Week 4: Earth's Interior Structure & Evidence

Grade 7 Science | Rosche

MS-ESS2-1

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The Phenomenon: The Unreachable Mystery

Anchoring Context & Focus Question

How do we know what's inside Earth if no one has ever been there?

Animated cross-section of Earth showing four concentric layers: the thin gray crust at the surface, the dark red mantle beneath it, the liquid orange outer core, and the solid gold inner core at the center. Each layer pulses or rotates to show its dynamic nature.
Earth's interior layers — the inner core pulses, the outer core slowly rotates, showing the dynamic heat energy driving plate tectonics.

The Kola Superdeep Borehole: Humanity's Deepest Hole

12 km

Kola Borehole depth
(took 24 years to drill)

6,371 km

Earth's radius
(center of Earth)

0.2%

How far we've drilled
(like piercing an apple skin)

The puzzle: We can't drill to Earth's center — heat and pressure destroy equipment below 12 km. Yet scientists confidently describe every layer of Earth's interior, their temperatures, and compositions. How?

Connect to Prior Learning

From Weeks 1–3: We've studied plates at Earth's surface. Now we go deeper. The plate movement, earthquakes, and volcanism you've studied all have roots in Earth's interior — in the heat and convection happening thousands of kilometers below your feet.

Key Vocabulary (7 terms) — Practice Tool

Cognate Strategy: Many science words look similar in English and Spanish — use your Spanish to learn science!

Term Spanish Definition
seismic waves ondas sísmicas Energy waves generated by earthquakes that travel through Earth
compression compresión Force that squeezes or pushes material together
density densidad Mass per unit volume of a material
differentiation diferenciación Process of separation by density when Earth was molten
pressure presión Force applied over an area
composition composición What a material is made of (chemical makeup)
mantle manto Thick layer of solid rock between crust and core

Hook – The Unreachable Mystery

Compare deepest hole drilled vs Earth's radius.
Seismic P-waves and S-waves traveling through Earth's layered interior — P-waves pass through all layers while S-waves are blocked by the liquid outer core, creating a shadow zone
Seismic wave paths reveal Earth's internal structure. The S-wave shadow zone proves the outer core is liquid.

The Unreachable Mystery

What You'll Do

  1. Compare the Kola Borehole depth vs Earth's radius (scale model activity)
  2. Identify the problem: why can't we drill deeper?
  3. Explore the solution: what indirect evidence could reveal Earth's interior?
  4. Generate hypotheses about Earth's layers before looking at data

The Drilling Problem

DepthTemperatureProblem
0 km (surface)15°CNone — we live here
3 km~90°CHot water gushes up
12 km (Kola)~180°CRock becomes plastic, drill jams
6,371 km (core)~5,000–6,000°CSame as surface of the Sun
COMPLETE THE HOOK FORM

Explore indirect evidence about Earth's interior.

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Worked Example and Simulation – Using Seismic Waves to Map Earth's Interior

Cross-section of Earth showing crust, upper mantle, lower mantle, outer core, and inner core with labeled thickness and composition for each layer
Earth's layers differ in composition, density, and state of matter. Evidence comes from seismic waves, density calculations, and meteorite analysis.
Your Simulation Summary
Your prediction: —
Trial results: —
Average result: —

Worked Example and Simulation: Reading Seismic Wave Evidence

Scientists use seismic waves like X-rays for the planet. By measuring how waves speed up, slow down, bend, or stop, they infer what each layer is made of.

Wave TypeMotionTravels ThroughKey Evidence
P-waves
(Primary)		Push-pull (compress)Solids AND liquidsSlow down in outer core → liquid!
S-waves
(Secondary)		Side-to-side (shear)Solids ONLYStop at outer core → must be liquid!

How Scientists Concluded the Outer Core is Liquid:

  1. S-waves travel through solid mantle normally
  2. S-waves disappear on the far side of Earth — they hit a boundary and stop
  3. S-waves cannot travel through liquids
  4. Conclusion: the outer core must be liquid
  5. P-waves slow down (but don't stop) at same boundary → confirms liquid (liquids slow P-waves)

Simulation: Seismic Wave

PREDICT (before running the sim)

Look at the simulation controls. Before changing any variables, predict what will happen when you adjust them. Write your prediction down.

OBSERVE (while using the sim)

Change one variable at a time. Record what happens after each change. Use the data journal to capture at least 3 trials.

EXPLAIN (after collecting data)

Compare your observations with your prediction. Use scientific vocabulary to explain the patterns you found. What surprised you? What confirmed your thinking?

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Station 1 – Seismic Wave Investigation

Interpret P-wave and S-wave behavior through layers.

Seismic Wave Data Analysis

Use the wave travel-time data to figure out what each Earth layer is made of. Remember: S-waves stop in liquids; P-waves slow down in liquids.

