Lecture 5 Volcanoes

Volcanoes

What do volcanic eruptions include?

• Molten Rock (melt) flowing / spraying from vent

• Solid Debris blasting from a vent

Igneous Activity Includes

• Production and migration of melt

• Solidification of melt beneath earth’s surface

  • Eruption and Solidification of melt on Earth’s surface

Volcanic eruptions can cause two types of disasters, what are they?

o   Primary disasters

→ Eruptions have killed >250,000 people over last 200 years

o   Secondary Disasters

→ Famine, disease, mass migration following eruptions

What are the three ways melting occurs?

• Melting due to decrease in pressure

• Melting due to addition of volatiles

• Melting due to heat transfer

How does melting due to decrease in pressure work?

• rock deep in mantle under high pressure

• if deep mantle rock rises, pressure decreases

• lower pressure allows atomic bonds to break easilt

Decompression melting inclues

mantle plumes

tectonic rift areas

beneath mid ocean ridges

How does melting due to addition of volatiles work?

• volatiles are compounds that melt easily

  • H2O and CO2

• subducting plates melt and release volatiles which can cause melting in overlying mantle

How does melting due to Heat Transfer work?

• rising magma can pool at base of continental crust, which melts at relatively low temperaute, so when heat is transferred to it, continental crust begins melting

What is the difference between magma and lava?

→ Magma is molten rock beneath the Earth’s surface

→ Lava is molten rock on Earth’s surface

What is viscosity?

Melt’s resistance to flow

What are the four types of magma?

Felsic

Intermediate

Mafic

Ultramafic

Silica content in magma decreases in order of ____ magmas.

a.        Ultramafic, mafic, intermediate, felsic

b.       Felsic, intermediate, mafic, ultramafic

c.        Ultramafic, intermediate, mafic, felsic

d.       Felsic, mafic, intermediate, ultramafic

b.  Felsic, intermediate, mafic, ultramafic

With increasing silica content temperature of magma _________ and viscosity _______

• temperature decreases

• viscosity increases

With decreasing silica content, temperature of magma ______ and viscosity _______

• temperature increases

• viscosity decreases

T/F Felsic magma has higher silica content than mafic magma?

True.

Felsic Magma has higher temperature and lower viscosity than Ultramafic.

False.

Felsic magma has lower temperature and higher viscosity than ultramafic magma

Which of the following is incorrect?

a. Felsic magma has higher viscosity than mafic magma

b. Intermediate magmas have lower viscosity and silica content than ultramafic

c. Felsic Magma has lower temperature than intermediate magma

d. Mafic magma has higher viscosity than ultramafic magma

b. Intermediate magmas have lower viscosity and silica content than ultramafic

List the magmas from low to high temperatue

Felsic, Intermediate, Mafic, Ultramafic

List the magmas from low to high viscosity

Ultramafic, mafic, intermediate, felsic

What causes volatiles in magmas?

• subducting plates / soruce of rock may produce volatiles

• under high pressure and great depth volatiles remain dissolved in magma

• within 5km of surface, pressure decreases, volatiles come out of solution

When may lava eruptions be explosive?

• If volatiles (H2O, CO, SO2, H2S) are trapped in high viscosity lava

T/F The most violent eruptions are tied to gas in lava.

True

Intrusive v Extrusive Igneous Rock

Intrusive → Solidifies underground

Extrusive → Solidifies after coming out of volcano

What are the types of intrusive igneous rock?

Plutons → blob shaped

Dikes → wall shaped

Sills → magma squeezed between existing rock layers

What does extrusive igneous rock include?

→ pyroclastic debris

→ lava flows

What do textures of different rocks depend on?

Melting rate

→ Extremely fast → glassy (obsidian)

→ Slow cooling → coarse (granite)

→ Fast cooling → fine (rhyolite)

T/F: Felsic Ingeous Rocks are lighter than Mafic rocks, whihc are darker.

True

What moves material into the crust, hydrosphere, and atmosphere?

Volcanism

What are the geologic locations for volcanism?

• Divergent / Convergent Boundaries

• Continental Rifts

• Hotspots

T/F Volcanism is mainly seen at plate margins.

