EXTRUSIVE VOLCANIC

LANDFORMS

2 forms of lava:

1. BASALTIC – low silica, more fluid, so gas bubbles escape easily and so NOT so explosive

2. ANDESITIC AND RHYOLITIC – high silica(acidic) less fluid (high viscosity), gas does not escape

easily,pressure builds up and leads to violent explosions.

Types of extrusive volcanic

landforms.

• Lava plateaux – fissure eruptions eg

Iceland. Basaltic in nature, lava flows great

distances and creates flat, featureless

landscapes. Laki, Iceland.

Shield / Basic volcanoes

Free flowing lava, volcanoes

have gentle sides and cover a

large area eg Hekla

Acid/dome volcanoes

Steep sided, convex cones.

Viscous lava(rhyolite)eg Puy

region, France

Ash and cinder cones

Sides are steep and symmetrical

eg Paricutin, Mexico

Composite cones, consist of layers

of ash and lava, usually

andesitic.eg Mt Etna.

Calderas

Massive build up of gas causes

explosion and a crater to form eg

Krakatoa, Indonesia.

Location of Krakatoa, Indonesia.

The nature of volcanic eruptions

• Vulcanologists classify volcanoes according to the nature of their eruptions.

• Classification is based upon the degree of violence of explosion, which is a consequence of the pressure and amount of gas in the magma.

• Volcanic Explosivity Index (VEI) Used since 1982 to describe the relative size and magnitude of explosivr eruptions, 0-8 index of increasing explosivity. Each increase in number represents an increase of 10.

Forms of volcanic eruption

Minor volcanic forms

• GEYSERS- water heated by geothermal

activity reaches such a high temperature

that it becomes a gas and explodes out of

a fissure in the ground. Due to increased

pressure this may not occur until

temperatures reach over

120*c(superheated water).

STROKKUR GEYSER,ICELAND.

HOT SPRINGS/BOILING MUD

• If the heated water does not explode onto

the surface, but mixes with surface

deposits, boiling mud is formed. Iceland.

THE BLUE LAGOON, ICELAND

• Geothermal heating of groundwater

created lagoons, rich in silica, in a lava

field.

Positive impact of tectonic activity!

• TOURISM: AGRICULTURE:

SOLFATARA

• Small volcanic areas without cones,

produced by gas (initially sulphurous)

escaping to the surface. Eg Bay of Naples

in Italy.

FUMAROLES

• Areas where superheated water turns to

steam as it condenses at the surface.

• Very similar to Solfataras, but without the

high sulphur content.

THE IMPACT OF VOLCANIC

ACTIVITY

• Volcanic activity can affect the local,

national and global area.(spatial context).

• Volcanic activity can be classified as

having primary and secondary effects.

(temporal context).

PRIMARY EFFECTS.

• 1.VOLCANIC GASES-carbon dioxide,

carbon monoxide, hydrogen sulphide,

sulphur dioxide and chlorine. CO2 from

Lake Nyos in Cameroon in 1986

suffocated 1,700 people. Laki in Iceland in

1783 poisoned 40% of the population.

Affected harvests throughout Europe,

caused starvation and the French

Revolution!

TEPHRA

• Solid material of varying grain size from volcanic bombs to ash particles, ejected into the atmosphere.

• DUST-flour size

• ASH-sand size

• LAPPILLI-little stones

• CINDERS-rough clinker

• BOMBS-up to 1m in diametre

• Light materials such as dust and ash become airborne and travel great distances around the world. Larger materials accumulate around the crater and increade cone height.

Layers of tephra

Definition of Tephra

• Tephra, the Greek word for ash, is used to

describe any material that is ejected by a

volcano into the atmosphere. Tephra includes

dense blocks and bombs, and lighter materials

such as scoria, pumice and ash. As one moves

away from a volcano, the tephra deposits

become finer grained (the particles are smaller)

and thinner. This is because small particles stay

aloft longer and stay within the eruption cloud for

a greater distance from the volcano.

PYROCLASTIC FLOWS

• Very hot(800c),gas charged, high velocity flows made up of a mixture of gases and tephra.

• Montserrat.1995-97.

• Located in the Soufriere Hills on the island of Montserrat in the Caribbean.

• Began with ash emissions, steam and many earthquakes.

• Then in 1996 pyroclastic flows began! On 25th

June 1997 4-5 million cubic metres of pyroclastic material was emitted. See case study sheet.

Montserrat

Montserrat and Pyroclastic flows

• http://dsc.discovery.com/videos/ultimate-

guide-to-volcanoes-montserrat.html

Montserrat 1995-97

• http://www.google.co.uk/imgres?imgurl=http://w

ww.juicygeography.co.uk/images/PFJuly27.jpg&i

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sch:1

• Monitoring Volcanoes:

• Case study Montserrat.

http://www.mvo.ms/en/science/monitoring/science/monitoring

Definition of Pyroclasts

• Pyroclastic flows can form either from the collapse of an energetic eruption cloud or by the collapse of unstable lava domes that have oozed out of stratovolcanoes. The hot rock tumbles downhill as an avalanche then rapidly transforms into an expanding cloud of hot rock, "ash" and gas as it entrains ambient air and heats it, and as the hot volatiles within the lava violently decompress. The result is a self-supporting dense cloud of hot debris and gas that flows downhill as a fluid body. A cloud of "ash" rises above the flow, and it variously scours its path and/or deposits distinctive linear build ups of rock and debris, depending on the flow conditions.

