Transcript Lecture 17 Earth's Interior

Lecture 18 Earth's Interior
• Imaging Earth's interior
• Propagation of Seismic waves
• Major layers of the interior: crust, mantle,
core
• Dynamics of the Earth's interior: mantle
convection, geodynamo
Imaging Earth's interior
• How do we know what the Earth's interior looks
like?
• Seismic imaging: As seismic waves travel through
Earth, they carry information to the surface about
the materials through which they pass through.
Thus, seismic records can be used to image of
Earth's interior much like X-rays for medical use.
Seismic rays travel in straight
lines in a homogenous sphere.
The abrupt change in physical
properties (e.g. at the coremantle boundary) causes the
ray paths to bend sharply.
(Tarbuck and Lutgens)
Propagation of seismic waves
The propagation of seismic waves is similar to light in a
way -- concept of seismic RAYS:
 When a seismic wave passes through a homogeneous body,
it travels in a straight line.
 When it passes from one material to another, the ray is bent
(refracted), much like the bending of light from air to
water.
 In addition, energy is reflected from the discontinuity (a
boundary of different materials).
• Seismic energy travels in all directions from an
earthquake source (focus). The energy can be portrayed
as expanding wave fronts or as RAYS perpendicular to
the wave fronts. (Tarbuck and Lutgents)
A. The passage of P waves causes compressions and
expansions. B. The passage of S waves causes change in
shape but no change in volume. Liquid do not resist changes
in shape, it does not transmit S waves. (O.M. Phillips)
Reflection and refraction: Snell’s Law
sin A1 / V1 = sin A2 /V2
• Seismic rays travel in straight lines
in a homogenous sphere. (Tarbuck
and Lutgens)
If seismic velocity
increases with depth,
the rays will curve up
to the surface.
(Tarbuck and Lutgens)
• Examples of possible rays through a layered Earth.
Discovery of the Earth’s Central Core (Oldham, 1906)
• The abrupt change in physical properties at the core-mantle
boundary causes the ray paths to bend sharply, resulting in
a shadow zone for P waves between 105 and 140 degrees.
(Tarbuck and Lutgens)
• P and S wave paths through the Earth. No S waves
through the core because outer core is liquid.
• “Echoes” that bounced back from the boundaries
can be used to determine the depths of the central
core and inner core. (Tarbuck and Lutgens)
• P and S velocities through the depth of the Earth.
Major layers of Earth’s interior
• Chemical differentiation in early Earth: heavier elements
such as iron and nickel sank and lighter elements floated
upward.
• The principal layers include: the crust, the mantle, and the
core (including a fluid outer core and a solid inner core).
• The crust and mantle are made of rocks (silicate-rich
minerals).
• The core is made of iron-nickel alloy with light elements.
• Internal structure of the Earth. (Tarbuck and
Lutgents)
The crust
• The crust is the thin outer shell. The
thickness of continental crust is about 30
km on average, but exceed 70 km in some
mountain belts (such as Himalayas). The
oceanic crust is much thinner, about 3 km to
15 km.
The mantle
• The mantle extends to a depth of about
2900 km. The mantle is a solid (it can
transmit S waves), rocky (silica-rich) layer.
The core
• Formation: The core was formed early in Earth's
history as heavy molten iron alloy migrated
toward the center of the planet. High temperatures
(~5000K) keep the bulk of the core liquid. As the
Earth cooled through mantle convection, molten
iron began to solidify under enormous pressure
(over 3 million times atmosphere pressure) to
form the the solid inner core.
The core (continued)
• The outer core is made of iron (nickel) with some
other elements and the inner core is almost pure
iron (nickel).
• The core radius is 3400 km, larger than Mars; the
inner core radius is 1220 km, slightly smaller than
the moon.
• No S waves have been observed to traverse the
core, an indication that the outer core is liquid.
Dynamics of the Earth's interior
Earthquakes and volcanoes provide vivid displays
of the dynamic nature of our planet. What are the
driving engines of the dynamic system?
• Mantle convection
• Geodynamo and Earth’s magnetic field
• Inner core rotation
mantle convection
• Convection is a process of heat transfer by mass
movement.
• The mantle is convecting, despite slowly in human
time scale -- Warm, lighter rocks rise, and cooler,
denser materials sink.
• The energy sources of mantle convection comes (1)
radioactive decays; (2) heat from the core; (3) heat
converted from the gravitational energy of
colliding materials during the planet formation.
Fluid core convection and Earth's magnetic
field
• The Earth has a magnetic field.
• The magnetic field is generated by fluid
motions in the core (which is a good electric
conductor) through self-excited processes
(so called geodynamo).
• A snapshot of the
3D magnetic field
structure
simulated by
GlatzmaierRoberts.
Magnetic field
lines are blue
(inward) and
yellow (outward).
• Seismological observations suggest that the Earth’s solid
inner core is rotating relative to the mantle by about 1
degree per year (Song and Richards).
• Seismic waves
were used to
detect the
rotation of the
inner core by
Song and
Richards
(1996).