16  Surface Features & Plate Boundaries

Why do earthquakes and volcanoes form patterns on a map?

HS-ESS1-5 HS-ESS2-1 7–8 Days 🧠 Quiz & Evaluate ↓

🗺️ Why Do Earthquakes & Volcanoes Form Lines?

17 🔥 Engage — The Pattern on the Map

17.1 Earthquakes Aren’t Random

When we plot every major earthquake and volcanic eruption on a world map, an incredible pattern emerges: they don’t occur randomly. They form narrow bands that wrap around the planet, outlining the edges of about 15 major tectonic plates.

These patterns are the key to understanding plate tectonics — the unifying theory of geology.

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

earthquakeLocations = {
  const quakes = [];
  // Ring of Fire (Pacific)
  for (let i = 0; i < 80; i++) {
    const angle = (i / 80) * Math.PI * 1.5 - Math.PI * 0.75;
    quakes.push({
      lon: -170 + 90 * Math.cos(angle) + (Math.random() - 0.5) * 8,
      lat: 10 + 40 * Math.sin(angle) + (Math.random() - 0.5) * 5,
      mag: 5 + Math.random() * 3,
      region: "Ring of Fire"
    });
  }
  // Mid-Atlantic Ridge
  for (let i = 0; i < 30; i++) {
    quakes.push({
      lon: -30 + (Math.random() - 0.5) * 10,
      lat: -50 + i * 4 + (Math.random() - 0.5) * 3,
      mag: 4 + Math.random() * 2,
      region: "Mid-Atlantic Ridge"
    });
  }
  // Alpine-Himalayan Belt
  for (let i = 0; i < 30; i++) {
    quakes.push({
      lon: -10 + i * 5 + (Math.random() - 0.5) * 5,
      lat: 30 + (Math.random() - 0.5) * 8 + Math.sin(i * 0.3) * 5,
      mag: 5 + Math.random() * 3,
      region: "Alpine-Himalayan Belt"
    });
  }
  return quakes;
}

Plot.plot({
  title: "Global Earthquake Distribution (Simulated)",
  subtitle: "Earthquakes cluster along plate boundaries — they're not random!",
  width: 700,
  height: 400,
  projection: {type: "equirectangular"},
  color: {legend: true, domain: ["Ring of Fire", "Mid-Atlantic Ridge", "Alpine-Himalayan Belt"], range: ["#d63031", "#0984e3", "#f39c12"]},
  marks: [
    Plot.geo({type: "Sphere"}, {stroke: "#dfe6e9", fill: "#f0f3f4"}),
    Plot.dot(earthquakeLocations, {
      x: "lon",
      y: "lat",
      r: d => d.mag * 0.5,
      fill: "region",
      fillOpacity: 0.6,
      stroke: "white",
      strokeWidth: 0.5
    ,
        tip: true})
  ]
})

17.1.1 📝 Engage Activity

1. Describe the pattern you see in earthquake locations. Where do they cluster?
2. What do you think these “lines” of earthquakes represent?
3. Where on this map would you least expect an earthquake? Why?

18 🔍 Explore 1 — Types of Plate Boundaries

18.1 Three Ways Plates Interact

Tectonic plates move relative to each other in three fundamental ways. The type of interaction determines what geological features and hazards occur.

viewof boundaryType = Inputs.select(
  ["Divergent (pulling apart)", "Convergent (pushing together)", "Transform (sliding past)"],
  {label: "Select boundary type:"}
)