LAVA FLOWS

• BASALT

• ANDESITIC

• RHYOLITIC

• Depending upon silica content.

• Differing levels of viscosity.

• Lava flow during a rift eruption at Krafla,

Iceland in 1984

More detailed classification of lava.

• Igneous rocks, which form lava flows when

erupted, can be classified into three

chemical types; felsic, intermediate, and

mafic (four if one includes the super-

heated ultramafic). These classes are

primarily chemical; however, the chemistry

of lava also tends to correlate with the

magma temperature, its viscosity and its

mode of eruption.

MAFIC LAVA IS ALSO CALLED

BASALTIC!• Mafic or basaltic lavas are typified by their high ferromagnesian

content, and generally erupt at temperatures in excess of 950 °C. Basaltic magma is high in iron and magnesium, and has relatively lower aluminium and silica, which taken together reduces the degree of polymerization within the melt. Owing to the higher temperatures, viscosities can be relatively low, although still thousands of times more viscous than water. The low degree of polymerization and high temperature favours chemical diffusion, so it is common to see large, well-formed phenocrysts within mafic lavas. Basalt lavas tend to produce low-profile shield volcanoes or "flood basalt fields", because the fluid lava flows for long distances from the vent. The thickness of a basalt lava, particularly on a low slope, may be much greater than the thickness of the moving lava flow at any one time, because basalt lavas may "inflate" by supply of lava beneath a solidified crust. Most basalt lavas are of ʻAʻā or pāhoehoe types, rather than block lavas. Underwater they can form "pillow lavas", which are rather similar to entrail-type pahoehoe lavas on land. This can now be seen on the surface on Iceland.

2 types of basaltic lava

Pāhoehoe and Aa flows

• Aa is rough lava, whilst Pahoehoe is

smooth. Both are Basaltic and are found in

Iceland and Hawaii.

• http://en.wikipedia.org/wiki/Lava

SECONDARY EFFECTS

• LAHARS: volcanic mud flows eg eruption

of the Nevado del Ruiz in Nov 1985

caused the Colombian town of Armero to

be devastated.

Video of lahars

• http://www.bbc.co.uk/learningzone/clips/vo

lcanic-hazards-lahars-in-

indonesia/3069.html

• Complete ppq on primary and secondary

hazards/lahars.

SECONDARY EFFECTS.cont.

• Flooding- melting of icecaps such as

Eyjafjallajokull causes a lot of flooding.

Bridges had to be rebuilt.

• Tsunamis-From the eruption of Krakatoa in

1883 drowned 36,000 people.

• Volcanic landslides

• Climate change-volcanic debris reduces

global temperatures.

Human responses to natural

hazards.

• 1.This involves the assessment of RISK: the

exposure of people to a hazardous event.

• Why do people expose themselves to risk?

• Unpredictability- when or where and with what

magnitude/explosivity,

• Lack of alternatives- due to economic reasons

or lack of knowledge(LIC),

• Turn a blind eye!!

2. Vulnerability

• This relates to how capable the people are at predicting,preparing and modifying the loss!

• Poverty increases vulnerability,

• Wealth and higher levels of technical ability, as well as education make people less vulnerable to all kinds of natural hazard.

• See “Is California worth the risk?”

Human Responses

• 1. Prevention! Impossible with volcanoes and

Earthquakes,

• 2. Modification of vulnerability

a. prediction and warning, how much can be

given? (see later)

b.preparedness, public education,emergency

services provision, land use planning/zoning.

• 3. Modification of loss: a. aid, b. insurance (see

California case study).

Volcano hazard management.

• http://dsc.discovery.com/videos/volcano-video/

• Eruptions can not be prevented, but they can often be predicted and protection offered to the population.

• PREDICTION:

• Hazard mapping (previous lava flows and pyroclasts by studying geology),

• Analysing seismic shockwave patterns,

• Sampling gas and lava emissions,

• Remote sensing of changes in topography,

• Heat and gas emissions by satellite.

• Seismic shock waves alerted vulcanologists to the likely eruption of Popocatepetl, Mexico in 2000 and erupted 24 hours after the local population was evacuated.

Protection

• This involves reducing the risk of damage by preparing for an eruption. Warnings issued by the USGS in the US are supported by FEMA who instruct how to react before, during and after an eruption.

• http://volcanoes.usgs.gov/publications/2010/iceland.php

• In the longer term hazard mapping and land use planning may be used to avoid development in areas at risk.

http://volcanoes.usgs.gov/activity/

• http://www.fema.gov/hazard/volcano/index.shtm

• Check out the above web-sites to take notes on

how man can predict, prepare and protect

against volcanoes.

• Link this to your case studies from high-income

and low-income countries.

• Take specific notes on the volcano warning

systems.

• PPQ on prep/prediction of volcanoes.