15  Earth’s Interior

How do we know what’s inside Earth if we’ve never been there?

HS-ESS2-1 HS-ESS2-3 7–8 Days 🧠 Quiz & Evaluate ↓

🔬 What’s Inside a Planet We’ve Never Seen?

16 🔥 Engage — The Deepest Hole

16.1 The Kola Superdeep Borehole

The deepest hole ever drilled is the Kola Superdeep Borehole in Russia — 12,262 meters (12.3 km) deep. It took 24 years (1970–1994) to drill.

The result? They barely scratched the surface.

Earth’s radius is 6,371 km. The Kola borehole reached only 0.2% of the way to the center. It’s like scratching the skin of an apple.

So how do we know what’s inside Earth if we’ve never been there?

The answer: earthquakes. Every earthquake sends seismic waves through the entire planet. By analyzing how these waves travel, speed up, slow down, bend, and reflect, we can build a detailed picture of Earth’s interior — like an X-ray of the planet.

Plot = require("@observablehq/plot")
d3 = require("d3@7")

depthData = [
  {item: "Deepest ocean (Mariana Trench)", depth: 11, color: "#0984e3"},
  {item: "Kola Superdeep Borehole", depth: 12.3, color: "#636e72"},
  {item: "Deepest mine (Mponeng, SA)", depth: 4, color: "#fdcb6e"},
  {item: "Continental crust thickness", depth: 35, color: "#dfe6e9"},
  {item: "Oceanic crust thickness", depth: 7, color: "#74b9ff"},
  {item: "Upper mantle", depth: 410, color: "#e17055"},
  {item: "Lower mantle", depth: 2900, color: "#d63031"},
  {item: "Outer core", depth: 5100, color: "#f39c12"},
  {item: "Inner core (center of Earth)", depth: 6371, color: "#fdcb6e"}
]

Plot.plot({
  title: "How Deep Have We Gone? (km below surface)",
  subtitle: "Log scale — we've barely scratched the surface",
  width: 700,
  height: 350,
  marginLeft: 250,
  x: {label: "Depth (km) — log scale", type: "log", grid: true},
  y: {label: null},
  marks: [
    Plot.barX(depthData, {
      x: "depth",
      y: "item",
      fill: "color",
      rx: 8,
      sort: {y: "x"}
    ,
        tip: true}),
    Plot.text(depthData, {
      x: "depth",
      y: "item",
      text: d => d.depth.toLocaleString() + " km",
      dx: 20,
      fontSize: 11,
      fontWeight: "bold"
    })
  ]
})

16.1.1 📝 Engage Questions

1. If we can’t directly sample Earth’s interior, what kinds of indirect evidence might we use?
2. What could waves tell us about the material they pass through?
3. Why do you think the Kola borehole project was abandoned? What problems would you encounter drilling that deep?

17 🔍 Explore 1 — Seismic Waves

17.1 Earthquakes as Earth’s X-Ray Machine

When an earthquake occurs, it produces two main types of seismic waves that travel through Earth’s interior. These waves behave differently depending on what material they pass through.

17.2 P-Waves vs. S-Waves

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 380)
    .attr("viewBox", "0 0 700 380");

  svg.append("text").attr("x", 350).attr("y", 25).attr("text-anchor", "middle")
    .attr("font-size", 18).attr("font-weight", "bold").text("Two Types of Seismic Waves");

  // P-wave
  svg.append("rect").attr("x", 20).attr("y", 50).attr("width", 320).attr("height", 280)
    .attr("rx", 15).attr("fill", "#e17055").attr("opacity", 0.1);
  svg.append("text").attr("x", 180).attr("y", 80).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").attr("fill", "#e17055").text("P-Waves (Primary)");
  
  svg.append("text").attr("x", 180).attr("y", 110).attr("text-anchor", "middle")
    .attr("font-size", 13).text("Push-pull (compression)");
  svg.append("text").attr("x", 180).attr("y", 135).attr("text-anchor", "middle")
    .attr("font-size", 20).text("→ ← → ← → ← → ←");
  
  const pProps = [
    "⚡ Fastest (6–14 km/s)",
    "✅ Travel through solids AND liquids",
    "📏 Direction: same as wave travel",
    "🥇 Arrive first at seismometers"
  ];
  pProps.forEach((p, i) => {
    svg.append("text").attr("x", 40).attr("y", 175 + i * 30).attr("font-size", 12).text(p);
  });

