Volcanoes - Chapter 15

 

Including...

Introduction

Global Distribution

Classification of Volcanoes

Mafic eruptions

Intermediate eruptions

Felsic eruptions

Volcano lyrics (Jimmy Buffett)

 

Introduction

Very impressive events

Potential for great destruction

And construction!

What a deal - both earth processes in one event

"The book" discusses the intermittent activity of individual volcanoes

Be careful here!

Human time scales are not up to the task of understanding the timing of geological events

Named after Vulcan - Roman God of Fire and Metalworking

Also Mr. Spock's home planet

More than 500 "active" volcanoes identified

DIGRESS TO: active vs. dormant vs. extinct

Many cultures associated volcanoes with gods

Hawaiian - Pele

Greek - Hephaestus (the ugly son of Hera)

Europe in the middle ages - lots of crazy ideas (no surprise here)

Gateway to hell

Prisons of the damned

Noises associated with volcanoes were the "screams of tormented souls"

Proposed by the R.C. church, which incinerated thousands for worshiping "false" deities

 

Global Distribution

Very specific pattern (Merrill; fig. 15-3, pg. 391)

Usually associated with plate boundaries

DIGRESS TO: Pacific Ring of Fire

Also: Spreading centers are extremely active

 

Classification of Volcanoes

Generally based upon the composition of the magma

DIGRESS TO: magma vs. lava

REVIEW: Magma compositions

Mafic vs. intermediate vs. felsic

Composition definitely related to regional tectonic setting

Plate boundaries

REVIEW: Plate Margins

These compositional differences lead to different types of volcanoes

Each quite different from the others

Eruptive characteristics

Shape of the resulting pile of debris

 

Mafic eruptions

Regional tectonic setting

Tensional environments

Newly differentiated basaltic magma (blood of the earth)

Spreading centers

Also occur at Hot Spots - mantle plumes (Merrill; fig. 15-4, pg. 392)

Also referred to as "blowtorches"

Characteristics of mafic lavas and rocks

General description

Dark colored

Silica/Oxygen: 45% to 52%

High in iron, magnesium, calcium

Low in (or don't have) potassium, aluminum, sodium

Low volatile content

High temperature (>1000°C.)

Low viscosity - flow easily

Mineralogy

Olivine

Pyroxene

Plagioclase

Igneous rock formation

Basalt: extrusive, aphanitic

Diabase: medium texture

Gabbro: intrusive, phaneritic

Scoria: frothy

Various volcanic breccias

Wide range of clast sizes

Glassy rims commonly associated with submarine eruptions

These often shatter and break off

Eruptive characteristics

Relatively non-explosive

Due to high temperature, low silica, low volatiles

Flow like water in many cases

Put on your running shoes

DIGRESS TO: Pahoehoe vs. aa lavas

Flows cool and lose gas as they move away from the vent

Eruptions relatively quiet, but spectacular

Fissure eruptions

Extensional environment indeed!

Can cover immense areas

Columbia River Basalt

Fountains - of all sizes

Some incredible displays on Hawaii

Lava falls: Hawaii

Lava tubes: Lava Beds Nat. Monument

Lava lakes: Halemaumau, Mauna Loa

Pillow basalt: indicator of submarine eruption

Columnar jointing: indicator of subaerial eruption

Vesicular basalt (church rocks)

Cinder cones

Basalt flows commonly erupt from vents at the base of the cone

Morphology of mafic volcanoes

Shield volcanos - fluid lava

Examples of mafic volcanoes - describe each

Iceland: spreading center

Island of Hawaii: mantle plume

An incredible amount of rock has been produced!

Columbia River / Modoc Plateau basalt

Continental fissure eruptions

Parícutin: cinder cone

 

Intermediate eruptions

Regional tectonic setting

Compressional environments

Subduction zones

Re-melting of mafic crust, continental debris, and organic sediments

Characteristics of intermediate lavas and rocks

General description

Medium colored

Silica/Oxygen: 53% to 65%

Varying amounts of all the major rock-forming elements

Medium volatile content

Medium temperature

Mineralogy

Plagioclase

Amphibole

Muscovite/Biotite

Quartz

Igneous rock formation

Andesite: extrusive, aphanitic

Diorite: intrusive, phaneritic

Various volcanic breccias

Eruptive characteristics: (KaBoom)

