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Objectives
1. Identify igneous rocks by their physical and chemical
properties.
2. Tell which minerals are present in various types of
igneous rocks.
3. List and describe the characteristics of intrusive igneous
rocks. including textures.
4. List and describe the characteristics of extrusive
igneous rocks, including textures.
5. Explain the origin of various types of igneous rocks,
including cooling history.
6. Explain how igneous rocks are classified.
Origin of Igneous Rocks
The word igneous means "fire-formed".
Igneous rocks form by cooling and crystallizing from hot
molten magma or lava.


Magma - hot molten rock below the Earth's surface
Lava - hot molten rock that erupts onto the Earth's
surface through a volcano or crack (fissure)
Igneous rocks that cool and crystallize beneath the Earth's
surface are called intrusive igneous rocks.
Another name for intrusive igneous rocks is plutonic
igneous rocks (named for Pluto, Roman god of the
underworld).
Igneous rocks that cool and crystallize on the Earth's surface
are called extrusive igneous rocks.
Another name for extrusive igneous rocks is volcanic
igneous rocks (named for Vulcan, Roman god of the fire and
forge).
Cooling Rates
Cooling rates influence the texture of the igneous rock:


Quick cooling = fine grains.
Extrusive igneous rocks tend to be fine grained or
glassy.
Sometimes they are vesicular with tiny holes formed
by the release of trapped gases.
Other extrusive igneous rocks have a fragmental or
pyroclastic texture composed of pieces of volcanic
rock and ash welded together by heat. Lava cools more
quickly because it is on the surface.
Slow cooling = coarse grains.
Intrusive rocks tend to be coarse grained with
intergrown crystals ranging from several millimeters to
several inches in diameter.
Magma cools more slowly because it is deep within the
Earth where temperatures are high.
Igneous rocks are classified or named on the basis of their
texture and their composition.
Igneous textures:

Glassy - instantaneous cooling
o Obsidian = volcanic glass
Obsidian

Aphanitic - fine grain size (< 1 mm); result of quick
cooling
Rhyolite
Basalt
o Rhyolite
o Andesite
Phaneritic - coarse grain size; visible grains (1-10
mm); result of slow cooling
o

Granite - polished
Granite
o Diorite
o Gabbro
Pegmatitic - very large crystals (many over 2 cm)
o Granite pegmatite or pegmatitic granite
Porphyritic- Mixture of grain sizes caused by mixed
cooling history; slow cooling first, followed by a period
of somewhat faster cooling.
o Terms for the textural components:
 Phenocrysts - the large crystals
 Groundmass or matrix - the finer crystals
surrounding the large crystals. The
groundmass may be either aphanitic or
phaneritic.
o Types of porphyritic textures:
 Porphyritic-aphanitic
 Porphyritic-phaneritic
o Origin: mixed grain sizes and hence cooling rates,
imply upward movement of magma from a deeper
(hotter) location of extremely slow cooling, to
either:
 a much shallower (cooler) location with fast
cooling (porphyritic- aphanitic), or
 a somewhat shallower (slightly cooler)
location with continued fairly slow cooling
(porphyritic-phaneritic).
o Rock name = porphyry
o



Granite porphyry or porphyritic granite
(porphyritic-phaneritic) - phenocrysts usually
potassium feldspar
porphyry


Granite
Andesite porphyry or porphyritic andesite
(porphyritic-aphanitic) - phenocrysts usually
hornblende (amphibole)
Rhyolite porphyry or porphyritic rhyolite
(porphyritic-aphanitic)

Rhyolite Porphyry
Vesicular - contains tiny holes called vesicles which
formed due to gas bubbles in the lava or magma. Very
porous. May resemble a sponge. Commonly low
density; may float on water.
o Vesicular basalt - basalt with a vesicles, which
may be quite large. Sometimes lined with crystals
to form geodes.
Vesicular basalt
Vesicular basalt with olivine phenocrysts, building
stone at Hawaiian Volcano Observatory, Big Island
of Hawaii
o
Pumice - light in color; white to gray; may be
glassy or dull. Fully riddled with holes. Very
sponge-like. Floats. Used as an abrasive. (Pumice
stone, Lava Soap).
Pumice
Pumice
o
Scoria - dark in color; brown, black, or dark red;
similar to vesicular basalt but is fully riddled with
holes to form a spongy mass. (May find in
barbecue grills as lava rock).
Scoria

Pyroclastic or Fragmental - pieces of rock and ash
come out of a volcano and get welded together by
heat. May resemble rhyolite or andesite, but close
examination shows pieces of fine-grained rock
fragments in it. May also resemble a sedimentary
conglomerate or breccia, except that rock fragments
are all fine-grained igneous or vesicular.
o Tuff - made of volcanic ash
o Volcanic breccia - contains fragments of finegrained igneous rocks that are larger than ash.
Pyroclastic rock
Composition of Igneous Rocks
Igneous rocks can be placed into four groups based on their
chemical compositions:
1. Sialic (or granitic or felsic)
1. Dominated by silicon and aluminum (SiAl)
2. Usually light in color
3. Characteristic of continental crust
4. Forms a stiff (viscous) lava or magma
5. Rock types include:
1. Granite
Granite
2. Rhyolite
Rhyolite
6. Minerals commonly present include:
1. potassium feldspar (generally pink or white)
2. Na-plagioclase feldspar (generally white)
3. quartz (generally gray or colorless)
4. biotite
5. amphibole?
6. muscovite?
