How Can We Know What Is Inside the Earth Without Going There?
Earthquake Waves — Looking Inside a Planet We Cannot Enter
We can photograph distant galaxies, send spacecraft to other planets and examine individual cells under a microscope.
Yet we cannot travel very far into the planet beneath our feet.
No borehole has come remotely close to reaching the centre of the Earth. Nevertheless, science textbooks confidently show a rocky mantle, a liquid outer core and a solid inner core.
How can we know what is there when we cannot go and look?
One of the most powerful answers comes from earthquakes.
Earthquakes send waves through the planet. By measuring where those waves arrive, how long they take and which waves are missing, scientists can investigate regions thousands of kilometres below the surface.
The Earth’s interior becomes a problem we can solve using evidence.
For students, this brings together physics, geology and mathematical reasoning. And the investigation can begin with something as simple as a slinky.
Earthquakes Give Scientists a Signal to Follow
An earthquake releases energy, some of which travels through the Earth as seismic waves.
Sensitive instruments called seismometers detect the resulting ground motion. The recording, called a seismogram, contains information about the earthquake and the materials through which its waves have travelled.
Two particularly useful types are P waves and S waves.
Both travel through the Earth’s interior, which is why they are called body waves. Their different movements give us different clues. earthquake.usgs.gov
P waves: a travelling squeeze
P waves are longitudinal waves.
The material vibrates backwards and forwards, parallel to the direction in which the wave travels. Regions of compression and expansion move through it.
Sound travelling through air provides a familiar example of this type of motion.
P waves travel faster than S waves through the same solid material, so they usually produce the first identifiable arrival on a seismogram. The P stands for “primary”. earthquake.usgs.gov
S waves: a travelling sideways movement
S waves are transverse waves.
The material moves perpendicular to the direction of wave travel. Imagine sending a sideways pulse along a rope: the pulse travels towards the far end while individual sections of rope move sideways.
The S stands for “secondary”, because these waves arrive after the P waves. earthquake.usgs.gov
But the most revealing difference concerns what happens when the waves encounter a liquid.
The Crucial Clue: Liquids Cannot Carry S Waves Through Their Interior
P waves can travel through solids and liquids.
S waves require a material that can resist a sideways change of shape. Solids provide that resistance; liquids do not sustain it in the way needed to transmit these shear waves through their bulk.
Consequently, S waves cannot travel through the liquid outer core. P waves can enter it, but their speed and direction change. U.S. Geological Survey
This does not mean that liquids cannot carry waves. Sound travels through water, and waves move across its surface.
The distinction is between different kinds of wave.
That is a valuable teaching point: saying “waves travel through water” is not enough. We must ask which waves, and how the material is moving.
Practical 1: Make P and S Waves With a Slinky
This is an excellent starting point because students can watch the motion rather than simply memorising diagrams.
Equipment
- A long slinky or suitable demonstration spring.
- A smooth floor or long table.
- A small piece of coloured tape.
- A phone or camera for slow-motion recording, if available.
Keep the spring comfortably extended, avoid overstretching it and keep faces away from its ends.
Demonstrating a P wave
- Lay the slinky along the floor, with a person holding each end.
- Gather a few coils together near one end.
- Release them to send a compression along the spring.
- Watch the compression move towards the opposite end.
Attach a small tape marker to one coil.
Ask:
Does the marked coil travel all the way along the spring with the wave?
It does not. It moves locally while the disturbance travels onwards.
That observation helps separate two ideas students often confuse: the movement of the material and the movement of the wave.
Demonstrating an S wave
Move one end briefly from side to side, perpendicular to the spring’s length.
A sideways pulse travels along it.
Again, follow the marked coil. Its movement is across the direction of wave travel.
What the model shows—and where it stops
The slinky makes longitudinal and transverse motion visible. However, its speed, scale and mechanical behaviour do not reproduce the Earth.
I would use it to establish the movement first, then ask students which parts of the model are useful and which are unrealistic.
That question is part of the science.
Practical 2: Send a Wave Back Towards Yourself
A wave does not always continue straight through a material. At a boundary, some of its energy can be reflected.
Fix one end of a suitable rope securely and send a single sideways pulse towards it.
Watch the returning pulse. At a fixed end, it returns inverted: an upward displacement comes back as a downward displacement.
Slow-motion filming makes the outgoing and returning pulses much easier to distinguish.
The geological connection is that seismic waves can also reflect from boundaries inside the Earth.
An echo contains information about the surface that sent it back.
With suitable measurements and a model of wave speed, a reflected arrival can help reveal the position of a hidden boundary.
A simple calculation
For a deliberately simplified example, suppose a pulse travels straight down to a horizontal boundary and returns along the same route.
Assume:
- Wave speed = 6 km/s.
- Total return time = 10 seconds.
Then:
Total distance = speed x time
Total distance = 6 x 10 = 60 km
The wave travels down and back, so:
Depth = total distance / 2
Depth = 60 / 2 = 30 km
This is an invented teaching example, not a measurement of a particular location.
Real investigations must account for changing speeds, angled paths and more complicated structures. Nevertheless, the underlying question is accessible:
What can a travel time tell us about something we cannot see?
Practical 3: Explore Changing Speed and Refraction
A spring or mechanical wave model can demonstrate that wave speed depends on the properties of the system.
With a suitable rope, for example, increasing its tension increases the speed of a transverse pulse. Change one variable at a time and compare the travel time over a measured distance.
