IGNEOUS ROCKS AND VOLCANISM
Petrology is the study of rocks--All 3 families of rocks
have common occurrences including:
- a rock is an aggregate
of mineral(s) or mineral matter which occurs in massive deposits
- bedrock is a massive
unbroken solid rock directly overlain by soil in many locations
- an outcrop is a massive
unbroken solid rock deposit exposed at the Earth's surface
I. Igneous rock family
A. Definition of igneous rock
- rock which originates
from processes during the cooling of a molten rock body called magma
1. At a convergent plate boundary (subduction zone)--defines plates of the lithosphere which
converge
2. At a divergent plate boundary
(ridge or rift)--defines plates of the lithosphere which diverge
3. From an isolated magma plume (hot
spot) in mantle not associated with 1 or 2 above
click
here to see the origin of magmas
C. Assimilation
- incorporation of country
rock material into a magma causing the original composition of magma to
change
- an example is the mixing
of magma originating in the mantle or ocean basin crust with the
continental crust as the magma rises group
D. Intrusive and extrusive magmas and igneous rocks
- intrusive
refers to igneous rock formed below the Earth's surface or molten magma below the surface
- extrusive
refers to igneous rock formed below the Earth’s surface or molten
magma below the surface
E. Texture, mineral composition, and igneous rock names
- texture and composition are
very important in identifying igneous rocks by name
1. Texture
·
refers primarily to the grain size of mineral fragments in
the rock
·
depends primarily on cooling rate of magma
o
the faster a magma cools the finer or smaller the grain size
of the rock
o
the slower a magma cools the coarser or larger the grain
size of the rock
·
Kinds of textures
o
phaneritic (coarse
grained) texture---essentially all mineral fragments are visible to the naked
eye and essentially of the same size (example is granite)--phaneritic
rocks are intrusive
o
aphanitic (fine
grained) texture---essentially all mineral fragments are invisible to the naked
eye (example is basalt)--aphanitic rocks are
extrusive
o
glassy texture---mineral fragments are so small that the rock
looks like a glass (example is obsidian)--glassy rocks are extrusive
o
vesicular (glassy vesicular) texture---many holes (vesicles)
are formed from escaping gases resulting in a sponge like appearance of the
rock (examples are pumice and scoria)--vesicular rocks are extrusive
o
porphyritic
texture---more than one size mineral fragment group present in the rock caused
by more than one cooling rate for the magma---the minerals comprising the
larger(est) mineral fragment size are called phenocrysts and remaining size(s), the matrix- --(granite
porphyry is intrusive, while rhyolite porphyry and andesite porphyry are formed from an intrusive-extrusive
combination—more than one rate of cooling)
o
pegmatitic
texture---presence of super large mineral fragments and the rock is often
mistaken for granite porphyry (example is pegmatite)--pegmatitic
rocks are intrusive
o
pyroclastic
texture---containing many ejected rock fragments (example is tuff)--pyroclastic rocks are extrusive
·
click here for review and see pictures of textures
2. Mineral composition
·
igneous rocks are comprised of a combination of minerals
belonging to the silicate mineral class or group
o
describes an orderly sequence of mineral formation and
separation from a cooling magma and explains the specific mineral combination
or association in a rock
o
discontinuous series refers to the formation of different
minerals as a magma cools
o
continuous series refers to the formation of plagioclase
minerals with slightly different compositions at different magma temperatures
o
hydrothermal solutions--after almost all rock forming
minerals have formed (separated) from a cooling magma, hot watery (hydrothermal)
solutions remain with dissolved concentrations of precious metals which later
precipitate valuable ore deposits
o
click here to see Bowen's mineral
series
·
Magma composition
o
can change as cooling progresses and minerals separate---
also (as mentioned before) magma assimilation can cause a change in composition
o
gabbro/basalt,
diorite/andesite, and granite/rhyolite
are the 3 main categories of magma compositions
3. Igneous rock names
·
essential minerals
o
minerals used to classify rocks by name and include quartz,
