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IGNEOUS ROCKS
Igneous Rocks: Form from the cooling and solidification of molten rock material either on or below Earth's
surface.
I.
Liquid Rock
A. Magma:
1.
Magma vs. Lava
2.
a)
___________________ is the term used for all hot and liquid (_____________) rock.
b)
___________________ is generally used for magma that flows on Earth’s surface.
Where does magma form?
a)
b)
‘
3.
Magma forms in Earth’s ________________ and ___________________ .
Lava does not originate in Earth’s core.
Crystallization of minerals from a magma occurs between 1200o C and 600o C.
Sources of Heat for Melting
a.
____________________________ : Heat upward (by conduction and convection) from the very hot
(>5000/C) core through the mantle and crust.
b.
: Rate of temperature increase with increasing
depth.
i. 3o C /100 m (30o C/km)
ii. At great depth temperature alone would melt rock BUT high pressure may cause it to remain
solid.
iii.
Not the same everywhere (i.e., It’s higher in volcanic regions).
.
c.
i. Heat byproduct during decay.
ii. High concentration may cause temperature to increase with depth at a rate
geothermal gradient.
greater than the
d.
i. Rock grinding past rock
ii. Active Mountain building regions. Friction of moving and shifting rock masses in regions of
mountain building may combine with heat from other sources to melt rock.
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Igneous Rocks and Volcanism
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B. Factors That Control Melting Temperature
1.
a)
b)
Increased pressure increases the melting point.
Reduction in pressure may cause melting.
2.
Water under pressure (water vapor entrapped in magma) lowers the melting point.
3.
Different minerals have various melting temperatures. Some combinations of minerals may lower the overall
melting temperature.
C. Types of Magma (Chemistry of Igneous Rocks)
Approximately 99 percent of the total bulk of most igneous rocks is made up of only eight elements (oxygen, silicon,
aluminum, iron, calcium, sodium, potassium, and magnesium). Most of these elements enter into the crystal structure
of rock-forming silicate minerals and form feldspars, olivines, pyroxenes (ie. augite), amphiboles (hornblende), quartz,
and mica. These six minerals constitute over 95 percent of the volume of all common igneous rocks and are of great
importance in the study of their classification and origin.
1.
____________________________magmas (also referred to as sialic)
a.
b.
c.
2.
_____________ magmas
a.
b.
c.
3.
dark
Chemical content between that of felsic and mafic rocks
Color is often ______________ or _______________ gray.
____________________ magmas
a.
b.
c.
d.
II
Rich in iron magnesium, and calcium
Produce greater quantities of olivine, pyroxene, amphibole, and calcium plagioclase
They are generally _______________ colored because of the abundant
______________________________ minerals.
______________________ magmas
a.
b.
4.
Rich in silica and aluminum
They produce more quartz, potassium (orthoclase) feldspar, and sodium plagioclase.
The are generally _____________ colored and are usually intrusive)
Composed entirely or almost entirely of _________________________ minerals.
No _________________ or _____________________ are present.
Volcanic ultramafic rocks are very rare. They formed during Earth’s early history (Hadean) when
Earth’s internal temperatures were high enough for them to form.
Most come from the _____________ rather than the ____________.
Identification of Igneous Rocks
: This refers to the size, shape, and boundary relations between adjacent minerals in
A.
a rock mass.
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Igneous Rocks and Volcanism
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Textures of igneous rocks develop primarily in response to the magmas rate of cooling and its viscosity.
a
Generally, the
the cooling of the magma, the larger the grain size. Atoms in a very
viscous lava cannot move as freely as those in a very fluid lava.
b)
As a result, more viscous lavas are more likely to have larger grain sizes.
1.
2.
(PLUTONIC) IGNEOUS ROCKS:
Magmas deep within Earth's crust cool very slowly. Individual crystals are more or less uniform in size and
may grow to 2 centimeters or more in diameter.
a)
GRAINED: (phaneritic):
Individual minerals are large enough to be plainly visible to the naked eye (1 mm or larger).
b)
3.
COARSE GRAINED: Pegmatites consist of very large crystals, measured
in centimeters or even meters. Usually granitic.
(VOLCANIC) IGNEOUS ROCKS: A magma extruded out onto Earth's surface
cools rapidly and the crystals have only a short time to grow. Crystals from such a magma are typical so
small that they can rarely be seen without the aid of a microscope. The rock appears massive and
structureless.
a)
GRAINED (aphanitic): Cannot be seen without a microscope. While they appear
massive and structureless, they do contain small interlocking crystals.
b)
: Extremely rapid cooling (such as lava flow into the ocean or a lake) s t o p s
crystal growth. This supercooled solution has no time for minerals crystals to form. The appearance is like
that of ordinary glass and may occur in massive units or in a thread-like mesh similar to spun glass.