2. Intermediate (or andesitic)
1. Intermediate in composition between sialic and
mafic
2. Rock types include:
1. Andesite (aphanitic)
2. Diorite (phaneritic)
Diorite
3. Minerals commonly present include:
1. plagioclase feldspar
2. amphibole
3. pyroxene
4. biotite
5. quartz
3. Mafic (or basaltic)
1. Contains abundant ferromagnesian minerals
(magnesium and iron silicates)
2. Usually dark in color (dark gray to black)
3. Characteristic of Earth's oceanic crust, Hawaiian
volcanoes
4. Forms a runny (low viscosity) lava
5. Also found on the Moon, Mars, and Venus
6. Rock types include:
1. Basalt (aphanitic)
Basalt
2. Gabbro (phaneritic)
Gabbro
3. Diabase - texture intermediate between
basalt and gabbro; characteristic of Early
Mesozoic dikes in eastern North America.
7. Minerals commonly present include:
1. Ca-plagioclase feldspar
2. pyroxene
3. olivine
4. amphibole
4. Ultramafic
1. Almost entirely magnesium and iron silicates
(ferromagnesian minerals)
2. Rarely observed on the Earth's surface
3. Believed to be major constituent of Earth's mantle
4. Commonly found as xenoliths in basaltic lavas
5. Rock types include:
1. Peridotite (phaneritic)
1. dominated by olivine - the birthstone is
Peridot, which gives its name to
Peridotite
Peridotite
6. Minerals commonly present include:
1. Olivine is dominant. (Olivine is olive green).
2. may have minor amounts of pyroxene and
Ca-plagioclase
Other types of igneous rock:
Syenite
A polished syenite called larvikite with centimeter- to inchscale gray to blue plagioclase crystals. The industrial name
for the rocks is "blue pearl". Photographed in an aboveground cemetery in New Orleans, LA
Return to Earth & Space Science page
This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston Campus, Clarkston, GA
Know Bowens Reaction Series…. Be sure to be able to tell me what kind of
igneous rocks will form at different temperatures. What will these igneous
rocks look like and what is their mineral make up.
Volcano Notes are saved from the following website
http://www.geology.sdsu.edu/how_volcanoes_work/Home.
html
Please Visit this website for more tutorials as well as
practice exams
VOLCANO-TECTONIC ENVIRONMENTS
DISTRIBUTION OF ACTIVE VOLCANOES
The earth is a dynamic planet. Its rigid outer surface layer is broken
into several tectonic plates which are in constant motion relative to
one another. As demonstrated in the world map below, most of the
~550 active volcanoes on earth are located along the margins of
adjacent plates.
World map showing plate boundaries (blue lines),
the distribution of recent earthquakes (yellow dots)
and active volcanoes (red triangles). Courtesy of
NASA.
PLATE MOTION, MANTLE CONVECTION, AND MAGMA
GENERATION
Tectonic plates are composed of lithosphere, the rigid outer portion
of the earth. With a thickness of about 100 km, the lithosphere is
composed of an upper layer of crust (~7 km thick under the oceans,
and ~35 km thick under the continents) and a lower, denser layer of
the earth's upper mantle. The lithosphere is underlain by the
asthenosphere, a hot, mobile layer of partially molten rock lying
within the earth's upper mantle. (For detailed information, click the
Earth's Interior.)
The rigid
lithospheric
plates are
driven by
convection
within the
mobile asthenosphere.
Hot mantle rises
beneath mid-oceanic
ridges, and cold,
denser mantle
descends at oceanic
trenches. Lateral
motion of the
lithospheric plates
above these circular
convection cells is
analogous to rigid
blocks riding above a
rotating conveyor belt.
Images modified from
USGS.
Volcanic eruptions above these lithospheric plates are driven by the
ascent of magma (molten rock) from deep beneath the surface. The
various magma types are described in Physicochemical Controls on
Eruption Style. They vary from mafic, intermediate, to felsic as their
silica (SiO2) content increases. Mafic (basaltic) magmas are
generated directly from the mantle, either within the asthenosphere or
within the overlying mantle lithosphere. Many mafic-to-intermediate
(basaltic-to-andesitic) magmas appear to be derived from the melting
of hydrated lithospheric mantle. More differentiated, intermediate-tofelsic magmas, on the other hand, are partly derived from the melting
of continental crust by hot, mafic magmas that either pond at the
crust-mantle boundary, or intrude into the overlying continents where
they reside in magma chambers located at various crustal levels.
Volcanism is typically widespread along plate boundaries. Although
volcanism in the interior of plates is less common, these intraplate
regions can also generate voluminous eruptive products. The
regional volcano-tectonic processes associated with plate-boundary
environments and intraplate environments are described in more
detail below.
VOLCANISM AT PLATE TECTONIC BOUNDARIES
Plate boundaries mark the sites where two plates are either moving
away from one another, moving toward one another, or sliding past
one another. Adjacent plates are delineated by three types of
boundaries defined by this relative motion:



Divergent plate boundaries -- Plates diverge from one
another at the site of thermally buoyant mid-oceanic
ridges. Oceanic crust is created at divergent plate
boundaries.
Convergent plate boundaries -- Plates converge on one
another at the site of deep oceanic trenches. Oceanic
crust is destroyed at convergent plate boundaries.
Transform plate boundaries -- Plates slide past one
another.
Although volcanism is abundant at divergent and convergent plate
boundaries, there is a distinct lack of significant volcanism associated
with transform plate boundaries. Spreading center volcanism occurs
at divergent plate margins, and subduction zone volcanism occurs at
convergent plate margins. Intraplate volcanism describes volcanic
eruptions within tectonic plates. Each of these three volcano-tectonic
environments is depicted in the following diagram:
Volcanism at divergent and convergent plate
margins. Courtesy of USGS.
SPREADING CENTER VOLCANISM
Spreading center volcanism occurs at the site of mid-oceanic
ridges, where two plates diverge from one another. As the plates are
pulled apart, hot asthenosphere rises upward to fill voids of the
extended lithosphere. The rise of this hot mantle provides thermal
buoyancy to the ridge area and this is the reason that they stand as
high ridges in the center of ocean basins. This is demonstrated in the
colored relief images shown here.
