12a Earth's Interior

Geological and geophysical techniques are used to explore Earth’s interior.
Physical Samples:
- Magma brings up pieces of deeper rocks.
- Mines provide access to subsurface.
- Drilling allows direct sampling but is limited by depth.
- Exposures of deep, uplifted rocks reveal deep Earth materials.
The Deepest Borehole On Earth:
- The Kola (Scandinavia) superdeep borehole (SG3) reached a depth of $12,262$ meters ( $40,230$ ft) in 1989.
- Drilling stopped when temperatures at the bottom reached $180$ °C ( $356$ °F).
- The site is now abandoned.
Geophysical Surveys:
- Magnetic data provides information about magnetic properties.
- Gravity data reflects density variations.
- Electrical surveys measure electrical resistivity.
- Seismic-reflection data images subsurface structures.

Most knowledge of Earth’s interior comes from the study of earthquake waves.
Travel times of P (compressional) and S (shear) waves through the Earth vary depending on the properties of the materials.
Variations in the travel times correspond to changes in the materials encountered.

Seismic rays emerge from a hypocenter (focus) and travel through the Earth along bent paths.
Recorded by distant seismograph stations.
The character and travel times of seismic rays reveal important clues about the Earth’s interior.
Seismic Refraction: The bending of seismic rays passing through the Earth is called refraction.

Seismic waves can also reflect at the boundary between different materials.
Reflected seismic waves are used in the search for underground oil and gas reserves.
When seismic waves (rays) encounter a boundary between materials with different properties, the energy splits into reflected and refracted (bent) waves.
When the velocity of seismic waves decreases when passing from one layer into another, the waves refract (bend) downward away from the boundary separating the layers.
When the velocity of seismic waves increases when passing from one layer into another, the waves refract (bend) upward towards the boundary separating the layers.
P-waves bend outward when traveling through the mantle due to increasing velocity with depth.
Increasing velocity due to increasing mantle density with depth.

By composition:
- Crust (basalt/granite)
- Mantle (Mg-silicates)
- Core (iron)
By physical properties:
- Lithosphere
- Asthenosphere
- Transition zones
- Mesosphere
- D’’ layer
- Outer Core
- Inner Core

Seismic waves can reveal the thickness of Earth’s crust.
The Moho (Mohorovicic discontinuity):
- Discovered in 1909 by Andriaja Mohorovicic.
- Mohorovicic found that the average velocity of P waves increased ~ $200$ km from the earthquake source.
- He concluded that at ~ $50$ km depth, there was a change in physical properties that marked the base of the crust.
- Base of the crust is now defined as the Moho.
The depth to the Moho varies from place to place:
- Lies at an average depth of $22$ mi ( $35$ km) below continents and $4.5$ mi ( $7$ km) beneath oceanic crust.
The velocity of seismic waves increases rapidly at this boundary:
- Both P- and S-wave velocities increase when crossing from the lower crust (granite/basalt) to upper mantle (peridotite).
At station #1, slower direct wave arrives before refracted wave due to shorter distance.
At station #2, direct and refracted waves arrive at the same time.
At station #3, faster refracted wave overtakes direct wave and arrives first, even though it travels a longer distance.
Distance from the source where crossover occurs can be used to calculate crustal thickness (down to Moho).

$Xd = crossover distance$
$H = crustal thickness (km)$
$Vn = crustal P-wave velocity (e.g., 6 km/sec)$
$V1 = mantle P-wave velocity (e.g., 8 km/sec)$
Oceanic Crust:
- Ranges from $3$ to $15$ km thick.
- Consists primarily of basalt and gabbro.
Continental Crust:
- ~ $40$ to $65$ km thick.
- Average composition of granite.
- Less dense (more buoyant) than oceanic crust.
Seismic Wave Velocities Increase Abruptly At the Moho Due To Compositional Change From Crustal Rocks To Mantle Peridotite
Lithosphere (sphere of rock) includes the crust and solid upper portion of the mantle:
- Relatively cool, rigid layer
- Averages about $100$ km in thickness, but may be $250$ km or more thick beneath the older portions of the continents.
- Lithosphere broken into a series of plates
The Asthenosphere occurs in the upper mantle just beneath the lithosphere:
- Also known as the low velocity zone because it slows down seismic waves
- Small amount of melting in the upper portion of the asthenosphere makes this layer mobile
- The mobile asthenosphere is therefore mechanically detached from the overlying rigid lithosphere
- Deeper asthenosphere is significantly less mobile

Seismic Wave Velocities Decrease Within The Upper Asthenosphere Because Peridotite Contains A Few Percent Partial Melt, But Not Enough To Completely Stop S-waves
Two Transition Zones Marked By Increases in Seismic Wave Velocities
At ~ $400$ Km Depth, Olivine Changes To The Spinel Crystal Structure Of Higher Density
At ~ $660$ Km Depth, Spinel Changes To The Higher Density Perovskite Crystal Structure
Velocities Of S- And P-waves Increase With Depth In The Lower Mantle (Mesosphere)
Rocks in the Mesosphere, although solid, are very hot and capable of gradual flow
The D” Layer
- Comprises the bottom few hundred kilometers of the lower mantle, just above the outer core.
- Exhibits large horizontal variations in both temperature and composition.
- Possible graveyard of subducted oceanic lithosphere
- Birthplace of some mantle plumes

The Outer Core is Composed of Liquid Iron
- P-waves slow down.
- S-waves cannot pass through it.
- P-waves bend downward when entering the outer core due to a decrease in velocity
- Bending of P- waves in the outer core creates P- wave shadow zone, and they bend again when they leave.
- S-waves cannot travel through the outer core, which creates S-wave shadow zone, much larger than the P-wave shadow zone.
- Outer Core Slows Down P-waves And Stops S-waves, Indicating That The Outer Core Is Liquid
The Inner Core is Composed of Solid Iron and Nickel
- P-waves speed up again.