4.
TEXTURE:
Larger crystals (called phenocrysts) are surrounded by smaller crystals (the groundmass). An
igneous rock with a porphyritic texture (also referred to as a porphyry) appears rather like raisin bread. This
texture usually is a result of two stages of solidification. Slow cooling takes place while the magma is
underground and the minerals that form at higher temperatures crystallize and grow to form the phenocrysts
in the partially molten magma. When the entire mass is erupted, the remaining liquid portion cools rapidly
at Earth's surface forming the fine grained matrix (groundmass).
B. COLOR
a)
Color is based upon the mineral composition of the rock.
Generally the lighter colored igneous rocks are
are
.
1. Light Colors: White, tan, gray, pink, red
2. Intermediate (Medium) Colors: darker gray, green
3. Dark Colors: dark green, dark gray, black
b)
and the darker igneous rocks
Colors result from Magmas of Different Composition
1. __________________________________________________________________
•
•
ESC 101: Professor Vorw ald
The process by which different ingredients separate from an originally homogeneous
mixture.
_________________ Reaction Series (Refer to your textbook, pages 71 and 72)
Igneous Rocks and Volcanism
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2. __________________________
•
As a rock melts, the first portion of the rock melts.
•
Liquid with the chemical composition of quartz and potassium feldspar is “sweated out.”
•
It can accumulate into a pocket of felsic magma.
•
Mafic rock can also sweat out at higher temperatures.
3.
•
•
•
•
•
.
When only the original basaltic magma cools slowly.
The earliest formed minerals physically separate.
Being denser, these minerals settle to the bottom of the magma
The remaining liquid is now an intermediate magma.
Some of the intermediate magma may move upward to a smaller magma chamber that feeds
a volcano.
4.
•
•
•
•
Very hot magma comes into contact with surrounding older rock (called country rock”).
Some of the country rock melts and becomes part of the newly molten material.
If basalt from the mantle comes into contact with granite country rock it will become richer
in silica.
This may be how intermediate magmas form (andesite and diorite).
5. ___________________________
•
Two magmas meet and merge in the crust
•
The combined magma will be compositionally intermediate.
III Tectonic Settings Where Volcanism Occurs
A.
B.
Spreading Center Volcanism
1.
Oceanic
a)
Most coincide with mid-ocean ridges (crests of submarine mountain ranges)
b)
2.5 square kilometers of new sea floor per year is formed
c)
Estimated 20 submarine eruptions per year (not observed)
2.
Continental
a)
Tensional cracks develop creating a rift valley. These cracks extend down to magma
chambers that result from partial melting of the asthenosphere below.
b)
Magma erupts onto the floor of the rift valley.
Subduction Zone Volcanism
1.
Ocean-Continental Convergence
a)
Intermediate and silicic magmas are generated.
i. Melting occurs at the depth where the subducting oceanic plate is sliding under the
asthenosphere (approx. 100 km depth)
ii. Water released from the subducted crust lowers the melting point and initiates melting.
iii Ultramafic rock subducts, partially melts, and evolves into intermediate magma by
differentiation, assimilation of felsic crust, and by magma mixing.
2.
Oceanic-Oceanic Convergence
a)
Produce island arcs
b)
Includes Alaska’s Aleutian Islands and the Japanese Islands.
3.
Subduction Volcanoes include:
a)
The region of volcanoes that surrounds the Pacific Basin called the Pacific Ring of Fire
b)
Mediterranean volcanoes of Greece and Italy
c)
Cascade Mountains of the U.S. Pacific Northwest.
4.
There are on the average of 40 eruptions per year.
5.
400 of approximately 500 active volcanoes are subduction related.
6.
Subduction volcanoes differ from rift volcanoes.
a)
Form island arcs and high mountain chains rather than submarine ridges.
b)
Are more explosive and produce large volumes of ash and lava flows.
c)
Have more variable products
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Igneous Rocks and Volcanism
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C.
Intraplate Igneous Activity
1.
Volcanic activity at plate interiors and not associated with plate boundaries
2.
Hot spots above stationary mantle plumes
3.
Hawaiian volcanism
4.
Yellowstone National Park
5.
Columbia plateau flood basalts in Washington and Oregon.
Intrusive Activity and Igneous Rocks
IV
%
%
Country Rock: Any older, preexisting rock into which an igneous body has intruded.
Xenoliths:
Fragments of country rock often break off and are enclosed in the intrusion.
.
