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