Eruptions relatively explosive, VERY impressive

Mt. St. Helens is a SMALL example

Explosiveness due to high volatile content

Like shaking up a can of carbonated soda

Also due to "stickiness" of magma (high silica content)

Results in the formation of "pyroclastic" material

Pyro = fire; clast = small piece of broken rock

All different sizes can be formed

Get blown out by the force of the blast

Pyroclastic flows - nuée ardente

You CANNOT outrun one of these

Associated features

Volcanic gasses

Carbon dioxide: dense and will smother life when concentrated on surface

Sulfur dioxide: combines with water to make sulfuric acid

Tephra and ash deposits

Ashfalls: Just like snow, but don't melt in the spring

Will develop into good soil, with time (the earth's time frame)

Lahars

These types of volcanoes often form high mountain peaks

Collect snow and ice

Melt when an eruption occurs

Major flooding downhill

Can pick up massive amounts of pyroclastic (and other) debris

Houses, mobile homes, cows, slow cats

Whatever is loose and in the way

Extensive damage and loss of life

Morphology of intermediate volcanoes

Composite cones (stratovolcanoes)

Layered pyroclastics and flows

KaBoom then ooze; KaBoom then ooze; KaBoom then ooze

Examples of intermediate volcanoes

Mt. St. Helens

Mt. Pinatubo

Potential risk

Very great risk

From initial blast, pyroclastic flows, lahars, ashfalls

Several major urban areas in potential risk zones

The western flank of the Cascade Range

Mt. Shasta and the I-5 corridor

the Puget Sound in general

Portland, Oregon

Lucked out big time when St. Helens blew to the northeast!

DESCRIBE: difference in lahar distance related to a southern blast

What about Mt. Hood?

A west-directed blast would be a real problem

 

Felsic eruptions

Regional tectonic setting

Intra-plate hot spots

Tough to get granitic magma close enough to the surface to erupt

Eruptions are (fortunately) very infrequent (by the human time scale)

Origins VERY uncertain

Characteristics of felsic lavas and rocks

General description

Light colored

Silica/Oxygen: >65%

High in potassium, aluminum, sodium

Low in (or don't have) iron, magnesium, calcium

High volatile content

Low temperature (600°C. - 900°C.)

High viscosity

Mineralogy

Potash Feldspar

Quartz

Muscovite/Biotite

Amphibole

Igneous rock formation

Rhyolite: extrusive, aphanitic

Granite: intrusive, phaneritic

Pumice: frothy

Obsidian: glassy

Various volcanic breccias

Eruptive characteristics: (KaBoom)^2

Eruptions very explosive, MOST impressive

Explosiveness again due to very high volatile and silica content

DIGRESS TO: Most felsic magmas cool as intrusive igneous rocks

Obsidian and pumice: volcanic glass

Very quick cooling

No time for crystallization

Morphology of felsic volcanoes

Lava domes: very sticky

Caldera: very explosive

Examples of felsic volcanoes

Yellowstone National Park

Long Valley Caldera, California

Newberry Crater obsidian flows

Potential Risk

Ask the folks in Bishop, Lone Pine, or Wyoming how safe they feel

 

Volcano (Jimmy Buffett)

 

(chorus)

Now, I don't know

I don't know

I don't know where I'm a-gonna go

When the volcano blows

Let me say now

 

I don't know

I don't know

I don't know where I'm a-gonna go

When the volcano blows

 

Ground she's movin' under me

Tidal waves out on the sea

Sulfur smoke up in the sky

Pretty soon we learn to fly

Let me hear you now

 

chorus (1)

 

Now my girl quickly said to me

Mon, you better watch your feet

Lava come down soft and hot

You better love-a me now or love-a me not

Let me say now

 

chorus (1)

 

No time to count what I'm worth

Cuz I just left the planet Earth

Where I go I hope there's rum

Not to worry, man soon come

 

chorus (2)

 

But I don't want to land in New York City

Don't want to land in Mexico

Don't want to land on no Three Mile Island

Don't want to see my skin aglow

 

Don't want to land in Comanche Sky Park

Or in Nashville Tennessee

Don't want to land in no San Juan Airport

Or the Yukon Territory

 

Don't want to land no San Diego

Don't want to land in no Buzzard's Bay

Don't want to land on no Ayatollah

I got nothing more to say

 

chorus (2)