Mid-oceanic ridge of
the south Pacific.
Mid-Atlantic Ridge.
Courtesy of NOAA.
As the hot asthenosphere rises to shallow levels, it decompresses
and melts to produce basalt magmas. These magmas pond in crustal
magma chambers where they are periodically tapped by vertical
fractures that provide conduits for the rapid rise of magma to the
surface. The eruptions produced in this manner are typically fissure
eruptions. The erupting basalt can generate vast submarine lava
fields. Typically, the lava quenches quickly against the bottom waters
to produce characteristic bulbous shapes called pillow basalt.
The high heat content of mid-ocean ridges is evident from the
occurrence of numerous hydrothermal vents. These form from
surface water that seeps downward through cracks where it heated
by hot rocks lying above the magma chambers. These hot thermal
waters then ascend back through the overlying crust, where they
leach out silica and numerous metals from the basaltic lava. The hot
springs created at the surface are called a black smokers because
they are readily identified by billowing dark clouds composed of
metal-rich fluids.
Above: pillow basalt from the
south Pacific.
Right: black smoker from
the mid-Atlantic Ridge.
As basaltic lava erupts at the surface, more or less continuously for
millions of years, it is constantly accreted onto the edge of the
spreading plates as it cools into a hardened basalt layer. This
process generates oceanic crust. Oceanic crust is youngest near the
ridge, but it becomes progressively older away from the spreading
center due to divergence of the plates over time. This age
progression is demonstrated in the image below.
Age of the Atlantic
oceanic crust. The
crust near the
continental margins
(blue) is about 200
million years old. It
gets progressively
younger toward the
mid-Atlantic ridge,
where oceanic
crust is forming
today. Courtesy of
NOAA.
Whereas oceanic crust is generated at divergent plate margins, it is
consumed at convergent plate margins.
SUBDUCTION ZONE VOLCANISM
The most volcanically active belt on Earth is known as the
Ring of Fire, a region of subduction zone volcanism
surrounding the Pacific Ocean. Subduction zone
volcanism occurs where two plates are converging on one
another. One plate containing oceanic lithosphere
descends beneath the adjacent plate, thus consuming the oceanic
lithosphere into the earth's mantle. This on-going process is called
subduction. As the descending plate bends downward at the
surface, it creates a large linear depression called an oceanic
trench. These trenches are the deepest topographic features on the
earth's surface. The deepest, 11 kilometers below sealevel, is the
Mariana trench, which lies along the western margin of the Ring of
Fire. Another example, forming the northern rim of the Ring of Fire, is
the Aleutian trench shown here:
The Pacific plate
descends into the
mantle at the site of the
Aleutian trench.
Subduction zone
volcanism here has
generated the Aleutian
island chain of active
volcanoes. Courtesy of
NOAA.
The crustal portion of the subducting slab contains a significant
amount of surface water, as well as water contained in hydrated
minerals within the seafloor basalt. As the subducting slab descends
to greater and greater depths, it progressively encounters greater
temperatures and greater pressures which cause the slab to release
water into the mantle wedge overlying the descending plate. Water
has the effect of lowering the melting temperature of the mantle, thus
causing it to melt. The magma produced by this mechanism varies
from basalt to andesite in composition. It rises upward to produce a
linear belt of volcanoes parallel to the oceanic trench, as exemplified
in the above image of the Aleutian Island chain. The chain of
volcanoes is called an island arc. If the oceanic lithosphere subducts
beneath an adjacent plate of continental lithosphere, then a similar
belt of volcanoes will be generated on continental crust. This belt is
then called a volcanic arc, examples of which include the Cascade
volcanic arc of the U.S. Pacific northwest, and the Andes volcanic arc
of South America.
Island arc formed by
oceanic-oceanic subduction
Volcanic arc formed by
oceanic-continental
subduction
The volcanoes produced by subduction zone
volcanism are typically stratovolcanoes.
Incipient island arcs tend to be more basaltic in composition, whereas
mature continental volcanic arcs tend to be more andesitic in
composition.
INTRAPLATE VOLCANISM
HOT SPOTS AND MANTLE PLUMES
Although most volcanic rocks are generated at plate boundaries,
there are a few exceptionally active sites of volcanism within the plate
interiors. These intraplate regions of voluminous volcanism are called
hotspots. Twenty-four selected hotspots are shown on the adjacent
map. Most hotspots are thought to be underlain by a large plume of
anomalously hot mantle. These mantle plumes appear to be
generated in the lower mantle and rise slowly through the mantle by
convection. Experimental data suggests that they rise as a plastically
deforming mass that has a bulbous plume head fed by a long,
narrow plume tail. As the head impinges on the base of the
lithosphere, it spreads outward into a mushroom shape. Such plume
heads are thought to have diameters between ~500 to ~1000 km.
Many scientists
believe that mantle
plumes may be
derived from near the
core-mantle
boundary, as
demonstrated in this
computer simulation
from the Minnesota
supercomputing lab.
Note the bulbous
plume heads, the
narrow plume tails,
and the flattened
plume heads as they
impinge on the outer
sphere representing
the base of the
lithosphere.
Decompressional melting of this hot mantle source can generate
huge volumes of basalt magma. It is thought that the massive flood
basalt provinces on earth are produced above mantle hotspots.
Although most geologists accept the hotspot concept, the number of
hotspots worldwide is still a matter of controversy.
HOTSPOT TRACKS
The Pacific plate contains several linear belts of extinct submarine
volcanoes, called seamounts, an example of which is the Foundation
seamount chain shown here.
The Foundation
seamount chain
is located near
Easter Island in
the south Pacific.
Courtesy of
NOAA.
