Chapter 4; Igneous Rocks
>>> which magma is hotter, cooler, higher viscosity, lower viscosity??
(referring to 62-63)
cooling
how fast does magma cool? > depth (deeper is hotter, shallower is cooler), shape (spherical bodies cool slowly; tabular faster), groundwater (circulating water removes heat)
Dike
igneous body; geologist categorize intrusions by shape (tabular sheet: planar with uniform thickness) >>> cuts across rock layering inflate country rocks; thermally alter country rock
sill
igneous body; geologist categorize intrusions by shape (tabular sheet: planar with uniform thickness) injected parallels to rock layering (bedding)
pluton
igneous body; geologist categorize intrusions by shape balloon-shaped- blobs of melted rock >>>>>>large, irregular or blob shaped (tabular) intrusions
Laccolith
igneous body; geologist categorize intrusions by shape blister-shaped- a sill that domes upward mushroom shaped intrusion
Batholith
immense composite intrusion formed from aggregation of many plutons
xenolith
incorporation of wall rock fragments caused by magma invading colder rock wall
temperature
increases with depth (geothermal gradient)
Extrusive Igneous Rock
(melted rock can cool quickly above or below ground) cools quickly at surface extrusive settings: cool at or near surface (cool rapidly, chill too fast to grow big crystals) mostly mafic bc hot
viscosity
"thickness", or resistance to pouring >how thick is it?
volcano
(1) A vent from which melt from inside the Earth spews out onto the planet's surface; (2) a mountain formed by the accumulation of extrusive volcanic rock. (Example; Hawaii)
Crystalline Igneous Rocks
(Aphanitic, phaneritic, porphyritic) A rock that consists of minerals that grew when a melt solidified, and eventually interlock like pieces of a jigsaw puzzle.Indicates slow cooling, the larger the grain, the slower the cooling rate. Subcategorized into phaneritic (coarse-grained), aphanitic (fine-grained), and porphyritic (phaneritic surrounded by aphanitic).
Phaneritic
(Crystalline Igneous Rocks) crystalline texture: >course grained >slow cooling >crystals have a long time to grow >intrusive >can see grains with naked eye
Porphyritic
(Crystalline Igneous Rocks) crystalline texture >mixture of coarse and fine crystals >indicates a two-stage cooling history
Aphanitic
(Crystalline Igneous Rocks) crystalline texture: fine grained >rapid cooling >crystals do not have a long time to grow >extrusive >can't see with naked eye
if a body of magma becomes more felsic, what happens to it's viscosity?
(explosive) high viscosity
What does the addition of volatiles do to hot mantle rock?
(flux melting/wet melting) volatiles lower the melting temperature of a hot rock
Bowen's Reaction Series
(fractional crystallization: settling early formed crystals////felsic magma can evolve from mafic magma) experimental results of mineral growth in magmas; a mineral succession proceeds from cooling devised experiments cooling melts (1920s) >> with cooling, a mafic melt could become felsic >> discovered minerals solidify in specific series Bowen's Reaction Series The sequence in which different silicate minerals crystallize during the progressive cooling of a melt. Recognizes the difference between discontinuous and continuous series.
obsidian
(glassy classification) felici volcanic glass
pumice
(glassy classification) frothy felsic rock full of vesicles; it floats
scoria
(glassy classification) glassy, vesicular mafic rock
tabular (sheet)
(igneous bodies) geologists categorize intrusions by shape: planar with uniform thickness
Intrusive igneous rock
(melted rock can cool above or below ground) cool slowly underground cool @ depth >lose heat slowly >crystals often grow large mostly felsic bc cool
tuff
(pyroclastic) volcanic ash that has fallen on land
Fragmental igneous rock
(texture describes size, shape and arrangement of the minerals) pieces of preexisting rocks, often shared
Glassy igneous rocks
(texture describes size, shape and arrangement of the minerals) made of solid glass or glass shards
pegmatite
*exception* >exceptionally coarse mineral crystals >large crystals are not due to slow cooling
what is a pegmatite and what type of grains does it have?
