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Introduction- Igneous Rocks and Volcanism
Igneous rocks are formed form the cooling of molten rock, magma. They are crystalline,
which means they are made up of crystals joined together. There are many different
types of igneous rocks but they fall into two (very) broad categories; intrusive
(plutonic) and extrusive (volcanic). Intrusive rocks are igneous rocks which form at
depth. They cool slowly, taking tens of thousand of years to cool. They have large
(visible) crystals. Extrusive rocks are those which have erupted from volcanoes. They
have very small crystals, not visible to the naked eye, as they cooled quickly.
Igneous rocks are classified on their origin, texture and their composition.
Intrusive Igneous Rocks (Plutonic Rocks)

Cool more slowly

Develop larger crystals – visible to the naked eye
Extrusive Igneous Rocks (Volcanic Rocks)

Cool more rapidly

Develop smaller crystals – may not be readily distinguished without magnification
Texture - size, shape and arrangement of mineral grains in a rock.
Common textural terms applicable to igneous rocks:
Aphanitic - Individual crystals are small.; they may be hard to be detected with the
naked eye. Cooled rapidly before crystals had a chance to grow.
Phaneritic - Individual crystals large enough to be detected with the naked eye. May be
further described as fine grained (crystals < 1 mm), medium grained (crystals 1-5 mm)
or coarse grained (5-10 mm). Rock must have cooled slowly to allow large crystals to
develop.
Vesicular - contains cavities (vesicles) formed by gas bubbles escaping as the lava
cooled.
Glassy - lava or magma quenched abruptly, forming an amorphous (lacks crystalline
structure) mass, like a glass. No crystals evident even under high magnification.
All typical classification schemes rely on a combination of texture, particularly grain size,
and mineral composition. But, keep in mind they are process-oriented. Coarse grained
are intrusive, fine grained are extrusive.
See Table below that depicts a typical classification. Stress similar composition of
granite vs. rhyolite, just differ in grain size. Compare granite to gabbro which have the
same grain size, but different composition.
Notice from the table below how the three common fine-grained rocks, rhyolite,
andesite and basalt differ in their chemistry. Rhyolite is very rich in silica while basalt
has less silica, but more iron and magnesium. Andesite is intermediate.
Texture/Composition
Felsic
Intermediate
Mafic
Fine Grained
Rhyolite
Andesite
Basalt
Granite
Diorite
Gabbro
Ultramafic
(Aphanitic)
Coarse Grained
Peridotite
(Phaneritic)
Glassy
Obsidian
Vesicular
Pumice
Scoria
High silica ------------------------------------- Low silica
Low Fe/Mg ------------------------------------- High Fe/Mg
Low Temp--------------------------------------- High Temp
700°
1200°
We can combine mineral composition and igneous textures into a table to allow us to
determine the name of igneous rocks.
What is Bowen's Reaction Series?
Back in the early 1900's, N. L. Bowen (a Canadian Geochemist) and others at the
Geophysical Laboratories in Washington D.C. began experimental studies to
understand the order of crystallization of the common silicate minerals from a
magma. The idealized progression which they determined is still accepted as the
general model for the evolution of magmas during the cooling process. As with
everything else in geology, there are exceptions to this rule, but for the most part, it
works.
Bowen determined that specific minerals form at specific temperatures as a magma
cools. At the higher temperatures associated with mafic and intermediate magmas,
the general progression can be separated into two branches. The continuous branch
describes the evolution of the plagioclase feldspars as they evolve from being
calcium-rich plagioclase, Anorthite, to the more sodium rich plagioclase, Albite. The
discontinuous branch describes the formation of the mafic minerals olivine, pyroxene,
amphibole, and biotite mica.
The weird thing that Bowen found concerned the discontinuous branch. At a certain
temperature a magma might produce olivine, but if that same magma was allowed to
cool further, the olivine would "react" with the residual magma, and change to the
next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene
would convert to amphibole, and then to biotite. Mighty strange stuff, but if you
consider that most silicate minerals are made from slightly different proportions of the
same 8 elements, all we're really doing here is adjusting the internal crystalline lattice
to achieve stability at different temperatures. Finally, at the lowest temperatures the
two branches merge and we get the minerals that are common to felsic rocks muscovite mica, orthoclase (potassium feldspar), and quartz (3-D frameworks).
Explanation: The reason for this "stepped" evolution of minerals is that with dropping
temperature we have decreasing thermal vibration of molecules, and that allows
silica to form more complex structures. Thus, olivine with its isolated silica
tetrahedrons forms at the highest temperatures, and as temperatures drop silica
tetrahedrons first manage to join together in chains (pyroxenes), then in ribbons
(amphiboles), and then sheets (micas).