The formation of at least some of these intraplate seamount chains
can be attributed to volcanism above a mantle hotspot to form a
linear, age-progressive hotspot track. Mantle plumes appear to be
largely unaffected by plate motions. As lithospheric plates move
across stationary hotspots, volcanism will generate volcanic islands
that are active above the mantle plume, but become inactive and
progressively older as they move away from the mantle plume in the
direction of plate movement. Thus, a linear belt of inactive volcanic
islands and seamounts will be produced. A classic example of this
mechanism is demonstrated by the Hawaiian and Emperor seamount
chains.
The image on the left shows the Hawaiian and
Emperor seamount chains. The Hawaiian chain
begins at the Hawaiian Islands, to the southeast,
and continues to the bend located ~5000 km to the
northwest. From the bend, the Emperor chain
continues to the north-northwest until it
terminates at the Aleutian trench (Courtesy of
NOAA). The diagram on the right is a model
demonstrating how these chains form above the
stationary mantle plume, becoming progressively
older to the northwest (Courtesy of the USGS).
The "Big Island" of Hawaii lies above the mantle plume. It is the only
island that is currently volcanically active. The seven Hawaiian
Islands become progressively older to the northwest. The main phase
of volcanism on Oahu ceased about 3 million years ago, and on
Kauai about 5 million years ago. This trend continues beyond the
Hawaiian Islands, as demonstrated by a string of seamounts (the
Hawaiian chain) that becomes progressively older toward Midway
Island. Midway is composed of lavas that are ~27 million years old.
Northwest of Midway, the volcanic belt bends to the north-northwest
to form the Emperor seamount chain. Here, the seamounts become
progressively older until they terminate against the Aleutian trench.
The oldest of these seamounts near the trench is ~70 million years
old. This implies that the mantle plume currently generating basaltic
lavas on the Big Island has been in existence for at least 70 million
years!
The Hawaiians were very good at recognizing the difference in the
older, eroded volcanic islands and newer islands to the southeast,
where volcanic features are more pristine. Legend has it that Pele,
the Hawaiian goddess of fire, was forced from island to island as she
was chased by various gods. Her journey is marked by volcanic
eruptions, as she progressed from the island of Kaua'i to her current
home on the Big Island. The legend corresponds well with the
modern scientific notion of the age progression of these volcanic
islands.
PHYSICOCHEMICAL CONTROLS ON ERUPTION STYLE
There is a great range in the explosivity of volcanic eruptions. Many
eruptions are relatively quiescent and are characterized by the calm,
nonviolent extrusion of lava flows on the earth's surface. Other
eruptions, however, are highly explosive and are characterized by the
violent ejection of fragmented volcanic debris, called tephra, which
can extend tens of kilometers into the atmosphere above the volcano.
Nonexplosive eruption
with effusive lava flows
Explosive eruption with
voluminous plume of
tephra
Whether or not an eruption falls into one of these end-member types
depends on a variety of factors, which are ultimately linked to the
composition of the magma (molten rock) underlying the volcano.
Magma composition is discussed below, followed by a description of
the controlling factors on explosivity -- viscosity, temperature, and
the amount of dissolved gases in the magma.
MAGMA COMPOSITION AND ROCK TYPES
Only ten elements make up the bulk of most magmas: oxygen (O),
silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), titanium (Ti)
calcium (Ca), sodium (Na), potassium (K), and phosphorous (P).
Because oxygen and silicon are by far the two most abundant
elements in magma, it is convenient to describe the different magma
types in terms of their silica content (SiO2). The magma types vary
from mafic magmas, which have relatively low silica and high Fe and
Mg contents, to felsic magmas, which have relatively high silica and
low Fe and Mg contents. Mafic magma will cool and crystallize to
produce the volcanic rock basalt, whereas felsic magma will
crystallize to produce dacite and rhyolite. Intermediate-composition
magmas will crystallize to produce the rock andesite. Because the
mafic rocks are enriched in Fe and Mg, they tend to be darker colored
than the felsic rock types.
SiO2 CONTENT
MAGMA TYPE
VOLCANIC ROCK
~50%
~60%
~65%
~70%
Mafic
Intermediate
Felsic (low Si)
Felsic (high Si)
Basalt
Andesite
Dacite
Rhyolite
There also exists more unusual magmas that erupt less commonly on
the Earth's surface as ultramafic, carbonatite, and strongly
alkaline lavas.
For an historical note on rock terminology see: Basic/Acidic vs.
Mafic/Felsic
MAGMA VISCOSITY, TEMPERATURE, AND GAS CONTENT
The viscosity of a substance is a measure of its consistency.
Viscosity is defined as the ability of a substance to resist flow. In a
sense, viscosity is the inverse of fluidity. Cold molasses, for example,
has a higher viscosity than water because it is less fluid. A magma's
viscosity is largely controlled by its temperature, composition, and
gas content (see downloadable programs at the bottom of this page).
The effect of temperature on viscosity is intuitive. Like most liquids,
the higher the temperature, the more fluid a substance becomes, thus
lowering its viscosity.
Composition plays an even greater role in determining a magma's
viscosity. A magma's resistance to flow is a function of its "internal
friction" derived from the generation of chemical bonds within the
liquid. Chemical bonds are created between negatively charged and
positively charged ions (anions and cations, respectively). Of the ten
most abundant elements found in magmas (see above), oxygen is
the only anion. Silicon, on the other hand, is the most abundant
cation. Thus, the Si-O bond is the single most important factor in
determining the degree of a magma's viscosity. These two elements
bond together to form "floating radicals" in the magma, while it is still
in its liquid state (i.e., Si-O bonds begin to form well above the
crystallization temperature of magma). These floating radicals contain
a small silicon atom surrounded by four larger oxygen atoms (SiO4).
This atomic configuration is in the shape of a tetrahedron. The
radicals are therefore called silicon-oxygen tetrahedra, as shown
here.
