Igneous Environments

Intrusive

Igneous rocks that cool beneath surface are refered to as intrusive (plutonic) rocks.. These rocks lose heat slowly = large crystals. Most felsic magmas do not reach the surface. Intrusive and extrusive rocks commonly occur together.

Intrusive Characteristics

Magma invading colder country rock initiates…

• Thermal metamorphism and melting.
• Inflation of fractures pushing the rock aside.
• Incorporation of country rock fragments (xenoliths).
• Hydrothermal (hot water) alteration.

Intrusive contacts preserve evidence of high heat.

• Baked zone – Rim of heat altered country rock.
• Chill margin – Magma at contact that cooled rapidly.
• Xenolith - Country rock  fragment in magma.

Types of igneous intrusive features

• Plutons - Any body of intrusive igneous rock, regardless of size
  • Discordant – cuts across existing structures
  • Concordant – parallel to features such as sedimentary strata
• Batholiths - Largest intrusive body. Often occur in groups; surface exposure 100+ km² and frequently form the cores of mountains (like the San Gabriels to the north).
• Laccolith - Lens shaped mass that often crches overlying strata upward
• Sills - Horizontal body that forms when magma enters horizontal cracks or pushes aside bedding. SIlls often lift entire landscapes skyward. Usually intruded near the surface.
• Dikes - Forms in vertical cracks in the host rock, and spread rocks sideways. Common in extensional settings

 

Cooling Rates

The rate at which magma cools is dependant upon many conditions.

• Depth
  • Deep plutons cool slowly.
  • Shallow flows cool rapidly.
• Shape - Surface/volume ratio
  • Spherical bodies cool slowly.
  • Tabular bodies cool faster.
• Ground water - Ground water removes heat.

Visit https://courses.nus.edu.sg/course/geosnk/internet/animations/resources/anim/IgneousFeaturesLandforms_GG.html to see various igneous landscapes.