Lecture 2: Seismology & the Earth's Interior Flashcards
What is a body wave?
A seismic wave that travels through the body of the Earth
What is a surface wave?
A seismic wave that travels on the surface of the Earth
Which body wave is transverse?
S-waves
Which body wave is longitudinal?
P-waves
What 2 factors influence seismic wave velocity?
Density and elasticity
What is the shear modulus in liquids and gases?
0
What is a seismic discontinuity?
A boundary between 2 of the Earth’s layers that have different properties
What 3 simultaneous events occur when a seismic wave hits a discontinuity?
Wave refraction, reflection, and transformation (P-wave -> S-wave and vice versa)
If seismic wave velocity is constant, what happens to the motion of the seismic wave?
It travels in a straight path
If seismic wave velocity is gradually increasing, what happens to the motion of the seismic wave?
It travels in a curved path
When was the Moho discovered?
1909
How was the Moho discovered?
Shallow refraction surveying
What is the typical thickness of oceanic crust?
7km thick
What is the typical thickness of continental crust?
35km thick
What is the maximum thickness of continental crust?
80km under Himalayas
What layers of the Earth’s interior are considered the lithosphere?
Crust and rigid upper mantle
What is the typical thickness of oceanic lithosphere?
70km thick
What is the typical thickness of continental lithosphere?
150 km thick
Why is the asthenosphere considered a ‘weak’ layer?
It has low elasticity to be able to ‘flow’ and allow the vertical and horizontal movement of the lithosphere
Why can S waves pass through the asthenosphere?
It is partially molten, but a lot of their energy is absorbed because of this
What happens to the seismic velocity at the base of oceanic lithosphere?
It decreases
How thick is the ‘low velocity zone’?
About 100km thick
Why is the Mantle completely solid?
The melting point gradient is higher than the temperature gradient
Which rock type is the Mantle completely comprised of?
Peridotite
Which 4 main minerals is peridotite made from?
Olivine, clinopyroxene, orthopyroxene, and an aluminium rich mineral dependant on depth
How do we know that the mantle is comprised of peridotite?
Ophiolite suites, xenoliths, and meteorites
What are ophiolite suites?
Fragments of old oceanic crust uplifted onto continental crust
What are xenoliths?
Fragments of mantle peridotite contained within explosive volcanic rocks (torn from the mantle during a volcanic eruption)
How do meteorites suggest that the mantle is comprised of peridotite?
Many meteorites are comprised of peridotite and originate from asteroids which are thought to have a similar interior to Earth
What is the boundary between the lithosphere and asthenosphere called?
The Lithosphere-Asthenosphere Boundary (LAB)
How does seismic velocity change between 200-400km deep?
It increases steadily
How does seismic velocity change at 400km deep?
Increases more rapidly
At 400-450km deep, what happens to the main mantle mineral, olivine?
It changes to a higher presser form (same atoms but a more densely packed structure, mimics structure of spinel)
At 670-700km deep what happens to the main mantle mineral, olivine?
Structural change from 4-fold to 6-fold, even more densely packed, mimics structure of perovskite)
By how much does the density increase over the MTZ?
Around 11%
Why does the density increase in the MTZ NOT cause a decrease in seismic velocity?
The elasticity increases which allows the seismic velocity to increase
True or False? The lower mantle (below the Mantle Transition Zone) is homogeneous with a constant uniform structure and composition.
True
How deep is the Core-Mantle Boundary (CMB)?
2900km deep
How do P waves move?
Compressions and dilations
How do S waves move?
Peaks and troughs
What does seismic wave velocity depend on
Density and elasticity of the material its travelling through
How thick is the transition zone of the upper mantle?
400km