These floating tetrahedra are electrically charged compounds. As
such, they they are electrically attracted to other Si-O tetrahedra. The
outer oxygen atoms in each tetrahedron can share electrons with the
outer oxygen atoms of other tetrahedra. The sharing of electrons in
this manner results in the development of covalent bonds between
tetrahedra. In this way Si-O tetrahedra can link together to form a
variety shapes: double tetrahedra (shown here, C), chains of
tetrahedra, double chains of tetrahedra, and complicated networks of
tetrahedra. As the magma cools, more and more bonds are created,
which eventually leads to the development of crystals within the liquid
medium. Thus, the Si-O tetrahedra form the building blocks to the
common silicate minerals found in all igneous rocks. However, while
still in the liquid state, the bonding of tetrahedra results in the
polymerization of the liquid, which increases the "internal friction" of
the magma, so that it more readily resists flow. Magmas that have a
high silica content will therefore exhibit greater degrees of
polymerization, and have higher viscosities, than those with low-silica
contents.
The amount of dissolved gases in the magma can also affect it's
viscosity, but in a more ambiguous way than temperature and silica
content. When gases begin to escape (exsolve) from the magma, the
effect of gas bubbles on the bulk viscosity is variable. Although the
growing gas bubbles will exhibit low viscosity, the viscosity of the
residual liquid will increase as gas escapes. The overall bulk viscosity
of the bubble-liquid mixture depends on both the size and distribution
of the bubbles. Although gas bubbles do have an effect on the
viscosity, the more important role of these exsolving volatiles is that
they provide the driving force for the eruption. This is discussed in
more detail below.
VESICULATION
As dissolved gases are released from the magma, bubbles will begin
to form. Bubbles frozen in a porous or frothy volcanic rock are called
vesicles, and the process of bubble formation is called vesiculation
or gas exsolution. The dissolved gases can escape only when the
vapor pressure of the magma is greater than the confining
pressure of the surrounding rocks. The vapor pressure is largely
dependent on the amount of dissolved gases and the temperature of
the magma.
Gas escape through
Vesicle-rich flow top
vertical vesicle
cylinders
Explosive eruptions are initiated by vesiculation, which in turn, can be
promoted in two ways: (1) by decompression, which lowers the
confining pressure, and (2) by crystallization, which increases the
vapor pressure. In the first case, magma rise can lead to
decompression and the formation of bubbles, much like the
decompression of soda and the formation of CO2 bubbles when the
cap is removed. This is sometimes referred to as the first boiling.
Alternatively, as magma cools and anhydrous minerals begin to
crystallize out of the magma, the residual liquid will become
increasingly enriched in gas. In this case, the increased vapor
pressure in the residual liquid can also lead to gas exsolution. This is
sometimes referred to as second (or retrograde) boiling. Both
mechanisms can trigger an explosive volcanic eruption.
CONTROLS ON EXPLOSIVITY
The amount of dissolved gas in the magma provides the driving force
for explosive eruptions. The viscosity of the magma, however, is also
an important factor in determining whether an eruption will be
explosive or nonexplosive. A low-viscosity magma, like basalt, will
allow the escaping gases to migrate rapidly through the magma and
escape to the surface. However, if the magma is viscous, like rhyolite,
its high polymerization will impede the upward mobility of the gas
bubbles. As gas continues to exsolve from the viscous melt, the
bubbles will be prevented from rapid escape, thus increasing the
overall pressure on the magma column until the gas ejects
explosively from the volcano. As a general rule, therefore,
nonexplosive eruptions are typical of basaltic-to-andesitic magmas
which have low viscosities and low gas contents, whereas explosive
eruptions are typical of andesitic-to-rhyolitic magmas which have
high viscosities and high gas contents.
SiO2
~50%
MAGMA TEMPERATURE
GAS
VISCOSITY
TYPE
(centigrade)
CONTENT
mafic
~1100
low
low
ERUPTION
STYLE
nonexplosive
~60% intermediate
~70%
felsic
~1000
~800
intermediate intermediate intermediate
high
explosive
high
There are, however, two exceptions to this general rule. Andesitic-torhyolitic lavas that have been degassed often erupt at the surface
nonexplosively as viscous lava domes or obsidian flows. Similarly,
many of the so-called hydrovolcanic eruptions involve basaltic-toandesitic magmas that erupt explosively in the presence of
groundwater or surface water.
GENERIC FEATURES
A volcanic vent is an opening exposed on the earth's surface where
volcanic material is emitted. All volcanoes contain a central vent
underlying the summit crater of the volcano. The volcano's coneshaped structure, or edifice, is built by the more-or-less symmetrical
accumulation of lava and/or pyroclastic material around this central
vent system. The central vent is connected at depth to a magma
chamber, which is the main storage area for the eruptive material.
Because volcano flanks are inherently unstable, they often contain
fractures that descend downward toward the central vent, or toward a
shallow-level magma chamber. Such fractures may occasionally tap
the magma source and act as conduits for flank eruptions along the
sides of the volcanic edifice. These eruptions can generate coneshaped accumulations of volcanic material, called parasitic cones.
Fractures can also act as conduits for escaping volcanic gases, which
are released at the surface through vent openings called fumaroles.
