Exam 2

Volcanic activity is directly related to … 

plate tectonics

most volcanoes are located near…

plate boundaries

Where are the vast majority of Earth's active volcanoes located?

The Ring of fire

Melts

forms when solid materials become hot and transform into liquid

Molten rock is a…

type of melt

 igneous rocks are…

rocks made by the solidifying or crystallization of a melt

Magma

molten rock beneath the Earth’s surface, underground melt,

Magma is ____ dense than rock

less

Lava

molten rock that has flowed out onto the Earth’s surface,

Earth’s crust and mantle are primarily composed of

Solid rock

Where magma forms

 in the uppermost mantle

Where are the greatest amounts of magma produced?

divergent plate boundaries

 Lesser amounts of magma are produced at…

 subduction zones, hot spots/mantle plumes

 Three principal magma-generating processes:

Decompression melting, (lowering the pressure), Addition of volatiles (gas or water -seawater), Addition of heat

Decompression Melting 

The pressure exerted on hot rock is decreased. Because pressure prevents melting, a decrease in pressure can permit melting

Where does Decompression Melting occur?

divergent plate boundaries, mid-ocean ridges, Continental rifts, mantle plumes

Addition of Volatiles (Flux Melting)

If a rock is close to its melting point and some water is added to the rock, the melting temperature is reduced, and partial melting starts 

Example of Addition of Volatiles

Basaltic rock acts like a sponge, carrying seawater until it gets subducted into the mantle which lowers the melting point of rock

Where does the addition of volatiles occur?

subduction zones

Addition of Heat

•  When magma that was generated by some other mechanism intrudes into the cold crust, it brings with it heat 

•  Upon solidification, they lose this heat and transfer it to the surrounding crust 

•  Repeated intrusions can transfer enough heat to increase the local geothermal gradient and cause the melting of the surrounding rock to generate new magma 

Where does addition of heat occur?

continental hot spots, continental rift valleys, subduction-related environments

There are 3 “flavors” of magma (Increasing silica content)

Mafic, Intermediate, Felsic

The biggest volcanic boom comes from_____

high silica content

Viscosity

 Resistance to flow, Affected by temperature, Affected by composition, Affects The flow of lava, affects the Shape of the resulting volcano

the more viscous the material…

the greater its resistance to flow 

As magma cools, viscosity…..

increases

As silica content increases….

viscosity increases

Volatiles

components that behave as gases during volcanic eruptions

At depth, nearly all magmas contain

gases or volatiles

What gives magma its explosive character?

Gases/volatiles

gases expand as the pressure is ____

reduced

Water (H2O) vapor or steam 

most abundant volcanic gas

Carbon dioxide (CO2 )

second most abundant volcanic gas

Minor Volcanic gasses

SO2 (sulfur dioxide), HCl (hydrogen chloride), HF (hydrogen fluoride)

Gases/Volatiles Determine…

how explosive the eruption will be 

High concentrations of dissolved volatiles will…

explode violently

pyroclastic debris

Volcanic materials (like ash) that are explosively ejected 

Volatile-poor magma results in…

non-explosive eruptions

amount of gases/volatiles in magma is related to the…

chemical composition of the magma

higher silica content means…

higher percentage of gases/volatiles

Felsic magmas tend to have the ___ levels of gases/volatiles

highest

If the magma is felsic, and therefore too viscous for gases to escape easily, or if it has a particularly high gas content…

it is likely to be under high pressure

Viscous magma doesn’t flow easily, so even if there is a conduit for it to move toward the surface…

it may not flow out 

Under these circumstances _____ ___ _____ __as more magma moves up from beneath and gases continue to exsolve

pressure will continue to build

Are all volcaones the same?

No

Volcanoes vary in…

Size, shape, composition, Number of eruptions in formation, How and where magma is formed, Amount of magma evolution, Volatile content 

what controls eruption explosiveness?

Viscosity and volatile content

Volcanic explosivity index (VEI) 

Relative scale to compare explosions, 0-to-8 index of increasing explosivity, Each increase in number represents an increase around a factor of ten

Effusive eruptions

magma rises through the surface and flows out of the volcano as a viscous liquid called lava

Explosive eruptions

magma is torn apart as it rises and reaches the surface in pieces known as pyroclasts

Eruption Types 

 based on the eruption characteristics of a well-known volcano

How many eruption types are there?

