Geography 140
Introduction to Physical Geography

Lecture: Introduction to the Earth's Lithosphere

  I. In this last section of the course, we'll be concerned with the 
     lithosphere, which is the rock and mineral sphere of our planet (that is, 
     most of it!).  
     A. The branch of physical geography concerned with the lithosphere is 
        called "geomorphology."
     B. Remember how biogeography was the branch of physical geography 
        concerned with the biosphere and how it was shared with biology?  
        Well, geomorphology is shared with geology:  Some geomorphologists are 
        geographers and some are geologists.  Sometimes our tidy disciplinary 
        structures don't quite coïncide with what people are actually 
        doing!
     C. Anyhow, in this section, we will be concerned with:
        1. The structure and composition of the planet itself.
        2. Processes by which the earth's surface is built up (tectonism).
        3. Processes by which the earth's surface is worn smooth, as by 
           rivers, glaciers, waves, and winds (gradation).

 II. Structure of Planet Earth.
     A. Earth is structured in layers.  There are two different ways of 
        classifying these layers (there's that classification thing again).
     B. The first of these entails classification by the physical state of 
        matter and general chemical composition.  I'll start at the center of 
        the earth and work out to the surface in this discussion.
        1. The innermost part of the planet is believed to exist in a solid 
           state:  This is called the "solid inner core."
           a. It extends out some 1,250 km out from the center of the earth, 
              or just under 20 percent of the way out from the center to the 
              surface (the earth's radius is roughly 6,330 km:  Lecture 2).
           b. It is made up of iron (Fe) with some nickel (Ni), very dense and 
              heavy elements.
           c. This can be surmised from the following argument:
                i. The average density of the most abundant materials making 
                   up asteroids in our solar system is about 5.5 g/cubic 
                   centimeter (iron is the most common solid in outer space, 
                   particularly in the inner solar system, where Earth is 
                   located).  
               ii. The average density of Earth's crustal materials, however, 
                   is only 3.3 g/cubic meter.
              iii. Assuming Earth formed from the materials most common in the 
                   inner solar system, the heavy stuff has to be present 
                   somewhere in the earth.
               iv. Earth, like the other inner planets, went through a molten 
                   phase early in its history, which allowed gravity to 
                   stratify substances by density.  So, iron and nickel would 
                   settle at the "bottom" of the gravity well: the center of 
                   the young planet.
                v. The solidity of the inner core is surmised from the 
                   performance of iron and nickel in the laboratory and in 
                   massive computer models at pressures similar to those at 
                   the core of the planet:  Even though it's incredibly hot in 
                   the earth's interior (current estimates are that it's about 
                   5,500 K, which is close to the temperature of the surface 
                   of the sun!), the pressure is so great that the melting 
                   point of iron contaminated by nickel is elevated far beyond 
                   those high temperatures (6,500 K), leaving the nickel-iron 
                   as a solid.
        2. The next layer out is the liquid outer core.
           a. This, too, is dominated by nickel-iron.
           b. This is known from the earth's strong magnetic field, which can 
              be produced by currents in liquid iron.
           c. It extends out to some 3,450 km from the center of the earth, 
              which means it's about 2,200 km thick.
           d. It is known to exist in a liquid state because of the behavior 
              of earthquake waves, particularly shear body waves or secondary 
              waves.
                i. Liquid cannot respond to shear forces (lateral or side by 
                   side forces), so it can't transmit shear waves.
               ii. As a result, there is a seismic shadow on the side of the 
                   earth antipodal to an earthquake's epicenter:  Seismic 
                   stations there will not receive secondary waves, though 
                   they will get primary (compressional) waves from a given 
                   earthquake.  Primary waves, too, generate a shadow centered 
                   roughly 120° from the focus due to being refracted or 
                   bent inward as they slow down crossing into the liquid 
                   inner core.
              iii. It's not quite as direct as the size of the seismic shadow 
                   equalling the size of the liquid outer core, because 
                   seismic waves speed up as they travel through denser solids 
                   and refract or bend when they pass through different 
                   materials, and this curves their path back out to the 
                   surface.  Factoring that in, we can still compute the size 
                   of the outer liquid core's outer boundary from the size of 
                   the shear wave seismic shadow on the opposite side of the 
                   earth.

