1. Earth’s Interior “Seeing into the Earth”

2. Structure of the Earth I. From Seismic/Chemical classification Crust
Continental
Oceanic
“Moho” - seismic wave velocity discontinuity at crust--mantle boundary
Mantle
Core
Outer liquid core
Inner solid core

3. Crust Earth’s outermost layer 5-80 km thick
Composed of relatively low-density silicate rocks

4. Continental crust Makes up the continents
Predominantly granitic composition
20-80 km thick
Less dense than oceanic crust

5. Oceanic crust Makes up most of ocean floors
Predominantly basaltic composition
5-15 km thick
Denser than continental crust

6. Mantle Mostly solid (few % melt in upper mantle)
Extends from crust to core (~2900 km depth)
Represents ~80% of total volume of the earth
Upper Mantle
Extends to 670 km seismic discontinuity
Lower Mantle
670 km to 2900 km

7. Core Innermost region of earth
Believed to be composed of iron, nickel and sulfur
Outer core
Liquid portion of core extending from mantle to ~5150 km
Inner core
Solid portion of core extending from inner core to center (~6370 km)

8. Structure of the Earth II. From Mechanical Behavior Lithosphere
Cool, rigid (brittle) outer layer of the Earth
Extends to ~125 km
Includes crust and part of upper mantle
Asthenosphere
Hot, ductile, weak portion of upper mantle
Extends from base of lithosphere to ~350 km depth

9. LAYERS OF EARTH I. Seismic/Chemical II. Mechanical (Rheological)
Layer Depth Composition Density Characteristics
(km) (g/cm3)
Crust Continental Granitic 2.5 cool and rigid
Oceanic 5-10 Basaltic 3.0 cool and rigid
Mantle Upper to 670 ultramafic 3.5 partially 1-2% molten
Lower ultramafic 5.5 high pressure minerals
Core Outer iron + nickel 10 liquid
Inner iron + nickel 13 solid
II. Mechanical (Rheological)
Lithosphere to 125 varies cool, rigid, brittle
Asthenosphere ultramafic hot, weak, plastic

10. How do we know? Direct Sampling Deepest mines ~3 km (2 miles)
Deepest oil wells ~8 km
Deepest research drilling ~ km
Mantle nodules and diamond pipes provide samples from upper mantle ( km).
But, radius of Earth ~6370 km

11. How do [we think] we know?
Indirect Sampling
Seismic waves
Density (inertia) studies
Isostacy
Gravity
Magnetics
Heat

12. Seismic Waves
Velocity of seismic waves increases with increasing density of the rock. Hence, seismic wave velocities show general increase with depth into the Earth.
When seismic wave crosses a boundary between rock layers of different density it may be:
Reflected off of the boundary and back toward the surface, and/or
Refracted or bent due to the slowing down of the wave.

13. Seismic Wave Shadow Zones
P-wave shadow zone
No direct P-waves are detected at 103° to 142° from earthquake focus.
‘Shadow’ zone is due to refraction of P-waves at the core-mantle boundary.
Indicates dramatic increase in density

14. Seismic Wave Shadow Zones
S-wave shadow zone
No direct S-waves are detected at >103° from earthquake epicenters.
‘Shadow’ zone indicates that S waves do not travel through the core at all.
Implies that Earth’s core is liquid (or acts as a liquid).

15. Density Overall density of Earth = 5.5 g/cm3
Calculated from speed of earth’s revolution about the sun and the speed of its rotation on its own axis.
Density of crustal rocks = 2.7 to 3.0 g/cm3
From direct measurements
Density of mantle rocks = 3.3 g/cm3 upper mantle to 5.5 g/cm3 at base of mantle.
Some direct measurements and calculations from experimental data.
Density of core must be 10 to 13 g/cm3
By mass balance calculation.

16. Isostacy
Isostacy (“equal standing”) refers to the balance of the height of a less dense mass floating in a more dense material to acieeve hydrostatic equilibrium.
Isostatic adjustment - vertical movement of masses to achieve hydrostatic equilibrium.
Isostatic rebound - uplift of the crust in response unloading by erosion or melting of glaciers.
Provides a measure of the viscosity of the asthenosphere.

17. Force = constant [(massA x massB) / distance2]
Gravity
Gravitational forces are dictated by the masses of the objects and the distance between them.
Force = constant [(massA x massB) / distance2]
The gravitational force is greater for equal volume of denser rocks.

18. Gravity Anomalies Deviations from normal regional gravity.
Positive when greater than normal, negative when less than normal.
Useful for:
Determining isostatic equilibrium
Minerals exploration (ie, dense metal ores)

19. Magnetics The Earth has a dipole magnetic field.
Strength of the field is greatest at the poles.
Source of Magnetic Field:
Earth’s magnetic field is generated in the liquid outer core by convection currents.
Convection of metal generates an electrical current that creates magnetic field.

20. Magnetism in Rocks
Many rocks contain the direction and strength of the magnetic field present at the time they formed.
When iron-rich minerals like magnetite crystallize and cool through Curie point (`580°C), they acquire the direction of earth’s magnetic field at that time.

21. Magnetic Reversals
Studies of the magnetic field of stacked lava flows on continents show that some of the lava flows have a magnetic orientation opposite to the current magnetic field.
Explanation is that the direction of earth’s magnetic field has change through time:
Normal - aligned with current field direction
Reverse - aligned opposite to current field direction

22. Heat flow
Small but measurable amount of heat is gradually being lost through the earth’s surface.
Sources of Thermal Energy
Acquired during planetary accretion (“original” heat).
Decay of radioactive isotopes.

23. Heat flow
Average heat flow from continents is the same as from the seafloor.
Seafloor’s heat due to rising hot mantle rock.
Continent’s heat from great concentration of radioactive material.
Regional heat flow varies:
High heat flow at mid-ocean ridges
Low heat flow at deep ocean trenches.

24. Temperatures in the Earth
Geothermal Gradient
~25°C/km depth near the Earth’s surface
Drops to ~1°C in the mantle.
At core-mantle boundary T = 4800°C
At inner core - outer core boundary T = 6600°C
At center of Earth T = 6900°C (~1000°C hotter than surface of the sun!)