Chapter 12
Earth’s Interior
Dr. Irina Goll Natural Science Department, Blinn College, Bryan campus, TX Friday, April 9, 2010
Seismic waves & Earth Interior Most of our knowledge of Earth’s
interior comes from the study of earthquake waves
Friday, April 9, 2010
Friday, April 9, 2010
P-waves
S-waves
Figure 12.2 Friday, April 9, 2010
Earthquakes and the interior of the Earth ď °Waves
velocities & travel time of PPrimary (compressional) and SSecondary (shear) waves through the Earth vary depending on the properties of the materials ď °Velocity depends on the density and elasticity of the intervening material
Friday, April 9, 2010
Earthquakes and the interior of the Earth ď Ż Compare granite & mud. ď Ż Granite is more dense than mud ď Ż Travel time for P and S-waves (under
same temperature and pressure) through granite will be less than through mud
Friday, April 9, 2010
Seismic waves would travel through a hypothetical planet with uniform properties along straight-line paths and at constant velocities
Friday, April 9, 2010
Seismic waves and Earth’s structure Abrupt changes in seismic-wave
velocities that occur at particular depths helped seismologists conclude that Earth must be composed of distinct shells Because of density sorting during an early period of partial melting, Earth’s interior is NOT homogeneous
Friday, April 9, 2010
Seismic waves and Earth’s structure With increasing depth, Earth’s interior
is characterized by gradual increases in temperature, pressure, and density
Friday, April 9, 2010
Earth Model with increased density of rocks
In dense rocks seismic waves speeds up and their paths will be bent Friday, April 9, 2010
Seismic waves and Earth’s structure ď Ż Depending on the temperature and
depth, a particular Earth material may behave like a brittle solid, deform in a plastic–like manner, or melt and become liquid
Friday, April 9, 2010
Seismic waves and Earth’s structure The
nature of seismic waves
a given layer the speed generally increases with depth due to pressure forming a more compact elastic material
Within
Friday, April 9, 2010
P-waves vs. S-waves The nature of seismic waves P-waves = “compressional waves” are able to propagate through liquids as well as solids S-waves= “shear waves” cannot travel through liquids
Friday, April 9, 2010
P-waves vs. S-waves all materials, P waves travel faster than do S waves ď Ž When seismic waves pass from one material to another, the path of the wave is refracted (bent) or reflected ď Ž In
Friday, April 9, 2010
A few of many possible paths that seismic rays take through a real Earth reflected
refracted (bent)
discontinuity
propagated
Friday, April 9, 2010
Discontinuities – history of discovery Discontinuity 1 Lithosphere / Astenosphere
Author – A. Mohorovicic (1909) Discontinuity 2 Mezosphere / Outer Core
Author – R. Oldham (1906) Discontinuity 3 Outer Core / Inner Core
Author – I. Lehmann (1936)
Friday, April 9, 2010
Discontinuities
Friday, April 9, 2010
Discontinuity #1 lithosphere / asthenosphere Croatian meteorologist
Andrija MOHOROVICIC Friday, April 9, 2010
and seismologist Was born in January 23, 1857 He knew 6 languages Astronomy & Geophysics were his interests, besides meteorology One of most prominent earth scientists in 20th century
Layers defined by physical properties Lithosphere (sphere of rock) Earth’s outermost layer Consists of the crust and uppermost mantle Relatively cool, rigid shell Averages about 100 km in thickness, but may be 250 km or more thick beneath the older portions of the continents (like underwater part of an iceberg )
Friday, April 9, 2010
Layers defined by physical properties Asthenosphere (weak sphere)
Beneath the lithosphere, in the upper mantle to a depth of about 600 km Small amount of melting in the upper portion mechanically detaches the lithosphere from the layer below allowing the lithosphere to move independently of the asthenosphere
Friday, April 9, 2010
Discovering asthenosphere Discontinuity #1 (Moho discontinuity) Discovered in 1909 by Andriaja Mohorovicic Separates crustal materials from underlying mantle i.e. Lithosphere from Asthenosphere Identified by change in density of a matter and therefore velocity of P waves
Friday, April 9, 2010
A. Mohorovichich
discovered a crust-mantle boundary by comparing velocities of shallow and deep P-waves on seismograms from many different seismic stations Friday, April 9, 2010
