14  Unit 3: Opening — The Krakatoa Enigma

Why do massive volcanic eruptions, earthquakes, and tsunamis happen?

Unit 3 Time: 2 Days

🌋 What Forces Shape Earth’s Violent Surface?

15 The Anchor Phenomenon

15.1 💥 Krakatoa, August 27, 1883

The loudest sound in recorded history. The explosion was heard 4,800 km away — like hearing a bomb in New York from London.

What happened:

• 🌋 The volcanic island of Krakatoa (Indonesia) exploded with a force of 200 megatons — 10,000 times the Hiroshima bomb
• 🌊 Tsunamis up to 30 meters high killed over 36,000 people on nearby islands
• 🌡️ Ash and sulfur ejected into the stratosphere lowered global temperatures by 1.2°C for a year
• 🎨 The ash created brilliant red sunsets worldwide for months — scientists believe Edvard Munch’s painting The Scream was inspired by the sky
• 💨 The pressure wave circled the globe 7 times, detected on barometers everywhere
• 🔊 The sound wave traveled at 1,200 km/h and ruptured eardrums 60 km away

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

eruptionData = [
  {name: "Mt. St. Helens (1980)", vei: 5, deaths: 57, ejecta: 1, color: "#74b9ff"},
  {name: "Pinatubo (1991)", vei: 6, deaths: 800, ejecta: 10, color: "#0984e3"},
  {name: "Krakatoa (1883)", vei: 6, deaths: 36000, ejecta: 25, color: "#e17055"},
  {name: "Tambora (1815)", vei: 7, deaths: 92000, ejecta: 160, color: "#d63031"},
  {name: "Toba (74,000 ya)", vei: 8, deaths: 0, ejecta: 2800, color: "#636e72"}
]

Plot.plot({
  title: "Volcanic Eruptions by Scale (VEI = Volcanic Explosivity Index)",
  subtitle: "Each VEI level is ~10x more powerful than the previous",
  width: 700,
  height: 300,
  marginLeft: 180,
  x: {label: "Material ejected (km³) — log scale", type: "log", grid: true},
  y: {label: null},
  marks: [
    Plot.barX(eruptionData, {
      x: "ejecta",
      y: "name",
      fill: "color",
      rx: 8,
      sort: {y: "x"}
    ,
        tip: true}),
    Plot.text(eruptionData, {
      x: "ejecta",
      y: "name",
      text: d => `VEI ${d.vei} — ${d.ejecta} km³`,
      dx: 25,
      fontSize: 11,
      fontWeight: "bold"
    })
  ]
})

🧠 The Toba supervolcano eruption (74,000 years ago) ejected 2,800 km³ of material — enough to bury the entire state of Texas under 12 meters of ash. It may have caused a “volcanic winter” that nearly drove humans extinct, reducing our population to perhaps 10,000 individuals.

16 The Pattern

16.1 Where Do Disasters Strike?

hazardData = [
  {type: "Earthquakes (M6+, 2000-2023)", location: "Pacific Ring of Fire", percent: 81, color: "#e17055"},
  {type: "Earthquakes (M6+, 2000-2023)", location: "Alpine-Himalayan Belt", percent: 15, color: "#fdcb6e"},
  {type: "Earthquakes (M6+, 2000-2023)", location: "Rest of world", percent: 4, color: "#dfe6e9"},
  {type: "Active Volcanoes", location: "Pacific Ring of Fire", percent: 75, color: "#e17055"},
  {type: "Active Volcanoes", location: "Mid-ocean ridges", percent: 15, color: "#0984e3"},
  {type: "Active Volcanoes", location: "Hotspots (Hawaii, etc.)", percent: 10, color: "#2ecc71"}
]

Plot.plot({
  title: "Where Do Natural Hazards Occur?",
  subtitle: "The Ring of Fire dominates — but why?",
  width: 700,
  height: 300,
  marginLeft: 200,
  x: {label: "Percentage of global total (%)", grid: true, domain: [0, 100]},
  y: {label: null},
  fy: {label: null},
  marks: [
    Plot.barX(hazardData, {
      x: "percent",
      y: "location",
      fy: "type",
      fill: "color",
      rx: 5
    ,
        tip: true}),
    Plot.text(hazardData, {
      x: "percent",
      y: "location",
      fy: "type",
      text: d => d.percent + "%",
      dx: 15,
      fontSize: 12,
      fontWeight: "bold"
    })
  ]
})

16.1.1 🌍 Earthquakes on a 3D Globe

This interactive globe uses the Globe.gl library (built on Three.js) to show the locations of recent earthquakes. The spike height, radius, and color scale with magnitude so you can quickly spot the biggest events.

Data: USGS Earthquake Hazards Program · Drag to rotate · Scroll to zoom · Hover a spike to pause & inspect

16.1.2 🌋 Volcanic Eruptions on a 3D Globe

This globe shows notable volcanic eruptions from recorded (and geological) history. Each volcano is shown as a pulsing ring — the ring size, speed, and color all scale with the Volcanic Explosivity Index (VEI), a logarithmic scale where each step is ~10× more powerful than the last.

VEI 2-3 Minor VEI 4 Large VEI 5 Very Large VEI 6 Colossal VEI 7-8 Mega / Super

Smithsonian GVP / USGS data · Drag to rotate · Scroll to zoom · Hover a ring center to inspect

16.1.3 🎯 The Big Mystery

Earthquakes, volcanoes, and tsunamis don’t happen randomly. They cluster along narrow bands that wrap around the planet. These bands mark the edges of tectonic plates — the giant puzzle pieces of Earth’s surface.

In this unit, we’ll figure out:

1. What’s inside Earth that drives these violent events
2. Why disasters cluster along specific belts
3. What makes the plates move
4. Whether we can predict and prepare for these hazards

16.1.4 📝 Initial Model

Draw an initial model that explains:

1. Why do volcanoes, earthquakes, and tsunamis happen in certain locations?
2. What’s happening beneath Earth’s surface to cause these events?
3. Why did Krakatoa explode so violently?

Don’t worry about being “right” — this is your starting model. We’ll revise it as we learn more.

16.1.5 📋 Driving Question Board

Write at least 3 questions you want answered during this unit. Here are some starters:

• Why are there so many volcanoes around the Pacific Ocean?
• What causes an earthquake?
• Could a supervolcano like Toba erupt again?
• How do we know what’s inside Earth if we’ve never drilled more than 12 km?
• Why do some volcanoes explode violently while others flow gently?
• Can we predict earthquakes? Why or why not?