suggests, Rock A is similar to rocks in the Earth's mantle, while Rocks B, C, and D are representative of rocks common in the crust (though rocks of composition A are also found in the crust).  Also notice that the rocks are arranged in order so that SiO2, Na2O, and K2O increase from A to D, and MgO decreases from A to D. 

In many ways the composition of Rock A is very different from those of Rocks B-D.  It has the lowest amount of SiO2, the highest amount of MgO (by a wide margin), and only trifling concentrations of K2O and Na2O.  It represents the lowest concentration of SiO2 found in rocks on Earth (with a few very unusual exceptions) and is called ultramafic by geologists.

 Ultramafic rocks:

In many ways the compositions of Rocks B-D represent a continuum.  For example, SiO2, Na2O, and K2O increase progressively from B through D, while MgO, FeO, and CaO decrease progressively.  In fact, almost almost all of the rocks common in the crust fall somewhere in this continuum, and not outside of it.  In other words, if you took equal parts of Rocks C and D and mixed them up you would come up with a chemical composition (65% SiO2, 15% Al2O3, 2% MgO, 4% FeO, etc.) that actually corresponds to some rocks found on Earth.  Essentially all of the rocks in the crust have compositions that can be composed, to first order, by mixing Rocks B and D in various proportions!  This suggests as much easier way of remembering the chemical spectrum of rocks in the crust: as mixtures of two 'components' called mafic and felsic:

Mafic rocks:

Felsic rocks:

Essentially all rocks in the crust have compositions that fall between mafic and felsic, and can be represented as mixtures of these components in various proportions.  (You can almost consider ultramafic rocks just mafic rocks on steroids--ultra-enriched in MgO, ultra-depleted in SiO2, etc.  This works for everything but FeO, which is actually richer in mafic rocks than ultramafic rocks.  But to first order this is a fine way to incorporate ultramafic compositions into the general compositional spectrum.)

The following table summarizes what you need to know about the chemical composition of rocks in the crust and mantle:

  Ultramafic Mafic Intermediate Felsic
wt% SiO2 45% 50% 60% 70%
MgO ultra-enriched enriched . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . depleted
FeO   enriched . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . depleted
CaO   enriched . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . depleted
Na2O ultra-depleted depleted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . enriched
K2O ultra-depleted depleted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . enriched

Once these terms are defined, it's easy to describe the composition of the mantle and crust.  Remember that there are two kinds of crust, continental crust and oceanic crust.

To summarize:


Practice Quiz 2.6: A dacite is a rock with a composition between intermediate and felsic.  Approximately how much SiO2 is in a dacite? 

 

Practice Quiz 2.7: Which of the following is a characteristic of mafic rocks? 

 

Practice Quiz 2.8: Which of the following describes the oceanic crust?

 

 

Rheology of Earth Materials

Rheology describes how materials deform under pressure--whether they snap, bend, or flow.  The Earth does all three, and it is very important to understand which parts do what.  

Brittle, Ductile/Plastic, and Fluid

The first key idea for the Earth is the concept of mechanical strength.  Materials with strength can support a load, while those without strength will eventually fail.  The failure may be immediate or slow, but failure eventually occurs.  For example, consider what happens when a heavy weight is placed on a strong wooden table:


What happens?  Well . . . nothing!  Since the table is strong it can support the weight, and will continue to support the weight for eternity (assuming we ignore things like the wood rotting, etc.).  If the table were weak, however, it might fail instantly:


When the table fails the wood splinters and the weight plummets to the floor.  This kind of behavior is called brittle behavior.  The key features of brittle behavior are that the material has strength, but the strength is limited, and beyond that it fails catastrophically by breaking.  Rocks on the surface of the Earth are brittle: when you whack them with your rock hammer they break!

Not all strong materials are brittle.  For example, a mound of clay is not brittle--you cannot "break" clay, though it can certainly deform.  And a mound of clay has strength, because it can support a load indefinitely, as is shown in the diagram below:


In this case the mound of clay has deformed by changing its shape.  If you were to remove the weight after a day the mound would remain deformed; i.e., it doesn't "spring back" to its original shape.  This kind of behavior is called ductile.  Under very high temperatures all rocks can behave plastically, which is why we sometimes see rocks that are bent.  Bending (not breaking) is the result of ductile behavior.

