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Topic 3 A
Internal Structure of the Earth
Lecture by Dr. Ken Galli, Boston College
EESC116301 Environmental Issues and
July 7, 2016
Please do not distribute beyond the EESC116301 Class.
Case History: Two Major CA Cities
• San Andreas fault: a transform plate
boundary between the North American
and the Pacific plates
• Two major cities on the opposite sides
of the fault: Los Angeles and San
•Many major earthquakes
related to the fault system
•Loss of many lives and
billions of property damages
due to earthquakes
•New construction and
retrofitting of infrastructures
has become more expensive
•When will be the next “big
one” and what to do? How to
deal with the potential
Internal Structure of Earth
• The Earth is layered and dynamic: Interior
differentiation and concentric layers
• Chemical model by composition and density
(heavy or light): Crust, mantle, core, and Moho
discontinuity between the crust and mantle
• Physical property model (solid or liquid, weak or
strong): Lithosphere (crust and upper rigid
mantle), asthenosphere, mesosphere, liquid
outer core, inner solid core
Theory of Plate Tectonics
• The upper mechanical layer of Earth (lithosphere) is divided into rigid
plates that move away from, toward, and along each other
• Most (!) deformation of Earth’s crust occurs at plate boundaries
Earth’s Layers
3 main layers
defined by
• Crust - Outer
• Mantle - Middle
• Core - Center
Study of Earth’s Interior Structure
• Knowledge primarily through the study of
• Seismology: Study of earthquakes and seismic
• Examining the paths and speeds of seismic
waves through reflection and refraction
• Magma likely generated in the asthenosphere
• Slabs of lithosphere have apparently sunk deep
into the mantle
• Variability of lithosphere thickness reflects
changes in its age and history
Composition - How Do We Know?
Best Guess!
Whole Earth
• Meteorites - Fe, Ni (same age as Earth)
• Information from velocities of seismic waves indicate material
Crust (5-40 Km)
• Samples (mountain building helps!)
Mantle (5/40 to 2885 Km)
• Kimberlite pipes - intrusive igneous rock from the mantle
• Lava / volcanic rock
• Mountain building
Core (2885 to 6371 Km)
• Inference
– Earth’s mean density = 5.5 g/cm3
– Crust 2.5 to 3 g/cm3; mantle 3.3 g/cm3 to 5.5 g/cm3
– Density of core at least 10 to 11 g/cm3 (iron and nickel)
Seismic P Wave
• Primary or push-pull wave, travels like sound
• Direction of rock particle vibration parallel to
that of wave propagation
• Fastest rates of propagation, first arrival to
the seismograph
• Body wave travels through Earth interior and
all media—solid and liquid
Seismic S-Wave
• Secondary or shear waves
• The direction of particle vibration
perpendicular to that of propagation
• Propagates slower than P waves
• Body wave, propagating through Earth’s
interior, but not its liquid layers
Seismic Waves and Internal
• Earth’s interior boundaries: Sudden
changes in the speed of seismic waves
• Different characteristics: Different rates
and paths of wave propagation
• Asthenosphere: Low velocity zone, major
source of Earth magma
• Outer Core: Liquid, no S wave transmits
through it
Model of Earth’s
————— Moho
Figure 2.2b
• Two types of crust:
– Continental
• 30% of crust
• Granites and Diorites - rich in silicates
and feldspars (lighter materials)
• 35-40 Km thick
• Oldest is 3.8 billion years (90% solar
system age; missing ~700 m.y.)
– Oceanic crust
• Basalt - Mg, Fe (heavier materials)
• 5-10 Km thick
• 200 Ma oldest; 100 Ma average
Our deepest hole:
9 Kilometers ….. we have a
long way to go!
• MOHO - Mohorovicic Discontinuity
• Core mantle boundary - change in
• Density - getting heavier
• 3.3 - 5.5 g/cm3
• Probably material such as peridotite
(lots of heavy olivine - Fe, Mg)
• Samples from kimberlites, xenoliths
in volcanic eruptions, basalt
composition; lab experiments
Our deepest hole:
9 Kilometers ….. we have a
long way to go!
• Outer core
– Molten, near solid point (does not
transmit certain seismic waves)
– Density of pure iron or nickel/iron
– Includes ~ half of diameter of Earth
– 2x density of mantle
• Inner core
– Solid (higher pressure than outer core)
– Density of pure iron or nickel/iron
– ~ Size of moon
Our deepest hole:
9 Kilometers ….. we have a
long way to go!
Earth’s Layers
3 layers defined by
properties (strength):
• Lithosphere
• Asthenosphere
• Mesosphere
• Lithosphere
PLATES in Plate Tectonics
Upper 100 km
Crust and upper mantle
• Asthenosphere
– 100 km to ~350 km (to 660km in
– Upper mantle
– Near melting point; little strength;
ductile - NOT A LIQUID!
– Plates moving on this
– Magma generation
• Mesosphere
– Extends to core
– Also hot; strong due to pressure
Internal Dynamics of Earth
• Evidence
– Earth’s landscape
– Dynamic phenomena: earthquakes,
• Plate Tectonics: Hypothesis and Theory
– Continental drift
– Seafloor spreading
– Plate tectonics – a unifying theory
Dynamic Earth—Evidence
• Mountain belts (continental mountain
ranges and oceanic ridges)
• Earthquake distribution: Concentrated
• Earthquake occurrences over time
• Volcanism in space: Concentrated zones
• Volcanism over time