Chapter 12 Earth s Interior

1. Chapter 12 – Earth’s Interior Most of what we know about the Earth’s interior is from the study of seismic waves. From Chapter 11, remember that Body Waves occur as P-waves (Compressional Waves) and S-waves (Shear Waves). In all materials, P-waves travel faster than S-waves. Liquids do not transmit S-waves*. Velocity is dependent on density and elasticity of Earth materials. Pressure and compaction causes waves to travel faster with depth. When waves cross an interface between materials, refraction & reflection occurs. (*See slide 9).

2. The Earth’s principle layers as defined by Composition – iron-rich core, mantle, and crust. The Earth’s layers as defined by Physical Properties. From the center – Inner core of is “solid” because of intense pressure and has a radius of 3486 km. Outer core is a thick liquid that maintains a convective flow that generates the Earth’s magnetic field. Outer core is 2270 km thick and both parts consist of an Fe-Ni alloy. Mesosphere – Lower mantle – semi-solid from 660 km to 2900 km below surface.

3. Asthenosphere is the outer portion of the mantle from just below the crust to approx. 660 km (mesosphere boundary). The upper portion of the asthenosphere is semi-molten and structurally weak and is thought to host slow-moving vertical convection currents and isolated plumes. The lithosphere, the crust, averages approx. 100 km. in thickness, but may reach 250 km. beneath large mountain ranges.

4. Major Earth boundaries were revealed by the refraction and reflection of seismic waves as they crossed boundaries (interfaces). Because of the precise location of post-WWII nuclear tests, locations were precisely known. The “Moho” boundary was discovered by evidence of deep seismic waves (below 50 km) traveling faster than shallow seismic waves (see pp. 366 – 368). The refraction of P-waves through the Earth, that produced a zone of “No P-waves” from 105 to 140 degrees revealed the Mantle – Core boundary.

5. The Lehmann boundary (see Box article 12.1) marks the boundary between the inner and outer cores. The Mantle (pp. 370 – 371) text explains more about vertical changes within the mantle due to varying pressure conditions with increasing depth. The origins of the compositional layers is dicussed in pages 372 – 373. It is thought that while the Earth was molten, the more dense Fe-Ni sank to the core, while the other materials were separated by density contrasts as well. It is thought that the inner core is growing as it cools.

6. Box article 12.2 (p. 374) contains informa-tion about the current understanding as to the origins of the Earth’s magnetic field. When magnetic materials are heated, they lose their magnetism, thus there must be another reason for the Earth’s magnetic field, beyond the Fe-Ni core. The current concept is that interactions between the solid core and the slowing churning outer core, both of which are composed of an Fe-Ni alloy, contribute to an electrical field that reinforces the existing magnetic field.

7. Through study of the iron-bearing minerals in the basaltic seafloor and their magnetic orientations, we have learned that the Earth’s magnetic polarity reverses itself about every one million years. How or why this reversal takes place is unknown. Recent calculations suggest that the magnetic field has weakened about 5% in the last 100 years. At this rate, in about 1500 years, the magnetic field would become weak or non-existent (p. 374). Whether this is the prelude to a reversal is unknown.

8. The Earth’s internal heat is thought to be from: Radioactive decay of Uranium, Thorium, and Potassium isotopes. Heat released during the crystallization of the iron core. Heat released by colliding particles during “Planetary Accretion”. Heat from first two is less than in geologic past, thus Earth is getting progressively cooler.

9. Crustal Heat Flow is related to the: Thickness of the crust. Presence of young, shallow intrusions. Quantity of radioactive minerals undergoing decay. Mantle convection of heat is thought to drive Plate Tectonics. Though outer mantle is semi-molten, it does transmit S-waves due to confining pressure. The outer core does not transmit S-waves. Model on Fig. 12.14 (p. 375) is “Deep Con-vection Model”. Other models suggest more shallow mantle convection.