Finally we come to materials that have no strength.  For these materials deformation is instantaneous and continuous, as long as the load is present, and occurs as the material "flows away" from underneath the weight.  Eventually all of the material will flow away, leaving an infinitesimally thin film under the weight:

Notice that the arrows on the film present after one million years show that it extends far away in each direction.   Materials with no strength are called fluids, and include common liquids and gases.  A fluid will deform in response to any weight (or force), no matter how small.  For example, if we decrease the weight in the diagram above the fluid will still deform, but more slowly:

 


Viscosity

The second key concept in rheology is the viscosity of fluids.  Since fluids have no strength, and cannot support a load no matter how small, what matters is not whether a fluid will deform (it will) but how fast it deforms.  Viscosity describes the resistance to flow of fluids.  Fluids with a high viscosity are "sticky" and deform slowly, while fluids with a low viscosity are "runny" and deform rapidly.  For example, the fluid shown in red above might be considered a high viscosity fluid--after one week, the weight is still slowly sinking down and there is still a lot of fluid underneath it.  In comparison, this fluid would have a low viscosity:


Notice the pictures look the same but the times have changed!  For this material, after 3 seconds the fluid has nearly completely flowed away from underneath the weight and the weight has nearly fallen all the way down. 

We can rank the viscosity of many common fluids.  For example, from low to high:

    Water   <   Cooking oil  <  SAE30 motor oil  <  SAE40 motor oil  <  honey  <  road tar

 

Practice Quiz 2.9: What kind of mechanical behavior does a piece of chalk exhibit?

 

Rheology of Rocks

Rocks can be brittle, ductile, or fluid!  This may be surprising, in that our experiences with rocks on the surface of the Earth suggest they are decidedly brittle--whack 'em and they break.  However, as you go toward the center of the Earth both the temperature and the pressure increase.  The increasing temperature and pressure change the way rocks behave when they are stressed.  In general, rocks are brittle at low temperatures and pressures, and ductile and eventually fluid as temperatures and pressures increase.

There is one other very important factor that affects the way rocks deform: time.  At moderately high temperatures and pressures, rocks may be ductile if deformed slowly enough, but brittle if deformed rapidly.  By slowly we mean geologically slowly.  This means deformation that may occur over tens of millions of years--slowly and steadily.  Under these conditions, many rocks that otherwise might seem brittle can act like ductile.  Deeper in the Earth, materials can even act like a fluid if the deformation (flow) occurs slowly enough. 

One of the most important concepts in geology is that of a "fluid rock".  Fluid rock is solid rock that can flow because it is under extremely high temperatures and pressures, and because deformation deep inside the Earth tends to occur very slowly.  The viscosity of fluid rock is incredibly high--many millions of times higher than road tar.  It may take a fluid rock a million years to flow one meter.  But flow it does, and the flow of rock inside the Earth is one of the most important features of the deep Earth, and is one of key features of the unifying theory of plate tectonics.

 

Practice Quiz 2.10: How can a rock both break and bend?

 

Rheological Layering of the Earth

The following table summarizes the rheology of the Earth and the names of the rheological layers (which are shown in different colors).  There's a lot of information on this table so study it carefully.

Depth Composition (Compositional layers) Rheology Name of  Rheological Layer
0 (surface) to 15 km Mafic rock (oceanic crust)
Intermediate rock (continental crust)
Brittle under all conditions Lithosphere--brittle regime
15 - 100 km (under oceans)
15 - 250 km (under continents)
Mafic and ultramafic rock (oceanic crust and mantle)
Intermediate and ultramafic rock (continental crust and mantle)
Brittle when deformed rapidly; ductile when deformed slowly Lithosphere--ductile regime
100 - 200 km (under oceans)
250 - 350 km (under continents)
Ultramafic rock (mantle) Fluid rock with relatively low viscosity Sublithospheric mantle--asthenosphere
200/350 km - 2,900 km Ultramafic rock (mantle) Fluid rock with relatively high viscosity Sublithospheric mantle--lower part
2,900 km - 5,200 km Fe metal (core) Liquid--i.e., very low viscosity fluid; flows rapidly, turbulently Outer core
5,200 km - 6,350 km (center) Fe metal (core) Fluid rock with unknown viscosity Inner core

The Earth is divided into four main rheological layers:

Examine the figure below carefully.  It shows both the rheological and compositional layers of the Earth, and is drawn to scale.  The compositional layers are shown with the same color scheme used above, and the rheological layers are separated by heavy black lines.  Notice also that both continental and oceanic lithosphere and crust are shown.

 

Practice Quiz 2.11: How is "sublithospheric mantle" different from "mantle"?

 

Practice Quiz 2.12: Where is the asthenosphere located?

 

Practice Quiz 2.13: What does the lithosphere consist of?

 

Practice Quiz 2.14: The boundary between the crust and mantle is called the MOHO.  Where is the MOHO located?

 

Compositional vs. Rheological Layering

Students often confuse the well known compositional layering of the Earth (crust, mantle, core) with the less-well-known but equally important rheological layering.  It is very important  to keep the following points in mind when studying this section: