Physical Geology Chapter 17 Earth’s Interior

Earth Structure

 

Figure 17.5

 

  Interior layers of the Earth

 

Mean density of Earth =  5.5 g / cm³

 

 

Density of crust:  continents   =  2.7 g / cm³

 

ocean floor =  3.3 g / cm³

 

 

 this difference reflects mostly a change in composition, some effect from pressure

 

 

 

What does it imply that oceanic and continental crust

   are MUCH lower density than the mean?

 

 

 

Earth Structure: Layers (version I)

 

Three main layers

   by composition:

   crust

          mantle

          core

 

 

These are determined by seismic velocity and inferred rock composition

 

 

Earth layers by seismic velocity

Figure 17.7

 

 Velocity changes with material and density

 

 

 

 

How Can We Interpret the Internal Layers of the Earth?

Earthquake motion creates seismic waves that are transmitted through rock

Figure 16.4  Focus and epicenter of an earthquake

 

Pressure and Shear Waves

 Compression and shear are determined by:

     direction of pressure gradient and

     characteristics of medium

 

Example of diving into water

  Water has no resistance to shear, but significant resistance to compression

 

Figure 16.5   motion of seismic waves

P wave (Primary or “pressure”)

S wave (Secondary or “shear”)

 

 

Seismic waves propagate outward in all directions   (spherical)

   photo of battleship New Jersey firing a salvo

 

Energy transmitted by the wave:  each particle hits the next particle in line

 

Think about sound transmitted through:    air        water       rock

General principle:

    seismic velocity increases with increasing density

 

Primary (P) Waves – compression

 

  higher velocity - arrive first

 

  propagate through solid or liquid

 

  significantly lower velocity in liquid

 

 

 

Secondary (S) Waves – shear

 

  lower velocity

 

  propagate through solid, but not through liquid

 

  can result in “shadowing”

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Recording seismic waves

 

Figure 16.8    travel time and separation of P, S, and surface waves

 

Finding the epicenter  Figure 16.10

 

Triangulate – calculate the distance from each station

 

  find the intersection of the circles

Will this work with two stations?

 

Single raypath: Trace the path of one segment of the

  seismic wave as it moves away    Figure 17.4a

 

 

 

 

 

 

 

Multiple raypaths

:  Energy moves out in all directions    Figure 17.4b

 

Reflection of seismic waves:  Some energy is reflected off boundaries between different types of rock    Figure 17.1

 

Refraction of a seismic wave

:  The seismic wave ‘bends’

  as velocity increases

 

    Figure 17.2

 

Effect of refraction

any wave bends toward the slower medium

 

Example of marching in a line

 

Waves moving toward a beach, into shallow water

 

 

Examples of refraction

:  light through a prism, fish underwater

 

Why does light bend?

   Light wave slows moving from air into glass or into water

 

 

 

Critical refraction

Understanding critical refraction:  example of driving through Toledo – The fastest route depends on distance and speed

 

Velocity of seismic waves through:

 

                   V_p                    V_s

 

 

Crust          5 - 7.4            3 - 4 km / sec

 

 ------------- Moho separates layers -----------

 

Mantle        7.9 - 8.2          4.7 - 4.8  km / sec

 

 

 

 

 

 

 

 

Velocity typically increases with depth because of increasing density;  large increase at the Moho (boundary between crust and mantle) is caused by a difference in rock type

Figure 17.7

 

Seismic Shadow Zones

S-wave shadow zone – easier to understand; shear waves can not move through the liquid outer core    Figure 17.9

 

P-wave shadow zone

 – complicated because of refraction    Figure 17.8

 

Tomography of the Earth’s Interior

 

What is a CAT scan?

 

Cross section of mantle velocity

    Box 17.2 Figure 3

 

Earth Structure: Layers (version II)

 

Another way of looking at Earth’s interior: defined by strength and viscosity  (not composition)

 

Lithosphere    Asthenosphere    Mesosphere    Core

 

 

lithosphere – “rock” (or “hard”)

 

cool, rigid rock near surface

 

crust and upper layer of mantle

100 km thick on average

 

asthenosphere -- “soft”   (“hot Silly Putty”)

near melting point

mixture of melted, partially melted, and solid components
flows with pressure
100 - 700 km

 

mesosphere  --   “middle”

pressure dominates -- rocks are solid

most of the mantle  700 - 2900 km

Cross Section of Crust (and Upper Mantle)

Figure 17.6   Table 17.1

 

 

 

 

 

 

 

 

Difference in density among continental crust, oceanic crust, and mantle

 

Composed of different igneous rocks:

Granite – Diorite – Gabbro – [Peridotite]

 

 

Isostasy

 – buoyancy and plasticity

crust floating on a mantle capable of flowing

Figure 17.11   

comparison of wood floating on water with continental crust

 

Isostasy Example

:

(1)  Removal of sediment from the continent    Figure 17.12

(2)  Ice loading on continental crust

 

 

Chemical differentiation

 Of the Earth’s layers

 

Crust has more:   silicon  oxygen  aluminum

 

Whole Earth is enriched with: iron  magnesium  nickel (core)

 

Composition  -  density of compounds

         

Si, Al, O                           sialic  

         

Fe, Mg, SiO4                   mafic

         

Fe, Ni                              metallic

 

Why Differentiation?  Early evolution of the planet

 

Effects of melting & differentiation

Heavy elements and compounds sink to the center,