Inside Earth: Layers & Indirect Evidence · Grade 6 Science
Chapter: Earth Systems, Plates & Earth MaterialsInside Earth: Layers & Indirect Evidence
Learning goals
- I can explain how earthquake waves reveal what Earth is made of.
- I can explain why the inner core stays solid although it is hotter than the outer core.
- I can distinguish layers defined by composition from layers defined by how they behave.
Readings for this unit
Before this unit
Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.
Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.
The deepest hole ever drilled is the Kola borehole in Russia, which reached 12.3 km after nearly twenty years. Earth's radius is 6371 km, so that hole penetrated about 0.2 per cent of the way to the centre. Everything below it has been worked out without anyone going there.
The two main lines of evidence are earthquake waves and mass. Earth's average density is about 5.5 g/cm3, while the rocks at the surface are only about 2.7 g/cm3, so the interior must be made of something far denser, which points to iron and nickel.
Earthquake waves do the rest. Some kinds pass through liquids and some cannot, so the pattern of where waves arrive and where they do not, recorded at hundreds of stations, maps out the inside of the planet like a scan.
Build your understanding
Where this fits
The last lesson divided Earth into four systems. This one goes inside the largest of them. It is also the clearest example in the course of indirect evidence, where every conclusion rests on a chain of reasoning rather than on looking, and the next lesson uses these layers to explain why the surface moves.
Use three flexible 40-45-minute sessions: why the evidence has to be indirect; what earthquake waves show; the layers by composition and by behaviour, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · Nobody has been down there
The Kola Superdeep Borehole took from 1970 to 1989 to reach 12.3 km and stopped because the rock at the bottom was around 180 degrees Celsius, far hotter than predicted, and behaved more like plastic than like rock, closing in on the drill. Even deep mines reach only about 4 km. Against a radius of 6371 km, all of this is scratching the surface, and the deepest material anyone has ever held comes from volcanic eruptions that bring up fragments from perhaps 200 km down.
So the interior has to be deduced, and the first clue is weight. Earth's total mass can be calculated from the orbits of satellites, and dividing by its volume gives an average density of about 5.5 g/cm3. Surface rocks such as granite and basalt measure between about 2.7 and 3.0 g/cm3, barely half that. Since the average must lie between the extremes, there has to be material inside that is very much denser than anything at the surface, and iron at 7.9 g/cm3 compressed under enormous weight fits the figures. Meteorites, which are leftovers from the same material that formed the planets, are often iron and nickel, which supports the same conclusion from a completely different direction. Two independent lines reaching the same answer is worth noticing in its own right. The density argument uses the planet's mass and volume and knows nothing about meteorites; the meteorite evidence is a sample of leftover material and knows nothing about Earth's mass. When two methods with no shared assumptions agree, the conclusion is far stronger than either alone, which is the reasoning chapter 1 described for a surprising claim.
Pause and explain
Earth's average density is 5.5 g/cm3 and surface rocks are about 2.7. What does this prove about the interior?
Check your reasoning after answering
That it contains material much denser than anything at the surface. An average must fall between the values making it up, so if every rock we can reach is around 2.7 g/cm3 and the whole planet averages 5.5, the deep interior has to be considerably denser than 5.5 to pull the average up. Iron and nickel under great pressure fit this requirement.
Part 2 · Reading the waves
An earthquake sends out two kinds of wave that travel through the planet. P-waves are pushes and pulls along the direction of travel, like a sound wave, and they can pass through solids and liquids alike. S-waves shake the rock sideways, across the direction of travel, and they cannot travel through a liquid at all, because a liquid has nothing to grip when it is pushed sideways; it simply flows. That single difference is what revealed Earth's structure.
After a large earthquake, seismometers around the world record what arrives. Stations on the far side of the planet receive no S-waves whatever, across a huge region called the shadow zone, and P-waves arriving there are both delayed and bent off course. The conclusion is forced: there is a region deep inside that S-waves cannot cross, so it must be a liquid outer core, and the bending of the P-waves shows where its boundary lies. That boundary sits about 2890 km down, and it was identified in 1914 from wave arrival times alone. The scale of the detective work is worth appreciating. A single earthquake is recorded by thousands of seismometers worldwide, each noting the exact arrival time of each kind of wave, and because wave speed depends on the density and stiffness of the rock it passes through, the pattern of arrivals maps the interior. Thousands of earthquakes over a century have built a picture detailed enough to show structures only a few tens of kilometres across, all from instruments sitting on the surface.
