Igneous rocks Flashcards
What are igneous rocks?
Rocks formed from the solidification and cooling of magma or lava
What are the two categories of igneous rock?
Extrusive
Intrusive
What is an intrusive igneous rock?
A rock formed by magma below the surface
- cools slowly because it is insulated by surrounding rock
- forms large crystals because they have more time to grow
What is an extrusive igneous rock?
A rock formed by lava above the surface
- cools v fast because lava comes into contact with air (cool)
- forms small crystals because rapid growth doesn’t allow time for large crystals to grow
Where would you find intrusive/extrusive igneous rock?
Pluton
Dyke
Sill
Lava flow
What is a sill?
a tabular sheet intrusion that has intruded so it is concordant to the country rock
What is a dyke?
What do we call a dyke that has split?
a sheet intrusion that has intruded so it is discordant to the country rock
- bifurcated
What is a pluton?
a cooled and solidified magma chamber that is discordant to the country rock
What is a baked margin?
the area of country rock that has been thermally altered by the heat from an igneous intrusion
- affects the COUNTRY ROCK
What is a chilled margin?
a part of the igneous intrusion that cools very quickly upon contact with the cooler country rock
- forms a fine grained, glassy texture
- affects the IGNEOUS INTRUSION
What is a lava flow?
outpouring of lava during a volcanic eruption that spreads out on the surface
What are the igneous textures and cooling history?
EQUICRYSTALLINE
- all crystals similar size
- 1 stage cooling
PORPHYRITIC
- 2 diff sizes
- 2 stage cooling
- larger grained phenocrysts in a finer grained groundmass
e.g. porphyritic basalt
VESICULAR
- bubbles
- formed when reduced pressure causes dissolved gases to exsolve out of solution
What 3 crystallinities (grain shapes) happen in igneous rocks?
Euhedral
Anhedral
Subhedral
Describe euhedral crystals
- perfectly formed, well defined crystal faces
- all faces are flat, with sharp and straight edges
How do euhedral crystals form?
- plenty of space and time to grow in underground while everything else is liquid
- phenocrysts are often euhedral
Describe subhedral crystals
- partially developed crystal faces
- some formed, some less distinct/incomplete
- some recognisable crystal faces
How do subhedral crystals form?
- forms in limited space so growth in restricted and crystals may obstruct each other
Describe anhedral crystals
- no visible or poorly developed crystal faces
- irregular shape/structure and lack of geometric structure (mineral-like)
- rounded/irregular shapes, no straight edges or planar faces
How do anhedral crystals form?
- forms in crowded environments, space is limited
- crystals are forced to grow into gaps between other minerals
- packed tightly together, lots of obstruction
Why is crystallinity (grain shape) important to geologists?
infers the conditions under which the rocks formed
What are the igneous structures?
Pahoehoe lava
Aa lava
Columnar joining
Pillow structures
Xenoliths
Cooling joints
What are the crystal sizes?
COARSE: over 3mm
MEDIUM: 1-3mm
FINE: under 1mm
