EOSC 114 - Volcanoes

Why should we care about Volcanoes

• There is a high socioeconomic cost when volcanoes erupt

• Many communities are very close to volcanoes (including BC)

  • Volcanoes are associated with fertile soil

  • Harvest of coffee beans

  • Most volcanoes are near water and mineral deposits

Volcanoes in BC

• There are 15-20 major volcanoes in BC

  • Hundreds of inactive ones

• More than half of Canada’s volcanoes are in BC

• Most volcanically active place in Canada

• Experiences hundreds and thousands of volcanic eruptions, but we haven’t been here to witness any

• We are on a convergent boundary, which is why we have so many explosive stratovolcanoes

  • Subduction occurs at convergent boundaries

Volcanoes

Where magma and gas leak out from the earth’s crust and/or mantle

How do volcanoes form?

• The heat from the earth’s core can melt rocks, called magma

• Magma is created by melting pre-existing rock below the earth’s surface and reaches 

• If it reaches the surface, it extrudes as lava or it explodes as pyroclastic material 

• When magma forces its way up to the volcanic vent or breaks through weak parts of the crust, an eruption happens

  • gases, ash, and lava escape

Magma

• Melted rock in the earth below the earth’s surface (typically in earth’s crust or mantle)

• Melting liquid molten rock, and then turns into a solid (crystals- igneous rock) that crystallize out of the melt as the magma cools

• Creates bubbles (gas phase) and are volatiles

  • As the magma rises and pressure decreases, these gases come out (H2O, CO2, SO2)

Lava

• Melted rock exposed at the earth’s surface

• melt (becomes lava, hot and molten) + crystals (solid component of the lava now visible) + bubbles (rapid expansion gas bubbles or become trapped if the lava is too viscous)

Igneous Intrusive Rocks

• The rock that forms when magma cools and solidifies beneath the earth’s surface (underground)

• Called plutonic rocks because they never made it to the surface

  • I.e granite (felsic)

• Very slow cooling

• Large crystals

• Intrusive igneous never make it to the surface because the magma feeding them cools and solidifies underground before it can complete its ascent and erupt

Igneous Extrusive Rock

• Formed when magma escapes the volcanic vent

• Happens when lava cools quickly on the earth’s surface 

• Quick cooling

• Tiny crystals

• I.e. rhyolite

Magma Source

• Primary magma source is the melting of the mantle (always begins as mafic, but rises and becomes felsic)

• Secondary source is the melting of the crust

  • When magma reaches the crust, it tends to stay there and begins to melt the crust

Magma Transport

• Magma moves along fractures in the earth’s crust to form dikes and sills

• Dikes: Near vertical intrusion of magma. Cuts across existing layers

• Sills: Near horizontal intrusion of magma that conforms to existing layers

Properties of Hot Magma

• Low viscosity

• Low silica

• Low gas content

• Mafic

Properties of Cool Magma

• High viscosity

• High silica

• High gas content

• Felsic

Effusive Eruption

• Happens with mafic magma because of the low gas content and low viscosity

• Gentler, flowing of lava

Explosive Eruption

• Happens with felsic magma because of the high gas content and high viscosity

• Results in ash clouds, fragments

Where do volcanoes occur?

• Plate boundaries

  • Divergent (on continental crust between oceanic plates) and convergent plates (subduction zones)

    • Mid-oceanic ridges (most volcanism on earth and is not exposed on the surface except for in Iceland because of the presence of a hot spot)

  • I.e. the Ring of Fire (plate edges)

• Hotspots

  • Under continental or oceanic crust

Continental Volcanic Arc

• A type of subduction volcano

• Forms at convergent plate boundary (oceanic-continental)

• Forms on the edge of a continent as a curved mountain belt

• Felsic magma

• Cascadia 

• Garibaldi belt is the northernmost segment of the Cascadia volcanic arc

• Subduction of the Juan de Fuca Plate under the North American Plate 

Oceanic Volcanic Arcs

• A type of subduction volcano 

• Long, curved chain of volcanic islands formed in the ocean above a subduction zone

• Mafic magma

• Forms at a convergent plate boundary (oceanic-oceanic)

