Volcanoes

What is a volcano?

• A point on the earth’s crust where magma forces its way to the surface

• Ash and gases may also escape

Where do they occur?

• On subduction zones• On constructive plate boundaries• On hot spots

What types of volcanoes are there?

Acid volcanoes• Highly explosive• Magma/lava is viscous

(thick)• Found where oceanic

crust is subducted under continental crust

Basaltic volcanoes• Less explosive*• Magma/lava less viscous

(runny)• Found at rift zones

(constructive boundaries) and hotspots

* Continental hotspots are basaltic but potentially highly explosive

Acid (rhyolitic) volcanoes

Lava domes• Formed of layers of lava

high in silica• Lava is viscous and does

not run very far• Rounded form• composed completely of

lava

Stratovolcanoes • Also called composite

volcanoes• Formed of layers of lava

and ashes• Lava is viscous• Distinct cone shape

Example – lava dome

• Mount St Helens - Washington state, USA

beforeduring

after

Example - stratovolcano

• Mount Pinatubo, Luzon, Philipines

before

During…

after

Other famous stratovolcanoes

Fuji,dormant

Krakatoa,active

Vesuvius,active

Kilamanjaro,dormant

Location of stratovolcanoes

• Along subduction zones

• Often found in volcanic arcs*

• E.g. Cascade range, USA

*not to be confused with Island chains formed over hotspots

Basic (basaltic) volcanoes

• Also known as shield volcanoes• Formed of widespread layers of lava low in

silica – low viscsity, lava travels very far• Low form spread over a great distance

Example – basic volcano

• Eyjafjallajökull, Iceland

Example 2

• Kilauea, Hawaii

Calderas

• Collapsed volcanoes• Magma chamber has emptied and the

ground has sunk• Often becomes a lake• New volcanoes can form, or pressure can

build from below, lifting the ground• If acidic, this can cause a catastrophic

eruption in the form of a ”super-volcano”

Caldera - Santorini

• Destroyed the Minoan civilization due to tsunamis circa 1600BC

• May have given rise to the Atlantis myth

Yellowstone national park

Yellowstone is a hotspot

Hotspots

• Tectonic plate moves over a magma ”plume”

• Occasionally erupts, creating a volcano

• Hawaii – basic• Yellowstone - acid

Major hotspots

Volcanic Explosivity index (VEI)

• 1 – Hawaiian gentle (Kilauea)• 2 – Strombolian explosive (Stromboli)• 3 – Vulcanian severe• 4 – Peléan cataclysmic (Mt.Pelée)• 5 – Plinian paroxysmal (St.Helens)• 6 – plinian/ultra-plinian colossal (Krakatoa)• 7 – ultra-plinian super-colossal (Tamb.)• 8 – supervolcanic mega-colossal (Y.st)

Number 8 has never been experienced in human history

The list has been simplified. See correct details here:http://www.unc.edu/~rowlett/units/scales/VEI.html

Volcanic hazards

• Most hazards during eruptions are caused by what comes out of a volcano

• In worst case scenarios then a volcano may violently explode

Volcanic hazards – lava flows

• Mostly associated with basic volcanos

• Slow moving 1 – 5 kmh

• Low risk to human life

• Cover large areas and destroy large amounts of infrastructure

Hazards – pyroclastic flows

• Hot clouds of ash and gas• Move downhill, close to the ground,

following valleys• High speed – up to 500 kmh• As far as 30 km• Up to 7000 C• Highly dangerous

Hazards - lahars

• Occur on high volcanos covered in snow and ice, or wet mud/soil

• Eruption causes snow to melt, or lava mixes with mud

• Flows downhill like wet concrete

Hazards – ash clouds• Slow moving• Weight of ash can

collapse buildings• Destroys crops, pollutes

water• Affects air traffic• Can enter high

atmosphere and cause cooling – disrupting climate

Hazards – lava bombs• Molten rocks thrown out of volcano• Pummice – smaller rocks• Travel short distances• Can start fires

The Decade Volcanoes

• 16 volcanoes identified as potentially highly hazardous to humankind

So why live near a volcano?

• Aesthetic beauty e.g. Mount Fuji, Japan

• Mining of minerals and diamonds

• Geothermal energy and hot springs

So why live near a volcano?

• Highly fertile soils – soft rocks and high mineral content

So why live near a volcano?

• Poverty – cheap marginal land

• Overcrowding and population pressure – nowhere else to live

• Common problems in LEDCs

So why live near a volcano?

• Complacency or naivete– ”nothing will happen”

• Confidence– ”we will be warned and have time to get away”

• Tradition/family– ”we have always lived here”

Predicting volcanic eruptions

• Monitoring seismic disturbances (tremors)• Changes in volcano profile (shape)• Chemical changes in groundwater• Emissions of gases• Thermal monitoring (temperature)

Vesuvius

Mount Vesuvius

Mount Vesuvius is an active volcano in Italy

It is the only volcano on the European mainland to have erupted in the last 100

years

It is not currently erupting

It is dormant, not extinct

• Vesuvius is in southern Italy near Naples

• Location:

40°49′N 14°26′E

Naples

Vesuvius

Vesuvius is a composite volcano(strato-volcano)

Vesuvius - data

• Height (elevation) 1,281 metres (4,203 ft) above sea level

• Has a collapsed caldera from a previous eruption

Formed at the collision of two tectonic plates – the African and

the Eurasian

• A converging, destructive boundary, with very little movement

• Several volcanoes nearby, none active

An acid volcano

• Potential for violent eruptions

• Slow moving lava• Explosions• Hot ash/cinders• Pyroclastic flows• Real danger!

History of eruptions

• 1800 BC – destruction of Bronze age settlements – then several more times

• 79 AD – destruction of Pompeii• At least 40 times until the last eruption in

1944 – witnessed by allied troops towards the end of WWII

• = once every 40-50 years

Pompeii Ad79

Pompeii AD79

• Massive eruption of Vesuvius buries the towns of Pompeii under a thick layer of burning ashes and pumice up to 2.8 metres deep and Herculaneum under 23 metres of ash and pumice

• Eruption lasted for two days• Deaths unknown – estimates from 5000 to

25000

Causes of deaths

• Burial under ash and pumice• Killed by pyroclastic flows• Collapsing building• Rain of lava bombs• Starvation• Disease• Drowning (many fled to the sea for rescue)

Buried alive!

The population of Naples is 960,000(as of 2012)The centre of Naples is 15km from Vesuvius

the population of the Naples suburban region is nearer 3 million

Reducing the risks

• Round-the-clock monitoring of the volcano– Tremors, gasses, changes in water

• Identifying hazard areas– Weak spots, secondary vents, predicting

routes of pyroclastic flows• Creating an evacuation plan

– Zoning, warning systems, public education

Evacuation plans?

• Red zone – immediate risk from pyroclastics• Blue zone – secondary zone of floods and lahars• Yellow zone – mostly ash and other debris

Useful links

• http://greenfieldgeography.wikispaces.com/IGCSE+Plate+Tectonics+and+GCSE+Plate+Tectonics

• http://volcano.oregonstate.edu/

• http://www.physicalgeography.net/fundamentals/chapter10.html

• http://volcanoes.usgs.gov/hazards/index.php