2.1 Earthquakes and volcanoes

What is a volcano

opening in the Earth’s crust through which hot molten magma (lava), molten rock and ash are erupted onto the land

Magma

molten materials inside the Earth’s interior

Lava

when molten material is ejected at the Earth’s surface through a volcano or crack

Types of volcano

shield volcanoes - produced by very hot, runny lava

associated with constructive boundaries and hotspots

cone volcanoes - produced by thick material

associated with destructive plate boundaries

chamber

the reservoir of magma located deep inside the volcano

crater

depression at the top of a volcano following a volcanic eruption. may contain a lake

vent

channel that allows magma within the volcano to reach the surface in a volcanic eruption

active volcanoes

those that have erupted in recent times, such as Mt Pinatubo in 1991 and Montessart in 1997, and could erupt again

dormant volcanoes

volcanoes that have not erupted for many centuries, but may erupt again

e.g. Mt. Rainier in the USA

Extinct volcanoes

not expected to erupt again

e.g. Kilimanjaro in Tanzania

What is an earthquake

an earthquake involves sudden, violent shaking of the Earth’s surface

occur after a build-up of pressure causes rocks and other materials to give way.

most of this pressure occurs at plate boundaries when one plate is moving against another.

focus

the place beneath the ground where the earthquake takes place

Deep-focus vs. shallow-focus earthquakes

deep-focus earthquakes are associated with subduction zones.

shallow-focus earthquakes are generally located along constructive boundaries and along conservative boundaries.

epicentre

the point on the ground surface immediately above the focus

what human activities could cause earthquakes

• nuclear testing

• building large dams

• drilling for oil/natural gas

• coal mining

The Richter scale

measures the magnitude (strength or force) of earthquakes

measured on a seismometer and shown on a seismograph

logarithmic scale

means that an earthquake of 6.0 is ten times grater than one of 5.0, and one hundred times greater than one of 4.0

the Mercalli scale

relates ground movement to things that you would notice happening around you

its advantage is that it allows ordinary eyewitnesses to provide information on the strength of the earthquake

Distribution of earthquakes

very uneven

mostly along plate boundaries which are regions of crustal instability and tectonic activity

500,000 earthquakes are detected every year

most occur in linear chains along all types of plate boundary

some appear in areas away from plate boundaries, such as in the mid-west of the USA

Distribution of volcanoes

there are over 1300 active volcanoes in the world

many of them are under the ocean

three quarters of the world’s active volcanoes are located in the ‘Pacific Ring of Fire’, the area around the Pacific Ocean

e.g. Mt Pinatubo, Krakatoa

plate tectonics

set of ideas that describe and explain the global distributino of earthquakes, volcanoes, fold mountains and rift valleys.

the cause of earth movement is huge convection currents in the Earth’s interior, which rise towards the Earth’s surface, drag continents apart and cause them to collide

these events happen because the Earth’s interior consists of semi-molten layers (magma), so the crust moves around on the magma

the main plates are: Pacific, Indo-Australian, Antarctic, North American, South American, African and Eurasian

structure of the Earth

there are four main layers

1. the inner core: solid, 5 times denser than surface rocks

2. outer core - semi-molten

3. mantle - semi-molten and about 2900 km thick

4. crust - solid and is divided into two main types: oceanic and continental. the depth of the crust varies between 10km and 70 km

continental crust is mostly formed of granite, it is less dense than the oceanic crust.

the oceanic crust (more denser) plunges beneath the continental crust when they come together

constructive (divergent) boundaries

the two plates move AWAY from each other

new oceanic crust is formed

volvanic activity is common

destructive (convergent) boundaries

the oceanic cruts moves towards the continental crust and sinks beneath it due to its greater density

deep sea trenches and island arcs are formed

the continental crust is folded into fold mountains

volcanic activity is common

collision zones

two continental crusts collide

as neither can sink they are folded up into fold mountains

plates are folded and crumpled

conservative plates

two plates slip sideways past each other but land is neither destroyed nor created

earthquakes can occur

causes of earthquakes

caused by the build-up of pressure that results from plate movement

so many earthquakes are found close to plate boundaries

what is the shit that comes out of volcanoes

many diff. types of material

pyroclastic flows are superhot (700*) flows of ash and pumice moving at speeds of over 500km/hour

cinders are small rocks and coarse volcanic materials

volcanic strength

measured by the volcanic explosive index (VEI)

based on the amount of material ejected in the explosion, the height of the cloud it causes and the amount of damage caused

any explosion above level 5 is considered very large and violent

the scale is logarithmic, so VEI8 is ten times more powerful than VEI7

supervolcanoes

have VEI8

tend to be much larger than normal volcanoes

the likely impacts of a VEI8 eruption include:

• almost complete loss of life within about 1000 km of the eruption

• destruction of all crops and livestock, leading to a global famine

• economic and social devastation

natural hazard

a natural event that causes damage to property and/or disruption to normal life, and may caues loss of life

involve hydrological, atmospheric and geological events

caused by the impact of natural events on the social and economic cenv. in which people live

some groups of people are more vulnerable to them/have greater exposure

since the 1960s, more people have been affected by natural hazards. What are the reasons for this?

• a rapid increase in population, especially in developing countries

• increased levels of urbanisation, including more shanty towns, which are often located in hazardous env.

• changing land use in rural areas, which results in flash floods, soil erosion and landslides

• inc. number of people living in poverty who lack the resources to cope w natural hazards

• changes in the natural env. causing inc. frequency and intensity of storms, floods and droughts

hazard, disaster, risk

hazard: potentially dangerous event or process

disaster: when the hazard affects people and their proerty

risk: there is possibility of loss of life or damage

risk assessment is the study of the costs and benefits of living in a particular env.

two ways of looking at people’s vulnerability

1. people choose to live in hazardous env. because they understand the env. people choose to live in an area because they feel the benefits outweigh the risks (wrong)

2. people live in hazardous env. because they have very little choice over where they live, as they are too poor to move (right)

why are volcanic eruptions useful

• some countries were created by volcanic activity

• volcanic soils are rich, deep and fertile, allowing intensive agriculture to take place

• volcanic areas are imp. for tourism

• some areas are seen by people as symbolic and part of the national identity

what affects the extent of earthquake damage

• strength of the earthquake and number of aftershocks

• population density

• type of buildings

• time of day

• distance from the epicentre of the earthquake

• types of rocks and sediments

• secondary hazards

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managing volcanoes

• spraying lava flows with water to cool them down and solidify

• digging diversion channels to divert lava flows away from settlements

• adding ‘cold’ boulders to lava flow in attempt to cool the lava and stop it moving

but if the eruption is a pyroclastic flow, there is little that can be done to prevent the impacts apart from evacuation

predicting volcanoes

main methods:

• seismometers, to record swarms of tiny earthquakes that occur as the magma rises

• chemical sensors, to measure increased sulfur levels

• lasers, to detect the physical swelling of the volcano

• measurement of small-scale uplift or subsidence, changes in rock stress and changes in radon gas concentration

• ultrasound, to monitor low-frequency waves in the magma, resulting from the serge of gas and molten rock

how do people cope with earthquakes

three basic options

• do nothing and accept the hazard (LMAO???)

• adjust to living in a hazardous env. - strengthen the home

• leave the area

how to deal with earthquakes

• better forecasting and warning

• building design, location and emergency procedures

how to predict and monitor earthquakes

• measuring crustal movement - small-scale movement of plates

• recording changes in electrical conductivity

• noting strange and unusual animal behaviour, e.g. among carp (fish)

• checking historical evidence - there are possibly trends in the timing of earthquakes in some regions

building design for earthquakes

a single-story building responds quickly to earthquake forces.

high-rise building responds slowly and shock waves are increased as they move up the building

if buildings are too close together, vibrations may be amplified b/w buildings and increase damage

weakest part of the building is where diff. elements meet

elevated motorways are vulnerable in earthquakes bc. they have many connecting parts

certain areas are at risk from earthquake damage - areas with weak rocks, broken rocks and soft soils

many oil and water pipelines in tectonically active areas are built on rollers so that they can move with earthquake rather than fracture

safe houses

safe houses can be built cheaply using straw, adobe or old tyres

e.g. in India, a concrete house reinforced with bamboo has been successfully tested by researchers