Earth and Environmental Science 11–12 · Year 11
The S-wave shadow zone: modelling why the outer core must be liquid
Module 1: Earth’s Resources
This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.
The idea
S waves cannot pass through liquid, so stations more than about 103 degrees from an earthquake record no direct S waves, which fixes the size and state of the outer core.
What you need
- white paper with two concentric circles, one the diameter of a glass beaker (the outer core) and one twice that diameter (the surface), and a radius line from an epicentre (Earthlearningidea Shadowlands)
- an empty 500 mL glass beaker, water, a phone torch
- drawing compass, protractor, ruler
- slinky spring for P and S wave motion
How to do it
- Show P (push) and S (side-to-side) pulses on the slinky; note that a liquid cannot carry the side-to-side pulse.
- Stand the empty beaker on the inner circle and shine the torch through it from the epicentre along the radius line; mark on the outer circle where the shadow falls.
- Add water to the beaker and mark the new pattern, which Earthlearningidea uses to model the P-wave shadow zone.
- Measure the angular distance from the epicentre to each shadow with the protractor and compare with the real Earth: no direct S waves beyond 103 degrees, and no direct P waves between 103 and 142 degrees.
- Run the ray-path model with a straight-ray mantle, then with velocity rising with depth, and read where the S shadow begins in each case; list why the beaker’s angles differ (light, glass, a core half the Earth’s radius, no inner core).
What you should see
In the straight-ray model the last ray that clears the core (radius 3480 km in a 6371 km Earth) reaches the surface at 2 × arccos(3480/6371) = 113.8 degrees; for the paper model’s core of half the Earth’s radius the same straight-line geometry gives 120 degrees, and refraction through the glass changes what the paper shows. The observed S shadow begins near 103 degrees (Earthlearningidea Shadowlands), earlier than the straight-ray value, because seismic velocity rises with depth and bends rays upward, so rays that reach the core leave the source more steeply; the model shows this shift when the velocity gradient is switched on. Beyond 103 degrees no direct S wave is recorded, which requires a liquid outer core.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Earthquake waves travel along the surface only.
- The whole core is liquid (the inner core is solid).
Safety card
Hazards
No hazard is listed.
Controls
No control is listed.
Note
No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-8EES11/12-3EES11/12-4EES11/12-5
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-01
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.