(4) Intrusive Rocks

Question 1

Intrusive rocks are

Answer 1

Magma

Question 2

How to recognize

Answer 2

• Compositionally the same as extrusives, but have textures expected from slower cooling
• Evidence that ‘country rock’ (rock of the land already there) has been intruded by a liquid
• Evidence that country rock may have been ‘baked’ near the contact
• Inclusions/xenoliths of country rock in the intrusive material
• ‘Chilled margin’ near contact

Question 3

Structures (Shallow)

Answer 3

• Shallow
• Cylindrical = volcanic
• Tabular = dike is discordant to features in surrounding rocks (crosscuts)
• Tabular = sill is concordant (goes along features)
• Lenticular = laccolith/lopolith

Question 4

Structures (Deep)

Answer 4

• Deep >~2km
• Irregular, large mass = Stock ~100km^2 (large area)
• Veins can be shallow or deep
• Joints= fractures/breaks

Question 5

Classification

Answer 5

• Composition (Rock Name)
  o Felsic (Granite)
  o Intermediate (Diorite)
  o Mafic (Gabbro)
  o Ultramafic (Dunite, peridotite, anorthosite)
• UM rocks are almost always plutonic because they solidify at 2000 and wont reach the surface NOW