Pause and explain
Why does the absence of S-waves on the far side of Earth show there is liquid inside?
Check your reasoning after answering
Because S-waves shake material sideways and a liquid cannot resist a sideways push; it flows instead of transmitting the wave. So S-waves stop at a liquid boundary. Stations on the far side receive none at all, which means the waves met a region they could not cross, and a liquid layer deep in the interior is what produces exactly that shadow.
Part 3 · A solid centre, held by pressure
In 1936 the Danish seismologist Inge Lehmann noticed faint P-waves arriving inside the shadow zone where none should have been. The only explanation that fitted was a further boundary deep within the core, reflecting waves back out, and that is how the inner core was discovered. It begins about 5150 km down and runs to the centre at 6371 km.
The inner core is solid, and it is hotter than the liquid layer above it, at something like 5200 degrees Celsius, which is close to the surface temperature of the Sun. That sounds contradictory until you add the missing quantity, which is pressure. At the centre the weight of everything above produces a pressure around 3.6 million times atmospheric pressure, and squeezing a material that hard raises the temperature at which it melts. The iron there is above the melting point it would have at the surface and far below the melting point it has at that pressure, so it stays solid. Temperature alone never decides a state of matter; temperature and pressure together do. You have met a smaller version of this already. In chapter 2 the boiling point of water fell from 100 degrees Celsius at sea level to about 71 degrees Celsius on the summit of Mount Everest, where the pressure is lower. The inner core is the same relationship pushed to an extreme: lower the pressure and a melting point falls, raise it enormously and the melting point climbs far above anything reachable at the surface.
Pause and explain
How can the inner core be solid when it is hotter than the liquid layer above it?
Check your reasoning after answering
Because melting point depends on pressure as well as temperature. At the centre the pressure is about 3.6 million times atmospheric, which raises iron's melting point far above the value it has at the surface. The inner core is hot enough to melt surface iron and nowhere near hot enough to melt iron squeezed that hard, so it remains solid.
Part 4 · Two ways to slice the same planet
The layers named so far, crust, mantle, outer core and inner core, are divided by what they are made of. There is a second and equally useful division based on how the material behaves, and confusing the two causes a great deal of trouble. The rigid outer shell, made of the crust plus the cool uppermost part of the mantle, is called the lithosphere, and it is roughly 100 km thick. Beneath it lies the asthenosphere, mantle rock that is still solid but hot enough to flow slowly, like extremely stiff putty, over thousands of years.
The distinction matters because the lithosphere is what is broken into the moving plates of the next lesson, and the asthenosphere is what they move over. Notice that the lithosphere crosses the compositional boundary: it includes all of the crust and the top slice of the mantle, which are different materials behaving the same way. A boundary drawn by composition and a boundary drawn by behaviour answer different questions, and a sentence that mixes them, such as saying the plates are the crust, is a mistake that will produce wrong predictions in the next lesson. Numbers make the proportions clearer than any diagram. Oceanic crust is only about 5 to 10 km thick and continental crust between roughly 30 and 70 km, while the mantle below runs to 2890 km, so the crust is well under 1 per cent of the distance to the centre. The lithosphere at about 100 km is a little more than 1 per cent. Everything humans have ever built, mined or drilled sits on the thinnest possible skin of the planet.
Pause and explain
Why is it wrong to say that Earth's plates are simply the crust?
Check your reasoning after answering
Because the plates are the lithosphere, which includes the crust and the rigid uppermost part of the mantle, roughly 100 km in total. The crust and that mantle layer differ in composition but behave the same way, rigidly, while the hotter mantle below them flows slowly. The plates are defined by behaviour, not by what the rock is made of.
Apply the whole unit · Build the case
A student is told that Earth has a liquid outer core and a solid inner core, and asks how anyone could possibly know, given that the deepest hole reaches only 12.3 km.
Your task
Set out the chain of evidence, naming what each observation rules out.
What your response should show
- Uses the density comparison and says what it rules out
- Explains the S-wave shadow zone in terms of how liquids respond to sideways shaking
- Cites the inner core reflection as separate evidence from the shadow zone
- Resolves the hot-but-solid problem using pressure rather than asserting it
Response guidance — after your attempt
Give the evidence that the deep interior is denser than surface rock, and what it rules out.
Give the evidence for a liquid layer, and explain what a liquid does to one kind of wave.
Give the evidence for a solid centre, and explain how it can be solid while so hot.