Igneous Intrusions (Plutons)
An igneous Intrusion is a subsurface intrusive feature that was formed from the
solidification of emplaced magma below the Earth's surface. Over time, igneous
intrusions may become exposed at Earth's surface due to weathering and erosion as
well as crustal movement and uplift.
There are 5 common igneous intrusions, they are explained with respect to: shape, size,
depth characteristics and orientation. Orientation refers to whether the pluton is
discordant or concordant with respect to surrounding, preexisting rock strata (country
rock)
In the illustration below, note the general shapes and sizes of the following plutons:
batholith, stock, laccolith, dike and sill. Each of these igneous intrusive bodies is further
detailed in the data table within this document.
A Discordant Igneous Intrusion cuts across preexisting rock layers, often
along zones of structural weakness such as faults or fractures. E.g. dike
A Concordant Igneous Intrusion solidified from magma that flowed parallel to
and in between preexisting rock layers, often along zones of structural weakness
such as faults or fractures. E.g. sill
Igneous Plutons - DataTable
Name
Shape and Characteristics
Individual intrusions tend to display a
Batholith characteristic teardrop shape.
Commonly composed of felsic or
intermediate granitic rock types
though occasionally intermediate
varieties are found.
Size and Depth
Massive in size, greater
than 100 sq. km.
Orientation
Discordant
Solidify at depth - 5 to 30
km underground
Typically contain many separate
intrusive plutons that form over time,
solidifying into one large irregularly
shaped mass. Batholiths exposed at
Earth's surface often form long
mountain ranges extending up to
1,800 kilometers. One well known
structure is the Sierra Nevada
Batholith.
Stock
Irregular and massive in shape.
Typically the subsurface conduit that
fed molten material to a volcano,
however an exposed stock may
actually be part of a larger,
subsurface igneous intrusion.
Composition is generally granitic;
occasionally intermediate varieties
are found.
Laccolith Mushroom-shaped; flat at the bottom
and domed at the top. The general
trend of the floor of the laccolith is
horizontal to country rock strata.
Pressure exerted by magma forces
the overlying strata upward creating a
characteristic domed shape on
Earth's surface. Typically formed by
intermediate to relatively viscous,
granitic magmas.
Dike
Tabular intrusive body that is vertical
in orientation unless disturbed by
Smaller than a batholith,
less than 100 sq. km
Discordant
Typically form at
shallower crustal depths.
Thickness ranges from
hundreds to a few
thousands of meters and
the diameter is generally
less than 15 km
Concordant
Typically form at
shallower crustal depths.
Thickness is variable and
may range from a few
Discordant
Sill
crustal movement which may rotate
surrounding rock strata changing the
dike's orientation to a more horizontal
position. Composition and texture is
variable; from gabbro, or mafic
magmas, to granitic. If the length of a
dike extends to the Earth's surface,
volcanic activity can occur.
centimeters to greater
than 100 meters in
thickness. Laterally a dike
can extend over a
distance of many
kilometers.Typically form
at shallower crustal
depths.
Tabular, nearly horizontal intrusive
mass that lies parallel with the
preexisting country rock strata into
which it has intruded. Crustal
movement of rocks in the area may
force the horizontal intrusion upwards
into a more vertical and angled
orientation. Composition is variable
and includes rock compositions of
many types.
Thickness is variable and
may range from a few
centimeters to greater
than 1 kilometer. May
extend laterally for
hundreds of
miles.Typically form at
shallower crustal depths.
Concordant
Structure of a Volcano
A volcano constitutes a magma chamber, a vent (pipe or passage way), a crater,
and a cone.
The vent is an opening at the Earth's surface leading from the magma chamber.
The vent is a passageway in the volcano in which the magma rises through to the
surface during an eruption.
The crater is a bowl-shaped depression at the top of the volcano where volcanic
materials like, ash, lava, and gases are released.
TYPES OF VOCANIC CONES
Cinder Cones, are a common type of volcano. They are also the smallest type, with
heights generally less than 300 meters. They can occur as discrete volcanoes on
basaltic lava fields. Cinder cones are composed of ejected basaltic fragments
(pyroclastic material). The fragments are most commonly of cinder size, although bombsize fragments may also be present. The fragments typically contain abundant gas
bubbles (vesicles), giving the cinders and bombs a spongy appearance. The tephra
accumulates and is build up around the vent to form the volcanic cone. The cone has
very steep slopes, up to 35-40 degrees. Unlike the other two main volcano types, cinder
cones have straight sides and very large summit craters, with respect to their relatively
small cones.
