Melt composition changes as a result
Fe, Mg, Ca are removed
Igneous Environments
o
Two major categories
Extrusive settin—cool at or near the surface
Cool rapidly
Chill too fast to grow big rystals
Intrusive settings—cool at depth
Lose heat slowly
Crystals often grow large
o
Extrusive rocks are mostly mafic
o
Intrusive rock are mostly felsic
Extrusive Settings
o
Lava flows cool as blankets that often stack vertically
o
Lava flows exit volcano vents and spread outward
Intrusive Settings
o
Magma invades colder wall rock, initiating:
Thermal (heat) metamorphism and melting
Inflation of fractures, wedging wall rock apart
Detachment of large wall rock blocks (stopping), and
Incorporation of wall rock fragments (xenoliths).
o
Magma that doesn’t reach the surface freezes slowly
o
Geologists categorize intrusions by shape
Tabular (sheet)—planar with uniform thickness
Blister-shaped—a sill that domes upward
Balloon-shaped—blobs of melted rock
Tabular intrusions
Have two major subdivisions
o
Sill
—injected parallels to rock layering
o
Dike
—cuts across rock layering
o
Dikes and sills:
Cause the rock to expand and inflate
Thermally alter surrounding rock
o
Both dikes and sills exhibit wide variability in:
Size
Thickness (or width).
Lateral continuity
Intrusions…
o
Dikes:
Cut across preexisting layering (bedding or foliation)
Spread rocks sideways
Dominate in extensional settings
o
Sills:
Are injected parallel to preexisting layering
Are usually intruded close to the surface
Plutonic Activity
o
Immense volumes of intrusives
o
Form above subduction zones
o
May add magma for tens of Ma.
o
Batholiths mark former subduction
Intrusive and Extrusive
o
Intrusive and extrusive rocks commonly co-occur
o
Magma chambers feed overlying volcanoes
o
Magma chambers may cool to become plutons
o
Many igneous geometries are possible
Influence on landscape
o
Continued uplift and erosion exposes the pluton
Intrusive rocks are usually more resistant to erosion
Often stand high on the landscape
Describing Igneous Rock
o
Igneous rock is used extensively as building stone
Office building
Kitchens
o
Why?
Durable (Hard)
Beautiful
o
Often called “granite”; it is not always true granite
o
The size, shape, and arrangement of the minerals
Interlocking—mineral crystals fit like jigsaw pieces
Fragmental—pieces of preexisting rocks
Glassy—made of solid glass shards
o
Texture directly reflects magma history
Crystalline Igneous Textures
o
Texture reveals cooling history
Aphanitic (fine-grained).
Rapid cooling
Crystals do not have time to grow
Extrusive
Phaneritic (course-grained)
Slow cooling
Crystals have a long time to grow
Intrusive
Crystalline Textures
o
Porphyritic texture—a mixture of coarse and fine crystals
Indicates a two-stage cooling history
Initial slow cooling
Subsequent eruption cools remaining magma more
rapidly
Fragmental Textures
o
Preexisting rocks that were shattered by eruption
o
