Geography · Lesson g06

Plate Tectonics & Hazards

The ground feels solid, but it is moving. Very slowly, and sometimes not so slowly, giant plates push, pull and grind against each other, and that is where earthquakes and volcanoes come from.
A girl and her horse on a grassy ridge looking at a smoking volcano and a cracked rift valley at sunset, with the ocean in the distance

If you ride across Iceland on an Icelandic horse, you cross a place where two continents are slowly pulling apart. If you walk along the Pacific coast of Mexico, you stand above a plate sinking into the Earth. In g05 you learned how to give an exact address for any place on Earth. Now we learn why some addresses are shaken by earthquakes and others almost never are, and how a speed calculation you already know can buy people precious warning time.

🌋 Big idea 1: the Earth's skin is cracked into plates

Earth's rocky outer shell is broken into about a dozen large tectonic plates, plus smaller ones. They float on the hot, slowly flowing mantle beneath and move a few centimetres a year, about as fast as your fingernails grow. Almost all the action happens at the boundaries where plates meet. The Pacific Ring of Fire, a horseshoe of boundaries around the Pacific Ocean, has most of the world's earthquakes and many of its volcanoes.

A. Convergent ocean continent mantle Plates move TOWARD each other. Heavy ocean plate sinks under the continent: quakes, volcanoes. B. Divergent magma Plates move APART. Magma rises and cools into new crust: volcanoes, mild quakes. C. Transform plate 1 plate 2 Plates slide PAST each other (map view from above). They stick, then slip: big quakes, no volcanoes.
Red arrows show which way each plate moves. A and B are side-on cross-sections; C is a map view from above. Examples: the Andes (A), Iceland and the Mid-Atlantic Ridge (B), the San Andreas Fault in California (C).
📈 Big idea 2: earthquakes, volcanoes and tsunamis

When stuck plates suddenly slip, an earthquake sends out waves. P waves arrive first because they travel fastest; slower, more damaging S waves arrive later. Where plates collide or pull apart, rock can melt and reach the surface as a volcano. A big undersea earthquake can push up the water and start a tsunami, a series of waves that crosses the deep ocean as fast as a jet airliner.

Physics connection (s03 and s05): waves obey the same rule as a horse at a gallop: $\text{speed} = \dfrac{\text{distance}}{\text{time}}$, so $\text{time} = \dfrac{\text{distance}}{\text{speed}}$. The bigger the distance a wave travels, the more time people have. That time is called warning time. (For this lesson, use rounded figures: P waves about 6 km/s and S waves about 3 km/s in rock.)
Worked Examples
Worked Example 1: which boundary?

Two plates are moving toward each other, one dense ocean plate and one lighter continental plate. Name the boundary type and say what happens to the ocean plate.

Moving toward each other is convergent. The denser ocean plate is forced down beneath the lighter continent (it subducts). That causes strong earthquakes and, as the sinking plate melts, volcanoes on the continent above, like the Andes.

Worked Example 2: tsunami arrival time

A tsunami crosses the open sea at 600 km/h. A coast lies 900 km away. How long until the first wave arrives?

$$t = \frac{d}{v} = \frac{900 \text{ km}}{600 \text{ km/h}} = 1.5 \text{ h}$$

That is 1 hour 30 minutes. Distance and speed must be in matching units: kilometres with kilometres per hour.

Warm-Up
  1. Problem 1
    The Eurasian and North American plates under Iceland are moving apart at about 2 cm a year. Name this boundary type.
    toward, apart, or sliding past? →
  2. Problem 2
    A plate moves about 4 cm a year. How many kilometres does it travel in 25 million years? (Rounded numbers for this exercise.)
    cm → m → km →
  3. Problem 3
    Which boundary type can have big earthquakes but no volcanoes: convergent, divergent or transform?
    where does magma come from, if at all? →
Core Problems
  1. Problem 4
    A quake starts underground. P waves travel at 6 km/s and S waves at 3 km/s. (a) A seismometer is 180 km from the quake. How many seconds after the P wave does the S wave arrive? (b) Another one is 600 km away. How many seconds is the gap there?
    time $= \dfrac{\text{distance}}{\text{speed}}$ for each wave, then compare
    P time · S time · gap →
  2. Problem 5
    A tsunami moves across deep water at 200 m/s. (a) A city is 3 600 km from the quake. How many hours until the wave arrives? (b) A town only 90 km from the quake needs 20 minutes for the alarm to sound and 45 minutes for people to reach high ground. Do they have enough time? Show the numbers.
    same units first (m/s and km) · then compare times →
  3. Problem 6
    When an ocean plate and a continental plate collide, why does the ocean plate sink rather than the continent?
    think density →
  4. Problem 7 Challenge
    A seismometer in Mexico City records a P wave at 12:00:00 and the S wave exactly 48 seconds later. Using P at 6 km/s and S at 3 km/s, (a) how far away was the quake? (b) At what clock time did the S wave arrive?
    gap depends on the distance: write it, then solve →
  5. Problem 8 Open
    Millions of people live near plate boundaries, in places like Tokyo, Reykjavik and Lima. Give two reasons (think back to g01 and g04) why people stay in or move to such places, and two ways communities can prepare for earthquakes or volcanoes. There is no single right answer.
    pull factors · preparation →
Think like a geographer 🧭: Hazards are natural, but disasters depend on people: how many live nearby, how well buildings are made, how much warning there is. Our wave-speed sums show that one second of warning is worth something, and that is why geographers care about both the science and the people.
Show answers
Problem 1
Divergent boundary (the plates move apart).
Problem 2
4 cm × 25 000 000 = 100 000 000 cm = 1 000 000 m = 1 000 km.
Problem 3
Transform (plates slide past; no magma reaches the surface).
Problem 4a
P: 180 ÷ 6 = 30 s. S: 180 ÷ 3 = 60 s. The S wave arrives 30 seconds after the P wave.
Problem 4b
P: 600 ÷ 6 = 100 s. S: 600 ÷ 3 = 200 s. The gap is 100 seconds (farther away gives a longer warning).
Problem 5a
3 600 km = 3 600 000 m. 3 600 000 ÷ 200 = 18 000 s = 5 hours.
Problem 5b
90 km = 90 000 m. 90 000 ÷ 200 = 450 s = 7.5 minutes. They need 20 + 45 = 65 minutes, so no, they are about 57.5 minutes short.
Problem 6
The ocean plate is denser (heavier) than the continental plate, so it is forced underneath (subduction).
Problem 7a
Gap = d/3 - d/6 = d/6 = 48, so d = 288 km. (Check: 288 ÷ 6 = 48 s for P; 288 ÷ 3 = 96 s for S; gap 48 s.)
Problem 7b
The S wave takes 96 s, which is 1 min 36 s, so it arrives at 12:01:36.
Problem 8
Open. Reasons might be fertile volcanic soil, geothermal energy, jobs, trade or family. Preparation might be earthquake-proof buildings, warning systems and drills, evacuation routes, or tsunami signs.
Next up → g07 (planned): Living with Hazards & Trade We have seen where hazards strike and how much warning there is. A future lesson turns to how people respond: how communities prepare and rebuild, and how trade routes link far-away places so help, goods and ideas can flow when disaster hits.
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