  // S-wave
  svg.append("rect").attr("x", 360).attr("y", 50).attr("width", 320).attr("height", 280)
    .attr("rx", 15).attr("fill", "#0984e3").attr("opacity", 0.1);
  svg.append("text").attr("x", 520).attr("y", 80).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").attr("fill", "#0984e3").text("S-Waves (Secondary)");
  
  svg.append("text").attr("x", 520).attr("y", 110).attr("text-anchor", "middle")
    .attr("font-size", 13).text("Side-to-side (shear)");
  svg.append("text").attr("x", 520).attr("y", 135).attr("text-anchor", "middle")
    .attr("font-size", 20).text("∿∿∿∿∿∿∿∿∿∿");
  
  const sProps = [
    "🐢 Slower (3.5–7.5 km/s)",
    "❌ CANNOT travel through liquids",
    "📏 Direction: perpendicular to travel",
    "🥈 Arrive second at seismometers"
  ];
  sProps.forEach((p, i) => {
    svg.append("text").attr("x", 380).attr("y", 175 + i * 30).attr("font-size", 12).text(p);
  });

  // Key insight
  svg.append("rect").attr("x", 100).attr("y", 340).attr("width", 500).attr("height", 30)
    .attr("rx", 15).attr("fill", "#d63031");
  svg.append("text").attr("x", 350).attr("y", 360).attr("text-anchor", "middle")
    .attr("font-size", 13).attr("font-weight", "bold").attr("fill", "white")
    .text("🔑 S-waves can't pass through liquid → this reveals Earth's liquid outer core!");

  return svg.node();
}

17.3 Wave Speed Changes with Depth

Seismic wave speeds change as they pass through different materials. Denser materials = faster waves.

waveSpeedData = [
  {depth: 0, pSpeed: 6.0, sSpeed: 3.5, layer: "Crust"},
  {depth: 35, pSpeed: 8.1, sSpeed: 4.5, layer: "Upper Mantle"},
  {depth: 200, pSpeed: 8.5, sSpeed: 4.6, layer: "Upper Mantle"},
  {depth: 400, pSpeed: 9.5, sSpeed: 5.1, layer: "Transition Zone"},
  {depth: 660, pSpeed: 10.3, sSpeed: 5.6, layer: "Lower Mantle"},
  {depth: 1000, pSpeed: 11.4, sSpeed: 6.2, layer: "Lower Mantle"},
  {depth: 2000, pSpeed: 12.8, sSpeed: 7.0, layer: "Lower Mantle"},
  {depth: 2900, pSpeed: 13.7, sSpeed: 7.3, layer: "Lower Mantle"},
  {depth: 2901, pSpeed: 8.1, sSpeed: 0, layer: "Outer Core (liquid!)"},
  {depth: 3500, pSpeed: 9.5, sSpeed: 0, layer: "Outer Core"},
  {depth: 4500, pSpeed: 10.3, sSpeed: 0, layer: "Outer Core"},
  {depth: 5100, pSpeed: 11.0, sSpeed: 0, layer: "Outer Core"},
  {depth: 5150, pSpeed: 11.3, sSpeed: 3.6, layer: "Inner Core (solid!)"},
  {depth: 6000, pSpeed: 11.2, sSpeed: 3.7, layer: "Inner Core"},
  {depth: 6371, pSpeed: 11.3, sSpeed: 3.7, layer: "Inner Core"}
]