Summit Crater
Parasitic Cones
Fumarole
MAIN VOLCANO TYPES
Although every volcano has a unique eruptive history, most can be
grouped into three main types based largely on their eruptive patterns
and their general forms. The form and composition of the three main
volcano types are summarized here:
VOLCANO
TYPE
SCORIA CONE
SHIELD VOLCANO
VOLCANO
SHAPE
Straight sides
with steep
slopes; large
summit crater
Very gentle
slopes; convex
upward
COMPOSITION
ERUPTION
TYPE
Basalt tephra;
occasionally Strombolian
andesitic
Basalt lava
flows
Highly variable;
alternating
Gentle lower
basaltic to
slopes, but
rhyolitic lavas
STRATOVOLCANO steep upper
and tephra with
slopes;
an overall
concave
andesite
upward; small composition
summit crater
Hawaiian
Plinian
SUBORDINATE VOLCANO TYPES -- Lava and tephra can erupt
from vents other than these three main volcano types. A fissure
eruption, for example, can generate huge volumes of basalt lava;
however, this type of eruption is not associated with the construction
of a volcanic edifice around a single central vent system. Although
point-source eruptions can generate such features as spatter cones
and hornitos, these volcanic edifices are typically small, localized,
and/or associated with rootless eruptions (i.e., eruptions above the
surface of an active lavaflow, unconnected to an overlying magma
chamber) . Vent types related to hydrovolcanic processes generate
unique volcanic structures, discussed separately under hydrovolcanic
eruptions.
For a description of each of the main volcano types, see:



SCORIA CONES
SHIELD VOLCANOES
STRATOVOLCANOES
SCORIA CONES
Scoria cones, also known as cinder cones, are the most 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, or as parasitic cones generated by
flank eruptions on shield volcanoes and stratovolcanoes. Scoria
cones are composed almost wholly of ejected basaltic tephra. The
tephra is most commonly of lapilli size, although bomb-size fragments
and lava spatter may also be present. The tephra fragments typically
contain abundant gas bubbles (vesicles), giving the lapilli and bombs
a cindery (or scoriaceous) appearance. The tephra accumulates as
scoria-fall deposits which build up around the vent to form the
volcanic edifice. The edifice has very steep slopes, up to 35 degrees,
although older eroded scoria cones typically have gentler slopes,
from 15 to 20 degrees. Unlike the other two main volcano types,
scoria cones have straight sides and very large summit craters, with
respect to their relatively small edifices. They are often symmetric,
although many are asymmetric due to (1) the build up of tephra on
the downwind flank of the edifice, (2) elongation of the volcano above
an eruptive fissure, or (3) partial rafting of an outer wall of the volcano
due to basalt lava oozing outward from beneath the volcano edifice.
Where scoria cones have been breached, they typically reveal redoxidized interiors.
Scoria cone on Mauna Kea
Sunset Crater scoria cone
La Poruna scoria cone
Scoria cones are generated by Strombolian eruptions, which produce
eruptive columns of basalt tephra generally only a few hundred
meters high. Many scoria cones are monogenetic in that they only
erupt once, in contrast to shield volcanoes and stratovolcanoes. An
exception is the Cerro Negro volcano in Nicaragua, which is the
Earth's most historically active scoria cone. It is one of several
parasitic cones on the northwest flank of Las Pilas volcano. Cerro
Negro has erupted more than twenty times since it was born in 1850.
Its most recent eruptions were in 1992 and 1995.
SHIELD VOLCANOES
Shield volcanoes 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 (2-3 degrees), but
the middle slopes become steeper (~10 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 extrusion of very fluid (low viscosity) basalt lava that spreads
outward from the summit area, in contrast to the vertical accumulation
of airfall tephra around scoria-cone vents, and the build-up of viscous
lava and tephra around stratovolcanoes. Cross-sections through
shield volcanoes reveal numerous thin flow units of pahoehoe basalt,
typically < 1 m thick. Pyroclastic deposits are minor (< 1%) and of
limited dispersal, generally from flank eruptions associated with
parasitic scoria cones, or from rare, localized hydrovolcanic
eruptions.
Very thin
pahoehoe flow
units (< 0.5 m
thick) on the flank
of a shield
volcano in
western Saudi
Arabia. Photo by
Vic Camp.
The Mauna Loa volcano on the Big Island of Hawaii, seen here in
both a oblique and satellite views, is the world's largest shield
volcano:
Mauna Loa from the
southeast
Mauna Loa satellite view
Shield volcanoes are generated by Hawaiian eruptions. However,
there is some variability in their eruptive style, which translates into
variations in shield morphology and size. The almost perfect
symmetry and small volume (~15 km3) of Icelandic shields, for
example, stands in marked contrast to the elongation and huge
volume (thousands of km3) of Hawaiian shields. These variations
are largely attributed to the monogenetic, small-volume, centralized
summit eruptions, typical of icelandic shields, and the polygenetic,
large-volume, linear fissure eruptions, typical of most hawaiian
shields. Still different are the symmetrical Galapagos shields, shown
below, which have steep middle slopes (>10 degrees) and flat tops
occupied by large and very deep calderas. These shield types appear
to be generated by ring-fracture eruptions, which delineate the sides
of the caldera and mark the site of caldera collapse.
Coalesced shield volcanoes
of the Galapagos Islands
Three-dimensional
image of
the Alcedo shield
volcano on
Isabella Island,
Galapagos
STRATOVOLCANOES
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. 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. Mayon in the
Philippines, and Mt. Agua in Guatemala.
Mt. Mayon
Mt. Agua
In detail, however, stratovolcano shapes are more variable than these
classic examples, primarily because of wide variations in eruptive
style and composition. Some may contain several eruptive centers, a
caldera, or perhaps an amphitheater as the result of a lateral blast
(e.g., Mt. St. Helens).
Typically, as shown in the image to the left,
stratovolcanoes have a layered or stratified
appearance with alternating lava flows, airfall
tephra, pyroclastic flows, volcanic mudflows
(lahars), and/or debris flows. The
compositional spectrum of these rock types
may vary from basalt 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. Mt. Fuji and Mt. Etna, for example, are dominanted by
basaltic lava flows, whereas Mt. Rainier is dominated by andesitic
lava, Mt. St. Helens by andesitic-to-dacitic pyroclastic material, and
Mt. Lassen by dacitic lava domes.
