IGNEOUS ROCKS (Part 2):
Characteristics of Igneous Rock Bodies:
There are two questions we want to answer:
(1) What are the shapes and sizes of igneous rock bodies?
(2) For extrusive rocks: what are the dangers of eruptions?
> to answer these questions, we first need to consider the nature of MAGMA of different
compositions:
(a) Felsic magmas: high viscosity, high Si content, and high dissolved water content
(generally greater than 5%)
(b) Intermediate magmas: intermediate viscosity, intermediate Si content, intermediate
dissolved water content (1 to 5%)
(c) Mafic magmas: low viscosity, low Si content, low dissolved water content (less than
1%)
>> the higher the Si content of a magma, the higher the viscosity. This is because, even
when the magma is entirely a liquid, the Si in the magma is trying to form Si-O polymers
(the beginnings of crystals). This creates little strands of Si-O polymers that act like little
Velcro pieces that will snag each other as the magma flows. For a felsic magma, that will
ultimately have a high proportion of framework-structure crystals of feldspar and quartz,
these Velcro-like pieces have a high amount of polymerization and are quite big:
therefore the magma is quite "sticky" and can't flow easily (high viscosity). In the mafic
magma, there will ultimately be a high proportion of minerals with low or no
polymerization: therefore, the Velcro-like pieces will be small, and the magma can flow
easily (low viscosity) since the small pieces don't snag on each other as easily.
So, back to our two questions:
First, (1) what are the shapes and sizes of igneous rock bodies?
A. Intrusive rocks:
(see diagrams in the book)
> dykes and sills are relatively small, and are in the form of planar sheets
> they are simply cracks that magma has moved along and then crystallized in
> dykes cut across the layers they cut through, and sills are parallelt o them
> stocks and batholiths are large to very large (kilometers to tens of kilometers and more
across), and irregular to sub-spherical in shape
> they are crystallized magma chambers
> batholiths are the really, really big ones, and the stocks are the smaller ones (stocks are
most commonly protrusions upwards from the huge batholiths below)
Generally, mafic magma (i.e., gabbroic) is more likely to reach earth's surface than is
felsic magma (i.e., granitic)
WHY? Two reasons:
(a) mafic magma has a lower viscosity, and hence flows more easily. The "stickier" felsic
magma has a harder time moving upwards
(b) felsic magma has a higher water content than mafic magma. As felsic magma rises
towards earth surface to lower pressures, the melting temperature of the magma goes up
(look back at earlier discussions about this). This will encourage crystallization before
magma reaches the surface. Furthermore, dry mafic magma has the opposite behaviour.
At lower pressures, the melting point drops, and that keeps the magma above the melting
point, inhibiting crystallization. This allows the liquid to keep rising.
B. Extrusive Rocks:
> magma, when it comes out of the ground, will be in one of two forms:
(1) LAVA (i.e., liquid) or (2) PYROCLASTIC MATERIAL ("hot" particles)
> magma reaches the surface by coming through either (a) fractures (fissures) or (b)
volcanic cones
(a) fractures (fissures):
> a fracture or fissure is nothing more than a crack at the surface of the earth
>> a fissure eruption is magma coming out of the crack
> most fissure eruptions involve mafic lava (i.e., magma that crystallizes to form basalt)
>> because of the low viscosity of mafic magma, the lava can flow long distances once it
reaches the surface, and extensive LAVA SHEETS result (e.g., much of Iceland has been
built up that way)
>> because mafic magma has little dissolved water, and has a low viscosity, such fissure
eruptions are generally quite calm (that is, they are not violently explosive)
(b) volcanic cones:
> in a volcanic cone, there is a pipe-like conduit and vent that channels the magma from
the magma chamber at depth up to the surface
>> the volcanic cones, at surface, look like upside-down ice cream cones
> eruptions from cones range from calm to exceedingly violent, depending on the
composition of the magma
Volcanic Cones:
> there are three main types of volcanic cones:
>> (1) Cinder Cones: made entirely of pyroclastic material. These are steep-sided cones
(steep because the pyroclastic material, when it falls and piles up, produces a steep angle
of repose), but the cones are relatively small.
