Topic 6:  THE FIERY FURNACE: Plutons and Volcanoes

First, Volcanoes:

What determines the explosivity associated with volcanic activity?
Why can we state that volcanoes in Hawaii pose only slight danger 
to humans, when violent explosions might occur in the 
Cascade Mountains (Mt St Helens), and are expected on Montserrat?


The main factors are:
--  the amount of gas dissolved in the lava or magma (mainly 
water vapor, carbon dioxide, sulfur dioxide, but lots of others too).
--  the ease or difficulty affecting gas escapes to the atmosphere.
Gas comes out of solution as pressure decreases in the rising magma, forming
gas bubbles (vesicles).  The gas then tries to escape.

Viscosity - resistance to flow. In a magma or lava, viscosity determines 
how easily (or how hard) the gas escapes.  The more viscous the magma,
and the greater the volume of gas exsolving from solution,
the greater the pressure buildup and the more violent the resulting 
eruption will be.  Viscosity is controlled by silica content,
and by temperature. 

There are three common types of magma which are as follows from high 
temperature to low temperature:  basaltic (around 50% SiO2); andesitic 
(around 60% SiO2); and rhyolitic (around 70% SiO2).  Basaltic magma has 
low silica content, and therefor a low viscosity; rhyolitic magma has a high 
silica content, and thus very high viscosity.  Also basalt has a low
water content; rhyolites have a high water content.
Thus, rhyolites commonly explode; basalts generally do not.

Volcano - generally a hill or mountain formed by the extrusion of lava or 
explosive ejection of tephra (rock fragments, ash) from a vent.  

Shield volcanoes - broad, gently sloping cones constructed of successive 
solidified lava flows. Typically many flows, with soil layers in between.
The eruption from a shield volcano is generally nonviolent 
because the lavas are rather fluid (low viscosity).  The slope of a shield 
volcano is gentle. One characteristic surface of a basalt flow 
is called pahoehoe, which has a ropy surface.  Examples include Moana Loa
in Hawaii (the Homepage cover photo), Kilauea, some Icelandic volcanoes.

Cinder cones - a volcano constructed of tephra -- loose rock fragments ejected 
from a central vent.  Most of the ejecta lands near the vent to form a 
cone with a slope of up to 30 degrees. Commonly formed by hydro-volcanic
explosions, e.g. explosive eruptions involving mixtures of magma and water.
Some good examples found in Iceland (see Hverfjall, p.65).

Composite volcano  -  constructed of alternating layers of 
tephra and lava flows.  Examples Mt Shasta, Rainier, St Helens; Fuji;
Vesuvius.

Lava dome - steep-sided mound (sometimes spiny) of viscous lava that 
solidifies above a volcanic vent.  The most viscous lavas form volcanic domes. 
They are usually rich in silica, or are rich in crystals when they extrude.
They are commonly rhyolite or andesite; rhyolite-composition domes can
be made of obsidian if no (or few) minerals crystallize.  Mt St Helens
has a dome, squeezed out in increments after the 1980 blast.  Montserrat
has a dome, that is still growing; it occasionally collapses, or explodes,
to give pyroclastic flows.  Thus, MonsterRat.

Rhyolitic eruptions are most likely to be explosive.  The material blown 
to bits out of a volcano is called pyroclastic debris, or tephra.  Bubbles that 
form quickly in a huge mass of depressurized, sticky rhyolitic magma can shatter 
into a froth containing innumerable tiny glass-walled bubbles. Pieces of
this froth harden to form pumice.


Crater -  Hole at the summit of the volcano; where the vent is exposed.

Flank Eruption - lava from a vent on the side of a volcano.

Caldera - a volcanic depression much larger than the original crater,
resulting from the collapse of the volcano into a magma chamber below,
after the cham,ber has been largely emptied by a large eruption.  

NuŽees ardentes - (hot cloud) clouds of red-hot ash and dust moving 
at high speed down volcano slopes for large distances.
Another term for "pyroclastic flow".
In 1902, such a gas cloud descended Mt Pelee on Martinique, the Caribbean,
engulfing the port town of St. Pierre and incinerated everything in its path 
-- including 27,000 people.  Pompeii: same result, Vesuvius AD 79.
Montserrat: 25 killed in June; the major reason for the evacuations.

Pyroclastic flows - see above. Also called, Ash hurricane.

Tuff  -  Consolidated tephra. Rock at Yucca Mountain nuclear repository.

Welded tuff  -  Consolidated tephra so hot it has partly remelted. Dense.

Plateau basalts -  produced during the geologic past by vast outpourings 
of lava.  The Columbia plateau area of Washington, Idaho, and Oregon 
is constructed of layer on layer of basalt lava, in places as thick as 3,000 
meters.

Pillow basalts -  elongate blobs of lava break out of a thin skin of 
liquid basalt submerged in water. Pillows form the base
of most underwater volcanoes.  High permeability rock mass.


Next, Plutons:

What is a pluton?  Be sure you know.

What are the pluton-rock equivalents for all the volcanic lava types.
Do you know them?

What are the various forms of plutons?  Do ypou know the names and
definitions? sill, dike, stock, batholith, etc.

Where might you look at plutons in the USA?