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.
Change the depth and watch where each layer and discontinuity lies.
How do we know the interior?
A hole you can actually drill stops in part of the crust; even the deepest borehole is around 12 km, so it never crosses an Earth whose radius is 6371 km. Deeper boundaries are read where seismic waves speed up, slow down, or cut off, and those places are named the Moho, Gutenberg, and Lehmann discontinuities. P-waves bend as they cross solids; S-waves do not cross a liquid, so the interval where they vanish in the outer core is the evidence for a liquid state. Set the 12 km borehole next to the Moho at 35 km and the reachable depth does not even reach the first boundary. You pick the cell where wave type and state lock together before you recite layer names, because a drill hole cannot reach that cell.
🌍 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
If you treat the whole mantle as liquid, that claim collides with the evidence that the outer core is the liquid layer. The mantle is closer to solid and only deforms plastically on long timescales; it also holds about 80% of Earth's volume. The inner core can be iron and nickel and still solid because the pressure is different, not because the same composition must mean the same state. When you read an interval that S-waves do not cross as liquid evidence, first match that interval to the outer-core band below 2891 km, and it is safer to overlay the P-wave shadow from 103° to 142° with the S-wave shadow beyond 103° on the same liquid layer. Separate state and makeup by layer first (silicates in the crust, iron and nickel in both cores).
100 km
seegongsik.com
Earth's cross-section: crust, mantle, outer/inner core and discontinuities
On the left, crust, mantle, outer core, and inner core are drawn as circles stacked from the outside in, and dashed circles mark the seats of the Moho, Gutenberg, and Lehmann. A gold line goes down from the surface to the depth you chose and ends in a dot; the sentence at the top writes depth n km together with that layer's name. The depth slider runs from 0 km to 6371 km in steps of 100 km: past 35 km the name becomes mantle, past 2891 km outer core, past 5150 km inner core. The legend on the right keeps the four layer colors and the three discontinuity names fixed. The circles do not spin and the dot does not trace a path; only the layer name that matches the depth is restamped.
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
Comparison 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.
exam-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).