1. How do you measure something you can’t observe directly?
Earth’s Interior
How do you measure something you can’t observe directly?

2. 300 years ago, Isaac Newton used observations of the planets and his law of gravity to calculate that the density of the Earth was double that of the surface rocks.
His density value is essentially unchanged today.
Great hair. Greater scientist.
Newton

3. Therefore, since 1700 we have known that the interior of the Earth was composed of different material to the crust.
We now differentiate the interior into two types of mantle (asthenosphere and lower mantle) and two types of core (outer and inner).
The key to measuring the size of each of these regions is the study of seismic waves.
Differentiation

4. Two ways to divide into layers
By chemical composition (what it’s made of):
Crust
Mantle
Core
By physical properties (density, state of matter, etc.):
Continental crust
Oceanic crust
Asthenosphere (plastic part of upper mantle)
Lower mantle
Outer Core
Inner Core
Two ways to divide into layers

5. Waves change speed when they pass from one medium to another. Why
Waves change speed when they pass from one medium to another. Why? Well, look at this lawnmower:
Refraction

6. Longitudinal / Compression Transverse
P-waves
S-waves
Faster
Slower
Longitudinal / Compression
Transverse
Pass through all states of matter
Only pass through solids
Go to YouTube and search for “Curving Seismic Paths”

7. Seismic waves refract when they pass through the mantle/outer core boundary.
No P-waves can be detected between 104 and 140 degrees from the focus of an earthquake.

8. Why can’t we detect any P-waves in the shadow zone?
Why can we detect P-waves opposite the earthquake focus?

9. S-waves aren’t completely blocked by the outer core, they are attenuated. This means they gradually fade to nothing.
Another example: sunlight is attenuated by sunglasses.
So only P-waves are detected on the opposite side of the Earth.
Attenuation