Mohorovicic discontinuity lithosphere / asthenosphere ď Ż A. Mohorovicic has found that the
seismographic stations located more than 200 km from an earthquake obtained appreciably faster average travel velocities for refracted (deep) Pwaves, than did stations located nearer the quake
Friday, April 9, 2010
Mohorovicic discontinuity lithosphere / asthenosphere ď Ż Direct (shallow) P-waves that
reached the closest stations first had velocities that averaged ~ 6 km/sec, by contract the seismic energy recorded at more distant stations showed that refracted (deep) P-waves traveled at speeds that approached 8 km/sec
Friday, April 9, 2010
Discontinuity lithosphere / asthenosphere From the data , A. Mohorovicic concluded that
between 50-660 km there exists a layer – asthenosphere - with properties markedly different from those of Earth’s outer shell lithosphere. Explanation: At a depth of about 410 km, magnesium-rich olivine is thought to collapse into spinel - a mineral that exhibits a more compact crystalline structure and hence greater density Friday, April 9, 2010
In Asthenosphere (under great pressure & temperature) Olivine
Friday, April 9, 2010
converts into
Spinel
•Moho discontinuity is marked by abrupt decrease in P-wave velocity
Friday, April 9, 2010
Discontinuity #2 core-mantle boundary The core-mantle boundary Discovered
in 1906 by a British geologist Richard D. Oldham In 1914 Beno Gutenberg calculated the depth to the core boundary as 2900 km (a value that has stood the test of time!)
Richard D. Oldham Friday, April 9, 2010
“Geology stars” (from left to right):
Fr. Press, B. Gutenberg, H. Benioff & Ch. Richter
Friday, April 9, 2010
Discontinuity #2 core-mantle boundary Mesosphere - lower mantle Rigid
layer between the depths of 660 km and 2900 km Rocks are very hot and capable of very gradual flow
Friday, April 9, 2010
Core Origin Most accepted explanation is that the core formed early in Earth’s history As Earth began to cool, iron in the core began to crystallize and the inner core began to form
http://www.youtube.com/watch?
v=H5h5ubAiiWE&feature=related
Friday, April 9, 2010
Discontinuity #2 core-mantle boundary Core Composed mostly of an iron-nickel alloy Our knowledge about core composition is based on:
o density measurements & calculations, o composition of meteorites stony, iron-nickel carbonaceous chondrite with organic compounds,
o existence of magnetic field
Friday, April 9, 2010
Discontinuity #2 core-mantle boundary Characterized by refracting (changing the
angular of a waves trajectory) of the P-waves
Based on the observation that P waves die out
at 105 degrees from the earthquake and reappear at about 140 degrees
35 degree wide belt is named the P-wave
shadow zone
Friday, April 9, 2010
P-wave shadow zone Figure 12.8
Friday, April 9, 2010
P-wave shadow zone Figure 12.8
Friday, April 9, 2010
•Discontinuity#2 is marked by abrupt decrease in P-wave velocity
Friday, April 9, 2010
Discontinuity #2 core-mantle boundary ď Ż S-waves don’t pass the mantle-
core boundary & are reflected from it http://ircamera.as.arizona.edu/NatSci102/ NatSci102/lectures/earth.htm
Friday, April 9, 2010
How S waves travel through Earth interior
Friday, April 9, 2010
How S waves travel through Earth interior
Friday, April 9, 2010
How S waves travel through Earth interior
Friday, April 9, 2010
How S waves travel through Earth interior
http://www.classzone.com/books/earth_science/terc/content/visualizations/es1009/ es1009page01.cfm Friday, April 9, 2010
S-wave shadow zone Friday, April 9, 2010
Discovery of the inner core • •
Friday, April 9, 2010
Predicted by Inge Lehmann in 1936 P-waves passing through the inner core show increased velocity suggesting that the inner core is solid
Discontinuity #3 outer core / inner core ď ŻOuter core ď Ż The fact that S-waves do NOT
travel through the core (S-wave shadow zone) provides evidence for the existence of a liquid layer outer core - beneath the rocky mantle
Friday, April 9, 2010
Discontinuity #2 core-mantle boundary Outer core Liquid layer 2270 km (1410 miles) thick Convective flow within generates Earth’s magnetic field
Friday, April 9, 2010
Discontinuity #3 outer core / inner core Inner core Sphere with a radius of 3486 km (2161 miles) Stronger than the outer core Behaves like a solid