{
  const info = {
    "Divergent (pulling apart)": {
      title: "↔️ Divergent Boundaries",
      motion: "Plates move APART from each other",
      features: ["Mid-ocean ridges", "Rift valleys", "New ocean floor", "Volcanic islands (Iceland)"],
      examples: ["Mid-Atlantic Ridge", "East African Rift", "Iceland"],
      hazards: ["Mild earthquakes", "Volcanic eruptions (effusive, less explosive)", "Rift formation"],
      process: "Magma rises to fill the gap as plates separate, creating new oceanic crust. This is called SEAFLOOR SPREADING.",
      color: "#2ecc71"
    },
    "Convergent (pushing together)": {
      title: "→←  Convergent Boundaries",
      motion: "Plates move TOWARD each other",
      features: ["Deep ocean trenches", "Mountain ranges (Himalayas)", "Volcanic arcs (Cascades)", "Subduction zones"],
      examples: ["Cascadia Subduction Zone", "Himalayas", "Andes Mountains", "Japan Trench"],
      hazards: ["Powerful earthquakes (M8-9+)", "Explosive volcanic eruptions", "Tsunamis", "Landslides"],
      process: "When oceanic crust meets continental crust, the denser oceanic plate SUBDUCTS (dives under). This creates deep trenches, explosive volcanoes, and the most powerful earthquakes on Earth.",
      color: "#d63031"
    },
    "Transform (sliding past)": {
      title: "⇄ Transform Boundaries",
      motion: "Plates slide PAST each other horizontally",
      features: ["Linear fault valleys", "Offset streams", "Crushed/deformed rock zones"],
      examples: ["San Andreas Fault (California)", "Alpine Fault (New Zealand)", "North Anatolian Fault (Turkey)"],
      hazards: ["Sudden, powerful earthquakes", "Surface rupture", "No volcanism"],
      process: "Plates grind past each other. Friction locks them in place for years, stress builds, then they suddenly slip — causing earthquakes. NO volcanism because no plate is being consumed.",
      color: "#6c5ce7"
    }
  };

  const b = info[boundaryType];
  const div = d3.create("div").style("font-size", "1em");
  
  div.append("h3").style("color", b.color).style("font-family", "Space Grotesk, sans-serif").text(b.title);
  div.append("p").style("font-size", "1.2em").style("font-weight", "bold").text(b.motion);
  div.append("p").style("background", `${b.color}15`).style("padding", "12px").style("border-radius", "8px")
    .style("border-left", `4px solid ${b.color}`).text(b.process);
  
  const grid = div.append("div").style("display", "flex").style("gap", "15px").style("flex-wrap", "wrap");
  
  const featCard = grid.append("div").style("flex", "1").style("min-width", "200px")
    .style("background", "#f8f9fa").style("padding", "15px").style("border-radius", "10px");
  featCard.append("h4").style("margin-top", "0").style("color", b.color).text("🏔️ Features");
  const ul1 = featCard.append("ul");
  b.features.forEach(f => ul1.append("li").text(f));

  const hazCard = grid.append("div").style("flex", "1").style("min-width", "200px")
    .style("background", "#fff5f5").style("padding", "15px").style("border-radius", "10px");
  hazCard.append("h4").style("margin-top", "0").style("color", "#d63031").text("⚠️ Hazards");
  const ul2 = hazCard.append("ul");
  b.hazards.forEach(h => ul2.append("li").text(h));

  const exCard = grid.append("div").style("flex", "1").style("min-width", "200px")
    .style("background", "#f0f4ff").style("padding", "15px").style("border-radius", "10px");
  exCard.append("h4").style("margin-top", "0").style("color", "#0984e3").text("📍 Examples");
  const ul3 = exCard.append("ul");
  b.examples.forEach(e => ul3.append("li").text(e));

  return div.node();
}

19 💡 Explain — Seafloor Spreading

19.1 The Proof That Plates Move

In the 1960s, scientists discovered a pattern in the ocean floor that proved plates move apart at mid-ocean ridges: magnetic stripes.

19.2 Magnetic Striping Evidence

When lava erupts at a mid-ocean ridge and solidifies, iron minerals in the rock align with Earth’s magnetic field — like tiny compass needles frozen in place. Since Earth’s magnetic field reverses every few hundred thousand years, the rock records alternating “stripes” of normal and reversed polarity.