5

Names of eruption types (least to most violent)

Hawaiian, Surtseyan, Strombolian, Vulcanian, Plinian (Krakatoan)

Continental volcanoes

Most Violent

Ocean-continent

intermediate

Ocean Crust

least violent

Volcanic Explosivity Index uses several factors to assign a number, including:

volume of erupted pyroclastic material, height of eruption column, duration in hours, qualitative descriptive terms

Supervolcanoes

 volcanic center that has had an eruption of magnitude 8 on the Volcano Explosivity Index (VEI) 

eruption classified as a VEI 8 if…

the measured volume of deposits is greater than 1,000 cubic kilometers (240 cubic miles)

Examples of supervolcaones

Yellowstone (Montana, and Wyoming), Long Valley (eastern California), Toba (Indonesia), Taupo (New Zealand),  large caldera volcanoes of Japan, Indonesia, South America, and Alaska (e.g. Aniakchak, Emmons, Fisher)

Conduit

fissure that magma moves through to reach the surface

 Vent

surface opening of a conduit

Crater

funnel-shaped depression at the summit of most volcanic cones, generally less than 1 km in diameter 

Caldera

large circular depression (diameter less than1 km) produced by a collapse following an eruption

 Parasitic cone

flank vent that emits lava and pyroclastic material 

Fumaroles

lank vent that emits gases

Different styles of volcanism (i.e. volcanoes and eruption types) occur at …..

different locations on Earth

Most eruptions occur along ____ ____, but major eruptions also occur at hot spots

plate boundaries

 “Gentle Giants”, Largest volcanoes ,Associated with mafic magma 

– Low viscosity, low volatile content 

– Gently flowing lava with non-explosive (effusive) eruptions

 – Generally develop a caldera 

•  Thin mafic (basaltic) lava flows build up volcanoes with gentle slopes 

– Wider than they are tall 

•  Still among the tallest mountains on Earth (measured from bases) 

•  Common at hot spots (oceanic lithosphere), divergent plate boundaries, continental rifts

Shield Volcanoes

•  “Mass Murderers” 

•  Beautiful conical shapes 

– Result of high-viscosity silica-rich magma 

•  Lava does not flow far resulting in steep sides (slopes) 

•  A mixture of explosive activity and lava flows 

– Produce a combination of lava flows and pyroclastic deposits 

• Can be extremely explosive 

– Responsible for over 80% of eruptions

 – Responsible for most of the death and destruction 

•  Common in subduction zones 

• – “Ring of Fire”

Stratovolcanoes (Composite Volcanoes)

•  Small dome with steep sides 

– Often forms in the vent of a stratovolcano after an explosive eruption 

– Can grow in a single event or over decades 

•  Made from highly viscous felsic magma 

•  Common in “Ring of Fire”

Lava Domes

Examples of Lava Domes

Lassen Peak(California), Unzen (Japan)

Cinder Cones (Scoria Cones) are?…

Small - typically only a few 100 m in diameter; few are more than 200 m high

Cinder Cones are ____ shaped with summit crate

Cone

Cinder Cones are built from an accumulation of

tephra

Cinder Cones are often formed when…

magma meets groundwater 

Cinder Cones are associated with which type of eruption?

mafic eruptions

Cinder Cones have ___ ___ ___ explosivity

Low to intermediate

Where are cinder cones commonly found?

larger volcanoes, normal faults, along cracks and fissures

Continental Caldera  “Serial Killers”  

 Large summit depression (caldera) 

How are these calderas formed?

Collapse of the land surface or volcanic edifice

Continental Calderas are associated with what type of eruption?

felsic (silica-rich) eruptions

Continental Calderas produce ______ explosions 

violent

Continental Calderas are ___ _____

Very rare

The largest are termed supervolcanoes

Continental caldera

Continental Caldera eruptions extrude a great deal of ______ ____ (mainly ash)

pyroclastic debris

Calderas are circular, steep-sided depressions with a diameter ___ ____1 km

less than

 one of the largest calderas on Earth

 – ~20 miles (32 km) long (east-west)

 – ~11 miles (18 km) wide (north-south) 

•  formed ~760,000 years ago following a huge volcanic eruption 

– The magma chamber under the now-destroyed volcano significantly emptied to the point of collapse 

•  collapse itself caused an even larger secondary eruption of pyroclastic ash that burned/buried thousands of square miles 

Long Valley Caldera, CA 

– Subduction Zones 

• Ring of Fire: surrounds the Pacific Ocean basin 

• Caribbean 

• Indonesia 

– Hot spots: Hawaii, Yellowstone, etc.

 – Mid-ocean ridges: Iceland (also a hot spot)

 – Rift valleys: East Africa 

• Indirect volcanic risk 

– Ash fall and ash clouds: all locations in the path

 Direct volcanic risk 

Ash fall and ash clouds: all locations in the path

Indirect volcanic risk

______ produce a wide variety of hazards that can kill people and destroy property

Volcanos

Large explosive eruptions can: 

endanger people and property hundreds of miles away, affect global climate

… … …volcanoes erupt each year worldwide

50 to 60

___ million people live close to volcanoes

 500

In the US, _ - _per year, mostly in Alaska/sparsely populated areas

2 to 3

 1. Lava Flows 

2. Tephra – Ash, lapilli, blocks, bombs 

3. Pyroclastic flows 4. Release of volcanic gasses

Primary Effects

Lava Flows

Results when magma reaches the surface through crater or from vent 

What are the three types of lava flows?

 basaltic, andesitic, rhyolitic

One of most familiar products of volcanic activity are?

Lava Flows 

Lava Flows can move ____ or more rapidly

slowly