Intrusive Igneous Rocks/Activity

  1. Intrusive igneous rock
  2. Form from magma that solidifies underground.
  3. Rate of cooling determines crystal size
  4. slow cooling = larger crystals
  5. rapid cooling = smaller crystals
  6. Crystal size or texture classifies igneous rocks
  7. Evidence that suggests that intrusive rocks solidify underground from magma
  8. Mineralogically and chemically = to volcanic rocks (Rhyolite and Granite).
  9. Volcanic rocks small grained/intrusive are larger grained.
  10. Experiments have shown that most minerals in intrusive rocks require high temp. and high pressure, indicating deep burial.
  11. Preexisting solid rock (country rock) appears to have been forcibly broken by an intruding liquid. Country rock = any older rock into which an igneous body intruded.
  12. Country rock appears “baked” or metamorphosed close to contact point.
  13. Rock types of the country rock often match xenoliths – fragments of rock that are distinct from the body of igneous rock, in which they are enclosed.
  14. Intrusive rocks next to country rock have chill zones – finer – grained rocks indicating rapid cooling because of loss of heat to surrounding rock.
  15. Intrusive Igneous Activity
  16. Magma emplaced at depth
  17. Underground igneous body is called a pluton.
  18. Plutons are classified according to
  19. form at depth or shallow
  20. particular shape (tabular of massive)
  21. does it follow layering of country rock or not (discordant, concordant)
  22. is body large or small
  23. Shallow intrusive structures

1. Form at depth of probably <2km

2. These form the “plumbing systems” of volcanoes.

3. These tend to be small (compared to those at depth).

4. Tend to chill quickly because they are near the surface. This makes them fine grained.

  1. Dike – tabular or sheet-like discordant intrusive structure (deep or shallow). Injected into fractures that cut across rock layers.

Discordant = the body is not parallel to any layering of the country rock. Range from <1cm to >1km. Horizontal or vertical depending on strata.

  1. Sill – tabular intrusive structure that is

concordant or parallel to any planes or layering in the country rock.

  1. Formed when magma is injected along sedimentary bedding surfaces.
  2. Occur shallower because they must lift overlying layers.
  3. Ex. Palisades Sill New York City
  4. Sill resembles buried lava flows (horizontal to layers).

Evidence that a sill is not a lava flow

  1. lava flows contain void spaces of escaped gas.
  2. Only rock below lava flow shows metamorphic alteration.
  3. Sills will contain fragments of overlying alteration rock.
  4. ‘Baked’ zones both above and below sills.
  1. Laccolith
  2. Forms the same way as a sill
  3. Forms when more viscous magmas, collect as a lens shaped mass that arches overlying strata upward (detected by the dome it creates). Opposite to laccolith is a lopolith.
  1. Large intrusive igneous bodies(plutons)

Large, discordant and form deeper.

Classified according to the area of rock exposed at the surface.

  1. Stock
  2. Small discordant pluton with an outcrop area (the area exposed) of less than 100 sq. km.
  3. Batholith
  4. a massive discordant pluton almost always composed of granite.
  5. greater than 100 square miles of it are exposed to the atmosphere.
  6. forms from numerous coalesced plutons called diapers.
  7. irregular roof
  1. How magmas of different composition form
  2. Differentiation and Bowen’s Reaction series.
  3. The process by which different ingredients separate from an originally homogenous mixture.
  4. Shows that those minerals with the highest melting temperatures crystallize from the cooling magma before those with lower melting points. This is called

Crystal settling = the downward movement of minerals that are denser than the magmas from which they crystallized.

  1. Partial Melting – produces most magmas.
  2. When rock is heated those minerals with the lowest melting temperatures melt first (quartz and potassium feldspars).
  3. These melted minerals can separate from rock and collect into pockets and form diapers.
  4. This can explain how a magma high in silica can form from a mafic parent rock.
  5. The lower part of the continental crust is a plausible source for felsic magma by partial melting.
  6. Assimilation – process called stoping where xenoliths are broken off and incorporated into the magma changing its composition (forms intermediate magma).
  7. Mixing of magma – two magmas might meet or merge within the crust to combine to form intermediate magma (convergent boundaries).
  1. Explaining igneous activity by plate tectonics
  2. Generation of Basaltic Magma
  3. Believed to be generated at divergent boundaries by partial melting of the athenosphere, which is mainly composed of the ultramafic rock peridotite (olivine,pyroxene, and plagioclase).
  4. Process of basaltic magma goes as follows
  5. the athenosphere moves up and out – the crust splits causing a reduction in pressure and causes melting.
  6. if only part of the mantle melts and most of the olivine is left behind, you have a basaltic magma.
  7. Generation of Granitic Magma
  8. Believed to occur at convergent boundaries where there is partial melting of the lower crust.
  9. Continental crust contains the high amount of silica needed for a silica rich magma.
  10. The partial melting of a subducting oceanic plate supplies heat to the lower crust. As the plate descends it begins to melt. As the mafic material melts it rises and is trapped underneath the crustal plate causing the felsic crustal plate to melt- called magmatic underplating. The melted silica rich magmas work their way upward in diapirs to higher levels and solidify as a pluton (batholith).
  11. Classification of Igneous Rocks
  12. Magma that solidifies and forms at depth is called intrusive or plutonic igneous rock.
  13. Coarse – grained (slow cooling)

a. grains > than 1 mm.

  1. porphyritic – two stage cooling process. Large crystals called phenocrysts are embedded in a matrix of fine- grained crystals.
  1. Those that cool and crystallize above ground called extrusive or volcanic rocks
  2. fine grained
  3. no grains = glass
  4. Mineralogically (chemically) equivalent rocks

Intrusive Extrusive

Granite ______Rhyolite

Diorite ______Andesite

Gabbro ______Basalt