We cannot drill deep into Earth, but changes in seismic-wave speed and path let us map the interior like a CT scan.
With depth come crust, mantle, outer core (liquid iron), and inner core (solid iron), bounded by discontinuities such as the Moho, Gutenberg, and Lehmann.
The disappearance of S-waves in the outer core is evidence that the outer core is liquid.
Here you can move the depth slider and explore where each layer and discontinuity lies.
How do we know the interior?
🌍 Looking inside Earth with seismic waves
①We can't directly drill into Earth (deepest borehole: ~12 km)
②Seismic waves change speed and path as they pass through the interior
③Analyzing those changes reveals interior structure (similar to CT scans!)
Explore the Earth's cross-section
100 km
seegongsik.com
Earth's cross-section: crust, mantle, outer/inner core and discontinuities
S-waves (transverse) cannot pass through liquid → S-waves vanish at outer core → outer core = liquid
key evidence for determining the state of Earth's interior
Earth's internal materials
Layer properties
ChartComparison of internal layers
Layer
Depth (km)
State
Composition
Crust
0~35
solid
silicate minerals (SiO₂)
Mantle
35~2,891
solid (ductile)
olivine, pyroxene
Outer core
2,891~5,150
liquid
iron + nickel
Inner core
5,150~6,371
solid
iron + nickel
Source of Earth's magnetic field
convection of liquid iron in outer core → generates magnetic field (geodynamo theory)
solid inner + liquid outer core combination is essential to the field
Worked Examples and Exam Practice
Example 1
The Gutenberg discontinuity, where S-waves suddenly vanish, is the boundary between which two layers, and what does that tell us about the layer below?
1
The Gutenberg discontinuity is the boundary between the mantle and the outer core.
2
S-waves (transverse) cannot pass through liquid, and they vanish here, so the outer core is liquid.
▸ Mantle/outer-core boundary; the outer core is liquid
The disappearance of S-waves is decisive evidence the outer core is liquid. Convection of that liquid iron generates Earth's magnetic field.
Example 2
After a quake, P-waves barely reach the angular range 103°~142° from the epicenter, and S-waves reach nothing beyond 103° (shadow zones). Why is the S-wave shadow zone wider?
1
P-waves are refracted by the liquid outer core and miss the 103°~142° range.
2
S-waves cannot pass the liquid outer core at all, so they miss everything beyond 103°, making a wider shadow zone.
▸ Because S-waves cannot pass the liquid outer core at all (shadow beyond 103°)
The difference in shadow-zone width is another piece of evidence that the outer core is liquid.
CSAT-style
Which statement about Earth's interior is correct?
①The Moho is the boundary between the mantle and the outer core
②S-waves cannot pass the outer core because it is liquid
③The inner core is liquid
④The mantle makes up about 10% of Earth's volume
⑤Earth's magnetic field arises from the magnetism of the solid inner core
▸ ② S-waves cannot pass the outer core because it is liquid
1
S-waves are transverse and cannot pass liquid; they vanish below the Gutenberg discontinuity, so the outer core is liquid.
2
The Moho is the crust/mantle boundary (①), the inner core is solid (③), the mantle is about 80% of Earth's volume (④), and the magnetic field comes from convection of the liquid outer core (⑤).