Plot.plot({
  title: "Seismic Wave Speed vs. Depth",
  subtitle: "The dramatic drop in P-wave speed and disappearance of S-waves reveals the liquid outer core",
  width: 700,
  height: 450,
  grid: true,
  x: {label: "Depth (km)", domain: [0, 6371]},
  y: {label: "Wave speed (km/s)", domain: [0, 15]},
  color: {legend: true},
  marks: [
    // Layer shading
    Plot.rectY([{x1: 0, x2: 35, label: "Crust",
        tip: true}], {x1: "x1", x2: "x2", y1: 0, y2: 15, fill: "#dfe6e9", fillOpacity: 0.3}),
    Plot.rectY([{x1: 35, x2: 2900,
        tip: true}], {x1: "x1", x2: "x2", y1: 0, y2: 15, fill: "#e17055", fillOpacity: 0.1}),
    Plot.rectY([{x1: 2900, x2: 5150,
        tip: true}], {x1: "x1", x2: "x2", y1: 0, y2: 15, fill: "#f39c12", fillOpacity: 0.15}),
    Plot.rectY([{x1: 5150, x2: 6371,
        tip: true}], {x1: "x1", x2: "x2", y1: 0, y2: 15, fill: "#fdcb6e", fillOpacity: 0.2}),
    // P-wave line
    Plot.line(waveSpeedData, {x: "depth", y: "pSpeed", stroke: "#e17055", strokeWidth: 3, curve: "natural",
        tip: true}),
    Plot.dot(waveSpeedData, {x: "depth", y: "pSpeed", fill: "#e17055", r: 3,
        tip: true}),
    // S-wave line
    Plot.line(waveSpeedData, {x: "depth", y: "sSpeed", stroke: "#0984e3", strokeWidth: 3, curve: "natural",
        tip: true}),
    Plot.dot(waveSpeedData.filter(d => d.sSpeed > 0), {x: "depth", y: "sSpeed", fill: "#0984e3", r: 3,
        tip: true}),
    // Labels
    Plot.text([{x: 1500, y: 14}], {x: "x", y: "y", text: d => "P-waves", fill: "#e17055", fontSize: 14, fontWeight: "bold"}),
    Plot.text([{x: 1500, y: 8}], {x: "x", y: "y", text: d => "S-waves", fill: "#0984e3", fontSize: 14, fontWeight: "bold"}),
    Plot.text([{x: 4000, y: 1.5}], {x: "x", y: "y", text: d => "S-waves = 0\n(LIQUID!)", fill: "#d63031", fontSize: 12, fontWeight: "bold"}),
    // Boundary markers
    Plot.ruleX([2900], {stroke: "#d63031", strokeWidth: 2, strokeDasharray: "5,3"}),
    Plot.ruleX([5150], {stroke: "#f39c12", strokeWidth: 2, strokeDasharray: "5,3"})
  ]
})

17.3.1 💡 How Seismic Waves Reveal Earth’s Interior

1. P-waves drop sharply at 2,900 km → material changes from solid to liquid (mantle → outer core)
2. S-waves disappear entirely from 2,900 to 5,150 km → outer core is liquid (S-waves can’t travel through liquid)
3. S-waves reappear below 5,150 km → inner core is solid again
4. Wave speeds increase with depth in the mantle → denser material = faster waves

This is how we know Earth has a liquid outer core and solid inner core — without ever going there!

18 💡 Explain — Earth’s Layered Structure

18.1 Why Is Earth Layered?

When Earth was young (~4.5 billion years ago), it was almost entirely molten. In this liquid state, dense materials (iron, nickel) sank to the center while lighter materials (silicon, oxygen, aluminum) floated to the surface. This process is called differentiation.

18.2 Earth’s Layers by Density

viewof layerView = Inputs.select(
  ["By Composition", "By Physical Properties"],
  {label: "View Earth's layers:"}
)

{
  const compLayers = [
    {layer: "Inner Core", depth: "5,150–6,371 km", thickness: 1221, density: "12.8–13.1", composition: "Solid iron & nickel", temp: "5,400°C", color: "#fdcb6e"},
    {layer: "Outer Core", depth: "2,900–5,150 km", thickness: 2250, density: "9.9–12.2", composition: "Liquid iron & nickel", temp: "4,400–5,400°C", color: "#f39c12"},
    {layer: "Lower Mantle", depth: "660–2,900 km", thickness: 2240, density: "4.4–5.6", composition: "Silicate rock (Mg, Fe)", temp: "1,900–3,700°C", color: "#d63031"},
    {layer: "Upper Mantle", depth: "35–660 km", thickness: 625, density: "3.3–4.4", composition: "Silicate rock (olivine)", temp: "500–1,900°C", color: "#e17055"},
    {layer: "Crust", depth: "0–35 km", thickness: 35, density: "2.7–3.0", composition: "Granite (cont.) / Basalt (ocean)", temp: "0–500°C", color: "#dfe6e9"}
  ];

  const physLayers = [
    {layer: "Inner Core (solid)", depth: "5,150–6,371 km", thickness: 1221, density: "12.8–13.1", composition: "Rigid solid — immense pressure keeps iron solid despite extreme heat", temp: "5,400°C", color: "#fdcb6e"},
    {layer: "Outer Core (liquid)", depth: "2,900–5,150 km", thickness: 2250, density: "9.9–12.2", composition: "Liquid iron — convection here generates Earth's magnetic field", temp: "4,400–5,400°C", color: "#f39c12"},
    {layer: "Mesosphere (rigid mantle)", depth: "660–2,900 km", thickness: 2240, density: "4.4–5.6", composition: "Solid but slowly flows over millions of years", temp: "1,900–3,700°C", color: "#d63031"},
    {layer: "Asthenosphere (ductile)", depth: "100–660 km", thickness: 560, density: "3.3–3.6", composition: "Partially molten — plates 'float' and slide on this layer", temp: "1,300–1,900°C", color: "#e17055"},
    {layer: "Lithosphere (rigid)", depth: "0–100 km", thickness: 100, density: "2.7–3.4", composition: "Rigid crust + upper mantle — the tectonic plates", temp: "0–1,300°C", color: "#dfe6e9"}
  ];