Stratovolcanoes typically form at convergent plate margins, where
one plate descends beneath an adjacent plate at the site of a
subduction zone. Examples of subduction-related 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. In the Americas, the Ring of Fire includes stratovolcanoes
forming the Aleutian islands in Alaska, the crest of the Cascade
Mountains in the Pacific Northwest, and the high peaks of the Andes
Mounains in South America. A satellite view of three stratovolcanoes
from the Andes is shown here:
Three Andean
stratovolcanoes in
northern Ecaudor
The eruptive history of most stratovolcanoes is delineated by highly
explosive Plinian eruptions. These dangerous eruptions are often
associated with deadly pyroclastic flows composed of hot volcanic
fragments and toxic gases that advance down slopes at hurricaneforce speeds. Like shield volcanoes, stratovolcanoes are polygenetic;
however, they differ from shield volcanoes in that they erupt
infrequently, with typical repose intervals of hundreds of years
between eruptions. Most active stratovolcanoes worldwide appear to
be < 100,000 years old, although some, like Mt. Rainier, may be
more than 1 million years old.
PYROCLASTIC FLOWS
GENERAL CHARACTERISTICS
A pyroclastic flow is a fluidized mixture of solid to semi-solid
fragments and hot, expanding gases that flows down the flank of a
volcanic edifice. These awesome features are heavier-than-air
emulsions that move much like a snow avalanche, except that they
are fiercely hot, contain toxic gases, and move at phenomenal,
hurricane-force speeds, often over 100 km/hour. They are the most
deadly of all volcanic phenomena.
Pyroclastic flow
Pyroclastic flow
Pyroclastic flow
Mt. Augustine (1996) Mt. St. Helens (1980) Mt. Pelée (1902)
FLOW FLUIDIZATION
The extraordinary velocity of a pyroclastic flow is partly attributed to
its fluidization. A moving pyroclastic flow has properties more like
those of a liquid than a mass of solid fragments. Its fluid behavior can
only be described as spectacular, as evidenced by the 6000-year-old
Koya flow in southern Japan, which traveled more than 60 km from its
source, ten of which were over open water! The Koya flow left a
deposit that was only two meters thick over its 60 km extent. Such
mobility comes from the disappearance of inter-particle friction. A
fluidized flow is best described as a dispersion of large fragments in a
medium of fluidized fine fragments. A constant stream of hot,
expanding gases keeps the smallest of the fragments (ash and lapilli
size particles) in constant suspension. This solid-gas mixture can
then support larger fragments that float in the matrix. The expanding
gas component is derived from a combination of (1) the constant
exsolution of volcanic gas emitted by the hot pyroclasts, and (2) from
the ingestion, heating, and rapid expansion air during movement of
the flow.
TERMINOLOGY
The terminology of pyroclastic flows and pyroclastic flow deposits can
be complex and confusing. In general, there are two end-member
types of flows:


NUÉE ARDENTES -- these contain dense lava
fragments derived from the collapse of a
growing lava dome or dome flow, and
PUMICE FLOWS -- these contain vesiculated,
low-density pumice derived from the collapse of
an eruption column.
A nuée ardente deposit is called a block-and-ash deposit (see
below), whereas a pumice flow deposit is called an ignimbrite. Each
end-member type of pyroclastic flow is discussed below, followed by
a description of ignimbrites.
NUÉE ARDENTES
The French geologist Alfred Lacroix attached the name nuée ardente
(glowing cloud) to the pyroclastic flow from Mt. Pelée that destroyed
the city of St. Pierre in 1902. The flow was generated from the
explosive collapse of a growing lava dome at the summit of the
volcano, which then swept down on the city. Thus, nuée ardente
eruptions are often called Peléen eruptions. However, this term
cannot be as narrowly defined as the other eruption types, because
nuée ardentes are often linked with both Plinian and Vulcanian
activity.
Although the term nuée ardente is now applied to all pyroclastic flows
generated by dome collapse, it is somewhat of a misnomer to
describe these features as a "glowing cloud." A more precise term
would be glowing avalanche. The bulk of these hot block-and-ash
flows hug the ground surface, but are disguised by an overlying cloud
of fine ash particles that are winnowed out of the flow by a processes
called elutriation. Nuée ardentes, therefore, are composed of two
related parts: a pyroclastic avalanche largely hidden from view by an
overlying ash cloud, sometimes called a co-ignimbrite ash.
Mt. Unzen nuée ardentes -- The
diagram here demonstrates the
sequence of events associated with
the 1991-95 nuée ardente eruptions
from Mt. Unzen, Japan. Collapse of
a growing lava dome generates the
nuée ardente. Within seconds a
faster-moving cloud of smaller ashsized fragments (the ash-cloud
surge) forms above and in front of
the nuée ardente. In some cases,
dome collapse is attributed to
explosive eruption at the summit
Courtesy of B. Meyers, crater. Explosive collapse may clear
the throat of the volcano, thus
USGS
generating vertical eruption columns.
Over a four-year period, hundreds of
nuée ardentes erupted from Mt.
Unzen's summit area. Many of these
swept down the populated
Mizunashi River valley displacing
thousands of people and destroying
several hundred homes and
precious farmland.
Nuée ardente deposits are composed of dense, non-vesiculated,
blocky fragments derived from the collapsed lava dome. They
therefore differ significantly from the highly vesiculated ignimbrites
which are derived from eruption column collapse. Nuée ardente
deposits contain blocks in a fine-grained matrix of ash. The deposits,
therefore, are called block-and-ash deposits. They are denser than
ignimbrites, and typically are less extensive.
PUMICE FLOWS
Pumice flows are pumice-rich pyroclastic flows derived from the
collapse of an eruption column. The lowermost part of the eruption
column is called the gas thrust region. Here, the density of the
eruption column is greater than the density of the surrounding air.