Friday, April 9, 2010
Recent nuclear tests used to measure the depth of the inner core accurately
Friday, April 9, 2010
Dynamo theory ď Ż The Earth magnetic field is believed to
be caused by the convection of molten iron and nickel, within the outercore, along with Coriolis effect caused by the overall planetary rotation
Friday, April 9, 2010
Earth Magnetic field The requirements for the core to produce
Earth’s magnetic field are met in that it is made of material that conducts electricity and it is mobile Inner core rotates faster than the Earth’s surface and the axis of rotation is offset about 10 degrees from the Earth’s poles
Friday, April 9, 2010
Dynamo theory
Friday, April 9, 2010
Friday, April 9, 2010
Convection patterns within Earth’s liquid iron outercore
Friday, April 9, 2010
Dynamo theory ď Ż When conducting fluid flows across
an existing magnetic field, electric currents are induced, creating another (secondary) magnetic field. ď Ż When this secondary magnetic field reinforces the original magnetic field, a dynamo is created which sustains itself Friday, April 9, 2010
Possible origin of Earth’s magnetic field
Figure 12.C Friday, April 9, 2010
Friday, April 9, 2010
NORMAL orientation of Earth magnetic field Friday, April 9, 2010
Magnetic field weakens and poles begin to wonder Friday, April 9, 2010
Reversal complete with North Pole pointing South Friday, April 9, 2010
Earth’s Temperature Earth’ internal heat engine Earth’s temperature gradually increases
with an increase in depth at a rate known as the geothermal gradient Varies
considerably from place to place Averages between about 20°C and 30°C per km in the crust (rate of increase is much less in the mantle and core)
Friday, April 9, 2010
Earth’s Temperature
Friday, April 9, 2010
Earth’s Temperature Earth’ internal heat engine Major processes that have contributed to
Earth’s internal heat
Friday, April 9, 2010
Heat emitted by radioactive decay of isotopes of uranium (U), thorium (Th), and potassium (K) Heat released as iron crystallized to form the solid inner core Heat released by colliding particles during the formation of Earth
Earth’s Temperature Heat travels by three different
mechanisms:
Radiation Conduction Convection
Within a planet all three are active, but
are more or less significant within different layers
Friday, April 9, 2010
Earth’ internal heat engine Heat flow in the crust
called conduction Conduction is a flow of heat through material Process
Friday, April 9, 2010
Earth’ internal heat engine Heat conducts in two ways:
Through the collision of atoms (domino effect) Through the flow of electrons (in metals)
Rates of heat flow in the crust varies
Friday, April 9, 2010
Earth’ internal heat engine Heat flow in the mantle
Process called mantle convection Convection is the transfer of heat by moving material Convection occurs because of
Friday, April 9, 2010
Thermal expansion Gravity Fluidity
Convective flow in boiling water (convection cycle)
Friday, April 9, 2010
Earth’ internal heat engine Heat flow in the mantle There is not a large change in temperature with depth in the mantle Mantle must have an effective method of transmitting heat from the core outward Chemical convection – occurs when changes in density result through chemical and not thermal means
Friday, April 9, 2010
Model of convective flow in the mantle
http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/earth.htm Friday, April 9, 2010
Earth’ internal heat engine Mantle convection
Important process in Earth’s interior Provides the force that propels the rigid lithospheric plates across the globe Because the mantle transmits S waves and at the same time flows, it is referred to as exhibiting plastic (both solid and fluid) behavior
Friday, April 9, 2010
Seismic tomography of the Mantle
Friday, April 9, 2010
A map of the rate of heat flow
Friday, April 9, 2010
What did you learn today? Seismic waves are used
to probe Earth’s interior Seismic waves reflect off of layers composed of different materials Discontinuity #1 (Moho) Lithosphere / Asthenosphere Discontinuity #2 Mantle / Outer core Discontinuity #3 Outer core / Inner core
Friday, April 9, 2010
Earth’s Temperature: Sources of Heat Heat flow - radiation, conduction, convection Earth’s temperature profile Seismic Tomography Earth’s Magnetic Field Geodynamo Magnetic reversals