For a clearer demonstration of refraction, use a ripple tank with a shallow region created by a submerged plate.
Send wavefronts towards the depth boundary at an angle. As their speed changes, their direction changes too.
Water-surface waves are not seismic body waves. Here, the model illustrates the relationship between changing wave speed and changing direction.
Inside the Earth, seismic waves likewise change direction as they encounter different conditions. Gradual changes in wave speed can produce curved paths, while boundaries can produce more abrupt changes.
That is why interpreting earthquake waves requires more than drawing straight lines through a globe.
The Missing Waves Reveal the Liquid Outer Core
Imagine placing earthquake detectors around the world.
After a large earthquake, you compare their recordings.
Some stations detect direct P and S waves. At greater angular distances, direct S waves disappear because their routes would require them to pass through the liquid outer core.
P waves behave differently. They enter the outer core, but strong refraction creates a region where direct P waves are absent: the P-wave shadow zone, approximately 104 to 140 degrees from the earthquake. These angles are measured around the Earth from the earthquake’s surface location. U.S. Geological Survey
“Shadow” does not mean that every instrument records complete silence. Reflected, diffracted and converted waves complicate real recordings.
The important point is that particular expected arrivals are missing.
What scientists do not detect can be as informative as what they do detect.
Become a Seismologist: What Does This Evidence Suggest?
Give students the following simplified evidence cards.
These are conceptual summaries, not a real earthquake dataset.
| Observation | What might it suggest? |
|---|---|
| P waves arrive before S waves at a nearby station. | The wave types travel at different speeds. |
| Some arrivals return after reflection from deep boundaries. | The Earth contains regions with different properties. |
| Direct S waves are absent where their paths would cross the outer core. | That region cannot transmit shear waves. |
| Direct P waves are missing over a particular angular range, then appear again farther away. | Strong refraction redirects their paths. |
| Additional P-wave arrivals do not fit a model with one uniform core. | Another internal region or boundary may be needed. |
Ask students to sketch an interior that could explain the observations.
Then challenge their reasoning:
- Which observation supports each feature of your drawing?
- Could another explanation fit?
- What further measurement would help you decide?
This is where a lesson about waves becomes a lesson about scientific argument.
The table alone cannot establish exact boundary depths. Those require measured travel times, many recordings and calculations that compare possible models.
How Did Scientists Discover the Inner Core?
A liquid outer core explains much of the evidence, but it does not explain every seismic arrival.
Danish seismologist Inge Lehmann examined waves appearing where the accepted model did not adequately explain them. In 1936, she proposed an inner core within the surrounding outer core.
An additional boundary could account for otherwise puzzling wave behaviour. Oxford University Museum of Natural History
This is an excellent example to discuss with students.
An unexpected result is not automatically a mistake to discard. It may expose a weakness in the explanation.
However, the conclusion also needs care: identifying an extra boundary and establishing that the inner core is solid are related but distinct questions. Our present understanding draws on accumulated seismic evidence, the Earth’s overall properties and laboratory investigations of materials under extreme conditions. pubs.usgs.gov
A Common Misconception: The Mantle Is Not a Giant Ocean of Magma
Pictures of the Earth often use bright orange and red layers. Those colours can give the impression that everything beneath the crust is molten.
Most of the mantle is solid rock.
It can transmit S waves, yet it can also deform and flow extremely slowly over geological timescales. “Solid” does not mean incapable of movement over millions of years.
The outer core is liquid, while the inner core is solid. Temperature alone does not determine whether material melts: pressure and composition matter too. pubs.usgs.gov
This gives students an opportunity to reconsider the simple categories they first encountered when learning about solids and liquids.
Why I Would Bring This Into My Teaching
What appeals to me about this investigation is how far it takes us from such simple equipment.
A spring on the laboratory floor introduces the movement of waves. A reflected pulse introduces hidden boundaries. A set of recordings raises questions about the structure of an entire planet.
It also changes the questions we ask.
Instead of only asking:
“What are the Earth’s layers called?”
we can ask:
“What evidence would persuade you that those layers exist?”
In my home laboratory, I would combine the practical demonstrations with close-up filming and a short evidence challenge. Students could make predictions, inspect the recordings and explain how their interpretation changed.
There is useful mathematics here too: speed, distance, time, angles, graphs and uncertainty all have a clear purpose.
Students are using them to investigate something.
The Bigger Lesson: Science Can Reach Beyond Direct Observation
We often associate scientific evidence with seeing something happen.
But much of science concerns things we cannot inspect directly.
We infer their properties from measurable effects, build explanations and test whether those explanations successfully predict further observations.
Seismology makes that process particularly vivid.
We cannot enter the outer core, but we can investigate how waves behave when they encounter it.
We cannot hold a sample of the inner core in a school laboratory, but we can examine the evidence that led scientists to propose it.
The strength of an explanation lies in how well it accounts for the evidence—and how well it survives further tests.
Conclusion: We Cannot Go There, but We Can Investigate
A diagram of the Earth’s interior can look like a finished answer.
Behind it lies a remarkable investigation involving earthquakes, sensitive instruments, unexpected arrivals, missing waves and generations of scientific reasoning.
That is what makes this topic so valuable beyond the syllabus.
A slinky does more than demonstrate a wave. It can begin a discussion about how knowledge is built.
We know about the Earth’s interior because the waves travelling through it carry clues—and scientists have learnt how to interpret them.