Chill Zone:
Finer-grained rocks adjacent to contacts with country rock are often
evidence magma solidified more quickly due to the rapid loss of heat to
cooler rock.
%
A.
Shallow Intrusive Structures
Solidified near Earth’s surface (most likely at depths less than 2 km).
Associated with subsurface volcanic activity
Are likely to be fine-grained because of shallow locations and relatively cool country rock into which they
intrude.
4. Smaller than bodies formed at great depths (also a reason for rapid cooling rates)
5. Types of Shallow Intrusive Structures
1.
2.
3.
a) ____________________
i
A tabular shaped structure
ii Cuts across older country rock (discordant)
b) ____________________
i
Tabular intrusive structure that is concordant (parallel to any planes or layers of older country
rock).
ii Originates as magma that has been squeezed in between older layers solidifies.
c) ______________________________
i
ii
Form from magma that solidifies in the pipe of a volcano
Shiprock, New Mexico is an example.
d) _____________________
i
Similar to a sill but the central portion is thick and domed upward.
ii “Mushroom-like” appearance
iii Not common
B Intrusions That Crystalize at Great Depths
1.
___________________
a) A body of magma or igneous rock that crystallizes at considerable depth within Earth’s crust.
b) Unlike dikes and sills, most plutons have no particular shape.
2.
Types of Plutons
a) ________________________
i
A large discordant pluton
ii Outcrop area of greater than 100 square km (most are larger)
iii Most are predominantly granitic although they often contain mafic and intermediate rocks
iv They are formed many coalesced plutons. These large blobs of magma, called diapirs, that melt
their way upward and collect 5 to 50 km below the surface.
ESC 101: Professor Vorw ald
Igneous Rocks and Volcanism
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b)
i
ii
V.
Small discordant pluton
Outcrop area less than 100 km
VOLCANIC PRODUCTS
•
•
A volcano is a vent of fissure through which solid, liquid (molten), and gaseous material pass upward to Earth’s
surface.
Common terminology indicates some kind of mountain or dome being formed but volcanic plateaus as well
mountains can form.
A. _________________________
1. Water Vapor
a) Most of the gases released during eruptions (50 % to 95 %)
b) Condenses as steam
2. Other gases include: carbon dioxide, sulfur dioxide, hydrogen sulfide (rotten egg smell) and hydrochloric
acid
B. ________________________ (Lava)
1.
Lava (pah-hoy-hoy)
a) Hawaiian name for basaltic lava that cools quickly and was fully liquid
b) Cools with a smooth ropy or billowy surface
2.
a)
b)
c)
d)
3.
(ah-ah) Lava
Hawaiian name for basaltic lava cool enough to have partially solidified
Moves as a slow, pasty mass
Solidified front of flow moves as a pile of rubble
Has a rough and jagged surface
Lava (Pillow structures)
Basaltic lava flows under water
Occur as pillow-shaped rounded masses that are closely fitted together
Blobs of lava are squeezed out of a thin skin of solid basalt over a lava flow.
The surface of the blob quickly chills and becomes solid while a new blob forms as more lava breaks
out.
e) Most pillow basalt forms at mid-ocean ridge crests although they can form in lakes and rivers.
a)
b)
c)
d)
4.
Types of Magma and Lava Affect the Type of Volcanic Eruption
a) The violence of an eruption depends on two components; __________ and ____________.
b) The concentration of silica determines the viscosity of the molten rock.
c) Water in magmatic solution provides the explosive potential of steam.
d) Types of Eruptions
I. Quiet Runny Lava
A. Low silica
B.
Low water content
II. Fire Fountains
A. Low silica
B.
High water (expanding steam bubbles surge upwards through running magma)
C.
Can be 100's of meters high
III. Gas Explosions
A. High silica
B.
High water
C.
Viscosity prevents steam from bubbling to the surface
D. Overlying pressure suddenly decreases (earthquake, landslide), trapped gas explodes
IV. Lava Domes
• High silica
• Low water
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Igneous Rocks and Volcanism
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A.
B.
C.
D.
E.
F.
G.
H.
Results in a thick and pasty lava
Steep-sided, dome or spine-shaped masses of volcanic rocks.
Formed from viscous lava that solidifies in or immediately above a volcanic vent. The lava
doesn’t flow because it’s so thick and pasty.
Often form in the craters of composite volcanoes
Built up in the crater of Mt. St. Helen’s after its catastrophic eruption in May 1980. New
domes have formed
In 2005 seven domes formed as a result of 70 milloion meters of lava being extruded.
Lassen Peak is a lava dome
Some of the most destructive volcanic explosions have been associated with lava domes.