Hot Spot Distribution

• Anomalously hot compared to the surrounding mantle

• Mafic magma from a pulme of hot mantle

  • Rises slowly through and reaches the lithosphere

• Plumes are stationary and pulsatory

Melting - Divergent Boundaries

• Mantle material rises in a dike

• Pressure decreases

• Mafic magma may melt overlying crust

Melting - Subduction Zones

• Hydration of the mantle by subducting plate

• Mafic magma melts overlying crust

Melting - Hot Spots

• Mantle material rises in a cylindrical plume

• Pressure decreases

• Mafic magma melts overlying crust

Magma Type - Divergent Boundaries

• On continental crust - mafic to intermediate to felsic

• Between oceanic plates: mafic

Magma Type - Hot Spots

• Under continental crust - mafic to intermediate to felsic

• Under oceanic crust - mafic 

Cinder Cone Volcano

• Mafic

• Occurs at any boundary

  • Parasitic (occurs within other types of volcanoes)

• Explosive

• Layers of pyroclastic ejecta from fire fountaining

• basically small volcanoes that never grow up

• Usually erupt for a few years than never again

• Frequent eruption

• Produce Strombolian eruption  

Shield Volcanoes

• Mafic

• Lava flow

• Not associated with any boundary (magma comes from deep within the earth)

• Occurs anywhere where a mantle plume comes to the surface

• Formed by continuous magma

• Frequent and continuous eruption

• Lava erupts from fissure, runs down gentle slopes, cooling

• Erupts often

Stratovolcanoes

• Intermediate/felsic

• Vulcanian/Plinian eruption

• Pyroclastic flows only here

• Looks like a mountain peak

• Only at a subduction convergent plate boundary

• Felsic

• Explosive, but infrequent

• Interbedded lava flows, pyroclastic flows, and laharsr

• Viscous magma

• May erupts many times and stay active for 100,000 years

Calderas (supervolcano)

• Felsic

• Explosive

• Created from large, explosive eruptions of felsic, pryoclastic material

• Created when the roof of the magma chamber collapses

• Up to 10km across (very large)

• Rarely erupts

• magma coming up to the surface and the surface does not move, but the magma does not come up all at once

Effusive Volcanic Eruption

Outpouring of molten magma from the vent (lava)

• May result in:

1. Lava flows (mafic-intermediate)

2. Lava domes (felsic-intermediate)

3. Gravitational collapse of lava flows/domes - pyroclastic flows

Explosive Volcanic Eruption

Gas driven violent eruptions (pyroclastic deposits)

• May result in:

1. Buoyant eruption column of ash

2. Pyroclastic airfall

3. Pyroclastic flows (column collapse)

4. Blocks and bombs proximal to vent 

Pyroclastic Flows

• Eruption columns: 10s of kms

• Widespread distribution of ash in the downwind direction

  • usually stay in valleys but if big enough can flow over ridges

  • Move even over water

• Ash blankets topography

• Develops when volcanic ash, rock fragments, and hot gases become too heavy to stay in the air and instead collapse and rapidly flow down the volcano’s slope

• gravity driven flows

• Flow down slop, channelled in valleys

• Velocity = 40-400 km/h

• Temperature = 100-600 degrees celsius

How do magma fragments form pyroclastic ash?

When liquid magma containing dissolved gas is suddenly decompressed during an eruption, gas bubbles expand rapidly and blow up the liquid magma, which freezes in mid air to form ash particles.

Mafic Explosion

• Low gas and fluid = effusive eruption

• Gas escapes

• Pressure released

• “Safe”

Felsic Eruption

• High gas content and gooey = explosive eruptions

• Gas kept under increasing pressure

• Dangerous

Hawaiian Eruption

• Low viscosity balsaltic magma

• Low explosivity (effusive)

  • Lava flows

  • Fire fountatining

Strombolian Eruption

• Balsaltic/andesitic magma

• Mildly explosive

• Bombs, lavas

Vulcanian Eruption

• Viscous andesitic/rhyolitic magma

• Very explosive

• Sustained explosions of ash

Plinian Eruption

• Andesitic/rhyolitic ash

• Violently explosive

• Sustained column of ash

• Pyroclastic flows

Volcanic Explosivity Index (VEI)

1. Volume of ash produced 

2. height of eruption cloud above the vent

3. Duration of eruption

Hazard

Any event or situation that could cause human (deaths, injuries) or economic harm (property and prosperity).