True

Fine Grained felsic rock and course grained felsic rock

Fine - Rhyolite

Coarse - Granite

Fine and coarse grained intermediate rock?

Andesite, Dionite

Fine and coarse grained mafic rock?

Basalt, Gabbro

What causes melting at divergent boundaries?

Mantle peridotite rises as seafloor spreading occurs

Peridotite partially melts due to decompression

Ring of Fire

Subaerial volcanoes surrounding pacific ocean

What causes melting at convergent plate boundaries?

Subducting oceanic plates release volatiles, causign melting in overlying mantle

What types of magma are created in magma chambers at convergent boundaries

Felsic and intermediate

Island Arcs vs Continental Arcs

Island arcs form on oceanic lithosphere (Aleutian Arc)

Continental arcs form on continental lithosphere (Andean Arc)

What causes igneous activty at continental rifts?

• thinning lithosphere causes decompression melting.

At continental rifts, magma can form, erupt, and solidify as _____

Basalt

Mafic magmas composition can transition to felsic magma at continental rifts

True

Eruptions at continental rifts can produce

Rhyolite and pyroclastic debris

Hotspots form atop very hot

peridotite mantle plumes

Decompression melting at hotspots forms

magic magmas

T/F On oceanic lithosphere, islands may rise form mafic lava and pyroclastic debris

True

______ sits atop a hotspot beneath the Mid-Atlantic Ridge

Iceland

Yellow stone sits atop a ______ lithosphere hotspot and Hawaii sits atop an______ lithosphere hotspot

Continental, Oceanic

Main characteristics of a shield volcano?

Broad, gently sloped

formed mainly from low viscosity, mafic lava flows

Example of shield volcano

• Kilauea on Hawaii

Far from shield volcano vent, edifice is mostly ____ lava flows. Near vent ____ _____ may occur between flows

Layered, pyroclastic debris

Stratovolanoes

cone shaped mountains of alternating lava and pyroclastic debris, typically occuring at convergent plate margins

T/F Intermediate composition lava does not flow far from stratovolcano vent.

True

What did Krakatau eruption result in?

• Stratovolcano in Sunda straigt

• Eruption greater than atomic bomb, resulted in tsunami killing 36000

Supervolcanoes

• Yellowstone

  • Sumantra’s Toba VolcanoT

T/F Flood basalts may be classified as supervolcanoes

true

Mt. St Helens is an example of a stratovolcano occuring at divergent plate margin.

False, located at convergent plate boundary.

North American plate migrates_______ with time relative to Yellowstone hotspot.

Southwest

Pacific plate moves with time relative to the Hawaiian hotspot

Northwest

Which of the following is mafic extrusive rock?

a.        Granite

b.       Gabbro

c.        Andesite

d.       Basalt

e.        Dionite

Basalt

What did the Lake Nyos, Cameroon disaster result from?

• Release of CO2 gas

T/F Increasing pressure with depth of earth may cause melting of rocks to form volcanoes

False

Identify the volcanic hazard orignating from Mt Pelee Eruption for the 1902 catastrophe in Saint Pierre

Pyroclastic Flow

Pyroclastic flow

reach 100km/h, contain 1000 degree gas and ash

Identify the volcanic hazard responsible for the 1985 catastrophe in Armero Columbia

Lahar Flow

Lahar

muddy slurries of ash and water

T/F viscosity and density of lahar is much higher than water

True - flows like wet concrete, burning and destroying everything

Volcanic Hazards

Lahar Flow

Pyroclastic Flow

CO2 release

What happened in 1985 in Lake Nyos, Cameroon?

• volcanic vents beneath water filled craters may emit CO2

landslide disturbed crater’s lake water, releasing CO2

Long term preductions for eruptions depend on

Recurrence Intervals

Short term predictions for eruptions are possible within days to months looking at signs of

Unrest

Why are lahar flows dangerous to communities near volcanos?

travel fast, carry heavy debris, and cover areas with meters of sedimentW

Why is seismic data important when prediciting volcanoes?

Seismic data indicates rising magma and breaking rocks within a volcano