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 300)
    .attr("viewBox", "0 0 700 300");

  svg.append("text").attr("x", 350).attr("y", 25).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").text("Magnetic Striping at a Mid-Ocean Ridge");

  // Ridge center
  svg.append("line").attr("x1", 350).attr("y1", 50).attr("x2", 350).attr("y2", 230)
    .attr("stroke", "#d63031").attr("stroke-width", 3).attr("stroke-dasharray", "5,3");
  svg.append("text").attr("x", 350).attr("y", 250).attr("text-anchor", "middle")
    .attr("font-size", 12).attr("font-weight", "bold").attr("fill", "#d63031").text("Ridge axis");

  // Stripes (symmetric)
  const stripeWidth = 30;
  const stripes = [
    {offset: 0, color: "#2d3436", label: "Normal", age: "0"},
    {offset: 1, color: "#dfe6e9", label: "Reversed", age: "0.8"},
    {offset: 2, color: "#2d3436", label: "Normal", age: "1.8"},
    {offset: 3, color: "#dfe6e9", label: "Reversed", age: "2.6"},
    {offset: 4, color: "#2d3436", label: "Normal", age: "3.6"}
  ];

  stripes.forEach(s => {
    // Right side
    svg.append("rect")
      .attr("x", 352 + s.offset * stripeWidth)
      .attr("y", 60)
      .attr("width", stripeWidth - 2)
      .attr("height", 160)
      .attr("fill", s.color)
      .attr("opacity", 0.8);
    // Left side (mirror)
    svg.append("rect")
      .attr("x", 348 - (s.offset + 1) * stripeWidth)
      .attr("y", 60)
      .attr("width", stripeWidth - 2)
      .attr("height", 160)
      .attr("fill", s.color)
      .attr("opacity", 0.8);
  });

  // Age labels
  svg.append("text").attr("x", 350).attr("y", 280).attr("text-anchor", "middle")
    .attr("font-size", 11).attr("fill", "#636e72").text("← Older                    Youngest                    Older →");

  // Arrows
  svg.append("text").attr("x", 250).attr("y", 140).attr("text-anchor", "middle")
    .attr("font-size", 24).text("⬅");
  svg.append("text").attr("x", 450).attr("y", 140).attr("text-anchor", "middle")
    .attr("font-size", 24).text("➡");

  // Legend
  svg.append("rect").attr("x", 20).attr("y", 60).attr("width", 15).attr("height", 15).attr("fill", "#2d3436");
  svg.append("text").attr("x", 40).attr("y", 73).attr("font-size", 11).text("Normal polarity");
  svg.append("rect").attr("x", 20).attr("y", 85).attr("width", 15).attr("height", 15).attr("fill", "#dfe6e9").attr("stroke", "#636e72");
  svg.append("text").attr("x", 40).attr("y", 98).attr("font-size", 11).text("Reversed polarity");

  return svg.node();
}

19.2.1 💡 Why Magnetic Stripes Are Proof

1. The stripes are perfectly symmetric on both sides of the ridge
2. The same pattern appears at every mid-ocean ridge worldwide
3. Rocks get older with distance from the ridge (confirmed by dating)
4. The pattern matches known magnetic reversal timelines

This is irrefutable evidence that new ocean floor is being created at ridges and pushed outward — seafloor spreading.

19.3 Seafloor Age Data

seafloorAge = [
  {distance: 0, age: 0, direction: "East"},
  {distance: 50, age: 2, direction: "East"},
  {distance: 100, age: 5, direction: "East"},
  {distance: 200, age: 10, direction: "East"},
  {distance: 500, age: 25, direction: "East"},
  {distance: 1000, age: 50, direction: "East"},
  {distance: 2000, age: 100, direction: "East"},
  {distance: 3500, age: 180, direction: "East"},
  {distance: 0, age: 0, direction: "West"},
  {distance: -50, age: 2, direction: "West"},
  {distance: -100, age: 5, direction: "West"},
  {distance: -200, age: 10, direction: "West"},
  {distance: -500, age: 25, direction: "West"},
  {distance: -1000, age: 50, direction: "West"},
  {distance: -2000, age: 100, direction: "West"},
  {distance: -3500, age: 180, direction: "West"}
]