The column continues to rise, however, because of the thrust
provided by the release and rapid expansion of volcanic gas.
Occasionally, the gas thrust region may become so chock-full of
debris that its high density cannot be supported by the thrust of
expanding gases. The column thus collapses downward under gravity
as a mass of vesiculating pumice that advances rapidly down the
flanks of the volcano.
Although both nuée ardentes and pumice flows are fluidized, pumice
flows are more energetic and mobile. This is partly attributed to their
lower densities, but also to their greater store of kinetic energy
generated by vertical drops up to several kilometers above the
volcano's summit. The further it falls, the greater its kinetic energy,
and the further and faster it will travel horizontally.
Pumice flows have a tripartite division. The main body hugs the
ground surface and is dominated by pumice fragments in an ash
matrix. Like nuée ardentes, this pyroclastic avalanche is overlain by
an ash cloud of elutriated fine ash. An additional component of a
pumice flow is the ground surge. These are forward-springing jets of
incandescent ash that occur in the advancing head of the flow. They
advance with a rolling and rapidly puffing movement which is thought
to be caused by the ingestion of air in the front of the flow. Air
ingestion produces strong fluidation in flow front, and explosive
heating of the air causes some of the material to be hurled forward as
a low-density, turbulent surge.
A single Plinian-type eruption may generate hundreds of pumice
flows which typically flow down valleys radiating outward from the
summit of the volcano. Individual flows may vary in length from a few
kilometers to tens of kilometers. These are miniscule, however, in
comparison to the massive pumice flows generated by caldera
collapse. Caldera-generated flows are not restricted to valleys, but
rather fill in valleys and adjacent low ridges to produce pumicedominated pyroclastic sheet flows that can obliterate an area the
size of Ohio in a few minutes. These huge eruptions can eject a
thousand cubic kilometers of material from ring fractures in just a few
hours. The last such eruption on earth took place at Toba, Indonesia,
about 74,000 years ago to deposit an ignimbrite with a volume of over
2000 cubic kilometers. Similar eruptions in the United States occurred
less than two million years ago at the Long Valley, Valles and
Yellowstone calderas.
LAHARS
GENERAL CHARACTERISTICS
Lahar is an Indonesian term for a volcanic mudflow. These lethal
mixtures of water and tephra have the consistency of wet concrete,
yet they can flow down the slopes of volcanoes or down river valleys
at rapid speeds, similar to fast-moving streams of water. These mud
slurries carry debris ranging in size from ash to lapilli, to boulders
more than 10 meters in diameter. Lahars can vary from hot to cold,
depending on their mode of genesis. The maximum temperature of a
lahar is 100 degrees Centigrade, the boiling temperature of water.
El Palmar lahar,
Guatemala (1989)
Mt. Pinatubo lahar,
Philippines (1991)
LAHAR GENERATION
Lahars are generated by a variety of mechanisms. The majority are
produced by intense rainfall during or after an eruption. A tragic
example of such an event was the 1991 eruption of Mt. Pinatubo in
the Philippines, which was contemporaneous with the arrival of a
major hurricane. An estimated 700 people died from buiral by the
ensuing lahars, together with the collapse of structures benearth the
wet ash. As demonstrated by the graph below, lahars can also be
generated directly from a volcanic eruption as massive amounts of
water are generated either by the rapid melting of ice and snow, or by
the disruption of crater lakes.
This graph
demonstrates the
number and type
of volcanic events
known to have
produced lahars. It
is based on a
USGS study of 108
historic eruptions
from around the
world. Modified
from USGS.
Pyroclastic flows are particularly efficient at generating lahars
because they have the capability to melt large quantities of snow and
ice in a just few hours. A tragic example of this mechanism occurred
in 1985 when pyroclastic flows erupted at the snow-covered summit
of the Nevado del Ruiz volcano in Columbia. In only a few hours, the
eruption generated a lahar that killed ~23,000 people in the village of
Armero and adjacent towns located several tens of kilometers
downslope from the summit. Lahars can also be generated by the
basal melting of glaciers by lava flows. Basal melting of glacial ice in
Iceland has produced largest historic lahars, in terms of discharge.
These water-rich, glacial outburst floods are called jokulhlaups.
Snow melt from
Eruption-generated
erupting
lahar
lava tiggers a lahar on
on Mt. St. Helens (1982) the Villarica volcano
(1984)
MT. RAINIER: The Future Site of a Catastrophic Lahar
The snow-covered peaks of the Cascade volcanoes in Washington,
Oregon, and northern California pose a clear threat to surrounding
towns and villages. Past events suggest that a catastrophic lahar
could lie in the future of Mt. Rainier, the largest of the Cascade
volcanoes.
The 4000 m high summit of Mt. Rainier
contains the largest system of alpine
glaciers in the Cascade Range. The
periodic melting of glacier ice from Mt.
Rainier has generated at least 50
major lahars over the past 10,000
years. The largest of these mudflow
deposits, one of the world's largest, is
the ~5700-year-old Osceola lahar,
shown in the adjacent map (courtesy
of USGS). The Osceola lahar travelled
down the White River, over 112 km
from its source. It then spread out at its mouth to cover an area of
over 300 square kilometers along the shoreline of Puget Sound. The
recent geologic history of Mt. Rainier demonstrates that a major
mudflow descends down the White River once every 600 years. The
younger 500-year-old Electron lahar (see map) was also generated
from Mt. Rainier. It flowed 56 kilometers down the Puyallup River to
within 15 kilometers of Tacoma, Washington. More than 300,000
people now live in the area covered by these extensive lahars! Unlike
floods, such catastropic mudflows can occur with little or no warning.
Some volcanologists have predicted that Mt. Rainier will be the site of
the next Cascade eruption. Therefore, the volcano is monitored
closely, with the hope that we can warn the local population before
the next lahar strikes.