Quakes and Volcanoes

Geology & Plate Tectonics Fundamentals

Course 1 · Chapter 4 · Earthquakes and Volcanoes

On 1 November 1755 an earthquake destroyed much of Lisbon, and the shock was felt as far away as Scotland, where the water in Loch Lomond is reported to have surged without any wind (check). Chapter 3 explained where such events happen: along the edges of plates. This chapter explains how. It covers why rock suddenly breaks, how seismic waves are used to measure and locate quakes, why some mountains erupt, what makes volcanoes dangerous, and the honest limits of prediction. Numbers and dates are from general knowledge and flagged "check" where they matter.

Why the Ground Shakes

Stress builds

Plates move steadily, but rock along a fault is rough and sticks. Stress builds for years or centuries as the rock bends slightly, like a plastic ruler held at both ends.

The rock breaks

When the stress exceeds the strength of the rock or friction on the fault, it slips. The stored energy is released as seismic waves in seconds. This is the elastic rebound idea, developed after the 1906 San Francisco earthquake (check).

Focus and epicentre

The focus (or hypocentre) is where the rock first slips, underground. The epicentre is the point on the surface directly above it. Shallower quakes tend to cause more surface damage for a given size.

Seismic Waves and Measurement

Wave (check)	Type	Notes
P-waves	Body wave, compressional	Fastest; arrive first; pass through solids and liquids
S-waves	Body wave, shearing	Slower; arrive second; cannot pass through liquid, which is how the liquid outer core was detected
Surface waves	Travel along the ground	Slowest, but usually cause the most damage

Locating a quake. The gap between the arrival of P- and S-waves at a station shows the distance to the source. Three stations give a position by triangulation.
Magnitude. A measure of the energy released, now usually given as moment magnitude. The scale is logarithmic: each whole number is about 32 times more energy (check). A magnitude 7 releases about a thousand times the energy of a magnitude 5.
Intensity. A description of the shaking and damage at a place, such as the Mercalli scale. A single quake has one magnitude but many intensities, depending on distance, ground and buildings.
Why buildings matter. Casualties depend heavily on construction and soil, so a smaller quake can kill more people than a larger one elsewhere (check).

Volcanoes

Setting	Why magma forms	Typical eruption (check examples)
Divergent boundary	Rock rises and partly melts as pressure drops	Runny basalt, mostly effusive; Iceland
Subduction zone	Water from the sinking slab lowers the melting point of the mantle above	Thicker, gas-rich magma; explosive; the Andes, Japan, Mount St Helens
Hot spot	A plume of hot mantle rises through a plate	Basalt shield volcanoes; Hawaii

Two features control how an eruption behaves: the viscosity of the magma and the amount of dissolved gas. Runny, gas-poor magma flows out; sticky, gas-rich magma traps gas until it blasts apart, like shaking a bottle of fizzy drink.

Hazards

Earthquakes: building collapse, landslides, liquefaction of soft ground and tsunamis when the sea floor moves (check).
Volcanoes: lava flows, ash falls, pyroclastic flows (fast hot clouds of gas and ash), lahars (volcanic mudflows) and gas. Ash can disrupt air travel, as in Iceland's 2010 eruption (check).
Climate effects: very large eruptions can cool the climate for a year or more by putting sulphur aerosols in the upper atmosphere (check).

Prediction and Preparedness

Forecast, not prophecy Scientists cannot predict the time, place and size of an earthquake in advance, though they can map where earthquakes are likely and estimate probabilities over decades (check). Volcanoes are easier: signs such as swarms of small quakes, ground swelling and gas changes can give days or weeks of warning, though not always exactly when or how big (check). Useful responses are building codes, warning systems that give seconds of notice, hazard maps and drills, because they work without precise prediction.

A British comparison Britain is far from plate boundaries, so serious earthquakes are rare, but not unknown: the Colchester quake of 1884 damaged hundreds of buildings, and small events are felt in Scotland, especially near Comrie in Perthshire, long known for tremors (check). Scotland's volcanoes are all extinct, from Arthur's Seat to the old centres of Mull and Skye, which belong to the time the Atlantic was opening (Chapter 3, check). Those rocks are a map of what a volcano's interior looks like once the top has worn away.

Check the details Death tolls, magnitudes and dates differ between sources and are often revised. Treat predictions in the media with caution, and check hazard statements against national agencies such as the British Geological Survey or the US Geological Survey. Advice on what to do in an earthquake varies by country.

Exercises

Exercise 1

A seismic station records the S-wave 40 seconds after the P-wave. Explain how this gives the distance to the quake, and why three stations are needed to locate it.

→ Solution

Exercise 2

Explain why a magnitude 6 earthquake can kill more people than a magnitude 7, using the ideas of magnitude, intensity and construction.

→ Solution

Exercise 3

Compare an Icelandic basalt eruption with a subduction-zone eruption in terms of magma, eruption style, main hazards, and warning time.

→ Solution

Quick Reference

Earthquake cause: stored stress released by sudden slip on a fault
Waves: P first, S second, surface waves last and most damaging
Magnitude vs intensity: energy of the quake vs shaking at a place
Volcano settings: divergent, subduction, hot spot
Prediction: hazard maps and probabilities for quakes; monitoring for volcanoes