*exception* >exceptionally coarse mineral crystals >large crystals are not due to slow cooling
what are the four major magma types and how are they different? (i.e. felsic to ultramafic)
1) Felsic (feldspar and silica) > very low temp (600-850 degrees C) >very low density >very high viscosity (explosive eruptions) 2) Intermediate >low temp >low density > high viscosity (explosive eruptions) 3) Mafic (Mg- and Fe- rich) >high temp >high density >low viscosity (thin, runny eruptions) 4) Ultramafic >very high temp (up 1300 degrees C) >high density >very low viscosity
Large Igneous Province (LIP)
A region in which huge volumes of lava and/or ash erupted over a relatively short interval of geologic time. Form when the bulbous head of a mantle plume first reaches the base of the lithosphere, where a large quantity of basaltic magma forms. The resulting lava has a low viscosity. Example: Ontong Java Oceanic Plateau.
Mafic magma
A term used in reference to magmas or igneous rocks that are relatively poor in silica and rich in iron and magnesium. (45-52% SiO2). Rocks tend to be black or dark gray. Examples: Basalt, Gabbro
ultramafic magma
A term used to describe igneous rocks or magmas that are rich in iron and magnesium and very poor in silica. (38-45% SiO2)
When are intrusive/extrusive igneous rocks formed?
Extrusive: (melted rock cooling quickly at surface) lava flows, pyroclastic debris Intrusive: (melted rock cooling slowly below the surface) large volume magma chambers, smaller volume tabular bodies or columns
What is the difference between lava and magma?
Magma is composed of molten rock and is stored in the Earth's crust. Lava is magma that reaches the surface of our planet through a volcano vent
Intermediate magma
Magma that has a composition between mafic and felsic. (52-66% SiO2). Rocks tend to be light gray or greenish gray.
Felsic magma
Magma that has a relatively high proportion of silica relative to magnesium and iron oxide (66-76% SiO2). Also called silicic magma. Rocks tend to be light tan, pink, or maroon. Examples: Rhyolite, granite
vesicle
Open holes in igneous rock formed by the preservation of bubbles in magma as the magma cools into solid rock
Igneous rock
Rock that forms when hot molten rock (magma or lava) cools and freezes solid (650-1100 C). It makes up all of the oceanic crust and much of the continental crust.
what is the difference between an aphanitic texture and a phaneritic one?
aphanitic: fine grained phaneritic: course grained
which magma/lava has the highest % of Fe and Mg? Lowest?
basaltic lava highest percentage of fe and mg
crystalline classification
based on composition and texture
volatile content
cause of melting (flux melting/wet melting) volatiles lower the melting T of a hot rock
heat transfer
cause of melting; rising magma carries mantle heat with it causing the raising of temperature in nearby crustal rock, which it then melts
what determines grain size of an igneous rock?
cooling rate of rock
stoping
detachment of large rock blocks caused by magma invading colder wall rock pieces of surrounding country rock fall into the magma (may melt and alter the material)
what is the difference between a dike and sill
dike: cuts across layering rocks sill: injected parallels to rock layering
Fractional crystalization
early crystals settle by gravity, melt composition changes as a result (remaining melt becomes enriched in Si, Al, Na, K) settling early formed crystals felsic magma can evolve from mafic magma
fractional crystalization
early crystals settle by gravity, melt composition changes as a result (remaining melt becomes enriched in Si, Al, Na, K) settling early formed crystals felsic magma can evolve from mafic magma
what kind of magma does partial melting create? (more mafic or more felsic, silica-rich? why?)
felsic magma produces a silica rich magma
how are the four types of major magmas different in terms of density, viscosity, temperature?
felsic, intermediate, mafic, ultramafic
How are igneous rocks formed in general?
forced by cooling from a melt
pyroclastic debris
fragments ejected during eruptions
pyroclastic rocks
fragments ejected during eruptions
what is a coarse-grained intrusive igneous rock similar in composition to a mafic basalt?
