Lecture 3: Earth’s Core and Geomagnetism Flashcards
When was the Core-Mantle Boundary discovered and by who?
Beno Gutenberg in 1914
How was the Core-Mantle Boundary found?
via the interpretation of the shadow zone on a travel time graph of seismic waves
What interval can we detect P & S waves from after an Earthquake?
0° and 105°
What interval do P & S waves disappear from after an Earthquake?
105° to 142° (shadow zone)
When do P waves reappear after an earthquake?
142° to 180°
What does P waves reappearing later than expected after an earthquake tell us?
that they’ve slowed down as well as been refracted
Do S waves reappear after an Earthquake?
No
Why do S waves not reappear after an Earthquake?
Likely because S-waves cannot go into the outer core (because it’s a liquid)
Where is the shadow zone in the interior of the Earth?
At the boundary between the solid mantle and liquid outer core
What happens at the CMB to P waves and why?
sharply refract vertically downwards due to the vast different in temperature
What happens at the CMB to S waves and why?
there is no refraction at CMB due to shear modulus being 0 → S-waves cannot be transmitted through liquid
What happens to the speed of P & S waves inside the Earth?
Constantly increasing because temperature is increasing
What happens to P-waves in the shadow zone?
They travel slower
What is the shadow zone the boundary between?
The core and mantle
When was the inner core discovered?
1936
How was in the inner core discovered?
by identifying the PKIKP phase (the small amount of energy that appears) in the shadow zone
What is the depth of the inner core?
5150km
How does P-wave velocities change in the inner core?
rapidly increase
What happens to S-waves in the inner core and what does this mean?
reappear transformed from P-waves, so the inner core must be solid
How does P-wave speed change from the crust to the mantle?
Increases from 7-8km/s
How does P-wave speed change in the mantle?
Increases from 8-13.5km/s
How does P-wave speed change in the liquid outer core?
Drops to 8.5km/s (because of its liquid state)
How does P-wave speed change in the solid inner core?
speeds up to 11.5 km/s.
How does S-wave speed change from the crust to the mantle?
Gradually increases from 4.5km/s to 7.5km/s
How does S-wave speed change when it hits the outer core?
Falls to 0 because it is liquid (SM is 0)
How does S-wave speed change when it hits the inner core?
after some P waves are transformed into S waves upon reaching the inner core, S wave speed increases to 4 km/s.
How does density change from the crust to the mantle?
Gradually increases
How does density change upon reaching the outer core (and why)?
Rapidly increases due to its high iron composition
How does density change upon reaching the inner core (and why)?
Continues increasing as the state changes to solid
How does iron content change from mantle to outer core?
Increases drastically
How does oxygen and silicon content change from mantle to outer core?
Drastically decreases
How does magnesium content change from crust to mantle?
Increases
How does aluminium content change from crust to mantle?
Decreases
What is the core mostly composed of?
Iron
What is the geochemical evidence for the core being made of iron?
The sun and meteorites contain a lot of iron, so Earth must have an abundance of iron (more than we know is in the crust and mantle)
What are the two main types of meteorite (and what are they similar to)
Stony meteorites (similar to Earth’s mantle) Iron Meteorites (similar to Earth’s core?)
What is the two main geophysical evidences for an iron core?
Seismology and Geomagnetism
How much faster do Seismic waves travel through the inner core N-S than E-W and why?
4% - Recent experiments/modelling show that Fe atoms might line up along N-S axis as they crystallise increasing rigidity along that axis
How is the inner core changing and why?
The inner core is growing slowly as the Earth cools as molten iron in outer core freezes and ‘snows’ onto surfact of inner core
Releases latent heat and iron-depleted liquid
What is the geomagnetic evidence for an iron earth core?
Earth has magnetic field generated by convection of electrically conducting liquid in outer core → iron is suitable for this
What is the earth’s magnetic field similar to (but why is it not the exact same and what does this mean)?
A bar magnet
But not exactly as when an object is heated hot enough (like the core) it loses its permanent magnetism → something else must be at play?
What degree is the Earth’s magnetic field tilted from the earth’s rotation axis?
10 degrees
Is geographic north magnetic north?
No
What is a dip meter?
a compass whose needle can move in vertical plane
What is magnetic dip measured relative to?
Horizontal
What does magnetic dip vary with?
Magnetic latitude
Where is dip 0 degrees?
At the magnetic equator
Where is dip 90 degrees?
At magnetic poles
What does magnetic field strength vary with?
Latitude
Where is magnetic field strength strongest/weakest?
Weakest at magnetic equator
Strongest at magnetic poles
Why does the earth’s tilt change overtime?
because of the movement of liquid within the core
Can the earth’s interior produce permanent magnetism?
The Earth’s interior is too hot for permanent magnetism to be possible.
How does the earth generate a magnetic field?
The Earth’s interior is too hot for permanent magnetism to be possible. The geomagnetic field is therefore generated by electrically conducting metallic liquid in the outer core
What is the earth’s magnetic field likely affected by?
The Earth’s inner core and rotation, explaining why it is generally aligned with the Earth’s spin axis
What does the earth’s magnetic field occasionally do?
Reverse direction by 180 degrees
Why does the earth’s magnetic field sometimes reverse?
Due to change in pattern of convective fluid flow
How long does the magnetic field reversal process take?
Around 4000 years
