I. Evidence elucidating the structure and composition of Earth’s Interior

A. Evidence for internal structure of Earth has come from many sources – mainly “remote sensing” techniques since we have no samples from any but the outermost layer.

      1. Deepest drill hole ~ 10 km

      2. Magmas and lavas originate in the mantle so samples of these come from

            depths of many hundreds of kilometers, but we don’t know their

            exact depth of origin

B. Knowing something about the Earth's orbital parameters and the mass of the Earth, it is possible to determine the average densi­ty of the Earth (5.52g/cm3).

                                  MASS

              DENSITY  =  --------------------

                                UNIT VOLUME

1. We have direct measurements of samples of Earth's

         crust which has a density of approximately 2.8g/cc.

      2. In order to bring the average of the Earth's density to the high

         value of 5.52g/cm3 the material below the crust must have a density

         much greater than that of the crust.

      3. Also, Earth wobbles on its axis only very slightly so the internal

materials must be distributed uniformly as concentric rings.

C. Also a study of the variation of Earth's gravitational field yields the same conclusion.

  ---------------------------------------------------------------

                                 (M1) (M2)

                        F  =  G  ---------

                                    R2

 

         Force of attraction increases if the mass of either of

         the bodies involved increases.  Therefore, the gravita-

         tional pull that the Earth exerts on objects on the sur-

         face of the Earth varies depending on the subsurface 

         distribution of rocks of varying densities.

  ---------------------------------------------------------------

D. Also, we know the source of the Earth's magnetic field is in the core so the material there must be magnetic.

E. And the compositions of meteorites which representpieces of planets formed about the same time as the Earth, are composed of silicates, iron and nickel, suggesting that the material in our core is largely iron and nickel, and the material in the outer layers is largely silicate.

F. SEISMIC EVIDENCE

1. A seismic wave‚ is a vibration created in the Earth by an earthquake or an underground  explosion. These waves travel through the Earth just like sound travels through water or air.  They are detected and recorded at seismographic stations all over the globe.  This information has been used to gather information about the materials within the Earth (i.e., their density and the speed with which seismic waves travel through them)

        

2. TWO PROPERTIES OF SEISMIC WAVES, AND ANY WAVES FOR THAT MATTER, ARE THAT THEY CAN BE REFRACTED AND REFLECTED.

a. REFLECTION‚ - JUST LIKE water waves reflect off of a bulkhead and move back into th4e ocean, or like some of the light that strikes a piece of glass is reflected rather than passing through the glass, so seismic waves can be reflected off the various layers of rock they encounter as they move through the Earth.  If an earthquake occurs beneath you the seismic waves are reflected off these layers they encounter as they descend and it is possible to calculate the depths to these layers by the length of time it takes the wave to return to the surface.

1) As we'll sea later, geologic oceanographers generate their own seismic waves and record the reflected echoes to map the layers of sediments and rocks on the ocean floor.

b. REFRACTION‚ - Again just like water waves bend to parallel the beach as they move into an area where they are slowed down (i.e., shallow water), so seismic waves bend when they pass into rocks of differing densities in which the waves travel at different rates.  We'll talk about the refraction of wind-generated waves along the shore   in a few weeks when we study coastal processes.

1) Scientists cannot follow a seismic wave as it travels through the Earth.  They can only determine the time it takes to travel from its point of origin to the individual seismographic stations and , therefore,  determine its speed and path.  This information allows them to determine the properties of the materials through which the waves pass.

 

3. Two important types of seismic waves travel through the Earth and are called body waves.

a. P WAVES - vibrations are parallel to direction of movement

                             Travel through solid, liquid & gas

b. S WAVES  - vibrate in a direction perpendicular to their

direction of travel

                             Can't travel through LIQUID or GAS.

1) This is how the liquid portion of the outer core was discovered when scientists determined that S waves were not detected by seismographic stations located more than ¼ of  the  distance around the globe from the source of a given set of seismic waves.

 

II. The structure and composition of Earth’s Interior

A.  Much  of the interior of the Earth is composed of  what  we think of as solid rock. However, during the course of geologic time (i.e., 4.6 billion years) and at the temperatures and pres­sures, which occur deep within the Earth it is not as unchanging and immobile as you might think.

1.  As temperature increases towards the center of the Earth rocks become much less viscous (viscosity is a measure of a substance’s resistance to flow).  That is, rocks can flow and bend more easily as temperature goes up.

a. By the way the source of most of the Earth's internal heat is the decay of radioactive elements similar to the way in which the radioactive fuel rods in a nuclear reactor release energy and heat.

2. However, at the same time, the pressure imposed on rocks increases as they are buried under a thicker and thicker (i.e., heavier and heavier) pile of rock towards the center of the Earth.  Increasing pressure has exactly the opposite effect on the viscosity of materials as does increasing temperature. Therefore, within the interior of the Earth temperature and pressure compete to determine how readily a given rock sample will melt, flow and bend.  At some places for some materials the effect of temperature is so overwhelming that rocks melt and move just like viscous liquids such as honey or tar even though they are buried 10's of kilometers within the Earth.

a. As you might expect the outer portions of the Earth are quite cool and therefore quite viscous.  The result is a very brittle, rigid substance making up the outermost surface of the Earth.  This

material breaks readily as these rigid blocks collide and grind past one another along faults.

b. In contrast, there is a region deep in the interior of the Earth that is completely molten and fluid.  Also closer to the surface of the Earth there is a region that is partially molten and plastic on which these rigid outer blocks float and move similarly to sheets of ice drifting on the surface of a partially frozen lake.

