Chapter 12: Earth's Interior

• Geophysics: The study of the physical properties of the Earth and its environment using methods such as seismic waves, gravity, and magnetic fields to understand the structure and dynamics of the Earth.

• P-waves (Primary waves): Seismic waves that compress and expand the material through which they pass. They are the fastest seismic waves and can travel through both solids and liquids.

• S-waves (Secondary waves): Seismic waves that move the ground up and down or side to side. They are slower than P-waves and can only travel through solids.

• Surface waves: Seismic waves that travel along the Earth’s surface. They are slower than P-waves and S-waves but cause the most damage during an earthquake.

• Reflection: The bouncing of seismic waves off a boundary within the Earth.

• Refraction: The bending of seismic waves as they pass through materials of different densities.

• Shadow zone: The area on the Earth’s surface where seismic waves are not detected due to the refraction of P-waves and the absorption of S-waves by the liquid outer core.

• Lithosphere: The rigid, outer layer of the Earth, consisting of the crust and upper mantle.

• Asthenosphere: The semi-fluid layer of the mantle beneath the lithosphere, responsible for the movement of tectonic plates.

• Moho (Mohorovičić Discontinuity): The boundary between the Earth's crust and the mantle, marked by a change in seismic wave speeds.

• Isostasy: The state of gravitational equilibrium between Earth's crust and mantle, where the lithosphere "floats" on the more fluid asthenosphere.

• Transition zone: The part of the mantle between 410 and 660 km deep where phase changes in minerals occur due to changes in pressure and temperature.

• Phase changes: Changes in the physical state (e.g., from solid to liquid) of minerals in the Earth's interior due to variations in pressure and temperature.

• Conduction: The process by which heat is transferred through a material from high to low temperature without the movement of the material itself.

• Convection: The movement of heat through a fluid (like the mantle) caused by differences in temperature and density.

• Geothermal gradient: The rate at which temperature increases with depth beneath the Earth’s surface.

• Heat flow: The transfer of heat from the Earth's interior to the surface, primarily through conduction and convection.

• Geodynamo: The process by which the Earth's magnetic field is generated by the movement of molten iron in the outer core.

• Dipole field: A magnetic field that has two opposite poles (north and south), which is the basic configuration of Earth's magnetic field.

• Paleomagnetism: The study of the Earth’s past magnetic field preserved in rocks, which provides evidence for plate tectonics and continental drift.

• Thermoremanent magnetization: The process by which minerals in rocks become magnetized by the Earth’s magnetic field during cooling, preserving a record of the field at the time of formation.

• Curie point: The temperature at which a material’s magnetic properties change. Above this temperature, the material loses its thermoremanent magnetization.

• Magnetic reversals: The periodic reversal of Earth's magnetic field, where the magnetic north and south poles switch places.

Concepts:

1. How do we “see” into the Earth?

   • We “see” into the Earth using seismic waves generated by earthquakes or artificial sources. By analyzing how these waves travel through the Earth, we can infer the structure and composition of different layers.

2. How is refraction and reflection of seismic waves used to image the Earth?

   • Refraction occurs when seismic waves change direction due to differences in the material properties of the Earth. Reflection happens when seismic waves bounce off boundaries between layers. These processes help scientists map the Earth's interior by analyzing the travel times and directions of waves.

3. How are the depths of boundaries in the Earth determined?

   • Depths are determined by analyzing seismic wave velocities and how they change as they pass through different layers. These changes help pinpoint the boundaries between the Earth's layers.

4. How do shadow zones help determine the structure of the Earth?

   • Shadow zones occur because S-waves cannot travel through liquids, and P-waves are refracted in the liquid outer core. The location and size of these zones provide evidence of the liquid outer core and help estimate the size and composition of the Earth’s interior.

5. What affects how waves move through the Earth? Why do waves speed up or slow down?

   • The composition, temperature, and state (solid or liquid) of the Earth’s layers affect seismic wave speeds. Waves speed up when passing through denser materials and slow down when passing through less dense or more fluid materials.

6. What are the major boundaries in the Earth?

   • The major boundaries are:

     • Crust-Mantle boundary (Moho)

     • Mantle-Core boundary (core-mantle boundary)

     • Outer core-Inner core boundary

7. Is the Moho the same depth everywhere? Why or why not? What is a good example?

   • No, the depth of the Moho varies. It is generally deeper beneath continental crust (30-50 km) and shallower beneath oceanic crust (5-10 km).

8. What is important about the core-mantle boundary? What ends and starts there?

   • The core-mantle boundary is where there is a dramatic change in material properties. The solid mantle transitions to the liquid outer core, and this boundary is important for understanding seismic wave behavior.

9. How do we know the composition of the core?

   • We know the core is composed mainly of iron and nickel through the study of seismic waves, which travel differently through solid and liquid materials. The way P-waves and S-waves behave at the core-mantle boundary provides indirect evidence of the core’s composition.

10. What are the sources of heat in the Earth?

    • Sources of heat include:

      • Radioactive decay of isotopes in the mantle and crust.

      • Primordial heat from the planet's formation.

      • Gravitational differentiation as materials separated during the planet's formation.

11. How is the Earth’s internal temperature controlled? Can conduction explain Earth’s heat flow?

    • Earth’s internal temperature is controlled by heat from radioactive decay, gravitational compression, and the heat left over from the planet’s formation. Conduction alone cannot fully explain the Earth’s heat flow; convection in the mantle plays a crucial role in transporting heat.

12. What is the geodynamo? What is it comparable to? How is it created?

    • The geodynamo is the process by which the Earth’s magnetic field is generated, similar to the working of an electromagnet. It is created by the motion of molten iron in the outer core, which generates electric currents, producing a magnetic field.

13. How is thermoremanent magnetization used? What is important about the Curie point?

    • Thermoremanent magnetization is used to study past magnetic fields in rocks. The Curie point is important because it is the temperature below which minerals retain their magnetic properties. This allows rocks to record the Earth’s magnetic field when they cool below this temperature.

14. Are magnetic reversals episodic or periodic? How are their time lengths measured?

    • Magnetic reversals are periodic but not regular. Their time lengths are measured by studying the pattern of magnetic stripes on the ocean floor, which provide a record of past reversals.