>> (2) Stratovolcanoes (also known as Composite Cones): this are large cones (up to
several kilometers high) with steep slopes. They consist of alternating layers of
pyroclastic material and lava (hence the name Composite cone). They result from
eruptions of felsic to intermediate magma. The pyroclastic material, and the "sticky"
high-viscosity magma builds steep slopes. These high volcanoes most often are snowcapped.
>> (3) Shield Volcanoes: these are huge cones, up to 10 km high and over a hundred km
in lateral extent, with very shallow slopes. They dwarf the stratovolcanoes in size. They
form from lava-flow eruptions only, because they involve mafic lave exclusively. This is
low-viscosity magma with low water content. Hence, the lava flows readily and spreads
out to yield very shallow slopes. Also, any water in the magma readily bubbles out of the
magma without needing to expand explosively, as happens in felsic magma. It is this
rapid water expansion in higher viscosity felsic magmas that produces so much
pyroclastic material when the stratovolcanoes erupt. Thus, shield volcano eruptions are
generally very calm, and of little or limited danger to humans.
Second question: (2) for extrusive rocks, what type of volcanoes are most dangerous?
Answer: STRATOVOLCANOES
Why?
> because their magma composition is FELSIC or INTERMEDIATE
>> the felsic/intermediate magmas have HIGH VISCOSITY and HIGH WATER
CONTENT, which can lead to:
(a) high, steep-sided volcanoes (because of the "sticky" magma and pyroclastic ash).
Their height leads to snow-cover on their tops and flanks (as we will see below, this can
lead to the development of lahars).
(b) volcanoes that can erupt violently. This is because the high water content means
that, as the magma nears surface and pressure is lowered, the steam in the magma
expands dramatically. However, the highly viscous nature of the magma prevents the
steam from escaping easily (the magma is too "sticky"), and so it literally blows the
magma apart (the steam "explodes") violently, producing the pyroclastic material.
Also, stratovolcanoes occur along continental margins (where people like to live!) Where
subduction zones are.
We can summarize the four main dangers of stratovolcano eruptions:
(1) AIR-FALL "ASH" (tephra): pyroclastic material that rains down out of the sky is
called tephra . Tephra ranges from house-sized blocks down to ash material.
> ash and pumice can bury towns (e.g., Vesuvius eruption in 79 A.D. that buried
Pompeii)
> ash in the atmophere can circle the globe and block the suns rays, leading to a decline
in global temperature for years after a major eruption (e.g., Tamboro in 1815; Pinatubo in
1991)
(2) ASH FLOW (Pyroclastic Flow): there are two types of such flow
> 1. Nuees ardentes ("glowing avalanches"): when a mass of pyroclastic material doesn't
have enough energy to be blasted up into the atmosphere, it can end up flowing
incredibly rapidly (100 km per hour or faster) down the steep volcano sides. It is so
highly charged with gases (steam) that it sort of floats on air. It is fiery hot, and so glows
at night. It can bury a town in minutes, and asphyxiate the people. (E.g., Mt. Pelee in
Martinique in 1902 A.D.)
> 2. Pyroclastic surge or blast: sometimes a stratovolcano erupts suddenly with a huge
sideways blast (e.g., Mt. St. Helens in 1980 A.D.), sending out a mass of hot pyroclastic
material at enormous speed (up to supersonic0 that can flatten trees, etc. for many
kilometers around
(3) LAHARS (volcanic-related mudflows):
> the heat from the eruption can melt snow on the volcano flanks, and this mixes with old
ash on the flanks and new ash from the eruption to make huge mudflows (called lahars)
that rush down valleys (e.g., in 1985 a lahar from Nevada del Ruiz volcano in Columbia
buried the town of Armero, killing 20,000 people)
(4) TSUNAMI (sea waves): (Note: we didn't actually get around to talking about this
volcanic danger in class)
> because many stratovolcanoes occur as volcanic islands in island arcs in the oceans
(e.g., around the western part of the Pacific Ocean basin), a tsunami can be triggered by a
sudden violent explosion (e.g., most deaths (20,000) from the eruption of Krakatoa in
1883 A.D. came from the ensuing tsunamis)
Last revision: 6 November 2000
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