• The dome keeps gases from escaping.
• If the plug is removed, gases and magma suddenly and violently escape usually as a
pyroclastic flow
C. Solids
VI
1.
2.
(also called tephra or ejecta) are solid fragments formed by volcanic explosions.
Pyroclasts are classified by ______________.
a) Dust:
Smaller than 1/8 m
b) Ash:
1/8 to 2 mm
c) Cinders: 2 to 64 mm
d) Blocks: greater than 64 mm and are solid
e) Bombs: greater than 64 mm and is plastic
I. Form as a molten blobs are ejected and become streamlined as they fall and solidify
II. Spindle- or lens-shaped
3.
Rocks Formed from Pyroclastic Material
a) Tuff:
Formed from fine-grained pyroclastic particles (dust and ash)
b) Breccia: Includes larger pieces of volcanic rock
4.
Pyroclastic Flows Combined with Gases
a) A mixture of pyroclastic debris and gas
b) Fluidizes the debris so that it rapidly flows close to the ground
c) Two Types of Origin
I. Exploding froth of gas and magma blasts out from a solid or viscous plug
II. Vertically blasted gas and pyroclastic debris collapses due to gravity
A. Very dense and hot
B.
Travels over 100 km/hr
III. Called a glowing avalanche or nuée ardente
5.
Lahars
a) Pyroclastic flows combined with water
b) Form from heavy rains wash down loose ash from eruptions or more frequently from an eruption that
ejects a crater lake or melts snow and ice on a volcanoes high summit
c) Mudflows are formed as the floods of water surge down the flanks of the volcano
d) Larger rocks a boulders can be picked up as the entire mass crashes down the valleys like a torrent of
wet concrete.
VOLCANIC LANDFORMS
A.
: Opening through which an eruption takes place
B. The
typical of volcanoes originates from volcanic material deposited and ejected
around a central vent
C.
: Basin-like depression over a vent at the summit of a volcano
D.
: A volcanic depression much larger than the original crater with a diameter of at least
one km.
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Igneous Rocks and Volcanism
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E.
Eruption: When lava flows from a vent on the side of a volcano instead of from a central vent.
F. Types of Volcanoes: (Differ in size, shape, and usually composition)
1.
a)
b)
c)
d)
2.
a)
b)
c)
d)
e)
f)
g)
3.
a)
b)
c)
d)
e)
f)
Volcanoes
Also called an Hawaiian Volcano
Gentle slopes (between 2 and 10 degrees)
Low viscosity lava flows from relatively nonviolent eruptions.
Layers of solidified lava flows.
Volcanoes
Also called stratovolcano
Alternating layers of solidified lava and pyroclastic material
Slopes less than 33o (steep cones built by pyroclastics but later lava flows flatten them)
Build up over long periods of time and can become very large
Mostly intermediate composition magma
Found along the “Ring of Fire” (circum-Pacific belt) and the Mediterranean belt
Cones
Steep slopes (maximum of 33o )
Composed of pyroclastic materials ejected from a central vent
Smallest of the three types (up to 500 meters in height)
Associated with mafic or intermediate lava
Frequently found on the flanks and in the calderas of Hawaii’s shield volcanoes
Unconsolidated pyroclasts are easily eroded.
G. Lava Floods
1. Massive outpourings of lava that leaves behind flood basalt (plateau basalt).
a) Mafic composition (very running with consistency of liquid water)
b) Created rock formations 1,000's of meters thick (each individual flood adds a layer between 15 to 100
meters in thickness)
c) Columnar Structure (Jointing)
I. Basalt contracts after solidifying
II. Cooling rock can’t pull in the edges and tension fractures the rock into an hexagonal pattern.
2. Within the last 200 million years there have been at least six major events
a) Deccan Traps along the western coast of India
I. 66 million years ago (coincides with extinction of the dinosaurs)
II. 500,000 cubic miles of lava covered a third of the peninsula
III. Called Deccan Traps because the hundred or so individual flows exposed by erosion resemble giant
steps
IV. Decaan is from the Sanskrit for “southern” and trap is Dutch for “staircase.”
b) The last lava flood last episode:
I. Ended about 15 million years ago
II. Created the Columbia Plateau of Washington, Idaho, and Oregon.
VII. Volcanic Hazards
A. Fatalities
1. Increased during recent centuries
2. Not caused by increased volcanic activity
3. Result of increasing population and more people living near volcanoes
4. Most fatalities have been caused by seven major eruptions
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Igneous Rocks and Volcanism
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B. Hazards
1. Pyroclastic flows
a) Hot gas laden with pyroclastic debris
b) High temperatures and suffocating ash cause fatalities.