gabbro
what does a porphyritic texture tell us about history of cooling?
indicates two-stage cooling history (initial slow cooling creates large phenocrysts; subsequent eruption cools remaining magma more rapidly)
if a basalt underwent extremely slow cooling, as opposed to being extrusive, what would the resulting rock be?
intrusive
if a body of magma becomes more mafic, what happens to it's viscosity?
lower viscosity aka runny/thin
what kind of residue is left? (more mafic or felsic?)
mafic residue
as magma cools, Hi Temp rystals settle out of melt which changes remaining composition of the melt. (i.e. removes the Fe, Mg and increases the amount of Si, Al. Ca, Na and K). So, what kind of magma is generated with fractional crystallization? (i.e. what evolves to what?) Think about what the composition of a mafic magma vs composition of a felsic magma!
magma generated with fractional crystallization: felsic magma, can evolve to mafic magma Mafic lava, before cooling, has a low viscosity, in comparison to felsic lava, due to the lower silica content in mafic magma. Higher silica content in felsic aka high viscosity
Assimilation
magma melts the wall rock it passes through (a reason magmas differ chemically) mafic magmas from the mantle become intermediate by assimilation
assimilation
magma melts the wall rock it passes through (a reason magmas differ chemically) mafic magmas from the mantle become intermediate by assimilation
magma
melted rock below ground
Lava
melted rock once it has reached the surface
partial melting
occurs in the crust and upper mantle; caused by pressure release, volatile addition, heat transfer only portion of rock melts; si-rich minerals melt first then si-poor minerals melt last reason chemical makeup of magma changes
what is the difference between pluton and a batholith?
pluton: large, irregular or blob shaped intrusions batholith: immense composite intrusion formed from aggregation of many plutons
magma forms from partial melting of lower crust and upper mantle. Melting occurs due to what 3 things?
pressure release (decompression), volatile addition, heat transfer
Decompression
pressure release: cause of melting >>>> drop in pressure initiates "decompression melting" >>>> taking pressure off >>>> pressure drops when hot rock is carried to shallower depths (mantle plumes [hot spots], beneath rifts, beneath MORs)
As you remove the magma from the source material as melting occurs, what do you have and what is left?
reason magma chemical composition varies: (source rock dictates initial magma composition; mantle source: ultramafic/mafic magmas; crustal source: mafic, intermediate, felsic magmas) [continental crust: felsic/thicker; ceanic crust: mafic/thinner]
What were the sources of heat in early earth?
remnant heat >planetesimal and meteorite accretion >gravitational compression >iron differentiation >early collision with planetoid ongoing heat generation >decay of radioactive elements > magma doesn't stay put; it tends to rise upward
volcanic breccia
rock consisting of angular fragments cemented together.
SiO2
silica
silica content
silica rich: magma think and viscous silica poor: magma thin and runny
partial melting
silica-rich minerals melt at lower temp, so these melt first, then are removed only a portion of the rock melts reason magma chemical makeup differs
what are the main controls on magma viscosity and how does each of these affect the magma? 146
speed of magma governed by viscosity: > lower viscosity eases by movement > lower viscosity is generated by: ...... higher V ......... lower SiO2 content ......... higher volatile content Viscosity depends on temp, volatiles, silica >>>temp: hot- lower viscosity; cool- higher viscosity >>>volatile content: more volatiles: lower viscosity less volatiles: higher viscosity >>>silica (SiO2) content less of SiO2: mafic...lower viscosity more of SiO2: felsic...higher viscosity
Geothermal gradient
temperature increases with depth
texture
texture describes size, shape and arrangement of the minerals: interlocking (mineral crystals fit like jigsaw puzzle pieces) fragmental: pieces of preexisting rocks, often shared glassy: made of solid glass or glass shards directly reflects magma history; reveals cooling history
silica-rich magmas
thick and viscous
silica-poor magmas
thin and runny
volcanic ash
tuff
magma gas
variable amounts of dissolved gas occur in magma
