OCR A-Level Geology Module 6 Geohazards overview quiz quiz

A tsunami

Ground shaking

Liquefaction

A fire from a broken gas main

Lava cooling so quickly that it turns straight into a solid natural glass such as obsidian

A type of long-wavelength ocean wave generated by sudden displacement of the seabed during strong shaking

Water-saturated loose sediment behaving like a liquid when shaken, so buildings sink or tilt

The partial melting of the outer core triggered by the energy released during a large quake

Magnitude has no effect whatsoever on the amount of damage that an earthquake actually causes on the ground at all

A larger surrounding population always acts to reduce the damage, because the overall load is shared across many more people

Deeper-focus earthquakes always cause more damage than shallow ones, regardless of where the population lives

The hazard may be similar, but exposure and vulnerability (for example building quality) differ, so the risk differs

The exact time, place and size cannot be reliably predicted, so mitigation is the main defence

Earthquakes can already be predicted reliably to the exact minute, so warning everyone is straightforward

Earthquakes never actually cause any real damage, so there is simply no need to defend against them at all

Predicting earthquakes is illegal in most countries, so scientists are not permitted to attempt it

Slow basaltic lava flows

Pyroclastic flows

Long-term ground inflation

Steam vents

A fast basaltic lava flow; it needs only a steady supply of runny magma to form

A volcanic earthquake; it needs an active fault running directly beneath the cone

A volcanic mudflow; it needs ash to mix with water (rain or melted snow and ice)

A dense cloud of volcanic gas; it needs no water at all in order to develop

Its magma is much runnier and erupts gently, so it spreads quietly as fast-flowing lava

It has no magma chamber at all beneath it, so the pressure can never build up to dangerous levels

It only ever produces slow, cool lava flows that creep downslope and threaten property alone

Its viscous, gas-rich magma erupts explosively, producing pyroclastic flows, ash and lahars

Magma is accumulating in the chamber and pressure is building, possibly before an eruption

The volcano is now completely extinct and is therefore very slowly sinking back down again

The regional sea level is rising and is pushing the whole flank of the volcano gently upward

A large earthquake has just ended nearby and the ground is rebounding back to its old level

They erupt strictly on a fixed calendar, so the exact date of the next eruption is always known

Magma movement produces measurable precursors (seismicity, inflation, gas, heat) that allow forecasting

They give no measurable warning at all before erupting, so monitoring instruments are of little use

They are physically much smaller events than earthquakes, so their effects are always far simpler to anticipate

When resisting forces exceed driving forces

When the slope is perfectly horizontal

When driving forces exceed resisting forces

Only during an earthquake

It simply cools the rock down, which makes the whole slope mass contract and grip together more firmly

It increases the friction between the grains, which acts to lock the slope material more securely in place

It has no measurable effect on slopes at all, since water drains straight through without changing anything

It raises pore water pressure, reducing the friction holding the grains together (as well as adding weight)

Rotational slide (slump)

Rockfall down a vertical cliff face

Translational planar slide

Saturated debris flow

It makes the slope considerably steeper, which somehow increases the friction that is available to resist failure

Water is so often the trigger, so removing it lowers pore pressure and weight, restoring friction and stability

It adds a useful amount of extra weight to the slope, which presses the loose material firmly into place

It removes the need for any other stabilisation measure, since a drained slope can never fail again

The slope steadily becoming flatter on its own without any human intervention at all

A fall in the water table on its own, with no cracks or bulging appearing anywhere

Tension cracks at the top of the slope and bulging at the toe

Vegetation on the slope growing noticeably thicker and healthier than it was before