Eve cone is a young, well preserved cinder cone at Mount Edziza, British Columbia.
(Photograph by C.J. Hickson (Geological Survey of Canada))
Shield Cones, are broad, low-profile features with basal diameters that vary from a few
kilometers to over 100 kilometers (e.g., the Mauna Loa volcano, Hawaii). Their heights
are typically about 1/20th of their widths. The lower slopes are often gentle (3-5
degrees), but the middle slopes become steeper (10-12 degrees) and then flatten at the
summit. This gives shield volcanoes a flank morphology that is convex in an upward
direction. Their overall broad shapes result from the release of very fluid (low viscosity)
basalt lava that spreads outward from the summit area, in contrast to the vertical
accumulation of airfall tephra around cinder cone vents, and the build-up of viscous lava
and tephra around stratovolcanoes. Cross-sections through shield volcanoes reveal
numerous thin flow units of basalt, typically < 1 m thick.
Mauna Loa
Mauna Loa Volcano,
Hawaii, measured from its 4170 meter-high summit to its base nearly 5000 meters
below sea level, is one of the highest mountains on Earth.
Stratovolcanoes, also known as composite cones, are the most picturesque and the
most deadly of the volcano types. Their lower slopes are gentle, but they rise steeply
near the summit to produce an overall morphology that is concave in an upward
direction (35-40 degrees). The summit area typically contains a surprisingly small
summit crater. This classic stratovolcano shape is exemplified by many well-known
stratovolcanoes, such as Mt. Fuji in Japan, Mt. Vesuvius in Italy, and Mt. St. Helens in
USA.
Typically, as shown in the image ontop, stratovolcanoes have a layered or stratified
appearance with alternating lava flows, ash, pyroclastic flows, volcanic mudflows
(lahars), and/or debris flows. The compositional spectrum of these rock types may vary
from andesite to rhyolite in a single volcano; however, the overall average composition
of stratovolcanoes is andesitic. Many oceanic stratovolcanoes tend to be more mafic
than their continental counterparts. The variability of stratovolcanoes is evident when
examining the eruptive history of individual volcanoes.
Stratovolcanoes typically form at convergent plate margins, where one plate descends
beneath an adjacent plate at the site of a subduction zone. Examples of subductionrelated stratovolcanoes can be found in many places in the world, but they are
particularly abundant along the rim of the Pacific Ocean, a region known as Ring of
Fire.
Lava Domes, are formed by the extrusion of viscous, silica-rich lava that accumulates
above the volcanic vent.Dome extrusion often follows explosive eruptions, which
decrease the gas content of the remaining magma. Dome growth, however, is
commonly accompanied by explosive activity and pyroclastic flows. Lava domes can
form within the summit craters of existing volcanoes or on their flanks. They may build
solitary, dome-like masses, or a complex of overlapping domes.
Lava Dome at Mount St. Helens
Volcanic Hazards
Lava Flows
-Lava flows are common in Hawaiian type of eruptions, which consist of fluid basaltic
lava and the least explosive. Although lava flows have been known to travel as fast as
35 km/hr, most are slower than 20 km/hr and this give people time to move out of the
way. Thus, in general, lava flows are most damaging to property, as they can destroy
(burn) anything in their path.
Pyroclastic Activity from Violent Eruptions
-Pyroclastic activity is one of the most dangerous aspects of volcanism.
-Hot pyroclastic flows (nuee ardente) cause death by suffocation and burning. They
can travel so rapidly, up to 200 km/hr, making it impossible to escape.
Historic examples include: Mt. Pelee on Martinique (1902) - 29,000 dead. Mt Vesuvius
(79 A.D.) - thousands in Pompeii and Herculaneum died.
Cinders and ash can cause the collapse of roofs and can affect areas far from the
eruption. Ash falls like a blanket over an area like snow, they are far more destructive
because they have a density that is three times that of snow and ash deposits do not
melt like snow.
Poisonous Gas Emissions
-Volcanoes emit gases that are often poisonous to living organisms. Among these
poisonous gases are: Carbon Dioxide (CO2), Sulfur dioxide,Hydrogen Chloride,
Hydrogen Sulfide, Ammonia, Methane, etc.-In August 1986, a large CO2 gas emission
from Lake Nyos in Cameroon killed more than 1700 people and 3000 cattle.
Mudflows (Lahars)
-Volcanoes can emit voluminous quantities of loose, unconsolidated ash and
pyroclastics which become deposited on the landscape. Such loose deposits are
subject to rapid removal if they are exposed to a source of water.