                   Shear wave seismic shadow:
                   


                   Primary wave seismic shadow due to velocity change:
                   


        3. The earth's mantle is the third layer out.
           a. It covers everything from the liquid outer core to the bottom of 
              the earth's crust.
           b. It extends, then, from about 3,450 km out from the center all 
              the way to within anywhere between 5 and 100 km of the surface.  
              It's not quite 2,900 km thick.
           c. The mantle is rather variable, but it is dominated by actual 
              rock, not just iron alloys.  
                i. This is largely silicate in basic structure (built around 
                   the silica and oxygen combination, SiO2).
               ii. These rocks are, however, highly enriched in iron and 
                   magnesium, so they are called "ultramafic" rocks ("ma" from 
                   magnesium and "f" from the chemical symbol for iron, Fe).
              iii. So, these rocks are extremely heavy and dense compared with 
                   typical surface rocks, and they are usually very dark as 
                   well because of the iron and magnesium in them.
               iv. The mantle varies in its state of matter, from a soft and 
                   nearly liquid condition near its inner boundary with the 
                   liquid outer core and again near the top, a few kilomters 
                   under the earth's crust.  In other areas, it may show 
                   nearly brittle solidity.  The softest parts of the mantle 
                   can flow in response to heat, kind of like "silly putty"?

                   

        4. The fourth and final layer out is the crust.
           a. This is very, very thin, ranging from maybe 5 km under the 
              oceans to as thick as 100 km under certain mountainous areas of 
              the continents (usually, it's about 40 km thick under the 
              continents).
           b. The Mohorovicic Discontinuity (often called, simply, "the Moho" 
              by its friends) marks the transition from the top of the mantle 
              to the bottom of the crust:  It's an area where there is a sharp 
              increase in the velocity of seismic waves as they pass into an 
              area of different density and rigidity.  Andrija Mohorovicic 
              first noticed this effect back in 1909, and it has since been 
              tied in theory with the mantle-crust transition.
           c. Rocks in the crust are solid to the state of rigid and brittle.
           d. They are also highly variable, including rocks of molten origin, 
              rocks of sedimentary deposition origin, and rocks that have 
              undergone all sorts of structural and chemical alterations.  
           e. The crust itself can be divided into one or two sublayers, 
              depending on where you are:
                i. One kind of layer is found everywhere, under the oceans and 
                   way under the contintents.
                   a. This layer is dominated by relatively heavy, dark, dense 
                      rocks of "mafic" composition.  Most of these mafic rocks 
                      are of volcanic origin and are called "basalts." 
                   b. This dense, heavy mafic layer is sometimes called the 
                      "sima" (dominated by silicate rocks of mafic 
                      composition) and sometimes the "mafic layer" and 
                      sometimes the "basaltic" layer.
                   c. It tends to be relatively thin, usually from about 5-12 
                      km in thickness.
               ii. A second layer is normally found only on the continents.
                   a. It is made up of light rocks primarily composed of 
                      silicates enriched in the lighter elements, such as 
                      aluminum (Al), potassium (K), and sodium (Na).
                   b. This layer is sometimes called the "sial" (dominated by 
                      silicate rocks with lighter elements mixed in, such as 
                      aluminum) or the "silicic layer" (overwhelmingly 
                      silicic) or the "felsic layer" (for potassium-rich 
                      versions of feldspar).  These rocks commonly occur in 
                      the earth's crust as "granite," so this layer is 
                      sometimes called the granitic layer.
                   c. This layer is considerably thicker than the basaltic 
                      lower layer, around 40 km.


                      