B. Structure of the Earth

1.  When the Earth originally formed denser material  began  to sink towards the center of the Earth while the lighter  materials moved upwards towards the surface.

a.    Density =  mass  /  unit of volume

b.  Intuitively  you  are  all aware  of  the  effects  of differing densities.  When you throw a stone in a pond it sinks down through the water towards the bottom under the influence of the force of gravity.  It does this because the stone is more dense than the water.  That is, if you had a glass filled with stone and a glass  of the same volume filled with water, the glass filled with stone would be much heavier than the glass filled with water.

c. Well the same thing happens whenever any materials of differing densities are mixed.  The denser materials sink down through the less dense materials.

2. This gravitational separation of materials has created three general regions of different composition within the  Earth:  the crust, mantle, and core.

Depth below

Region            the Earth's             Density (gm/cm3)

                  Surface (km)         Range        Average

Crust             0 to 30           2.5 to  3.3       2.8

Mantle            30 to 2900        3.3 to  5.7       4.5

Outer core        2900 to 5100      9.4 to 14.2       11.8

Inner core        5100 to 6370      16.8 to 17.2      17.0

Whole Earth       0 to 6370         2.5 to 17.2       5.5

a. The core

1) Innermost region of the Earth comprising half

the radius of the Earth and about 16% of its

volume.  Divided into two regions: the outer core

which is molten and the solid inner core.

2) Composition

a) Inferred by two methods –

1) estimate the density of the core and identify common materials that have such a density and

2) assume the Earth's core has a similar composi-

tion to other astronomical objects such as

meteors.

b) Result: Fe-Ni alloy plus minor amounts of S, O, Si or C

3) Physical properties

Density: 9.4 - 17.2

b. The mantle

1) Extends from outer edge of core nearly all the way to the surface.  Outer half of Earth and 84% of its volume.

2) Primarily made of Mg, Fe, Si, and O in the form

of oxide and silicate minerals.

3) Outer mantle fairly uniform containing the partially molten material on which the rigid crustal blocks 'float'. Structure and density (3.3 to 5.7) of mantle changes with depth.

 

c. The Mohorovicic Discontinuity ( "Moho")

1) Boundary between mantle and overlying crust.

2) Corresponds to a sudden change in density.

d. The crust

1) Very thin outer shell of the Earth (Averages

10-20 km)

Less dense, cooler, more rigid and much more
      brittle than the mantle

2)  Two  kinds of crust both dominated by a kind of rock called igneous. There are 3 major rock types: igneous, metamorphic

and sedimentary.  We will spend a lot of time talking about the sediments in the bottom of the ocean and how they form by deposition out of the air or water.  Metamorphic rocks form

when sedimentary rocks (or sometimes igneous rocks) are heated and subjected to pressures much greater than those under which they

formed.  The mineralogical and chemical composition and the texture of rocks change under these conditions.  Metamorphic rocks are not very important volumetrically so we will not mention them very often.

C. Igneous rocks form more than 95% of the outer 10 km of the globe.  They form when molten rock (magma) solidifies (freezes) either within the crust (plutonic rocks) or when extruded on the surface during volcanism (extrusive or volcanic

rocks). Igneous rocks are classified on the basis of chemical composition, in particular SiO2 content and texture which is a function of whether they are extrusive or plutonic.  The three types of igneous rocks with which we will be concerned in this course are basalt and andesite (volcanic) and granite (plutonic)

Basalt  --  Andesite  --  Granite

Ô

Increasing silica content

Decreasing Fe and Mg content

Decreasing density

Decreasing temperature of formation

 

1. Continental crust - granitic - Rich in O,Si, Al, Na, K, and Ca.

Density = 2.8 gm/cc

Lighter in color, thicker and older than

oceanic crust (10-70 km) – plutonic

2. Oceanic crust - basaltic - Rich in O, Si,Ca, Fe, Mg, Al.

Density = 2.9 gm/cc

Darker in color, thinner, younger than

continental crust (Up to 7 km)-extrusive 

3. These two different kinds of crust are reflected in two different dominant

 elevations on the surface of the Earth.

0-1 km above sea level = continental
      4-5 km below sea level = oceanic

4. These two different kinds of crusts have nonrandom distribution of thicknesses

a) Thicker under large continental masses

b) Thinner under oceans

c) Under the continents the crust is thicker under high mountain ranges and less thick under areas of lower elevations.

D. Lithosphere and asthenosphere

1. Lithosphere is the term used for the cool, rigid block composed of crust and upper mantle that "floats" on a hot, plastic, partially molten 

portion of mantle called asthenosphere.

2. The lithosphere makes up the plates we will discuss when we talk about Plate Tectonics and the formation of the ocean basins.