-They have properties that vary between thick water and wet concrete.
-The source of water can be derived by melting of snow or ice during the eruption,
emptying of crater lakes during an eruption, or rainfall that takes place any time. Thus,
mudflows can both accompany an eruption and occur many years after an eruption.
- Mudflows are a mixture of water and sediment which have the consistency of wet
concrete. They move rapidly down slope along existing stream valleys, and they may
easily top banks and flood out into surrounding areas.
-On November 13, 1985 a mudflow generated by a small eruption on Nevado del Ruiz
volcano in Columbia flowed down slope and devastated the town of Armero, 40 km east
of the volcano, 23,000 people died in the mudflow that engulfed the town.
Prediction of Volcanic Eruptions
In recent years, with the eruptions of Mount St. Helens and Mount Pinatubo many
advances have been made in the study of volcanoes particularily in eruption prediction.
When scientists study volcanoes, they map past volcanic deposits and use satellites to
look at volcanic features, ash clouds, and gas emissions. They also study and monitor
seismic activity, ground deformation, gas emissions, thermal changes and water level
and pH of streams and lakes near the volcano.
•
Seismic Monitoring - Seismic detection of small (M<4), shallow earthquakes
(volcanic tremors) beneath the volcano. As the magma rises though the vent it pushes
against the rocks deforming them and releasing vibrations. Thus, there is usually an
increase in seismic activity prior to a volcanic eruption. Movement of magma can
sometimes be tracked as it is moving below the surface.
•
Ground Deformation - As magma moves into a volcano, the structure may
inflate. This will cause deformation of the ground which can be monitored. Instruments
like tilt meters measure changes in the angle of the Earth's surface. Other instruments
track changes in distance between several points on the ground to monitor deformation.
•
Changes in Groundwater System - As magma enters a volcano it may cause
changes in the groundwater system, causing the water table to rise or fall and causing
the temperature of the water to increase. By monitoring the depth to the water table in
wells and the temperature of well water, quality of spring water, changes can be
detected that may signify a change in the behavior of the volcanic plumbing system.
•
Changes in acidity (pH) - the acidity of lakes and streams in the area within the
volcano will increase during the emission of gases from the volcano. Gases become
dissolved in the water as they are released. Increasing levels of acidity are indicative of
increased activity and may signal an impeding eruption.
•
Changes in Heat Flow - Heat is everywhere flowing out of the surface of the
Earth. As magma approaches the surface or as the temprature of groundwater
increases, the amount of surface heat flow will increase. Although these changes may
be small they be measured using infrared remote sensing and thermal probes which
have been inserted into the ground around the volcano.
•
Changes in Gas Compositions - The composition of gases emitted from
volcanic vents and fumaroles often changes just prior to an eruption. In general,
increases in the proportions of sulfur dioxide (SO2) are seen to increase consistently
prior to a volcanic eruption.
•
In general, no single event can be used to predict a volcanic eruption, and
thus many events are usually monitored so that taken in total, an eruption can
often be predicted.
Hazard Prevention
There is no known method of preventing volcanic eruptions and no known defense from
the primary threat from pyroclastic flows. Comparatively little can be done about ashfalls
but strengthening of buildings and roofs can help prevent their collapse on the
occupants due to ash accumulating on the roof.
Lava flows moving at comparatively slow speeds are the primary volcanic hazard over
which most physical control can be exerted. Several ways have been implemented for
diverting and controlling lava flows.
Retaining walls or barriers can be constructed to divert lava flows away from valuable
property and villages. Barriers must be constructed from resistant materials. Water
sprays to cool the lava rapidly thus solidifying it. This rapid solidifying of the lava
causes the flow to slow and creates a new barrier for the lava to overcome. This method
was first used experimentally in the 1960's in Iceland and they successfully stopped the
lava from destroying buildings.
As with many natural hazards the only totally effective way of reducing or preventing the
hazard is to remove all human presence from the area in which the hazard may occur.
Of course with volcanoes this is not viable and very unlikely to happen due to the
advantages of living on the rich agricultural soils of the volcano. This means that a
volcano hazard is very difficult to prevent as it is impossible to prevent an actual
eruption.
The best way of reducing the danger of the hazard is to monitor volcanoes to give
warning time for evacuation. This awareness will hopefully prevent loss of life and
property when an eruption occurs. It is important that scientists communicate with local
government officials and the general public about hazards produced by the volcanoes in
their area. This interaction and the development of an emergency evacuation plan
with established lines of communication will hopefully save lives and encourage
better land use planning.