Igneous Rocks Flashcards
What is an igneous rock?
A rock that is formed from the cooling and crystallising of magma or lava
What is an intrusive igneous rock?
A rock that forms from magma that has cooled within the earth’s crust
What is an extrusive igneous rock?
A rock that forms from magma that has reached the earth’s surface and has cooled from lava
What are the 4 categories of crystal size and what sizes are the crystals?
Glassy - no present minerals
Fine (aphanitic) - <1mm crystals
Medium - 1-5mm. Seen with naked eye
Coarse (phaneritic) - >5mm. Easily seen with naked eye
What are phenocrysts?
Large crystals surrounded by smaller matrix
How are phenocrysts formed?
Two stage cooling. First cools slower.
Porphyritic textures
What is felsic?
A.k.a silicic
Felsic –> feldspar + silica
What is the most common mineral on earth?
Feldspar
What can be said about the appearance of quartz and feldspar in igneous rocks?
Only present in silicic and intermediate rocks
What is mafic?
Contain magnesium and iron
Dark and dense
Mafic —> Magnesium + ferric (iron)
What can be said about the appearance of hornblende in igneous rocks?
Mostly found in intermediate rocks.
Often confused with augite (a.k.a hard to identify)
What can be said about the appearance of augite in igneous rocks?
Type of pyroxene
Found in mafic and ultramafic rocks
What can be said about the appearance of olivine in igneous rocks?
Sometimes in mafic, definitely in ultramafic
What is silica?
SiO²
Often described quartz or sand grains.
% silica in a rock includes the quartz (SiO²) and any other silicon and oxygen present
What is the silica content required for a rock to be identified as silicic/felsic?
> 66%
What is the silica content required for a rock to be identified as intermediate?
52-66%
What is the silica content required for a rock to be identified as mafic?
45-52%
What is the silica content required for a rock to be identified as ultramafic?
<45%
How does the silica percentage relate to colour?
The more silica, the lighter the colour
(Except obsidian)
How does the silica percentage relate to colour?
The more silica, the lighter the colour
(Except obsidian)
In the igneous rock classification table, what rock can be found in ‘silicic’ ‘glassy’?
Obsidian
In the igneous rock classification table, what rock can be found in ‘silicic’ ‘fine’?
Rhyolite
Pumice
In the igneous rock classification table, what rock can be found in ‘silicic’ ‘medium’?
Microgranite
In the igneous rock classification table, what rock can be found in ‘silicic’ ‘coarse’?
Granite
In the igneous rock classification table, what rock can be found in ‘intermediate’ ‘fine’?
Andesite
In the igneous rock classification table, what rock can be found in ‘intermediate’ ‘medium’?
Microdiorite
In the igneous rock classification table, what rock can be found in ‘intermediate’ ‘coarse’?
Diorite
In the igneous rock classification table, what rock can be found in ‘mafic’ ‘glassy’?
Scoria
In the igneous rock classification table, what rock can be found in ‘mafic’ ‘fine’?
Basalt
In the igneous rock classification table, what rock can be found in ‘mafic’ ‘medium?
Dolerite
In the igneous rock classification table, what rock can be found in ‘mafic’ ‘coarse’?
Gabbro
In the igneous rock classification table, what rock can be found in ‘ultramafic’ ‘coarse’?
Peridotite
Which crystal sizes (G,F,M,C) can be found in which intrusion type (E,I)
Glassy & Fine are extrusive.
Medium & coarse are intrusive.
What is flow banding?
Formed by friction as the magma or lava slows down near an interface, aligning the minerals as it moves
What is conchoidal?
A fracture which results in a curved surface
What is vesicular?
Textural term for a rock containing vesicles. Vesicles were bubbles of gas which came out of solution as a result of pressure release
What is porphyritic?
A texture where large crystals, called phenocrysts, are completely surrounded by smaller crystals
What is equigranular?
When crystals in an igneous rock are approximately the same size
What is amygdaloidal?
Where there are large vesicles which have been filled with a secondary mineral
What are ophiolites?
Sections of the earth’s oceanic cryst that have been tectonically moved (obducted) onto continental crust
What are the formation and characteristics of silicic magmas?
> 66% silica
Generated by the melting of continental crust
Magma is viscous
Form at lower temperatures than other magmas (600-900°C)
What are the formation and characteristics of intermediate rocks?
52-66% silica
Most common is andesite
Less viscous than silicic, but still a little
What are the formation and characteristics of mafic magmas?
Contain pyroxene and sometimes olivine
There will be no quartz
52-45% silica
Low viscosity
Starts solidifying at around 1000°C
What are the formation and characteristics of ultramafic magmas?
<45% silica
Pyroxenes and some olivine
Extremely high melting points
Peridotites
What determines crystal grain size?
Rate of cooling. Volume of magma
What is the formation of glassy texture?
Lava extruded onto the surface. Cools too quickly - no crystals. Forms volcanic glass. Cooling occurs in hours
What is the formation of fine texture?
Hard to see with naked eye. Lava extruded onto the surface. Cooling time usually takes weeks, sometimes months. Can occur in intrusive settings, but only at chilled margins
What is the formation of medium texture?
Easily seen with naked eye. Intrusive. Form in minor intrusions. A thousand years cooling required for these rocks
What is the formation of coarse texture?
Major intrusions - batholithes and plutons. Millions of years for cooling
What is texture?
General character or appearance of a rock shown by arrangement of minerals with crystal shape and size. It does NOT refer to the roughness or smoothness of the surface
What is nucleation?
The growth of crystals from a magma building onto another piece of crystal growth
What is euhedral and how is it formed?
Crystals that are well formed with crystals faces. Form at depth slowly, grew unimpacted
What is subhedral?
Some well formed faces, some pooly formed faces
What is anhedral?
Poorly formed crystal faces
What is pegmatite?
Common plutonic rock
Coarse crystals. Interlocking crystals.
Form from waters that separate from a magma in the late stages of crystallisation. From slow cooling of a low viscosity fluid phase rich in water
What is hypabyssal?
When igneous rocks form at relatively shallow depths below the surface
What is equant?
Crystals have all axes the same length (cubey)
What is prismatic?
Crystals have four or more sides but are elongated in one direction
