Lab 4: Magmatism and Igneous Rocks
rock vesicles:
bubbles frozen into a lava, tells that gas has been dissolved in magma.
example of an intermediate, intrusive rock
diorite
Felsite Porphyry
felsic, lighter/intermediate color, pophyritic and larger phenocrysts of k-feldspar and na-plagiocase
Pumice
felsic, lightweight, contains vesicles
low viscosity
flows quickly
Gabbro
mafic, phaneritic, has minerals; crystals of pyrocene and ca-rich plagiocase feldspar
Basalt Porphyry
mafic, porphyry, contains vesicles filled with phenocrysts of the mineral olivine
vesicles in intrusive igneous rock signifies:
magma was near surface before cooling
Stock
main tube through which magma exits the volcano
Pluton
small frozen magma chamber
The three major tectonic settings in which melting occurs
subduction zones, hot spots and mid-ocean ridges
example of a felsic, extrusive rock
rhyolite
When lava crystallizes it is known as:
extrusive igneous rock
Rhyolite
felsic, aphanitic and contains rock fragments
Graphic Granite
felsic, contains minerals plagiocase and quartz, looks striped (lines of quartz)
high viscosity
flows slowly
example of an mafic, intrusive rock
gabbro
Obsidian
glassy and conchoidal fracture
example of a felsic, intrusive rock
granite
Andesite Porphyry
has an intermediate color and porphyritic-phenocrysts of amphibole, variety hornblende
mafic lavas:
have higher melting temps and lower viscosity. they form broad shaped shield volcanoes
felsic lavas:
have lower melting temps, higher viscosity and form steep volcanic domes that are more likely to erupt explosively.
phaneritic texture
igneous rock texture in which minerals are easily visible without magnification
Diorite
intermediate color, phaneritic and minerals of amphibole (black) and light-colored/white plagiocase
When magma is erupted at the surface it is called:
lava
Intrusive igneous rock
when magma crystallizes before making it to the surface
Basalt
mafic, aphanitic
Porphorytic Texture
two or more distinct grain sizes, slow cooling forms large crystals called phenocrists, subsequent rapid cooling results in fine-grained crystals
Extrusive rocks cool and crystallize faster than intrusive rocks so:
very-fine grained or aphanitic texture
Where are mafic magmas formed?
Mid-ocean ridges
amorphous solid
A solid made up of particles that are not arranged in a regular pattern (glassy, obsidian)
What is the cause of melting at mid-ocean ridges and hot spots?
Decompression melting (where mantle material is upwelling).
Which minerals are first to melt in a rock?
Felsic minerals high in silica (those with the lowest melting temperature).
What happens to melting temperature when water is introduced?
It is lowered.
Is the temperature at which mantle peridotite melts higher in the upper or lower mantle?
Lower because the pressure is higher.
When a rock begins to crystallize, which minerals are the first to crystallize?
Mafic minerals high in iron and magnesium (those with the highest melting temperatures).
What is mantle rock also known as?
Peridotite
Sources of heat within Earth
Radioactive decay: heat produced by radioactive decay of elements like uranium and potassium, Primordial Heat: heat leftover from the formation of the planet
Where are felsic magmas formed?
Subduction zone volcanic arcs
Where are intermediate composition magmas formed?
Subduction zone volcanic arcs.
Why are pyroclastic flows so dangerous?
They move extremely fast and are extremely hot. Hot gases act as lubricant. Consist of lava, hot ash and gases.
Batholith
a large frozen magma chamber
Viscosity
a liquids resistance to flow
Caldera eruption
an explosive eruption resulting from the roof of a giant magma chamber collapsing in on its self (yellowstone)
example of an intermediate, extrusive rock
andesite
example of a mafic, extrusive rock
basalt
Scoria
mafic and aphanitic, contains vesicles
mafic rocks are:
more rich in iron and magnesium and are generally dark in color
felsic rocks are:
more rich in silica and are generally light in color
Most intrusive rocks are:
phaneritic because they cool slowly allowing large crystals to form
Fine-Grained Granite
phaneritic, light-colored and contains quartz, k-feldspar and mica
Coarse-Grained Granite
phaneritic, light-colored, contains quartz, k-feldspar, and mica.
Dike
planar intrusion of magma cross cutting surrounding rock layers
Sill
planar intrusion of magma parallel to surrounding rock layers
During fractional crystallization:
the liquid portion is always more silica rich than the solid portion.
Fractional crystallization:
the process by which minerals crystallize in a cooling magma chamber and are separate from the remaining magma by density.
What causes melting in subduction zones?
water is released from the down-going slab which creates water vapor and lowers the melting temperature of the overlying mantle.