Earthquakes send clues through Earth
You can learn about a closed box by how sound travels through it. Scientists study earthquake waves recorded at many stations to investigate Earth’s interior. The coloured layers are a model built from evidence, not a photograph from a tunnel.
Work it out, one step at a time
Separate observation from interpretation
ground motion → arrival times → interior model
Seismometers record motion. Comparing many records helps infer the materials inside.
Compare familiar drilling depth with Earth’s size
12 km ÷ 6,371 km × 100 ≈ 0.19%
A roughly 12 km borehole reaches only a small fraction of the distance to the centre.
Words made simple
- Model
- A simplified explanation or calculation constrained by observations.
- Pressure and gravity
- Pressure is a push per area; local gravity is a net gravitational acceleration.
Think about it: Does zero net gravity at the centre mean zero pressure?
No. In a symmetric model the pulls cancel at the centre, while the overlying material still compresses it.
Where the numbers come from: USGS · Dziewonski & Anderson (1981). The illustration and worked model are made for this lesson; assumptions are stated above.
Go further: definitions, formulas and a second example
A cutaway of Earth looks like a picture of a known interior. In reality it is a compact explanation built from many observations and physical models. Understanding the chain from recorded waves to inferred layers makes the familiar diagram far more informative.
Seismometers record motion, not layer colours
An earthquake sends elastic disturbances through Earth. Seismometers record motion at the surface, including arrival times and different phases. Scientists compare these records across stations and earthquakes. A missing arrival, a delayed phase or a conversion can constrain the material and boundaries encountered along a path.
P waves involve compression, while S waves involve shear. A liquid can carry compression but lacks static shear rigidity for the simple transmitted S-wave model. This contrast is an important part of the evidence for a liquid outer core; a complete Earth interpretation also considers ray paths, conversions and additional observations.
A reference Earth is a documented approximation
PREM is a published radial reference model rather than a map of every rock beneath every location. ScaleTrip uses its density profile in a spherical calculation. Local geology, three-dimensional structure and physical uncertainties belong to a more detailed problem than this educational one-dimensional profile.
The familiar boundaries near 410 km, 660 km and the core illustrate changes in physical behaviour. Some mantle changes concern mineral crystal structure under pressure, not a universal transition to liquid. A colour change in the journey should prompt a question about the underlying property rather than serve as evidence by itself.
High central pressure and zero central gravity can coexist
In a spherically symmetric model, gravitational pulls balance at the centre, making net local gravity zero there. Pressure describes the mechanical state supported by the overlying material. The centre can therefore have high pressure even though an ideal test particle at the exact centre has no preferred direction of gravitational acceleration.
The calculation first integrates density to obtain enclosed mass, then evaluates gravity and integrates the weight of overlying layers to estimate pressure. Surface pressure is set to zero in this model. Numerical convergence checks the integration method; it does not certify that the assumed radial Earth captures every real feature.
Change one physical question at a time
Use the existing seismic experiment to compare P and S particle motions in solid and liquid. Then move the depth slider and follow pressure and gravity separately. Predicting the direction of each change before looking at the curve turns an attractive cutaway into an evidence-based activity.
Finish with three sentences: what a sensor observed, what physical property was inferred, and what your calculation assumed. That distinction is a powerful way to explain Earth’s interior without claiming direct access to places no drilling project has reached.
Reproduce one result
Calculate this result using the definitions and assumptions in the article. At the centre use symmetry, not direct division by zero.
- Expression
- g = G × M(r) / r²; symmetry at r = 0
- Calculated result
- 0 m/s²
Check the numbers and their sources
Open a source to see what it supports. The picture is our explanation; the original evidence is linked below. These AI-assisted explanations have automated calculation checks, but no expert peer review.
USGS — Seismographs: keeping track of earthquakes
Teaching wave-strip is not a ray tracer, seismogram or velocity inversion. Source record checked:
Read the original sourceUSGS — P-wave and S-wave paths through the Earth
The original USGS Interior link was unavailable; this accessible official explanation supports the stated qualitative claim, not a new inversion. Source record checked:
Read the original sourceDziewonski & Anderson (1981) — PREM, Table I (PDF p. 12)
Original paper checked through Harvard public academic copy. Coefficients and numerical convergence checked; no expert review or physical-uncertainty certification. Source record checked:
Read the original source