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 core-mantle 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.

• 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)

Discovery of the Earth’s Central Core (Oldham, 1906)

• 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 Lutge

nts)

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 Glatzmaier-Roberts. 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).