Plot.plot({
  title: "Ocean Floor Age vs. Distance from Mid-Atlantic Ridge",
  subtitle: "Rock age increases symmetrically with distance — proving seafloor spreading",
  width: 700,
  height: 350,
  grid: true,
  x: {label: "Distance from ridge (km, negative = west)"},
  y: {label: "Rock age (million years)"},
  color: {legend: true, domain: ["East", "West"], range: ["#e17055", "#0984e3"]},
  marks: [
    Plot.line(seafloorAge.filter(d => d.direction === "East"), {x: "distance", y: "age", stroke: "#e17055", strokeWidth: 3,
        tip: true}),
    Plot.line(seafloorAge.filter(d => d.direction === "West"), {x: "distance", y: "age", stroke: "#0984e3", strokeWidth: 3,
        tip: true}),
    Plot.dot(seafloorAge, {x: "distance", y: "age", fill: "direction", r: 4,
        tip: true}),
    Plot.ruleX([0], {stroke: "#d63031", strokeWidth: 2, strokeDasharray: "5,3"})
  ]
})

🧠 The oldest ocean floor is only about 200 million years old — compare that to continental rocks that are up to 4 billion years old! Ocean floor is constantly being created at ridges and destroyed at subduction zones. The entire ocean floor is recycled every ~200 million years.

20 🔍 Explore 2 — Continental Drift Evidence

20.1 Wegener’s Wild Idea (That Turned Out To Be Right)

In 1912, Alfred Wegener proposed that all continents were once joined in a supercontinent called Pangaea. He was ridiculed by other scientists for decades — but he was right.

20.2 Evidence for Continental Drift

viewof driftEvidence = Inputs.select(
  ["Coastline fit", "Fossil distribution", "Rock types match", "Glacial evidence", "All evidence"],
  {label: "Explore evidence type:"}
)

{
  const evidence = {
    "Coastline fit": {
      desc: "The coastlines of South America and Africa fit together like puzzle pieces. When we match the continental shelves (not just the shorelines), the fit is nearly perfect.",
      strength: "Visual and geometric — continental shelves match within 50 km",
      counterarg: "Could be coincidence",
      verdict: "Suggestive but not conclusive alone"
    },
    "Fossil distribution": {
      desc: "Identical fossils of land animals (Lystrosaurus, Cynognathus) and plants (Glossopteris) are found on continents now separated by thousands of kilometers of ocean. These organisms couldn't have swum across.",
      strength: "Multiple species, same time period, on separate continents",
      counterarg: "Maybe land bridges existed?",
      verdict: "Strong evidence — too many species match across too many continents"
    },
    "Rock types match": {
      desc: "Mountain chains and rock formations that end abruptly at one coastline continue on another continent. The Appalachian Mountains (USA) match the Caledonian Mountains (Scotland/Scandinavia).",
      strength: "Same age, composition, and structure across ocean basins",
      counterarg: "Similar conditions could produce similar rocks",
      verdict: "Strong — the match is too precise to be coincidence"
    },
    "Glacial evidence": {
      desc: "300-million-year-old glacial deposits are found in South America, Africa, India, and Australia. These places are now tropical! The scratches on rocks (striations) point away from a common center — which was the South Pole when they were joined.",
      strength: "Glacial striations point to a single ice center that only works if continents were joined",
      counterarg: "None convincing",
      verdict: "Very strong — no alternative explanation works"
    },
    "All evidence": {
      desc: "All four lines of evidence converge on the same conclusion: continents were once joined and have drifted apart. This is now confirmed by GPS measurements showing plates moving 2-10 cm per year.",
      strength: "Multiple independent lines of evidence + direct GPS measurements",
      counterarg: "None — plate tectonics is one of the best-supported theories in science",
      verdict: "The theory of plate tectonics is as well-supported as evolution or gravity"
    }
  };

