• Hot mantle plumes breaching the surface in the middle of a tectonic plate

What are Hotspot Volcanoes?

Photo: Tom Pfeiffer / www.volcanodiscovery.com

The Hawaiian island chain are examples of hotspot volcanoes.

The Hawaiian Hotspot and its associated islands

• The Hawaiian hotspot is responsible for the formation of the Hawaiian Islands.

• The islands are a chain of volcanic islands in the middle of the Pacific plate located over 3,200 km from the nearest plate boundary.

• As the Pacific Plate passes over the hotspot, volcanoes burst through in succession, producing the islands.

• As the pacific plate moves over the hotspot, volcanoes grow, become dormant and eventually become extinct over a period of 500,000 years.

• The Pacific plate moves northwest over the hotspot at a rate of about 10 cm per year.

• The Hawaiian hotspot is about 500-600 km wide and lies about 100 km below the surface.

• The hotspot is approximately 86 million years old.

• The Hawaiian islands are the most recent volcanoes to form over it.

Chapter 3

• Volcanoes

Chapter 3: Volcanoes

What you will learn

1. How plate tectonics leads to volcanic activity2. The parts of a volcano and the products of a volcanic eruption3. External or extrusive landforms (volcanic)4. Internal or intrusive landforms (plutonic)5. Effects of volcanoes – positive and negative using case studies6. Predicting volcanoes using a case study. 7. Know how this topic appears on the exam paper

Volcano Intro Clip

• http://www.youtube.com/watch?v=uZp1dNybgfc

Common Exam Questions

1. Explain how the study of plate tectonics has helped us to understand the occurrence of volcanoes

2. Discuss the positive (and negative) impacts of volcanoes

3. Examine the processes that have led to the formation of any two volcanic landforms

Mind Map Volcanoes

• In groups think of everything you remember about volcanoes since Junior cert

• How they occur• Effects• Examples etc

Activity: Take down the mind map from the board

Activity: label the diagram of a volcano

Main Features of a Volcano

Formation of Volcanoes

• Volcanoes form when molten magma forces its way up to the earth’s surface either through a vent or through a fissure.

• As magma rises, gas bubbles expand and help force it up to the earth’s surface through the crust.

• Once the magma reaches the earth’s surface the sudden release of pressure causes a volcanic eruption.

The Distribution of Volcanic Activity

Volcanoes occur in three types of locations

1. Mid Ocean Ridges (at constructive plate boundaries)

2. Subduction zones (at destructive plate boundaries)

3. Hotspots in the middle of the plates

Volcanoes at DESTRUCTIVE PLATE BOUNDARIES Oceanic-continental plate collision – subduction zones

• Heavier oceanic crust collides with and slides under the lighter continental crust.

• As it slides into the mantle it melts and is recycled.

• The melted crust turns to magma and moves up through the continental crust above, AS IT RISES THERE IS A BUILDUP OF PRESSURE AND EVENTUALLY creates some of the most explosive volcanoes in the world, e.g. Mount Saint Helens in the U.S. (Juan de Fuca Plate and North American Plate)

• Nasca and South American (Andes fold mountains – the Andean Volcanism belt)

Volcanoes - Oceanic-Oceanic Plate Collision• Are also marked by curved lines of

volcanic islands known as island arcs• These volcanoes form when the sub-

ducted plate descends and melts creating rising bubbles of magma that break through the crust above.

• These active volcanoes have built up on the sea floor over millions of years until they appear above the surface of the water.

• The Aleutian Islands, the Philippine and Japanese Islands have all been formed this way.

Volcanoes at Constructive Plate Boundaries• Plates pulled apart by convection

currents in the mantle• Magma rises between the 2 plates the

lava cools and solidifies to form new crust (or a new layer to the volcano)

• Example: Iceland and Mid-Atlantic Ridge where the North American and Eurasian Plate are pulling apart.

• Iceland has over 200 active and dormant volcanoes.

Volcanoes at Hotspots

• Hot spots form when a rising column of very hot magma called a thermal plume forces its way through a fault in the oceanic crust; they do not drift with the plates; they have a fixed position

• Shield volcanoes form where the lava runs freely & quickly creating gentle slopes

HL Exam Question – Physical Geography – Long Questions

• Explain with the aid of an example which you have studied why volcanic activity happens at plate boundaries (30 MARKS)

Layout: 30 Marks (15 SRPS)

• Explain the formation of volcanoes

• Identify which Plate Boundaries Volcanoes occur at

• Destructive plate boundaries• Oceanic – Continental (What’s happening, Plates and example)• Oceanic – Oceanic (What’s happening, Plates and example)

• Constructive plate boundaries• (What’s happening, Plates and example)

• Labelled Diagrams

• Use notes, power-point slides and book to help you with your answer

Active, Dormant & Extinct Volcanoes• Active – almost constantly erupting• Dormant – no eruption for more than 100 years• Extinct – no eruptions in historic times

Products of Volcanic Eruptions:

• Lava (Acidic lava, Basic lava)• Pyroclasts and pyroclastic flows• Poisonous gases• Ash, pumice and dust• Water vapour

1. Lava

• Two types of Lava – Acidic and Basic

• Type of lava changes depending on the plate boundary

• Contains silica – The amount of silica determines how sticky the lava is.

