volcanoes: from inception to eruption

Volcanoes: from inception to eruptionDr Tom D. Pering

Department of Geography

University of Sheffield

The Plan

Part A: Heat and Plate Tectonics.

Part B: Volcano Fundamentals.

Part C: Volcanic Hazards.

• First – what is a volcano?

• “Surficial manifestation of a planet or moon’s internal thermal processes through emission of solid, liquid or gaseous products at discrete surface locations”

• Definition not geomorphological, volcanoes have a variety of shapes.

• Interweaved you will see snippets of a mini documentary (not yet released) on volcanoes and volcanic fieldwork.

• Aim: to cover common GCSE/A Level content, but also provide you with a little bit extra.

Part A: Heat and Plate Tectonics

A: Heat and Plate Tectonics

1. Heat Generation • Volcanism is powered through heat.• Heat is generated within a planetary body in two main ways:

Radiation Tidal Forces

Decay of radioactive isotopes ininterior liberates heat, like a nuclearpower station or bomb!

Astronomical objects don’t just movewater on Earth, but also deform theinterior, liberating heat, like kneadingdough.

NASA/JPL Image.


2. Heat Loss • Convection through plate tectonics and

mantle plumes generates volcanism.

Conduction

Heat travels from the interior andthen through the crust.

Convection – Plate Tectonics

Material cycles from the interior tothe surface through plate tectonics –this where most is heat is lost.

Convection – Mantle Plumes

Mantle plumes rise from the core-mantle boundary thus dissipating heat– like a lava lamp.


3: Plate Tectonics & Margins• Animation of earthquakes, eruptions, and SO2 emissions -

http://volcano.si.axismaps.io/

http://volcano.si.axismaps.io/


3: Plate Tectonics & Margins

Part B: Volcano Fundamentals

1. Classic Geomorphology

B: Volcano Fundamentals


Mount Fuji, a stratovolcano in Japan.

• Stratovolcanoes: layers of viscous lava flows and pyroclastic rocks.



• Shield volcanoes: layers of fluid lava flows, convex profile and can be enormous.

Above: Fernandina volcano, a shield volcano of the Galapagos

Islands (Chuck Wood, 1979).


Left: Kilauea summit area in 2020.


• 1: Low viscosity magmas such as Basalt, give rise to basaltic volcanism.• Most commonly produced at mid-ocean ridges and at hot spots.• These magmas create shield volcanoes!

2. Magma Type Overview


• 2. High viscosity magmas such as Andesite and Rhyolite give rise to silicic volcanism.

• Most commonly produced at subduction zone settings.• These magmas create stratovolcanoes!



• Eruptive style is determined primarily by magma type, specifically its viscosity, which is determined by the amount of SiO2 (silica) present.

• SiO2 polymerises into SiO4 (silcate tetrahedra lattices), which impede flow of magma, making it more viscous.


More Silica = Higher Viscosity

• Low silica (~ 50%) = Basalt• Medium silica (~ 50-65%) = Andesite• High silica (> 65%) = Rhyolite

• A magma is a complex mixture of:• Melt (molten rock), Crystals (solid), Gas


3. What is a magma?


4. Gases – Bob the Bubble


4. Gases


5. Basaltic (low viscosity) Volcanism


• Basaltic magmas have low viscosities.• The low magma viscosity allows the formation of larger bubbles during bubble

ascent. This leads to the formation of a number of different flow regimes which generated a number of different basaltic activity styles:

5. Basaltic (low viscosity) Volcanism


6. Silicic (high viscosity) Volcanism


• Higher viscosity magmas tend to restrict gas flow, leads to pressurisation, and the largest volcanic explosions!

• The lava dome case:

6. Silicic (high viscosity) Volcanism

Interlude: MAGATH

Interlude: MAGATH

• The key factors which act to control style of volcanic activity, how the activity may behave in the atmosphere, and how it is generated.

• Material (Molten Rock or Ice)

• Available Volatiles (Gases)

• Gravity

• Atmospheric Pressure

• Tectonic Environment

• Heat Generation Mechanism

Part C: Volcanic Hazards

C: Volcanic Hazards

• Local hazards:

C: Volcanic Hazards

• Overview:• Eruption column – aviation & climate.

• Tephra fall – localised.

• Collapse of lava dome or eruption column to form pyroclastic density current.

• Collapse of volcano in debris avalanche.

• Mobilisation of ash to form lahars.

• Volcanoes can cause tsunami.

• Volcanic gases.

• Sub-glacial volcanoes can cause massive floods (Jökulhlaups).

C: Volcanic Hazards

Left, gas unable to escape from a viscous magma. Right, a pyroclastic density current (PDC).

• Silicic volcanism - Pyroclastic Density Currents:• Gas can’t move freely within a magma and can find it difficult to escape (i.e., due

to high viscosity).• When gas can’t escape, pressure builds up, leading to large pyroclastic explosions

(i.e., vulcanian, sub-plinian, plinian).• Eruptions can lead to pyroclastic density currents (PDCs), where instability in

eruption columns causes column collapse.

C: Volcanic Hazards

• Silicic volcanism - Pyroclastic Density Currents:

C: Volcanic Hazards

• Silicic volcanism - Lahars:

C: Volcanic Hazards

• Precursors - notes on uncertainty…• Forecasting versus prediction.

Encyclopedia of Volcanoes, Chapter 67.

C: Volcanic Hazards

• Responding to risk:

What hazards? What intensity?What ‘assets’?

How vulnerable are they?

Resilience. Ability to Respond.

𝑅𝑖𝑠𝑘 =𝐻𝑎𝑧𝑎𝑟𝑑 × 𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒 × 𝑉𝑢𝑙𝑛𝑒𝑟𝑎𝑏𝑖𝑙𝑖𝑡𝑦

𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦

Where spatially?

C: Volcanic Hazards

• We can reduce risk, through monitoring:

Gas ReleaseY

Seismicity

Deformation

Thermal

+

+

+

C: Volcanic Hazards

• We can build sophisticated models:

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Pressurisation

StabilityMagma at surface

Magma movingat depth

Ground deformation

DOME COLLAPSERainfall

on dome

C: Volcanic Hazards

A summary of recent important eruptions:

Taken from: Sigurdsson, H., 2000. History of Volcanology. In: Encyclopaedia of Volcanoes. P15-40

Thanks & Summary

Part A: Heat and Plate TectonicsPart B: Volcano FundamentalsInterlude: MAGATHPart C: Volcanic Hazards

volcanoes: from inception to eruption

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