        5. So, that takes care of the first system for classifying the layers 
           of the planet, the one that classifies the layers by states of 
           matter (liquid, solid, and ductile or soft solid) and general 
           chemical composition (nickel-iron, ultramafic rock, mafic rock, and 
           felsic rock).  This system gives rise to the solid inner core, the 
           liquid outer core, the mantle, and the crust, with the crust 
           subdivided into sima and sial.
     C. The second classification of the layers within the planet is strictly 
        by states of matter only:  solid, liquid, or ductile solid.  It is 
        mechanical, then, rather than compositional.
        1. This classification was developed specifically for the needs of 
           plate tectonic theory (about which beaucoup later), for 
           understanding the drift of the continents in deep geological time.
           a. Since this one concerns the surface plates and the processes 
              driving them around, I'll start with the surface and work down 
              this time.
           b. In this system, the word "lithosphere" means the hardest, most 
              rigid layer of the outermost planet, the layer subject to 
              failure and division into plates (yes, this can be confusing, 
              because the word "lithosphere" also has a more generic usage, 
              describing the entire rock and mineral part of the planet).
                i. It extends down to about where compressional heating of 
                   rocks and minerals reaches roughly 1,300° C.  Past the 
                   immediate surface of the earth, temperatures of rock start 
                   climbing because of the compression of all the material 
                   lying above it.  Rocks cooler than 1,300° C in general 
                   behave in a rigid manner, while those warmer begin to 
                   weaken and behave in a soft, plastic manner (capable of 
                   flowing around).
               ii. The 1,300° C isotherm lies a few kilometers below the 
                   bottom of most of the ocean floor (except where ocean floor 
                   piles up at certain plate boundaries, where it can thicken 
                   to 100-150 km or so) and up to 250-300 km under the deep 
                   mountain roots and the shield areas at the heart of the 
                   continents.
              iii. As such, the lithosphere includes all of the crust in the 
                   other classification system, plus the most rigid part of 
                   the upper mantle.  This is the zone subject to breakage 
                   into new, rigid plates.
           c. In this mechanical system, the word "æsthenosphere" 
              (loosely rendered, the "wimpy sphere" or the "weak sphere") 
              refers to the softest, most plastic part of the upper mantle, an 
              area where rock weakens because it's awfully close to its 
              melting point (and localized pockets DO in fact melt, providing 
              magma for volcanoes and such).
                i. Interestingly enough, there is seismological evidence for 
                   the probable existence of the æsthenosphere.  Seismic 
                   shear waves generally slow down rather remarkably as they 
                   enter this zone of not-quite-liquid (remember shear waves 
                   can't pass through liquids, so it's not surprising that 
                   they would slow down as they pass through rocks just this 
                   side of liquid).  This "low velocity zone" is taken as 
                   empirical support for the existence of the 
                   æsthenosphere.
               ii. The æsthenosphere extends down to roughly 700 km, at 
                   which point the rocks are under enough pressure to 
                   overcompensate for the great temperature and resume a more 
                   rigid mechanical condition.
              iii. The lithospheric plates move about on top of the 
                   æsthenosphere, in response to currents of flow in the 
                   æsthenosphere that may reflect conduction of heat up 
                   from the core.  More on all that later.
           d. The next layer down is the "mesosphere."
                i. Yep, you're right, you DID hear that word before.  In a 
                   maddening disconnect between geology and climatology, 
                   between one end of physical geography and another, someone 
                   picked and popularized a word that already has a well-
                   established meaning in studies of weather and climate.  The 
                   mesophere you met already is the third layer up in the 
                   atmosphere; here it's the third layer down in the planet!  
                   Isn't that exasperating!?
               ii. Anyhow, the mesophere now refers to the most rigid part of 
                   the lower mantle, below about 700 km or so.  This is where 
                   pressure compensates for (compressional) temperature 
                   increases to return mantle rocks to a more rigid, solid 
                   condition.
           e. The fourth layer down is exactly the same one we bumped into 
              before:  the liquid outer core, extending down from about 2,900 
              km.  Here, we've passed beyond the lighter silicate rocks and 
              into the realm of nearly pure iron (iron with some nickel).  So, 
              the temperatures are high enough to transcend the melting point 
              of iron even at these pressures, and we get the liquid outer 
              core.
           f. The fifth and last layer down is the solid inner core we met 
              before, below about 5,100 km and extending right to the center 
              of the earth.  Once again the pressures are so great as to force 
              the melting point of even pure metal above the actual 
              temperatures down there, and so we believe the inner core is 
              solid nickel-iron. 
           g. Here are the two systems side by side:


              


Well, that's enough of the layered structure of the planet.  There are two 
different systems for classifying these gravity-induced layers:  States of 
matter plus general chemical composition and the strictly mechanical states of 
matter system.  

The first classification system really brings out the rôle of gravity in 
layering our once-molten planet:  The heaviest, densest (and locally most 
abundant) stuff, nickel-iron, settled out at the bottom of the planet's 
gravity well, its center.  This gave us the core. All rock materials were 
displaced towards the surface, giving us the mantle and crust.  These are also 
seen to reflect gravity layering:  The mantle is made up of the heavier 
ultramafic rock, while the basalt layer of the crust is made up of mafic rock, 
and the granitic layer of the crust (on top of the continents) is made up of 
felsic rocks, the lightest of them all.

The second classification is specific to the needs of understanding plate 
tectonics, so it focusses on mechanical properties and behavior:  strictly the 
states of matter (solid, plastic, and liquid).  This helps us conceptualize 
the rigid, brittle lithospheric plates sloshing around on the weaker 

æsthenosphere below and gives us room to think about mechanisms driving 
the currents in the æsthenosphere (such as convection of heat from the 
core, ultimately).

Next time, I'll go into the composition of the crust:  elements, minerals, and 
rocks, and how rocks transform from one type to another.

Document and © maintained by Dr. Rodrigue
First placed on web: 11/11/00
Last revised: 07/03/07