First, Earth's average density is about 5.5 g/cm3 while surface rocks are around 2.7, so the interior must contain much denser material; this rules out the planet being made of the same rock throughout. Iron, at 7.9 g/cm3 at the surface and denser under pressure, fits, and iron meteorites support it independently. Second, after a large earthquake no S-waves at all reach stations on the far side of the planet. S-waves shake material sideways and a liquid flows rather than transmitting that, so they cannot cross a liquid; their absence across the shadow zone rules out a wholly solid interior and places a liquid outer core boundary about 2890 km down. Third, Lehmann found faint P-waves arriving inside that shadow zone in 1936, which requires a further reflecting boundary about 5150 km down, marking a solid inner core. It stays solid at roughly 5200 degrees Celsius because pressure near 3.6 million atmospheres raises iron's melting point far above the value it has at the surface. Every step is a deduction from measurements taken at the surface.
Scientific references for this unit
- US Geological Survey: Earth's interior and what seismic waves reveal
- Incorporated Research Institutions for Seismology: P-waves, S-waves and shadow zones
- US Geological Survey: the Kola Superdeep Borehole and the limits of drilling
- NASA: iron meteorites and what they show about planetary cores
Parent / teacher teaching notes
Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.
The key idea
Nobody has ever been more than about 12 km into Earth, yet we know it has a liquid outer core and a solid inner one. Every bit of that knowledge is indirect, deduced from earthquake waves and from the planet's mass, and the reasoning is as worth learning as the answer.
Worked examples
Example 1 · Evaluate two interior explanations
- Distinguish mantle behavior from liquid state.
- Identify the two core states.
- Explain indirect evidence and sketch scale.
Example 2 · Transfer to a different case
- Identify the distinction and conditions in this new case.
- Use the relevant mechanism or quantity; preserve units and the chosen boundary.
- Explain why the answer fits and what further evidence would be needed.
Example 3 · Explain solid material that flows slowly · a focused example
- State the literal condition or observation in the example.
- Apply the distinction explained in this teaching part.
- Check whether the claim stays within the given evidence.
Avoid these mistakes
- Misleading claim: The entire mantle is a liquid ocean.
- Misleading claim: The crust and the whole lithosphere are identical layers.
- Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
- Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.
Check your understanding
Why should crust and lithosphere not be interchangeable?
Discuss the answer
Lithosphere includes crust plus rigid uppermost mantle. The categories use different criteria and boundaries.
Is the whole mantle liquid because it moves?
Discuss the answer
No. Most is solid; slow deformation and local melting are different descriptions.
Does one absent S-wave arrival uniquely map every layer?
Discuss the answer
No. It is evidence to interpret with geometry, multiple stations, and other records rather than a complete map by itself.
Can a readable cutaway be used for precise layer fractions?
Discuss the answer
Only if it is explicitly to scale. Otherwise it illustrates order and relationships rather than exact dimensions.
Sustained scientific explanation and revision
These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.
Activity 1 · Inside Earth: Layers & Indirect Evidence · Model and explain
Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Inside Earth: Layers & Indirect Evidence, study Parts 3–4 and attempt the worked case: Select the better supported basic model and explain two evidence or classification limits. Explain the steps before opening the model.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
B preserves the accepted basic state distinction: mostly solid mantle, liquid outer core, solid inner core. Slow mantle flow does not imply a liquid ocean. Seismic patterns and other evidence help infer these states, while a simple cutaway cannot provide exact thickness or composition at every point. Lithosphere and crust also should not be confused.
Activity 2 · Inside Earth: Layers & Indirect Evidence · Practice, revise and exit
Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Inside Earth: Layers & Indirect Evidence, complete practice questions 1–6 and both choice checks, then answer independently: A drawing places a solid inner core inside a liquid outer core. Is that impossible because the center is hotter? Save a correction explaining one changed claim or method.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
No. Pressure and composition as well as temperature affect state. The drawing should be evaluated through the relevant evidence, not a temperature-only rule.
Words to know
- lithosphere
- The rigid outer layer including the crust and uppermost mantle.
- indirect evidence
- Observations used to infer a feature that is not directly inspected.
- evidence
- A relevant observation, measurement, or record used to evaluate a claim.
- system boundary
- The chosen limit specifying what is included and which exchanges cross it.
- uncertainty
- A stated limit in measurement or knowledge, distinct from a careless mistake.
In the real world
Exit check
A drawing places a solid inner core inside a liquid outer core. Is that impossible because the center is hotter?
Check your explanation after answering
No. Pressure and composition as well as temperature affect state. The drawing should be evaluated through the relevant evidence, not a temperature-only rule.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.