  const e = evidence[driftEvidence];
  const div = d3.create("div");
  div.append("h3").style("color", "#e17055").style("font-family", "Space Grotesk").text(driftEvidence);
  div.append("p").style("font-size", "1.1em").text(e.desc);
  
  const grid = div.append("div").style("display", "flex").style("gap", "10px").style("flex-wrap", "wrap");
  
  const s1 = grid.append("div").style("flex", "1").style("min-width", "180px")
    .style("background", "#e8f5e9").style("padding", "12px").style("border-radius", "8px");
  s1.append("strong").text("✅ Strength: ");
  s1.append("span").text(e.strength);

  const s2 = grid.append("div").style("flex", "1").style("min-width", "180px")
    .style("background", "#fff3e0").style("padding", "12px").style("border-radius", "8px");
  s2.append("strong").text("🤔 Counter: ");
  s2.append("span").text(e.counterarg);

  div.append("div").style("background", "#ffecd2").style("padding", "12px").style("border-radius", "8px")
    .style("border-left", "4px solid #e17055").style("margin-top", "10px")
    .html(`<strong>Verdict:</strong> ${e.verdict}`);

  return div.node();
}

21 🔬 Elaborate — The Adirondacks & Intraplate Features

21.1 Not Everything Happens at Plate Boundaries

Most geological activity occurs at plate boundaries — but not all of it. The Adirondack Mountains in New York State are far from any plate boundary, yet they’re still rising.

21.2 Intraplate Features

Feature	Location	Plate Boundary?	Explanation
Adirondack Mountains	New York	No — middle of North American Plate	Crustal rebound from ancient orogeny; hot spot beneath
Hawaiian Islands	Pacific Ocean	No — middle of Pacific Plate	Mantle plume (hotspot) beneath moving plate
Yellowstone	Wyoming	No — middle of North American Plate	Mantle plume creating a supervolcano
New Madrid Seismic Zone	Missouri	No — middle of North American Plate	Ancient failed rift — weakness in the crust

21.2.1 💡 The Big Picture

Plate tectonics explains ~95% of earthquakes and volcanic eruptions. The other ~5% occur at intraplate locations due to:

• Hotspots/mantle plumes — columns of hot rock rising from deep in the mantle
• Ancient weaknesses — old rifts or fault zones that can reactivate
• Crustal rebound — the crust still adjusting from ancient tectonic events
• Stress transmitted through the plate from distant boundaries

22 ✅ Evaluate

22.1 Putting Plate Tectonics Together

22.1.1 🧪 Evaluate Questions

1. Explain why earthquakes and volcanoes cluster along narrow bands around the planet.

2. Compare and contrast the three types of plate boundaries. For each, describe the motion, features created, and types of hazards.

3. Evaluate Wegener’s evidence for continental drift. Why was he rejected at first, and what new evidence eventually proved him right?

4. Explain how magnetic striping on the ocean floor proves seafloor spreading. Include a diagram.

5. Apply: The Cascadia Subduction Zone (Pacific Northwest) is a convergent boundary that hasn’t had a major earthquake since 1700. Based on what you know about convergent boundaries, what hazards should people in Seattle and Portland prepare for?

6. Explain how the Adirondack Mountains can still be rising even though they’re far from any plate boundary.

22.1.2 📝 Model Update

Update your model from the Unit Opening:

• Add plate boundaries to your explanation of why volcanoes and earthquakes occur where they do
• Explain how seafloor spreading at divergent boundaries and subduction at convergent boundaries recycle Earth’s crust
• How does this connect to why Krakatoa (Indonesia) was so explosive? (Hint: What type of boundary is Indonesia on?)