orthoclase feldspar (K-feldspar), plagioclase feldspar, pyroxene (augite) and olivine
·
accessory minerals
o
minerals not used to classify rocks by name although on
occasions may be more abundant than essential minerals and include micas and
amphibole (hornblende)
·
felsic composition
rocks are light to pink-red in color and contain high concentrations of
feldspar and quartz; intermediate composition between felsic and mafic rocks are dark
gray in color , mafic rocks are black in color
and comprised of high concentrations of magnesium (Mg) and iron (Fe+3)
minerals; ultramafic composition rocks are
dark and green and contain high concentrations of iron (Fe+2) and magnesium
minerals such as olivine
·
igneous rock chart
o
phaneritic textured
rocks:---granite (essential minerals are orthoclase, white or sodic plagioclase and quartz);---diorite (essential mineral
is sodic plagioclase);---gabbro
(essential minerals are calcic plagioclase and
pyroxene);---dunite (essential mineral is olivine)
o
aphanitic textured
rocks:---rhyolite (essential minerals are same as
granite);---andesite (essential mineral same as
diorite);---basalt (essential minerals are same as gabbro
and it looks like a black chalkboard)
o
porphyritic textured
rocks:---granite porphyry (intrusive with essential minerals same as granite)
and rhyolite porphyry ( part intrusive and part
extrusive with same essential minerals as a rhyolite);---andesite porphyry (essential minerals same as andesite);---basalt porphyry (rarer rock with essential
minerals same as basalt)
o
glassy textured rocks:---obsidian (any essential mineral
composition combination--looks like glass)
o
vesicular textured rocks:---pumice (any essential mineral
combination as that for granite to diorite--looks like a sponge and some
samples may float in water);---scoria (mineral composition similar to gabbro or basalt)
o
pegmatitic textured
rocks:---pegmatite (any essential mineral composition as that for granite
and/or diorite)
o
pyroclastic textured rocks:---tuff
(any essential mineral combination as that for granite and/or diorite)
o
note that the igneous chart below which outlines the
previous rock compositions is related to Bowen's mineral series---those
minerals formed together at higher temperatures in Bowen's series include the
rocks in the right column(s) of the rock table---minerals forming at
intermediate temperatures comprise rocks in the center of table--and those at
lowest temperatures comprise rocks in the left column
o
click here to review igneous rocks
in the igneous rock chart
o
click here to see pictures of different igneous rocks
F. Igneous rock bodies
- 1.
Intrusive (plutons)
- a.
discordant---cuts across sedimentary rock contacts or boundaries
- batholith
- largest
of pluton bodies and irregular in shape
- often
ore deposits (gold, silver, copper, etc.) are associated with these
- click here to see batholiths
- stock
- also
irregular in shape but much smaller (a few square miles) than batholith
- smaller
concentrations of ore deposits may be associated with these bodies
- dike
- small
tabular shaped body
- b.
concordant--bodies which do not cut across sedimentary rock boundaries
- laccolith--large
lens shaped body
- sill--a
small tabular shaped body
- click
here to see and review pluton bodies discussed
above
- 2. Extrusive
- see
section on volcanism below
G. Important uses of igneous rocks
1. Used as building materials and
tombstones
2. Pumice is used as an abrasive
material in lava soap
II. Volcanism
(Volcanic activity)
·
Volcanology is the
study of volcanism
·
click here for a comprehensive reference of volcanic
terms and photo examples of volcanic features
·
click here
to see Volcano World
A. Some common terms
1. Volcanism
·
refers to the processes by which magma and/or gases or
volatiles are transferred to the Earth's surface
2. Volcano
·
is an elevated area or mountain formed from the accumulation
of lava and/or pyroclastic material
3. Active, dormant, and extinct
volcanoes or volcanic areas
·
active
o
eruption can occur in the near geologic future
o
an example is Mt. St. Helens and other Cascade Mountains
·
dormant
o
presently inactive but believed capable of future eruptions
·
extinct
o
expected not to erupt again
B. Origin and global distribution of
volcanism
1. Origin of volcanic materials
·
basically the same as for igneous activity (areas of subduction, ridges and magma plumes)
