Instructor’s Notes: Chapter 17 Earth’s Interior
Earth’s Interior
Indirect evidence- seismic
Nature of seismic waves
The velocity of a seismic wave is the result of the density and elasticity of the material
that it passes through
With depth the speed of seismic waves increase- more dense and elastic material
Deepest Wells:
1970: Kola super deep borehole in Kola Peninsula, Russia 7.6 miles planned 9.3 miles
1999: Hawaii Scientific Drilling, Project Hilo, Hawaii. 1.8 mile of planned 2.8 miles into flank of
Mauna Kea, Hawaii- magma movement and magnetic changes
2004: San Andreas Scientific drilling Parksfield California 2.5 miles into San Andreas Fault buried sensors
3-D visualization of San Andreas Fault in Parksfield (USGS)
2004: Integrated Ocean Drilling Project: Atlantis Massif Region of the Atlantic Ocean: .9 miles
goal to penetrate earth’s mantle, but crust proved thicker than anticipated
2006: 1.5 mile bore hole into an impact crater at the mouth of Chesapeake Bay, Cape Charles,
Virginia
Thickness of Earth’s Layers ( Plummer, and Carlson, 2009)
Earth Crust
Continental (30-50 km=18-31 miles)
Oceanic (5-8 km = 4.3 miles)
Lithosphere 70 km ocean to 125-250 continent
Mantle
Asthenosphere 200 km deep (124 miles)
Mesosphere -200 km- 6000 km
Core
Inner (6000 km depth)
Outer (2800 km-5100 km depth
Seismic Waves
P waves (compressional) travel through liquid
S waves (shear waves) do not travel through liquid- liquid has no shear strength-it flows
When seismic waves travel from one type of material to another, the waves are
refracted and/or reflected
Seismic Refraction
Geophysics- measurement of seismic wave velocity (travel time), refraction, and reflection to
interpreted the Earth’s interior
Seismic Reflection
The Compositional Layers of Earth:
Core
Mantle
The boundary between crust and mantle is the MOHO (discovered by Andrija
Mohorivicic)
Crust
Layers of Earth based on Mechanical StrengthsLayers Defined by Physical Properties (Mechanical Strength)
Inner Core
Outer Core
Mesosphere or Lower Mantle
Asthenosphere (1-5 % melting) weak zone
Lithosphere 100-250 km thick (62- 155 miles)
Determined by how seismic waves travel through different layers- rock composition plays a
role, but is also affected by pressure, temperature, phase changes, liquid or solid state.
Temperature and Pressure (Composition)
Increase in pressure and temperature with depth controls the mechanical behavior of earth
materials
Increased temp- chemical bonds weaken lowering mechanical strength- deformation
Melting- chemical bonds destroyed
Pressure increases with depth making rock more dense –increasing strength- pressure raises
the melting point because of confining pressure
Using Travel Times to Measure the Depth of the Inner Core
Earth Compositional Layers
The Crust
Crust varies in thickness and density, however:
Oceanic crust is thinner and denser – mafic volcanic rocks (as thin as 5 km (3 miles) in
some places)
Continental crust is thicker and less dense - average composition of felsic igneous rocks
Thickest portion of crust is found under mountain ranges (up to 70 km (45 miles) thick)
Mantle
Mostly indirect evidence – in the 1960’s, we did try to drill to the mantle – Project Moholenever made it- this was the precursor to deep sea drilling
The Mantle is 82% of the earth’s volume
The Mantle is made up of dense ultramafic rock- peridotite
Evidence:
inclusion in oceanic basalts
Kimberlites- peridotite and diamonds
partial melting of the mantle produces the mafic basalts at the mid oceanic ridges, and
mantle plumes – seismic evidence
stony meteorites- peridotite
Experimental data
Experimental Data Lawrence Livermore Labs
Multi Anvil Press 250,000 X atmospheric pressure (= 700 km 435 miles) depth 2,2000C
Mantle Subdivisions:
Crust- Mantle boundary (Moho) is compositional change
at 410 kilometers (255 miles), there is a phase change where less dense mineral due to
increased pressure collapse to denser formsOlivine- spinel
At 660 kilometers (410 miles) phase change where spinel- perovskite (Mg Fe) Sio3
D” layer -lower 200 km mantle – decrease in p wave velocity- partially molten- area of transfer
of heat from the core to the mantle
Transfer of Heat in the Earth
by Mantle Convection
Mantle Convection figure 19.37
The Core
Larger than the Planet Mars
1/6 earth’s volume, 1/3 of the earth’s mass
The Core is thought to be composed of heavy, dense mineral such as iron and nickel- which
sank to the center during the early part of earth history
Inner Core is Solid- Outer Core is Liquid
Evidence:
P wave, and S waves shadow zones
P wave shadow zone - 1914 Beno Gutenberg – refraction of p waves on
boundary of outer core
S waves shadow zones –1936 Inge Lehmann confirmed in 1960’s underground
Nuclear test in Nevada
P wave velocity increases through core- solid
P-wave Shadow Zone and S-wave Shadow Zone (figures 17.8 and 17.9)
Evidence for Composition of the Core
Density calculation of the earth indicate that the core is 11 g/cm3; 14 times denser than water
(crustal rock is 2.8 X denser than water)
Meteorites (metallic) high percentage of iron and nickel in solar system (crust and mantle have
relatively small amount of iron)- iron and nickel have same seismic properties as the coreEarth’s Magnetic Field
Evidence for solid inter and liquid outer core
Core is too hot for the magnetic field to be produced by magnetic material
The core must be made up of material that conducts electricity and it must be in motion
Inner core is rotating 10/yr faster than earth’s surface + 1 rotation every 400yrs
Axis of inter-core is offset by earth’s rotational poles by 100
Polar reversals
(Other planets have magnetic fields mostly larger one where core has not cooled)
Possible Origin of Earth’s Magnetic Field
Geothermal Gradient
Geothermal gradient - temperature increase with depth –
Three processes contribute to the internal heat of earth
Radioactive decay
Heat released as iron solidifies in core
Colliding particles during earth’s formation
Heat is transferred by conduction
Our plant is cooling
Transfer of Heat in the Earth by Mantle Convection
Seismic Tomography
Numerical Modeling ( predictive or forward modeling adhering to measured data)