  const layers = layerView === "By Composition" ? compLayers : physLayers;

  const div = d3.create("div");
  layers.forEach(l => {
    const card = div.append("div").style("padding", "15px").style("margin", "10px 0")
      .style("border-radius", "12px").style("border-left", `6px solid ${l.color}`)
      .style("background", `${l.color}15`);
    card.append("h4").style("margin", "0 0 8px 0").style("color", l.color).text(l.layer);
    card.append("p").style("margin", "3px 0").html(`<strong>Depth:</strong> ${l.depth} | <strong>Density:</strong> ${l.density} g/cm³`);
    card.append("p").style("margin", "3px 0").html(`<strong>Temperature:</strong> ${l.temp}`);
    card.append("p").style("margin", "3px 0").text(l.composition);
  });
  return div.node();
}

🧠 Earth’s inner core is as hot as the surface of the Sun (5,400°C) — but it’s SOLID because the pressure at the center is 3.6 million times atmospheric pressure. Pressure can keep things solid even at insane temperatures.

19 🔍 Explore 2 — Density and Differentiation

19.1 The Density Sorting Experiment

When Earth was molten, gravity sorted materials by density — like oil floating on water, but on a planetary scale.

viewof showDifferentiation = Inputs.range([0, 100], {
  label: "Time (% through differentiation):",
  step: 1,
  value: 0
})

{
  const t = showDifferentiation / 100;
  const svg = d3.create("svg")
    .attr("width", 400)
    .attr("height", 400)
    .attr("viewBox", "0 0 400 400");

  const cx = 200, cy = 200;
  
  // Earth circle
  svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 180)
    .attr("fill", "#dfe6e9").attr("stroke", "#636e72").attr("stroke-width", 2);
  
  // Layers appear as differentiation progresses
  if (t > 0.1) {
    const coreR = Math.min(50, t * 55);
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", coreR)
      .attr("fill", "#fdcb6e").attr("opacity", Math.min(1, t * 2));
  }
  if (t > 0.3) {
    const outerCoreR = Math.min(90, 50 + (t - 0.3) * 60);
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", outerCoreR)
      .attr("fill", "#f39c12").attr("opacity", Math.min(1, (t - 0.3) * 2));
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 50)
      .attr("fill", "#fdcb6e");
  }
  if (t > 0.5) {
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 150)
      .attr("fill", "#e17055").attr("opacity", Math.min(1, (t - 0.5) * 2));
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 90)
      .attr("fill", "#f39c12");
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 50)
      .attr("fill", "#fdcb6e");
  }
  if (t > 0.7) {
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 180)
      .attr("fill", "#dfe6e9").attr("opacity", Math.min(1, (t - 0.7) * 3));
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 150)
      .attr("fill", "#e17055");
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 90)
      .attr("fill", "#f39c12");
    svg.append("circle").attr("cx", cx).attr("cy", cy).attr("r", 50)
      .attr("fill", "#fdcb6e");
  }

  // Labels
  const phase = t < 0.1 ? "Homogeneous molten Earth" :
                t < 0.3 ? "Dense iron sinking to center..." :
                t < 0.5 ? "Core forming, outer core separating..." :
                t < 0.7 ? "Mantle solidifying around core..." :
                "Differentiation complete — layered Earth!";

  svg.append("text").attr("x", cx).attr("y", 30).attr("text-anchor", "middle")
    .attr("font-size", 14).attr("font-weight", "bold").text(phase);

  if (t > 0.9) {
    const labels = [
      {text: "Crust", y: 30, color: "#636e72"},
      {text: "Mantle", y: 70, color: "#fff"},
      {text: "Outer Core", y: 115, color: "#fff"},
      {text: "Inner Core", y: 200, color: "#636e72"}
    ];
    labels.forEach(l => {
      svg.append("text").attr("x", cx).attr("y", cy - 180 + l.y + 50)
        .attr("text-anchor", "middle").attr("font-size", 11).attr("font-weight", "bold")
        .attr("fill", l.color).text(l.text);
    });
  }

  return svg.node();
}

19.1.1 💡 Differentiation = Density Sorting

• Densest (iron, nickel: ~13 g/cm³) → sank to the core
• Medium density (silicates: ~4-5 g/cm³) → formed the mantle
• Least dense (granite, basalt: ~2.7-3.0 g/cm³) → floated to the crust

This is the same principle as oil (less dense) floating on water (more dense) — just happening at a planetary scale over millions of years while Earth was still molten.