Risk

• Probability that any given hazardous event might occur

• Chance of a potential loss

Hazard - Lava Flows

• Slow

• usually not dangerous

• easy to predict flow path, but hard to stop 

• (usually) mafic and low viscosity 

• usually don’t kill people 

• Fire fountaining:

  • If balsaltic lava is gas-rich

  • Small explosive eruptions form fire fountains

  • As some of the liquid drops fall back to the ground, they may coalesce to form a lava flow

Hazard - Pyroclastic Falls (Ash Fall)

• Breathing in ash can be deadly 

• Total darkness

• Roofs collapse

  • Most dangerous problem because most fatalities are people trying to shelter

• Avalanche of pyroclastic material, air and gas

• Driven by gravity

• 40-400km/h

• Hazards to aircraft:

  • Engines suck in ash and stop

  • Windshields are scratched and break

  • Turbulence

Origins of Pyroclastic Flows

• Common- explosive collapse of lava domes

• Silica rich magma

• Steep sized domes

Dome Collapse

• At Unzen volcano was followed by pyroclastic flows racing downslope

• The flow has a dense core of boulders which is hidden by the billows of ash

Lahars

• An Indonesian word for volcanic debris flow

• Flows of water and loose volcanic debris

• Prevalent at snow-clad and ice-clad volcanoes

• Can travel very far by existing river systems

• Move extremely fast

• Can grow with distance

Sector Collapse (Debris Avalanche)

• Volcanic edifice is weakened

• Collapse of part of the volcano may ensue

• During a collapse, a debris avalanche occurs, and a scalloped scar remains

Mt. St. Helens Eruption (1980)

• A landslide triggered an explosive eruption

• The lateral blast was comparatively cool (100-300 degrees)

• Speed approached 500km/h

• Devastating to a very large area up to 20km)

Volcanic Gases

• Highlight acidic and toxic

  • Kills plants and animals

• Major constituents include H2O, Co2, HCl, SO2, HF

Steps to Volcano Monitoring and Prediction

• Geology and mapping (hazard and risk maps)

  • Map volcanic deposits

  • Determine deposit flow (mud, pyroclastic, lava flow) and distribution

  • Determine age of deposits and frequency of eruption

  • Consider the risk

• Monitoring

  • ground deformation (GPS - measures changes in position and TM - measures changes in angle of slope and InSAR - measurements by satellites to detect changes in elevation)

  • gas emission (direct sampling but better to do it from distance and FTIR measures the absorption spectrum of gases in the plume to see which gases are present )

  • thermal imaging (measuring temp. and range)

  • lahar flow detection (monitoring lahar and debris flow channels by motion sensing systems and provides real-time warning)

  • satellite observation (ideal for early warning and remote areas)

  • seismology (earthquakes- rock cracks)

• prediction and hazard analysis

  • sarning systems and procedures

Magma Type - Subduction Zone

Beneath continental crust: mafic - int.-felsic

Beneath oceanic crust: mafic

What type of volcano is Mt. St. Helens?

Stratovolcano

What type of volcano is Yellowstone?

Caldera

Which type of volcano is the most devastating?

Calderas

Two Factors responsible for explositivity

Viscosity and gas content.

Granite

• Very felsic

• part of the continent (we are standing on continental crust)

• Made from very felsic secondary magma

Basalt

• What most oceanic crust is made of

• Mafic end-member

• Made from very mafic magma

Volcano Distribution 

• Around the pacific plate

• Pacific ring of fire

• Most of our explosive volcanoes

• Formed at a subduction zone 

• Most of the boundary is one massive subduction boundary

  • Create felsic magma = creates explosive eruptions

Divergent Boundary Volcanoes

Effusive, not explosive

Convergent Boundary Volcano

Highly explosive

Cinder Cone Hazards

• Lava flow

• Volcanic Bomb

• Ash fall

Stratovolcano hazards

• Lava flow

• Volcanic bombs

• Ash fall

• Pyroclastic flows

• Toxic gas