Volcanoes and Volcanic Landforms

VOLCANISM

THE ROCK CYCLE

I. Introduction

From: Roman god of fire, Vulcan

A. What is a volcano?

A conical mountain formed around a vent

where lava, pyroclastic materials, and

gases are erupted.

I. Introduction

B. Volcanic activity:

– Active

– Dormant

– Extinct

B. Volcanic Activity

1. Active volcanoes

• activity in the last few

centuries

Ex: Vesuvius, 79 A.D.

(50 times in 2000 yr)

Ex: Mt. St. Helens (1980)

B. Volcanic Activity

2. Dormant volcanoes

– “quiet” for the last

hundred to thousands

of years, but still have

potential to erupt.

Mt. Rainier

B. Volcanic Activity

3. Extinct volcanoes

– No eruption in historical times

– No signs of erupting again

I. Introduction C. Volcano Distribution

• Most volcanoes occur in one of three areas:

– Circum-Pacific (i.e. The Ring of Fire)

• 60%

– Mediterranean

• 20%

– Spreading centers

• 10 – 15%

Volcano Distribution

II. Volcanic materials

• Three types of material expelled from

volcanoes

Volcanic materials

A. Lava (“the liquid”)

– Molten rock

– Si (silica) affects

viscosity

II. Volcanic materials

A. Lava

1) Pahoehoe lava

– Basaltic lava

– Low viscosity

– Cools moderately

slowly

– Ropelike appearance

II. Volcanic materials

A. Lava

2) Aa lava

(pronounced aa-aa)

– Basaltic lava

– Higher viscosity

– Solidifies while

flowing

– Angular pieces

II. Volcanic materials

A. Lava

3) Pillow lavas

– Lava extruded

underwater

– Cools and contracts

– Spherical masses

– Ocean floor

II. Volcanic materials

A. Lava (“the liquid”)

B. Ash and pyroclastic material (“the solid”)

– Airborne material ejected by a volcano

– Classified based on size:

B. Ash and pyroclastic material (“the

solid”)

* Volcanic ash

– Fine ash - <0.06mm

– Coarse ash – 0.06mm

to 2mm

– Composition = rock,

mineral, and volcanic

glass

B. Ash and pyroclastic material (“the solid”)

* Cinders

• 2 mm and 64 mm

• Composition - same as ash

• Hazardous when falling

B. Ash and pyroclastic material (“the solid”)

C) Bombs

– Larger than 64mm

– Molten rock solidifies

in the air

– Shapes vary

II. Volcanic materials

C. Volcanic gases (“the gases”)

– Volatiles

• H2S – Hydrogen sulfide

• H2O – Water vapor

• SO2 – Sulfides

• CO2 – Carbon dioxide

• N2 – Nitrogen

• HCl – Hydrochloric Acid

These Gases have been shown to contribute to extinction events on Earth

II. Volcanic materials

A) Determines violence of an eruption

– High gas = violent eruptions

– Violent eruptions = felsic magmas

• High viscosity magma traps gas

• Expansion is prevented, pressure builds

III. Volcanic Landforms

• An erupting volcano will produce a number

of distinct landforms including:

A. Volcanic cones

B. Flood basalts

C. Calderas

A. Volcanic cones

1) Shield volcanoes

– Multiple layers of

basaltic lava

– Shallow sides due to

magma’s low viscosity

– Gentle eruptions

A. Volcanic cones

• Mauna Loa volcano,

Hawaii

A. Volcanic cones

2)Cinder cones –

– Smallest volcanic cone

– Layered ash and cinders

– Short, narrow cone,

– Steep sides

– Violent eruptions

Ex: El Paricutin

A. Volcanic cones

Lassen National

Monument, CA

A. Volcanic cones

3) Composite or

stratovolcanoes –

– Layered ash, lava, and

mud

– Intermediate to felsic

lava

– Steep sides, due to

lava’s high viscosity

A. Volcanic cones

• Mt. St. Helens,

WA

III. Volcanic Landforms B) Flood basalts

– Large outpourings of basaltic lava

– Multiple, quiet eruptions

– Lava plateau

B) Flood basalts

• A portion of the

Columbia Flood

Basalts in WA

III. Volcanic Landforms

C) Calderas

– Large depressions (>

1km) from violent

eruptions

– Ugashik Caldera, AK

C) CalderasTwo methods of formation:

Method 1:

Volcano rapidly empties

its magma chamber,

and support is lost

C) Calderas

Method 1 (cont.):

• Overlying material

collapses into magma

chamber

• Caldera forms

C) Calderas

Ex: Crater Lake,

OR

C) CalderasTwo methods of formation:

Method 2:

Volcano blows its top,

leaving behind a void

Inside the cone.

C) CalderasTwo methods of formation:

Method 2:

Volcano blows its top,

leaving behind a void

Inside the cone.

IV. Volcanic hazards

A) Lahars (hot mud

flows)

IV. Volcanic hazards

• Lahars

Sources of water

– Melting ice caps

– Excess rainfall

IV. Volcanic hazards

B) Nuee Ardentes (Glowing Ash flows)

– Clouds of dense gas and debris

– French for “glowing cloud”

– High speeds and high temperatures

IV. Volcanic hazards

• How does a Nuee

Ardente form?

– Volcano erupts

– Hot debris rises

– Gravity takes over

IV. Volcanic hazards

• How does a Nuee

Ardente form?

– Debris descends

rapidly (200 mph)

– Flows down mountain

slopes

– Travel up to 80 miles

IV. Volcanic hazards

C) Tsunamis

– Wave generated by

volcanic explosion

– Japanese for harbor

wave

IV. Volcanic hazards

• Tsunamis are

extremely hazardous

– Travel vast distances

– Strike with short notice

– Krakatoa (1883) -

36,000 people died

IV. Volcanic hazards

D) Lava flows

– Least dangerous

– Lava flows slowly

(<10 mph)

– Dangerous to property