20 🔬 Elaborate — The Rock Cycle Revisited

20.1 Connecting the Interior to the Surface

Earth’s interior doesn’t just sit there — it actively drives surface processes through the rock cycle, volcanism, and plate tectonics.

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 350)
    .attr("viewBox", "0 0 700 350");

  svg.append("text").attr("x", 350).attr("y", 25).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").text("Interior → Surface Connections");

  const connections = [
    {interior: "Mantle convection", surface: "Plate movement", arrow: "Drives", x: 120, color: "#e17055"},
    {interior: "Magma rising", surface: "Volcanic eruptions", arrow: "Creates", x: 300, color: "#d63031"},
    {interior: "Plate stress", surface: "Earthquakes", arrow: "Releases", x: 480, color: "#6c5ce7"},
    {interior: "Core convection", surface: "Magnetic field", arrow: "Generates", x: 620, color: "#f39c12"}
  ];

  connections.forEach(c => {
    // Interior box
    svg.append("rect").attr("x", c.x - 65).attr("y", 50).attr("width", 130).attr("height", 60)
      .attr("rx", 10).attr("fill", c.color).attr("opacity", 0.85);
    const iLines = c.interior.split(" ");
    iLines.forEach((line, i) => {
      svg.append("text").attr("x", c.x).attr("y", 75 + i * 16).attr("text-anchor", "middle")
        .attr("font-size", 11).attr("font-weight", "bold").attr("fill", "white").text(line);
    });

    // Arrow
    svg.append("text").attr("x", c.x).attr("y", 140).attr("text-anchor", "middle")
      .attr("font-size", 22).text("⬇️");
    svg.append("text").attr("x", c.x).attr("y", 165).attr("text-anchor", "middle")
      .attr("font-size", 10).attr("fill", "#636e72").attr("font-weight", "bold").text(c.arrow);

    // Surface box
    svg.append("rect").attr("x", c.x - 65).attr("y", 180).attr("width", 130).attr("height", 60)
      .attr("rx", 10).attr("fill", c.color).attr("opacity", 0.3);
    const sLines = c.surface.split(" ");
    sLines.forEach((line, i) => {
      svg.append("text").attr("x", c.x).attr("y", 205 + i * 16).attr("text-anchor", "middle")
        .attr("font-size", 11).attr("font-weight", "bold").text(line);
    });
  });

  // Section labels
  svg.append("text").attr("x", 20).attr("y", 85).attr("font-size", 12).attr("font-weight", "bold")
    .attr("fill", "#636e72").text("DEEP");
  svg.append("text").attr("x", 20).attr("y", 215).attr("font-size", 12).attr("font-weight", "bold")
    .attr("fill", "#636e72").text("SURFACE");

  return svg.node();
}

20.1.1 📝 Elaborate Activity

1. Explain how we know Earth’s outer core is liquid using seismic wave evidence.
2. Why did differentiation happen — why didn’t Earth stay homogeneous?
3. If Earth’s core cooled completely and solidified, what surface processes would stop? How would this affect life?
4. Mars’s core has mostly solidified. How does this connect to Mars having no magnetic field and a thin atmosphere?

21 ✅ Evaluate

21.1 Demonstrating Your Understanding

21.1.1 🧪 Evaluate Questions

1. Describe the two types of seismic waves and explain how each reveals information about Earth’s interior.

2. Explain why S-waves disappear between 2,900 km and 5,150 km depth. What does this tell us?

3. Construct a model of Earth’s interior that includes:

   • All major layers (composition AND physical properties)
   • Approximate depths and temperatures
   • State of matter (solid/liquid)
   • How we determined each layer’s properties

4. Compare Earth’s interior to that of a dead planet (like the Moon). What’s different and why does it matter?

5. Predict: If an earthquake occurs on one side of Earth, describe the path of P-waves and S-waves to a seismometer on the opposite side. What would the seismometer record?