c) 30% of volcanic fatalities
2. Lahars
a) Volcanic mudflows
b) Created when pyroclastic material mixes with snow melt or heavy rainfall.
c) 15% of volcano-related fatalities
3. Release of large amounts of gases
a) Include carbon dioxide and sulfur dioxide.
b) Lake Nyos, Cameroon in western Africa (August 21, 2986)
I. A crater lake
II. Carbon dioxide seeped into the lake.
III. It dissolved into the cold bottom water, saturating the water.
IV. A large cloud was released and the heavier than air carbon dioxide flowed down the slopes.
V. About 17,000 people suffocated and at least 3,500 cattle died.
4. Famine
a) Indirect cause
b) Accounts for 23% of fatalities
c) Widespread destruction of crops and farm animals.
5. Lightning
a) Volcanic lightning is generated by tiny ash particles ejected from the volcano.
b) Believed to be caused by friction that generates and electrical charge.
VIII. Monitoring Volcanoes
A.
“Age” of a volcano
1. Active
a) Currently erupting
b) Has erupted recently
2. Dormant
a) Has not erupted in thousands of years.
b) Has erupted within recorded history
c) Is expected to erupt at some point in the future.
3. Extinct
a) Have not erupted within recorded history
b) Not expected to erupt again
B.
United Geologic Survey Monitoring
1. Hazard Mapping
a) Study of past eruptions
b) Provides information on the frequency of eruptions and typical size and type of eruption
c) Allow nearby populations to create evacuation plans.
2. Monitoring
a) Seismicity
b) Gas emissions
c) Tiltmeters
d) Satellite monitoring of ground surface.
3. Alerts - Issued to populations and aviation
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Igneous Rocks and Volcanism
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IX. Landmark Volcanic Eruptions
A. Nevado del Ruiz Volcano (1985)
1. Columbia, Central America
2. Mudflows caused by hot pyroclastic flows killed about 1000 people
3. The town of Armero, located at the end of the Lagunillas Valley in which mudflows moved at 40 to 60
km/hour, was swept away.
4. The mudflows spilled out at about 30 meters high and spread out to dense floods 3 to 5 meters deep.
5. A one meter deposit of dried mud was left covering and area of about 40 km2 (15 mi2 ).
B. El Chichón Volcano (1982)
1. Southern Mexico
2. Ash and gas clouds rose to heights of 20 to 25 km.
3. Pyroclastic flows killed 2,000 to 3,000 people.
C. Mount St. Helens (1980)
1. Washington State, USA
2. Stream Explosion
D. Mt Pelèe (1902)
1. West Indies island of Martinique in the Caribbean Sea
2. Port City of St. Pierre was destroyed by pyroclastic flows after a period of dome growth.
3. The pyroclastic flows with temperatures of about 700o C (approx 1,290o F) burned or suffocated 28,000
people.
4. There were only four survivors (one a condemned prisoner in a poorly ventilated dungeon).
E. Krakatoa (1883)
1. Island in western Pacific
2. Composed of three volcanoes which blew apart the island (1/3 remained after the eruption)
3. 13 km3 (estimated) of rock collapsed into the emptied magma chamber creating a 300 m deep underwater
depression
4. Tsunamis were generated that killed over 34,000 people around the Pacific basin.
F. Mount Vesuvius (A.D. 79)
1. Near Naples, Italy
2. The Roman city of Pompeii and at least four other towns were destroyed.
3. Plinian explosion (named for the naturalist Pliny the Elder who was killed in the eruption)
a) extremely gas rich, viscous magma exploded deep within the volcano.
b) The vent acted like a gun barrel with a vertical blast twice the speed of sound. An ash column extended
an altitude of more than 30 km (20 miles).
c) Pompeii was buried under 5 to 8 meters (up to 26 feet) of ash
G. Crater Lake (6,600 years ago)
1. Oregon, USA
2. Original volcano (Mt. Mazama) collapsed creating a large caldera that has subsequently filled with fresh
water from melting snow
3. Volcanic debris from the eruption covered more than one million km2 in nearby states.
H. Iceland’s Heimaey Island (Jan. 23 to July 3, 1973)
1. Lava and ash erupted from fissures
2. 0.5 million m3 of ash blown onto the town
3. When the fissures closed the eruption changed to a lava flow that headed towards the harbor. Most of the
5000 inhabitants were evacuated by fishing boat
4. Seawater was sprayed onto the advancing lava
5. Some solidified, saving the harbor
6. Half the town was crushed
7. Area was expanded from 11. km2 (4.3 mi2 ) to 13.44 km2 (5.19 mi2 )
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