Lava

• Acid lava is high in silica which prevents gases escaping; is very explosive; found at convergent (destructive) plate margins

• Basic lava is low in silica; lava flows freely; found at divergent (constructive) plate margins. Gentle lava eruptions

Activity: In pairs fill in the following tables

Properties Acid Lava Basic Lava

Silica Content

Trapped Gases

Type of eruption

Typical location

Examples

2. Pyroclasts

• Hot ash, lava and rock fragments that are thrown out of the volcano. Sometimes called volcanic bombs.

• Pyroclastic Flows are boiling clouds of ash and rock which travel at great speeds (600km/hr) down the sides of the volcano. Also called Nuée ardentes.

3. Poisonous Gases

• Carbon dioxide• Chlorine • Sulpher dioxide

4. Ash, Pumice and Dust• Power of the eruption turns rock into a fine ash inside

the vent.

• The ash is thrown many kilometres into the sky and may be carried by winds across the world. Can disrupt air travel.

• When lava is thrown from the volcano it becomes full of air bubbles. If it cools quickly the bubbles become trapped in the rock and it is now called pumice. Some pieces contain so much air that they float in water.

Pumice

5. Water Vapour

• Volcanoes along subduction zones release huge amounts of water vapour

• (From the sea water that’s carried into the mantle by the sinking oceanic plates)

• As the water vapour rises into the air it cools quickly creating torrential rain.

• This can trigger lahars

Lahars

• Word that describes a mixture of water and rock fragments flowing down the slopes of a volcano.

• Looks like a large amount of wet concrete• Lahars vary in speed and size• Volcanic eruptions can trigger one or more lahar by quickly melting

snow and ice on a volcano.

Activity

• Work in pairs to discuss the positive and negative effects of volcanoes.

Good Effects of Volcanoes

• Energy – geothermal energy (hot water) is piped to many homes and businesses in Iceland

• Food production is abundant in volcanic regions; lava cools to form basalt which weathers to a mineral-rich soil

• Tourism – volcanoes & geysers attract tourists in the millions e.g. Mt Vesuvius, Italy & Old Faithful, Yellowstone National Park, USA

• New land is created Iceland and Hawaii are two examples• Minerals

Bad Effects of Volcanoes

• Toxic gases (carbon dioxide, hydrogen, carbon monoxide & sulphur) are released• Pyroclasts (fiery volcanic bombs) rain down on surrounding areas• Nuée Ardente are clouds of hot gases & pyroclasts running down the side of a

volcano at high speed• Lahar is a fast moving volcanic mudflow• Jokulhlaups are glaciers which are melted by an eruption beneath them• Climate change may occur if eruptions are particularly devastating• Can disrupt air travel

Volcanic Landforms

• Intrusive (plutonic): occur beneath the surface but are visible because of weathering and erosion.

• Extrusive (volcanic): Features found on the earth’s surface.

External/extrusive landforms (volcanic)• Volcanic features found on the earth’s surface

1. Lava plateaux, e.g. The Antrim Platuea2. Volcanic Cones, e.g. Mount Etna

Extrusive Landforms

• Volcanic Cones: magma rises from the crust. Gas bubbles expand and help the magma to force its way through the cracks. Once the magma reaches the surface an eruption occurs which gradually forms a volcanic cone. There are two types

• 1) shield volcanoes 2) volcanic domes

Shield Volcanoes• Broad, gentle, concave slopes e.g. Mauna Loa, Hawaii. • Associated with hotspots – highly fluid basic lava. • Basic lava flows easily over the ground for many kms

before it becomes solid. • Mount Loa is 4,160m above sea level but from its base

it is 10,000m high. 2km taller than Mount Everest.

Volcanic Domes

• Mounds that form when acidic lava is erupted slowly and piles up covering the vent rather than flowing away.

• Common at subduction zones. • Steep sided convex slopes that are very unstable and may callopse

causing massive landslides. • Mount St. Helen USA.

Lava Plateau

• Steep sided flat topped uplands of basalt that cover large areas• Formed by large but less explosive eruptions of highly fluid basic

lava that pours from long narrow openings or fissures in the crust. • E.g. Antrim Plateau in County Antrim. • Have the same characteristics1. Cover large areas of land2. Have a layered structure 3. Each lava eruption makes the plateau higher.

Name the features

Intrusive (plutonic) Landforms

1. Batholiths2. Sills3. Dykes4. Laccoliths5. Lopoliths

Intrusive Landforms

1. Batholith – a large mass of igneous rock (granite) that formed as magma from the mantle pushed into the crust and slowly cooled down and became solid.

Activity: Read the notes in your book about Batholiths. Summarise the main points anddraw the diagram in your copy.

• Sills: Magma forces its way between layers of rock and becomes solid as large flat areas of igneous rock.

• Dykes: Magma slices across rock layers and becomes solid and forms a wall of basalt or granite. It runs perpendicular to the rock layers.

• Laccoliths: Magma seeps between rock layers pushing them up to form dome like structures.

• Lopoliths: Form the same way but the weight of the magma causes the rock to sag downwards.

In pairs name the following intrusive features

Exam Question

• Discuss the formation of any two volcanic landforms.