·
see page 101 again
·
"ring of fire" or the circum-Pacific region is the
greatest concentration of volcanic activity on Earth
·
some individual volcanoes listed on page 100 will be cited
later as specific kinds of eruptions--- there is an interesting historical
eruption mentioned by Plato who suggested that Santorin, a group of islands in
the Mediterranean Ocean was once a continent which exploded and thought to be
the "lost continent of Atlantis"
·
click here to see the world
distribution of volcanoes
C. Volcanic rocks, volatile (gases
and vapors) emissions and manner of eruption
1. Volcanic rocks
·
are extrusive rocks and include basalt, andesite,
rhyolite, pumice, scoria, tuff, and obsidian
·
refer again to figure 1
2. Volatile emissions
·
steam or water vapor (H2O) is most abundant, carbon dioxide
(CO2) is next and sulfur dioxide (SO2) nitrogen (N2), and sulfur trioxide (SO3)
are others typically associated with volcanic eruptions
·
carbon dioxide additions to the atmosphere can add to the
"Greenhouse Effect" while SO2, N2, and SO3 can contribute to the
formation of acid rain
·
although the burning or decomposition of fossil fuels
produces carbon dioxide and the other gases emitted into our atmosphere we are
able to control only the synthetic emissions
3. Manner of eruption
·
QUESTION: why are some volcanic eruptions explosive or
violent while others are passive?
·
ANSWER: the manner of eruption depends largely on the
viscosity of the magma associated with the eruption --gases cannot escape as
readily from a more viscous (syrup or oil like) magma causing a gas pressure
buildup which eventually explodes
·
magma viscosity may depend on 2 factors
o
magma composition--the higher the silica (SiO2) content, the
more viscous the magma
o
magma temperature--the cooler the temperature, the more
viscous the magma
·
QUESTION:based on
composition alone a more explosive eruption would be associated with which
magma composition; rhyolitic, andesitic, or basaltic?
(HINT: which extrusive rock has the most quartz in it?). For the same reason
which associated magma composition would result in a passive eruption?
·
QUESTION: two magmas start out in the mantle with the same
composition, one magma moves through the ocean basin crust (basalt) and erupts on the ocean basin surface, while the other moves
through the continental crust and erupts on the continental surface. If there
is assimilation of rock in each magma, based on
composition at the time of eruption which magma would be more explosive?
D. Major categories of volcanic
deposits (Classification of volcanoes)
·
volcanic deposits are comprised of lava flows and/or pyroclastics
·
is a thick deposit of successive lava flows usually basaltic
in composition with little or no pyroclastics and
associated with ocean basin eruptions—source of magma to form islands and
rocks is a hot spot of magma located in the stationary upper asthenosphere
·
the Hawaiian island shield is comprised of basalt which
averages about 28,000 feet thick with the individual islands sticking above sea
level like pimples on the shield deposit— each island is progressively
older with distance from the hot spot source of magma
·
click here to see the shield
volcano
·
small deposit (volcanic cone) of pyroclastic
or ejected materials with steep slopes and usually less than 1000 feet tall
·
usually represents the last stage (cooler) of basaltic
eruption
·
an example is the eruption at Paricutin, Mexico (1943) which
accumulated pyroclastics about 120 feet tall in one
day
·
click here to see the cinder cone
volcano
·
click here to see the cinder
cone(s) in Arizona
3. Composite
or strato-volcano
·
is a very tall somewhat layered deposit of alternating lava
flow and pyroclastic deposits comprised of andesite or rhyolite composition
rocks
·
occur on continents and are usually explosive in manner
·
examples include: Mountains in the Cascade mountain range in
the western U.S. and includes Mt. St. Helens; Mt. Vesuvius, Italy; Mt.Fujiyama, Japan; Mt.Kilamanjaro,
Africa—often a volcanic dome
or solidified felsic or intermediate compostion magma can be piled up around a vent in Composite
volcanoes
·
click here to see the composite
volcano
·
click here to see more on volcanic types and manner of
eruption
E. Eruptions with special volatile
emissions
1. Strombolian
type
·
is a cloud of volatiles emitted during the eruption and may
