The landforms created when magma escapes Earth’s interior.

I. Factors controlling types of volcanic eruptions

A. Viscosity: resistance to flow

1. Temperature

2. Magma composition directly related to silica content - - - SiO2 chains even before crystallization begins

Introduction to Volcanoes

I. Factors controlling types of Volcanic Eruptions

B. Gases: 1-9% of magma

Volatiles: H2O, CO2 readily gas at Earth’s surface.

Dissolved underground due to pressure, as magmas rises pressure decreases and gas unmixes from the magma.

Gas less dense than liquid magma so it rises pushing the magma in front of it

Introduction to Volcanoes

I. Factors controlling types of Volcanic Eruptions

B. Gases: 1-9% of magma

Low viscosity magmas: the volatiles can escape more easily & don’t accumulate

High viscosity magmas: volatiles cannot escape easily, so gas pressure builds up within the magma and when it reaches the surface it explodes explosively (soda pop example)

Introduction to Volcanoes

If rock above/blocking magma, gas pressure builds up behind and finally overlying rock shatters (throat clearing)

II. Types of Volcanic Eruptions

1. Effusive (non-explosive): lava flows out fairly easily

a. primarily basalticb. low viscosityc. low gas contentd. more frequent non-explosive

events.e. Lava destructive to property,

but rarely kills people.f. mid-ocean ridges (oceanic divergent plate boundaries),

oceanic hot spotsg. Examples: Hawaii & Laki,

Iceland

Introduction to Volcanoes

II. Types of Volcanic Eruptions

2. Explosive / Pyroclastic

a. Primarily andesitic and some rhyolite.

b. High viscosity(rhyolite is often too viscous to even get out of the ground)

c. High gas contentd. Erupt less frequently, decades

to centuries—the longer the interval, the larger the eruption

e. Very dangerous to people and property

f. Over subduction zones, continental hot spots, continental rifting zones

g. examples: Cascades, Andes, Aleutian Islands

Introduction to Volcanoes

III. Volcanic Deposits

A. Lava: effusive deposits

1. Basaltic Lava: most likely to erupt because low viscosity8 cm to several meters thick

up to 20 mi/hr on slopes—usually 30-1000

feet/hrtravel up to 90 miles or more before cooling

Introduction to Volcanoes

III. Volcanic Deposits

A. Lava: effusive deposits

1. Types of Basaltic Lava

a. pahoehoe: ropy (in Polynesian dialect) lava

Surface cools to form and elastic skin which gets folded into ropes

b. ‘a’a: rough jagged basalt flows - - - more viscous than pahoehoe.

Resembles an advancing mass of lava rubble, travels 5-50 m/hr

Introduction to Volcanoes

Introduction to Volcanoes

Introduction to Volcanoes

Introduction to Volcanoes

III. Volcanic Deposits

A. Lava: effusive depositsNon-explosive1. Basalt2. Andesitic Lava

B. Pyrolcastic depositsExplosive deposits1. Andesitic2. Rhyolitic

Viscous: it tends to cool underground or explode violently.

Introduction to Volcanoes

III. Volcanic Deposits

2. Andesitic LavaCan flow like basalt (effusive)

or erupt explosively.

More viscous, flows do not travel as far and generally thicker flows.

Introduction to Volcanoes

III. Volcanic Deposits

3. Rhyolitic Lava: (rare) Rhyolite magma is so viscous it

tends to cool underground or explode violently.

If it has already been degassed—the magma left over after an eruption (at the bottom of a magma chamber), it may ooze out, but doesn’t get far

Forms domes -- Mt. St. Helens

Introduction to Volcanoes

Basaltic flow

Andesitic flow

Rhyolitic dome Rhyolitic spire

Introduction to Volcanoes

B. Pyroclastic Deposits: air borne deposits from explosive volcanic eruptions. (Greek Pyro = fire, klastos = fragments)

1. Consists of volcanic blocks, not from magma or lava flows

Pieces of the volcano that were ripped out during the explosion, up to the size of houses!

Throat clearing to remove all the stuff that was in the way.

When a volcano erupts, rapidly expanding gases carry small fragments of magma into the air, like champagne

Introduction to Volcanoes

Paricutin, Mexico, During its brief nine-year cinder cone soon after its birth in 1943 in a Mexican cornfield. Photograph: U.S. Geological Survey (K. Segerstrom).

B. Pyroclastic Deposits: air borne deposits from explosive volcanic eruptions. (Greek Pyro = fire, klastos = fragments)

1. Consists of volcanic blocks, not from magma or lava flows

Pieces of the volcano that were ripped out during the explosion, up to the size of houses!

Throat clearing to remove all the stuff that was in the way.

Often solidified magma from past eruptions

Introduction to Volcanoes

Paricutin, Mexico, During its brief nine-year cinder cone soon after its birth in 1943 in a Mexican cornfield. Photograph: U.S. Geological Survey (K. Segerstrom).

B. Pyroclastic Deposits: air borne deposits from explosive volcanic eruptions. (Greek Pyro = fire, klastos = fragments)

1. Consists of volcanic blocks and ejecta

2. Pieces of the volcano that were ripped out during the explosion, up to the size of houses!

Throat clearing to remove all the stuff that was in the way.

Often solidified magma from past eruption

When a volcano erupts, rapidly expanding gases carry small fragments of magma into the air, like champagne

Introduction to Volcanoes

Introduction to Volcanoes

Soufriere, St. Vincent, W. Indies. April 22, 1979

6 km eruption cloud rising into the early morning sky.

The cloud eventually rose to an altitude of 15 km.

Photograph: U.S. Geological Survey (F.C. Whitemore).

B. Pyroclastic Deposits : air borne deposits from explosive volcanic eruptions.

2. Tephra—pyroclastic material that cools and solidifies in the air

a. Volcanic dust particles: 1/1000 mm in diameter (cake flour) & can stay in atmosphere for up to two years after an eruption.

b. Volcanic ash: <2mm diameter, fine sand to rice size, remains in the atm for hours to days

c. Lapilli: (cinders), peas to walnuts in size (2-64 mm). Italian for “little stones”

d. Volcanic Bombs: (>64 mm), form from sizable blobs of lava erupt and solidify in mid-air

Introduction to Volcanoes

Introduction to Volcanoes

Size of tephra and thickness of layers varies as a function of distance from a vent

~1630 BC eruption of Santorini.

Introduction to VolcanoesChanges of tephra from volcanic source

Introduction to Volcanoes

Tephra layers

AshLapilliBombs

Relative volcano sizeShield and Composite volcano

Relative volcano sizeShield, composite, and cinder cone

Relative volcano size