Earth LayerP-wave SpeedS-waves Present?Inferred State
Crust6–7 km/sYesSolid rock
Mantle8–13 km/sYesSolid (plastic flow)
Outer Core8–10 km/s (slower)NO — shadow zoneLIQUID
Inner Core11–12 km/s (fastest)Yes (recent data)SOLID

Shadow Zone: There's a region on Earth's surface (103°–143° from an earthquake) where NO seismic waves arrive. This "shadow zone" is direct evidence of Earth's liquid outer core bending and blocking the waves.

COMPLETE THE STATION 1 FORM

Analyze how seismic waves reveal Earth's interior.

Complete Your Worksheet

Complete the "STATION 1 NOTES" section on your worksheet:

  • Record the P-wave speed and S-wave behavior for each Earth layer.
  • Use the outer-core shadow zone to infer where liquid material begins.
  • Explain why the seismic evidence supports a liquid outer core and solid inner core.
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Station 2 – Earth's Layers Analysis

Construct evidence-based model of Earth's interior.

Earth's Four Layers

LayerThicknessStateCompositionTemperature
Crust5–70 kmSolidBasalt / granite0–900°C
Mantle2,900 kmSolid (plastic)Silicate rock (olivine)900–3,700°C
Outer Core2,200 kmLiquidIron + nickel3,700–5,000°C
Inner Core1,220 km (radius)SolidIron + nickel (denser)~5,000–6,000°C

Key Misconception: The mantle is NOT liquid magma! It's solid rock under so much pressure it deforms slowly — like silly putty. Only a tiny fraction melts to form magma.

COMPLETE THE STATION 2 FORM

Analyze density and temperature data for Earth's layers.

Complete Your Worksheet

Complete the "STATION 2 NOTES" section on your worksheet:

  • Compare the thickness, state, composition, and temperature of all four layers.
  • Explain why the mantle can flow slowly even though it is mostly solid.
  • Combine density, meteorite, and magnetic-field evidence to infer the core's composition.
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Station 3 – Design an Earth Model

Apply indirect evidence to construct Earth's structure.
AUTONOMY SUPPORT: How to Ace Station 3 (25 pts)
Step-by-step approach to maximize your points.
  1. Represent all four layers in the correct order and show their relative boundaries.
  2. Connect each material or label in the model to seismic, density, meteorite, or magnetic evidence.
  3. Name at least one limitation: the model cannot reproduce Earth's scale, pressure, or temperature.

25-Point Success Criteria

Build an accurate layered model, justify boundaries and composition with multiple evidence sources, explain a model limitation, and describe how new data could refine the model.

Build an Evidence-Based Earth Model

Scientists never saw Earth's interior directly. They built their model from multiple independent lines of evidence — each one consistent with the others.

Evidence TypeWhat It ShowsLayer It Supports
Seismic wavesP/S wave behavior reveals solid vs liquid layersAll layers
Density calculationsEarth's average density (5.5 g/cm³) > crust rocks (2.7–3.3) → dense coreCore (iron/nickel)
MeteoritesIron meteorites match expected core compositionCore composition
Magnetic fieldEarth's field requires moving liquid conductorLiquid outer core

Model Building Task: Using all four evidence types, explain how scientists know the outer core is liquid iron-nickel. Your explanation must reference at least 2 evidence types.

COMPLETE THE STATION 3 FORM

Build a scale model using seismic and density evidence.

Complete Your Worksheet

Complete the "STATION 3 NOTES" section on your worksheet:

  • Label each model layer with its state and approximate composition.
  • Attach at least two evidence statements to the layer boundaries they support.
  • Describe one model limitation and one change you would make if new seismic data disagreed.
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Exit Ticket – Earth's Interior Integration

Synthesize understanding of Earth's interior structure and evidence.
AUTONOMY SUPPORT: How to Ace Exit Ticket (23 pts)
Step-by-step approach to maximize your points.
  1. Answer the two new-learning items with evidence about core composition and pressure.
  2. Use mantle behavior and magma origin when completing the two spiral-review items.
  3. For the integration item, combine at least three independent evidence types and explain why convergence matters.

23-Point Success Criteria

Complete both new-learning items, both spiral-review items, and the final evidence-based model argument. Name the evidence and explain what it reveals instead of listing facts alone.

Show What You Know

You've traced how seismic waves reveal Earth's four layers and built an evidence-based model of Earth's interior. Now apply your understanding to a real scientific question.

Sentence Starters:

  • "Scientists know the outer core is liquid because S-waves..."
  • "The inner core is solid despite being hotter than the outer core because..."
  • "The evidence I find most convincing about Earth's interior is ___ because..."
COMPLETE THE EXIT TICKET

Demonstrate mastery of Earth's interior concepts.

Complete Your Worksheet

Complete the "EXIT TICKET REFLECTION" section on your worksheet:

  • State the strongest evidence that Earth's outer core is liquid and explain why.
  • Explain how pressure keeps the hotter inner core solid.
  • Revise your Earth model using evidence from one new-learning and one spiral-review response.
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Enrichment & Extension
Optional content if you finish early or want to go deeper.
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Week 4 Complete!

Next Week: Synthesis & Assessment — bringing plate tectonics together!