Nobody can travel into the Earth, but piecing together the depths where seismic waves speed up, slow down, or stop reveals where the layers meet and what state each one is in.
From the outside in lie the crust, mantle, outer core, and inner core, and at the boundaries between them (the Moho at about 35 km, the Gutenberg discontinuity at about 2891 km, and the Lehmann discontinuity at about 5150 km) seismic speeds change abruptly.
The strongest evidence that the outer core is liquid is that S-waves do not continue below the Gutenberg discontinuity, and the rise in P-wave speed at the Lehmann discontinuity shows the inner core is solid.
As you change the depth, the cross-section on the left marks the layer you have reached and the graph on the right gives the P- and S-wave speeds there.
How can we see inside the Earth?
The deepest hole ever drilled, the Kola borehole in Russia, reaches about 12 km. With the Earth's radius at 6371 km, that scratches only about 0.2% of the way down, and since the continental crust alone averages around 35 km thick, no drill has reached even the first boundary, the Moho. So the interior is read with seismic waves. They spread faster through stiffer material, and when they meet material with different properties, their speed and direction change. S-waves travel by shaking the material sideways, and a liquid cannot resist a sideways push, so S-waves cannot pass through it; just finding the depth where S-waves cut off is enough to tell whether what lies below is liquid. That is why it pays to look at where the speeds change before memorizing the names of the layers.
🌍 Reading the interior with seismic waves
①Even the deepest borehole, about 12 km, stays in the outer skin of an Earth 6371 km in radius
②Seismic waves travel faster through stiffer material and change speed and direction at a boundary between different layers
③P-waves (longitudinal) cross solids and liquids; S-waves (transverse) cross only solids
④Arrival times at many stations let us work back to the speed at each depth, and from that to the boundaries and states of the layers
Reading the speeds on the way down
It is easy to picture the mantle as a sea of molten magma, but S-waves pass through the mantle without a break, so the mantle is solid. It does creep very slowly, a few centimeters a year over tens of millions of years, and that creep drives mantle convection and moves the plates above it. The other common mix-up is about temperature. The inner core is the hottest layer, yet it is solid, because pressure rises toward the center and raises the temperature at which iron melts. The outer core, even under its own pressure, is hotter than its melting point and stays liquid. That is also why, in the graph below, the red S-wave curve breaks off at the Gutenberg discontinuity and picks up again in the inner core.
1000 km
seegongsik.com
Earth's cross-section (left) and P- and S-wave speeds by depth (right)
The circle on the left is the Earth cut in half, and the end of the gold line running down from the surface marks the depth you have chosen. The center of the circle lies at 6371 km, so when the line reaches it, you are at the very middle of the planet. In the graph on the right, the vertical axis is the same depth and the horizontal axis is seismic speed; the blue line is the P-wave, the red line the S-wave, and the gold horizontal line your current depth. At the Moho both curves jump to the right together, and at the Gutenberg discontinuity the P-wave drops from 13.7 km/s to 8.0 km/s while the S-wave vanishes. Past the Lehmann discontinuity the P-wave speeds up again and the S-wave returns at about 3.5 km/s. The top line gives the layer name and both speeds at that depth; the speeds are rounded values simplified from the Preliminary Reference Earth Model (PREM).
💡 What changes with depth
①From 0 to about 35 km lies the crust, where P-waves are slowest at about 6 to 7 km/s
②From the Moho down to 2891 km lies the mantle, which is solid, so S-waves pass too, and both waves speed up with depth
③From 2891 to 5150 km lies the outer core, where S-waves vanish and P-waves drop from 13.7 km/s to 8.0 km/s
④From 5150 to 6371 km lies the inner core, where P-waves speed up again and S-waves reappear at about 3.5 km/s
The three discontinuities and the state of each layer
Major discontinuities
Boundaries where seismic speed changes abruptly
Mohorovičić discontinuity (Moho)
boundary between crust and mantle; P-waves speed up from about 7 km/s to 8 km/s
about 35 km deep (continents)
Gutenberg discontinuity
boundary between mantle and outer core; S-waves vanish and P-waves slow sharply → the outer core is liquid
about 2891 km deep
Lehmann discontinuity
boundary between outer and inner core; P-waves speed up again → the inner core is solid
about 5150 km deep
The depth where S-waves stop
S-waves (transverse) cannot cross a liquid → S-waves stop below 2891 km → the outer core is liquid
It is also why the S-wave shadow zone (beyond 103°) is wider than the P-wave shadow zone (103° to 142°).
What is each layer made of?
Layer properties
The Earth's layers compared
Layer
Depth (km)
State
Composition
Crust
0–35
solid
silicate rocks (granite, basalt)
Mantle
35–2891
solid (flows very slowly)
peridotite, rich in olivine and pyroxene
Outer core
2891–5150
liquid
iron and nickel (with some light elements)
Inner core
5150–6371
solid
iron and nickel
Source of Earth's magnetic field
convection in the liquid outer core + Earth's rotation → electric currents → Earth's magnetic field (geodynamo theory)
The inner core is far hotter than the temperature at which iron loses its magnetism (about 770 °C), so it cannot be a magnet; the field comes from the moving liquid outer core.
Worked Examples and Exam Practice
Example 1
Which two layers does the Gutenberg discontinuity, where S-waves suddenly disappear, separate? And what can you conclude from this about the state of the layer below it?
1
The Gutenberg discontinuity divides the mantle from the outer core at a depth of about 2891 km.
2
S-waves (transverse) move by shaking the medium sideways, and a liquid cannot resist that, so they cannot cross it. They stop below this boundary, so the outer core is liquid.
▸ Mantle/outer-core boundary; the outer core is liquid
Below 2891 km in the diagram, the red S-wave curve breaks off and the top line reads "no S-wave (liquid)". Convection in this 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 slow sharply at the boundary of the outer core and bend inward, so they hardly reach the surface between 103° and 142° of angular distance.
2
S-waves cannot enter the liquid outer core, so they cannot reach directly any place beyond 103°, which can only be reached through the outer core.
▸ Because S-waves cannot pass the liquid outer core at all (shadow beyond 103°)
P-waves are only bent, but S-waves are blocked outright; that difference makes the two shadow zones differ in width, and both are evidence that the outer core is liquid.
CSAT-style problem
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 cross a liquid; they stop in the outer core below the Gutenberg discontinuity, so the outer core is liquid.
2
The Moho is the boundary between crust and mantle (①); the inner core is solid, since P-waves speed up at the Lehmann discontinuity (③); the mantle takes up about 80% of Earth's volume (④). The inner core is far hotter than the temperature at which iron loses its magnetism, so it cannot be a magnet, and the field comes from convection in the liquid outer core (⑤).