hover for long periods of time over a volcanic cone--the molten material in the
volcanic vent reflects light off the cloud-- the prime example of this is the
incandescent cloud which hovers over Mt. Stromboli and acted as a navigation
guide for early mariners--Mt. Stromboli was known as the "lighthouse of
the Mediterranean"
2. Nuee Ardentes (fiery or glowing cloud—pyroclastic
flow)
type
·
is an emission
of a very hot cloud of volatiles and ash materials during eruptions--- the
prime example of this is an eruption of Mt. Pelee
near St. Pierre on the island of Martinique in the Caribbean. A 700 degree (C)
cloud killed about 30,000 people in a few minutes ---see the photo on page 152
in your text book
3. Lake Nios
type
·
carbon dioxide (CO2) trapped in subsurface rocks from
previous volcanic activity can be later emitted at the Earth's surface with
possibly disastrous results---the best example of this is Lake Nios, Cameroon (Africa) where a total of 1746 villagers
were killed in 1986 from an emission of CO2 from a caldera (collapsed volcanic
cone--see later notes) on which they were living
F. Special features, benefits and
mass wasting associated with volcanism
1. Special features
o
click here to see continental
lava flows
·
Ocean basin lava flows
o
mostly associated with the shield type of deposit
o
pahoehoe is a ropy
surface lava
o
aa is a blocky
surface lava
o
lava tubes or tunnels develop when lava inside the lava flow
continues to move while the outer lava stiffens and stays in place
·
Volcanic neck and radiating dikes
o
see photo on page 131 in the text book--one of the best
examples of a volcanic neck and radiating dikes is Shiprock
in New Mexico
o
is a volcanic cratered structure resulting from the collapse
of a volcanic cone
o
click here to see the stages in caldera
formation
o
best terrestrial examples are Crater Lake, Oregon and
Yellowstone and the best extraterrestrial example is Olympus Mons on Mars which
is 5 times the diameter of Crater Lake and covers over 45 times the area
o
click link on comprehensive reference on volcanic terms and
examples at the beginning of volcanism above or at the end of this section to
see pictures of different calderas
2. Benefits related to volcanism and
igneous activity
·
fertile soils result from weathering of many volcanic rocks
·
magma in contact with or in the vicinity of groundwater
produces steam which in turn can be converted to a clean source of energy
(geothermal energy)
·
water related to igneous activity and trapped below surface
of extraterrestrial bodies could be mined--this could influence colonization of
other celestial bodies in the future
3. Mass
wasting related with volcanism
·
severe damage can result from mudflows on the sides of
volcanoes--these mudflows are called lahars--lahars result when highly unstable
layers of volcanic ash and debris become saturated with water from heavy
precipitation and flow down slope
·
click here to see a house buried
in a lahar
G. Prediction of volcanic eruptions
Omens or precursor events related to
eruptions
·
the periodicity and magnitude of Earth tremors caused by
movement of rock materials below the surface can be used as precursors to a
volcanic event --the closer the tremors in time and the greater the magnitude,
the sooner the volcanic eruption
·
bulging of the Earth's surface can be used as an eruption
predictor: as magma rises prior to eruption the Earth's surface bulges
o
tiltmeters are used to
measure the magnitude of the surface bulge--the magnitude may be an indicator
of the eruption
o
laser beams reflected off mirrors can also determine the
magnitude of bulge
click
here for more on volcano monitoring techniques
H. Volcanism in our solar system
1. On our
moon
·
although there is no official citing of volcanism on the
moon the rocks collected show evidence of large scale igneous and volcanic
activities. Large abundances of gabbroic and basaltic type of rocks are present
on the moon
·
the lunar maria contain massive
lava flows
2. Mars
·
the presence of a large caldera, Olympus Mons (as mentioned
before), 17 miles high and covering the size of Arizona is a feature indicating
volcanic activity on Mars
3. Io, a moon of Jupiter
·
the only place other than on Earth where volcanism was seen
taking place---a scientist observed a volcanic plume on the horizon of Io as
the pioneer spacecraft passed by this moon